AN EXAMINATION OF 

MERCURY LEVELS AT CLEAN 

WATER AGENCIES

2003-2006 

NATIONAL ASSOCIATION OF CLEAN WATER AGENCIES

JANUARY 2008 


2 

ACKNOWLEDGMENTS

The staff of all of the participating agencies, who performed sampling, supplied data, answered 

questions, and provided editing. 

The Hampton Roads Sanitation District Central Environmental Laboratory 

Patty Lee, Hampton Roads Sanitation District 

Danny Barker, Hampton Roads Sanitation District 

Anna Rule, Hampton Roads Sanitation District 

Steve Anderson, Clean Water Services 

Elizabeth Toot-Levy, Northeast Ohio Regional Sewer District 

Seth Hothem, Northeast Ohio Regional Sewer District 

Keith Linn, Northeast Ohio Regional Sewer District 

Christopher Hornback, National Association of Clean Water Agencies 

The NACWA Mercury Workgroup Sampling Project Subgroup 

The American Dental Association (for draft report review) 
3 

An Examination of Mercury Levels at Clean Water Agencies: 2003-2006

ACKNOWLEDGMENTS..................................................................................................................... 2 

LIST OF FIGURES ............................................................................................................................ 4 

EXECUTIVE SUMMARY ................................................................................................................... 6 

1. INTRODUCTION ........................................................................................................................... 9 

2. METHODS.................................................................................................................................. 10 

2.1 Sampling Methods ............................................................................................................ 11 

2.2 Analytical Methods........................................................................................................... 12 

2.3 Statistical Methods............................................................................................................ 13 

3. INDIVIDUAL POTW RESULTS AND DISCUSSION...................................................................... 14 

3.1 POTW “A”......................................................................................................................... 14 

3.2 POTW “B” ......................................................................................................................... 19 

3.3 POTW “C” ........................................................................................................................ 23 

3.4 POTWs “D” and E”.......................................................................................................... 26 

3.5 POTW “F”......................................................................................................................... 31 

3.6 POTW “G” ........................................................................................................................ 34 

3.7 POTW “H” ........................................................................................................................ 38 

3.8 POTW “I” .......................................................................................................................... 41 

3.9 POTWS “J” AND “K” ........................................................................................................ 44 

3.10 POTW “L” ....................................................................................................................... 50 

4. COMBINED RESULTS AND DISCUSSION.................................................................................... 53 

4.1 INFLUENT, EFFLUENT, AND BIOSOLIDS ............................................................................. 53 

4.2 REMOVAL EFFICIENCY ...................................................................................................... 56 

4.3 DISSOLVED MERCURY........................................................................................................ 57 

4.4 TOTAL SUSPENDED SOLIDS................................................................................................ 59 

4.5 DENTISTS PER FLOW .......................................................................................................... 60 

4.6 DENTIST WITHOUT SEPARATORS PER FLOW.................................................................... 63 

4.7 PLANT FLOW ...................................................................................................................... 65 

4.8 NON-INDUSTRIAL FLOW .................................................................................................... 67 

4.9 FRESHWATER VS. MARINE DISCHARGERS ........................................................................ 71 

4.10 IRON SALTS....................................................................................................................... 73 

4.11 HAULED-IN WASTES......................................................................................................... 75 

4.12 POPULATION DENSITY ..................................................................................................... 77 

4.13 COMBINED OR SEPARATE COLLECTION SYSTEMS.......................................................... 79 

5. SUMMARY AND CONCLUSIONS ................................................................................................. 81 

6. IMPLICATIONS FOR FURTHER RESEARCH ............................................................................... 83 

7. APPENDICES.............................................................................................................................. 86 

7.1 APPENDIX A: DATA ............................................................................................................. 86 

7.2 APPENDIX B: SUMMARY OF STATISTICAL ANALYSES FOR COMBINED DATA................ 114 

7.3 APPENDIX C: STATISTICAL ANALYSIS ............................................................................. 117 

7.4 APPENDIX D: STUDY PLAN............................................................................................... 360 
4 

LIST OF FIGURES

Figure 3.1A: Effluent and Biosolids Mercury Concentrations at POTW “A” ............................. 17 

Figure 3.1B: Effluent Mercury and TSS Concentrations at POTW “A”...................................... 18 

Figure 3.1C: Influent and Effluent Mercury Load at POTW “A” ................................................ 18 

Figure 3.2A: Effluent and Biosolids Mercury Concentrations at POTW “B”.............................. 21 

Figure 3.2B: Effluent Mercury and TSS Concentrations at POTW “B” ...................................... 22 

Figure 3.2C: Influent and Effluent Mercury Load at POTW “B” ................................................ 22 

Figure 3.3A: Effluent and Biosolids Mercury Concentrations at POTW “C”.............................. 25 

Figure 3.3B: Effluent Mercury and TSS Concentrations at POTW “C” ...................................... 26 

Figure 3.3C: Influent and Effluent Mercury Load at POTW “C” ................................................ 26 

Figure 3.4A: Effluent and Biosolids Mercury Concentrations at POTW “D” ............................. 28 

Figure 3.4B: Effluent Mercury and TSS Concentrations at POTW “D”...................................... 29 

Figure 3.4C: Influent and Effluent Mercury Load at POTW “D” ................................................ 29 

Figure 3.4D: Effluent and Biosolids Mercury Concentrations at POTW “E”.............................. 30 

Figure 3.4E: Effluent Mercury and TSS Concentrations at POTW “E”....................................... 30 

Figure 3.4F: Influent and Effluent Mercury Load at POTW “E”................................................. 31 

Figure 3.5A: Effluent and Biosolids Mercury Concentrations at POTW “F” .............................. 33 

Figure 3.5B: Effluent Mercury and TSS Concentrations at POTW “F”....................................... 34 

Figure 3.5C: Influent and Effluent Mercury Load at POTW “F”................................................. 34 

Figure 3.6A: Effluent and Biosolids Mercury Concentrations at POTW “G” ............................. 36 

Figure 3.6B: Effluent Mercury and TSS Concentrations at POTW “G”...................................... 37 

Figure 3.6C: Influent and Effluent Mercury Load at POTW “G” ................................................ 37 

Figure 3.7A: Effluent and Biosolids Mercury Concentrations at POTW “H” ............................. 40 

Figure 3.7B: Effluent Mercury and TSS Concentrations at POTW “H”...................................... 40 

Figure 3.7C: Influent and Effluent Mercury Load at POTW “H” ................................................ 41 

Figure 3.8A: Effluent and Biosolids Mercury Concentrations at POTW “I”............................... 43 

Figure 3.8B: Effluent Mercury and TSS Concentrations at POTW “I” ....................................... 44 

Figure 3.8C: Influent and Effluent Mercury Load at POTW “I”.................................................. 44 

Figure 3.9A: Effluent and Biosolids Mercury Concentrations at POTW “J”............................... 47 

Figure 3.9B: Effluent Mercury and TSS Concentrations at POTW “J” ....................................... 48 

Figure 3.9C: Influent and Effluent Mercury Load at POTW “J” ................................................. 48 

Figure 3.9D: Effluent and Biosolids Mercury Concentrations at POTW “K” ............................. 48 

Figure 3.9E: Effluent Mercury and TSS Concentrations at POTW “K” ...................................... 49 

Figure 3.9F: Influent and Effluent Mercury Load at POTW “K"................................................. 50 

Figure 3.10A: Effluent and Biosolids Mercury Concentrations at POTW “L”............................ 51 

Figure 3.10B: Effluent Mercury and TSS Concentrations at POTW “L” .................................... 52 

Figure 3.10C: Influent and Effluent Mercury Load at POTW “L”............................................... 53 

Figure 4.1A: Influent Mercury Concentrations ........................................................................... 53 

Figure 4.1B: Influent Mercury Concentrations and Sample Collection Location....................... 54 

Figure 4.1C: Effluent Mercury Concentrations ........................................................................... 55 

Figure 4.1D: Biosolids Mercury Concentrations......................................................................... 56 

Figure 4.2A: Mercury Removal Efficiency .................................................................................. 57 

Figure 4.3A: Median Percent of Effluent Total Mercury that is Dissolved.................................. 58 

Figure 4.3B: Presence of Recycle flows and the Median Percent of Effluent Total Mercury that is 

Dissolved................................................................................................................................... 59 

Figure 4.4A Effluent Mercury Concentrations versus Effluent TSS Concentrations................... 60 
5 

Figure 4.5A. Influent Mercury Concentration and Dentists per Flow......................................... 61 

Figure 4.5B. Effluent Mercury Concentration and Dentists per Flow........................................ 62 

Figure 4.5C: Biosolids Mercury Concentration and Dentists per Flow....................................... 63 

Figure 4.6A: Influent Mercury Concentration and Dentists without Separators per Flow.......... 64 

Figure 4.6B. Effluent Mercury Concentration and Dentists without Separators per Flow.......... 64 

Figure 4.6C: Biosolids Mercury Concentration and Dentists without Separators per Flow ....... 65 

Figure 4.7A: Influent Mercury Concentration and Average Plant Flow ..................................... 66 

Figure 4.7B: Biosolids Mercury Concentration and Average Plant Flow................................... 66 

Figure 4.7C: Effluent Mercury Concentration and Average Plant Flow..................................... 67 

Figure 4.8A: Influent Mercury Concentration and Non-Industrial Flow .................................... 68 

Figure 4.8B: EffluentMercury Concentration and Non-Industrial Flow ..................................... 69 

Figure 4.8C: Biosolids Mercury Concentration and Non-Industrial Flow .................................. 69 

Figure 4.8D: Influent Mercury Concentration and Non-Industrial Flow in Separate Sewer 

Systems ..................................................................................................................................... 70 

Figure 4.8E: Effluent Mercury Concentration and Non-Industrial Flow in Separate Sewer 

Systems ..................................................................................................................................... 71 

Figure 4.8F: Biosolids Mercury Concentration and Non-Industrial Flow in Separate Sewer 

Systems ..................................................................................................................................... 71 

Figure 4.9A: Influent Mercury Concentration and Receiving Water Type................................. 72 

Figure 4.9B: Effluent Mercury Concentration and Receiving Water Type................................. 72 

Figure 4.9C: Biosolids Mercury Concentration and Receiving Water Type............................... 73 

Figure 4.10A: Effluent Mercury Concentration and Use of Iron Salts........................................ 74 

Figure 4.10B: Biosolids Mercury Concentration and Use of Iron Salts...................................... 74 

Figure 4.11A: Influent Mercury Concentrations and Hauled-in Waste....................................... 75 

Figure 4.11B: Effluent Mercury Concentrations and Hauled-in Waste ...................................... 76 

Figure 4.11C: Biosolids Mercury Concentrations and Hauled-in Waste .................................... 76 

Figure 4.12A: Influent Mercury Concentration and Population Density .................................... 77 

Figure 4.12B: Effluent Mercury Concentration and Population Density .................................... 78 

Figure 4.12C: Biosolids Mercury Concentration and Population Density .................................. 78 

Figure 4.13A: Influent Mercury Concentrations and Sewer System Type.................................. 79 

Figure 4.13B: Effluent Mercury Concentrations and Sewer System Type .................................. 80 

Figure 4.13C: Biosolids Mercury Concentrations and Sewer System Type ............................... 80 
6 

EXECUTIVE SUMMARY

The degree to which mercury concentrations at publicly-owned treatment works (POTWs) can 

be reduced by controlling potentially significant sources, such as dental clinics, is not well 

known. Therefore, the National Association of Clean Water Agencies (NACWA) funded a study 

to examine the effect of practical and reasonable measures of controlling mercury discharged to 

the POTW collection system, such as dental amalgam separator installation, on the ability of 

POTWs to reduce influent, effluent, and biosolids mercury levels. 

This study provides insight into the effect of amalgam separator installation and other factors on 

mercury concentrations at POTWs, but it does not, and was not intended to, address all the 

uncertainty surrounding such factors. Although the data collected during the course of the 

sampling project provide useful information, this report’s evaluations of the collected data 

illustrate the limitations of the study and, in many cases, highlight the need for further research. 

A considerable limitation is the inability to identify and account for all the variation that 

necessarily exists among POTWs. POTWs are designed to accomplish the same basic goals, but 

they achieve these goals differently, making some interfacility comparisons difficult. Among the 

potential sources of uncertainty are temporal variations in influent mercury concentrations, 

differences in sampling techniques, inconsistent methods of counting dentists and dental 

facilities, and varying chemical use by POTWs. It is these variables and others that lead to the 

implications for further research presented later in the report. Such additional research may 

allow a better understanding of the factors affecting mercury levels at POTWs, including the 

installation of amalgam separators. 

The influent, effluent, and biosolids mercury concentrations from twelve POTWs in the United 

States and Canada, in various stages of requiring dental facilities to install amalgam separators, 

were measured on a monthly basis from July 2003 through July 2006 to evaluate the impact of 

separator installation and other potential factors. Information on other source control efforts and 

operational activities at the sampled POTWs was also collected in an effort to shed light on 

numerous issues related to controlling mercury releases by POTWs to the environment. 

The influent mercury concentrations at the POTWs in the study were found to be highly variable 

within each facility as well as among facilities. Consistent with this variation, when considering 

facilities individually, some exhibited decreases in their influent mercury concentrations over the 

course of the sampling project, while others had apparent increases. Thus, a great deal of 

uncertainty surrounds the influent sampling data, and it may not be prudent to draw conclusions 

based solely on the limited influent data. Effluent mercury concentrations also exhibited 

variability between plants but less variability within plants. For the biosolids, there was some 

variability both within and among the plants. However, neither the effluent nor the biosolids 

mercury data were nearly as variable as the influent mercury data. 

The mercury removal efficiency from plant influent to plant effluent was generally greater than 

ninety percent. While the POTWs were similar in this aspect, in other ways they were not. For 

instance, the percentage of effluent mercury that was dissolved was found to vary based on the 

POTW. Additionally, at eight of the POTWs, there appeared to be a direct relationship between 
7 

effluent mercury concentration and effluent total suspended solids concentration while, at the 

remaining five POTWs, this relationship did not appear to exist. 

The study results strongly suggest that the density of dentists in a POTW’s collection system is 

positively correlated with mercury concentrations at the wastewater treatment plant. A higher 

number of dentists per flow was related to higher influent and effluent mercury concentrations. 

There was no relationship, however, between numbers of dentists per flow and biosolids mercury 

concentrations. This may be due to amalgam separators being installed by dentists in many of 

the sampled POTWs’ service areas. Importantly, though, it was found that, collectively, higher 

numbers of dentists without amalgam separators per flow were related to higher biosolids 

mercury levels. This relationship was highly significant statistically and among the most 

significant relationships observed in the study. 

Conversely, the effluent mercury concentration was found to be negatively correlated to the 

number of dentists without separators per flow, indicating that increasing amalgam separator 

installations does not necessarily lead to a decrease in effluent mercury concentrations. This 

result could be due, in part, to the limited number of amalgam separator installations occurring at 

the POTWs with generally lower effluent mercury concentrations at the study’s outset. 

For individual plants, the installation of separators did not always lead to a decrease in biosolids 

mercury. It may be that amalgam separator installation alone is not enough to result in 

permanent mercury reductions. Some of the plants that did show decreases in mercury have 

programs in place to ensure that the separators are well-operated and maintained, and this may be 

a factor. It is also thought that mercury reductions following separator installations can take 

several years or even longer to demonstrate significant trends. Therefore, this study may not 

have been long enough to determine significant trends for some POTWs whose dental facilities 

were in the process of installing amalgam separators during the course of the study. It is further 

possible that, on an individual-facility basis, apparent reductions or the lack thereof could be the 

result of other impacting factors such as previously elevated loadings related to sewer cleaning, 

construction activities, or historic depositions of mercury in the collection system. 

Some of the treatment plants exhibited mercury reductions even without an increase in amalgam 

separator installation. These reductions may have resulted from changes in treatment processes 

or the implementation of best management practices at dental facilities. Therefore, on a facilityspecific basis, the effectiveness of amalgam separators in reducing mercury should be examined 

in light of other activities that are occurring within the POTW and its service area to determine 

which factors are actually contributing to an observed reduction. 

In addition to the presence of dentists within a POTW’s collection area, several other factors 

were found to exhibit relationships with mercury levels at treatment plants. These included the 

discharge of effluent to marine waters and sewer cleaning projects occurring within the POTW’s 

collection system, which were each related to higher effluent mercury levels, and higher 

industrial flow and the use of iron salts, which were each related to lower effluent mercury 

levels. 
8 

Factors that were examined but not found to be related to mercury levels were higher plant flow, 

population density, whether the POTW accepts hauled-in wastes, and whether the collection 

system is comprised of combined or separate sewers. 

Overall, the results of this study indicate that many factors, not just amalgam separator 

installation, influence mercury concentrations. While separators have been shown to reduce 

mercury loadings at dental offices, it should be recognized that the installation and use of 

separators may not result in demonstrable and immediate reductions at the receiving treatment 

facility due, at least in part, to the legacy issues from historic mercury contributions. Therefore, 

a facility cannot predict with certainty that amalgam separators will decrease mercury 

concentrations without also exploring the other potential contributors to current mercury levels. 

Additionally, it should be noted that, even at those POTWs with demonstrably successful 

amalgam separator programs, effluent mercury concentrations were not low enough to 

consistently meet current and imminent effluent limits faced by some POTWs (e.g., 1.3 

nanograms per liter in the Great Lakes). 

According to the results, separator installation does not generally appear, at least within the 

timeframe of this study, to significantly reduce effluent mercury concentrations. However, 

amalgam separator installation does generally appear to result in reductions in biosolids mercury 

concentrations. Therefore, despite the variability and uncertainties described in this report, the 

data strongly support a conclusion that the use of separators can decrease the amount of 

particulate mercury entering POTWs, thereby decreasing the amount of mercury that would be 

removed by plant processes and deposited in the biosolids. 
9 

1. INTRODUCTION

The National Association of Clean Water Agencies (NACWA) represents the interests of more 

than three hundred of the nation’s publicly owned treatment works (POTWs) and other similar 

organizations. NACWA is recognized as a national leader in environmental policy and as a 

technical resource for many water quality issues. The organization has been crucial in the 

development of environmental legislation and has worked with Federal regulatory agencies in 

implementing environmental programs since the early 1970s. NACWA is currently involved 

with all aspects of water quality protection from point and nonpoint source control to the 

protection of endangered species and is a key stakeholder in both the legislative and regulatory 

arenas. 

Mercury has been identified as one of NACWA’s priorities. Therefore, the NACWA Mercury 

Workgroup began identifying information gaps regarding both the short-term and long-term 

abilities of POTWs to comply with anticipated mercury limits after implementing practical and 

reasonable measures to control sources of mercury at the treatment facilities and within the 

collection systems. This led to the realization that the relationship between mercury 

contributions to sewer systems and mercury concentrations in POTW effluents is not well 

understood. The degree to which influent, effluent and biosolids mercury concentrations can be 

reduced by controlling potentially significant sources, such as dental clinics, is unknown. 

Therefore, NACWA has funded an attempt to better understand the impact of controlling 

mercury discharged from dental clinics on the ability of POTWs to meet stringent effluent 

limitations and biosolids requirements. 

This study presents an opportunity to build a database depicting baseline mercury levels and 

trends that may occur as mercury control programs at POTWs progress. Due to their use of 

dental amalgam, both in the placement and removal of dental fillings containing mercury 

amalgam, dental facilities are significant sources of mercury to wastewater treatment plants. 

Amalgam separators are treatment devices that can be installed in the plumbing of a dental clinic 

to remove some quantity of dental amalgam from the wastestream tributary to a POTW. 

Amalgam separators remove more amalgam from the wastestream than the typical chairside 

traps and vacuum filters found in a dental office. 

This study was designed to examine the effect of practical and reasonable measures of 

controlling significant sources of mercury to the POTW collection system on the ability of 

POTWs to comply with anticipated effluent limitations and address concerns regarding mercury 

in plant influent and biosolids. In an attempt to quantify the amount of dental amalgam removed 

from the wastestream by dental amalgam separators, the influent, effluent, and biosolids mercury 

concentrations, as well as other parameters, from POTWs in the United States and Canada 

already having implemented regulations requiring dental clinics to install amalgam removal 

equipment, were evaluated on a monthly basis from July of 2003 through July of 2006. POTWs 

that do not have these regulatory requirements were also evaluated. 
10 

Twelve POTWs were each sampled at least one time per month for the duration of the study. 

The sampled POTWs were in various stages of requiring dental facilities to install amalgam 

separators to capture particulate amalgam waste. Table 1.1 shows the percentage of dental 

facilities/dentists with operating dental amalgam separators, i.e., amalgam separator units (ASU), 

for each POTW throughout the study. Samples from each POTW were analyzed for influent, 

effluent, and biosolids total mercury concentrations as well as influent and effluent total 

suspended solids concentrations. POTWs were also asked to monitor effluent turbidity and to 

collect a limited amount of effluent dissolved mercury concentration data. As some facilities 

joined the project partway through the course of study, not every POTW was able to provide 

these additional data. 

Table 1.1: Percentage of Dental Facilities/Dentists Operating Dental Amalgam Separators 

July 1, 2003 January 1, 2004 July 1, 2004 January 1, 2005 July 1, 2005 January 1, 2006 July 1, 2006

POTW "A" 0 0 0 98 100 100 100

POTW "B" 5678999

POTW "C" 6666666

POTW "D" 0.6 1.5 6.1 16 26 32 38

POTW "E" 0.6 1.5 6.1 16 26 32 38

POTW "F" 0 0 0 0 2 12 20

POTW "G" 44 86 95 98 100 100 100

POTW "H"* unknown 60 89 95 95 98 98

POTW "I" 93 93 93 93 93 93 93

POTW "J" 94 97 97 97 97 97 98

POTW "K" 94 97 97 97 97 97 98

POTW "L" unknown unknown unknown unknown unknown unknown 11

* The percentages for POTW "H" also include other approved control technologies.

Recognizing that such data can also be impacted by other source control efforts and operational 

activities at POTWs, NACWA collected key information about the agencies and communities 

participating in the study. This information was utilized in an effort to shed light on numerous 

issues related to controlling mercury discharges to the environment. It is intended that the 

findings of this report can be used to assist agencies in making informed management decisions 

about controlling mercury discharges to POTWs and the environment. 

 

2. METHODS

One of the most critical aspects of this study was the collection of data. It was necessary for the 

data to be sensitive enough to determine with confidence whether effluent concentrations can be 

reduced to very low levels. The sampling and analytical methods historically used for the 

quantification of mercury were inadequate for the measurement of mercury concentrations at the 

low levels typically found in wastewater treatment plant effluents. Therefore, the USEPA 

developed a more sensitive analytical method, Method 1631, for analysis of mercury 

concentrations at low levels. When analyzing for mercury at such low concentrations, there is 

significant potential for contamination of the samples. USEPA also provides guidance in 

Method 1669, clean sampling techniques for collecting samples to be analyzed with low-level 

methods. For the reliability of effluent data produced by this study, it was necessary to use these 

clean sampling techniques and low-level analytical methods. In most cases, the laboratory 

typically utilized by the POTW was not capable of analyzing for mercury at this low level. 

Therefore, a NACWA member laboratory was enlisted to perform the low-level mercury 
11 

analysis for the study, when needed. The following parameters were analyzed for and reported 

from each POTW: 

• Influent/Effluent Flow, million gallons per day (MGD); 

• Influent Total Recoverable Mercury, nanograms per liter (ng/L); 

• Influent Dissolved Mercury, ng/L; 

• Effluent Total Recoverable Mercury, ng/L; 

• Effluent Dissolved Mercury, ng/L; 

• Biosolids Mercury, mg/kg (dry weight); 

• Influent Total Suspended Solids (TSS), mg/L; 

• Effluent Turbidity, NTU; and 

• Effluent TSS, mg/L. 

2.1 Sampling Methods

Sample points were chosen to be representative of the influent and/or effluent. Samples were 

collected by POTW personnel at each plant during conditions that were most representative of 

normal operating conditions. Each facility was to identify normal operating conditions based on 

a 95% confidence interval of the previous two years of effluent flow, total suspended solids, and 

turbidity. It was vital to the integrity of the study that samples collected were representative of 

the process stream being sampled. Therefore, sampling was not to occur during the following 

conditions: 

• Atypical plant flow; 

• Unusual loss of solids in the effluent; 

• Plant upset conditions; 

• Receiving an unusual industrial discharge; 

• Significant diversion of flow; or 

• Atypical applications of chemical treatments. 

Because of differences in operating procedures between treatment facilities, single grab, multiple 

grab, or composite samples could be collected throughout the study. Once the decision was 

made on sampling protocol, the facility’s continuing to use the same protocol for the duration of 

the study was preferred. Composite samples for total recoverable and dissolved mercury were 

collected from the influent at POTWs A, D, E, F, G, and H; the remaining facilities collected 

influent grab samples. Composite samples for total recoverable and dissolved mercury were 

collected from the effluent at POTWs A, G, H, and L; the remaining facilities collected effluent 

grab samples. Samples were collected utilizing U.S. EPA Method 1669 and other clean sampling 

protocols to minimize potential contamination introduced during sampling and, where the 

NACWA member laboratory providing analytical services was utilized, the samples were 

shipped on ice overnight. A study plan and scope of work, which includes sampling protocols, 

was developed by this laboratory to be used by participating POTWs, as needed. This study plan 

is included as Appendix D. 

To reduce the potential for contamination, sampling protocols and equipment were specifically 

designed for each individual sample location. To reduce variability, the NACWA member 
12 

laboratory providing analytical services offered to provide necessary sampling equipment to the 

POTWs requiring outside laboratory services. In order to reduce contamination associated with 

grab sample collection, handling, and analysis, the “clean hands / dirty hands” technique was 

used. The individual designated as “dirty” hands provided support, and only the person 

designated “clean hands” could touch the sample bottles and anything that may have come into 

contact with the bottle. 

2.2 Analytical Methods

Influent and effluent samples were analyzed for total recoverable mercury, dissolved mercury, 

and total suspended solids by either the NACWA member laboratory providing analytical 

services or the laboratory typically used by the POTW. All effluent mercury analysis, both total 

and dissolved, was done according to U.S. EPA Method 1631, Revision E. This method was 

necessary to detect the extremely low concentrations of mercury often present in treated effluent. 

Method 1631, Revision E is for the determination of mercury in filtered and unfiltered water by 

oxidation, purge and trap, desorption, and cold-vapor atomic fluorescence spectrometry. For 

dissolved mercury analysis, the method specifies that samples are to be filtered through a 0.45μm 

capsule filter. Mercury analysis on influent samples was performed using the least sensitive 

method that resulted in a quantifiable result. Table 2.2A indicates the analytical methods and the 

detection limits most commonly used for the aqueous and solid samples. 

 

Table 2.2A: Analytical Methods 

PARAMETER REFERENCE METHOD METHOD DETECTION LIMIT

Hg (total recoverable 

and dissolved) 

EPA Method 1631, Rev. E 0.3 ng/L 

Hg (total recoverable 

and dissolved) 

EPA Method 245.7 0.7 ng/L 

Total Recoverable 

Hg 

EPA Method 245.2 30 ng/L 

Total Recoverable 

Hg 

EPA Method 245.5 or SW846 7441A 0.06 mg/L (TS values used 

for conversion to mg/Kg) 

TSS SM 2540D 1 mg/L 

Throughout the analytical process, clean protocols were adhered to at all times for the effluent 

mercury samples. All sample preservation, preparation, and analyses were performed in special 

clean areas. Appropriate measures were taken to limit potential contamination of samples and 

labware. All of the data from each POTW in the study are located in Appendix A. 

A total of 2,283 mercury analysis results were submitted for the study: 716 for influent total 

mercury; 120 for influent dissolved mercury; 458 for effluent total mercury; 151 for effluent 

dissolved mercury; and 838 for biosolids mercury. In each case, the entities assuming 

responsibility for sample collection and analysis were relied upon to comply with quality 

assurance/quality control (QA/QC) protocols specified in the study plan that was prepared at the 

study's outset by the NACWA member laboratory that provided analytical services (Appendix 

D) and/or specified in the respective method documents for the particular analytical methods 

utilized and listed in Appendix A. Data reported as not compliant with the applicable QA/QC 
13 

protocols were excluded with the sole exceptions of two (of the 458) effluent total mercury data 

points which were reported as lacking associated field blank data but which exhibited 

concentrations fully consistent with the relevant comparable data. 

2.3 Statistical Methods

To determine the strength of associations between pairs of variables measured at individual 

POTWs during the study, the non-parametric, Mann-Kendall test for trend was selected. Many 

of the distributions from the NACWA Mercury Sampling Project were skewed to the right (i.e., 

at higher mercury concentrations, there were fewer occurrences and outliers were very common. 

This indicates these variables were not derived from normal distributions. In addition, 

examination of plots of pairs of variables indicated that the relationships between many of them 

were not linear. Therefore the assumptions of parametric statistical tests (such as Pearson’s r) 

were often not met. This led to a choice between methods: the variables could be transformed to 

meet the assumption of the parametric tests or nonparametric tests, such as the Mann-Kendall 

test, could be used instead. The Mann-Kendall test, which uses Kendall’s τ (tau) as the test 

statistic, is a rank-based procedure that is resistant to the effect of outliers in the dataset. This test 

measures the strength of the monotonic relationship between two variables; therefore the 

relationship could be curved or linear and still be detected by this test. The use of the MannKendall test is well-suited to data from skewed distributions, such as the lognormal distributions 

often used to model water-quality variables. It was decided that use of nonparametric tests 

would be preferable to the extensive checking of assumptions and transformation of variables 

that would be required by use of the parametric techniques. A summary of the results of the 

Mann-Kendall tests for the combined data is located in Appendix B; more detailed, computergenerated results for all statistical analyses are located in Appendix C. Appendix C is provided 

for readers who are familiar with the software employed and are interested in further 

examination of the data. 

For the present analyses, α was set at 0.05. That is, the null hypothesis of no correlation (τ = 0) 

was rejected when the test resulted in a p-value that was ≤ 0.05. In such cases, the alternate 

hypothesis that the two variables are correlated (τ ≠ 0) was accepted. Using a value of 0.05 

means that there is a 5% probability that the null hypothesis will be rejected when it is actually 

true. There were a few instances in which relationships were not significant at the 0.05 level, but 

were so at the 0.10 level. Even though there is a slightly greater probability that the null 

hypothesis will be falsely rejected at this level, these instances are included because it is thought 

that they still identify meaningful relationships. 

Much of the data collected during the study are presented as box-and-whisker plots. These plots 

show the range of the data as well as the 25th, 50th, and 75th percentiles. While the data for the 

individual POTWs are plotted in six-month increments, the Mann-Kendall tests were performed 

utilizing every sample result. For the collective data, the Mann-Kendall tests were performed on 

the six-month median results to address sample variability, to correspond with the twice-yearly 

determinations of the number of dentists with separators installed, and to provide a more 

manageable quantity of data. 
14 

3. INDIVIDUAL POTW RESULTS AND DISCUSSION

Results from the study were analyzed in terms of both individual POTWs and as overall results 

combining all of the data from each POTW. For the individual POTW results, mercury 

concentrations at the treatment plant were examined and then related to individual plant 

characteristics and activities. The Mann-Kendall test was used to determine the statistical 

significance of trends; those related to effluent mercury, biosolids mercury, and TSS 

concentrations are presented. Influent and effluent mercury loading results are also presented, 

although the statistical significance of trends related to these two parameters was not determined. 

For the combined results, potential factors influencing mercury concentrations were examined to 

determine any overall relationships common to all or a portion of the treatment plants. 

3.1 POTW “A” 

PLANT PROCESSES

POTW “A” treats an average of 42.5 million gallons of wastewater per day (MGD). However, 

the capacity of the plant is such that 200 million gallons could receive preliminary and primary 

treatment and 65 million gallons could receive secondary treatment, if necessary. Treatment at 

POTW “A” consists of screenings removal, grit removal, primary treatment, aeration, secondary 

clarification, disinfection, and dechlorination. Hauled-in wastes are not accepted for treatment, 

at POTW “A”. Bar racks are employed for screenings removal and grit is removed via aerated 

grit chambers. The grit slurry is then separated using a screw degritter and disposed of in a 

landfill along with the debris removed by the bar racks. Approximately 1,000 wet tons of grit is 

removed annually. The primary and secondary treatments at POTW “A” include complete mix, 

fine-bubble diffused aeration and secondary clarification. The primary sludge and waste 

activated sludge are settled in gravity thickeners and then blended with fats, oil, and grease. This 

mixture is then dewatered by filter presses. The dewatered sludge and fats, oil, and grease 

mixture was either incinerated in a multiple hearth incinerator or stabilized with hydrated lime 

and disposed of in a landfill. The scrubber water effluent from the multiple hearth incinerator 

was discharged into the grit tank effluent. This biosolids incineration process ended on 

November 30, 2005. Thickened sludge blended with fats, oils, and grease is now collected and 

handled on site by a contractor for offsite disposal or composting. POTW “A” produces 

approximately 9000 dry tons of biosolids each year. Final effluent treatment includes 

disinfection with hypochlorite and dechlorination using sodium bisulfite. The following 

treatment chemicals are utilized at POTW “A”: 

• Sodium hypochlorite is used for disinfection of the final effluent, disinfection of the 

wet weather treatment tank influent, odor control in the gravity thickeners, odor 

control in the press room, and for the control of filamentous bacteria in the return 

activated sludge; 

• Sodium bisulfite is used for dechlorination of the final effluent; 

• Liquid cationic polymer is used to promote flocculation of the thickened sludge; 

• Hydrated lime is added to dewatered sludge intended for the landfill; 

• Caustic soda is used in unclogging grease lines. 
15 

SAMPLE COLLECTION

Composite influent samples were collected downstream of the bar racks, just prior to the grit 

removal chambers. Teflon suction tubing was used in a peristaltic pump along with clean 

methods. The effluent samples were flow-paced composite samples of dechlorinated final 

effluent. (Flow-paced composite samples are those in which the interval between samples is a 

specified volume of effluent that has passed a measuring point in the flow stream.) Biosolids 

were sampled by conducting manual grab samples that were composited to obtain a sample that 

represented a 24-hour period. 

COLLECTION SYSTEM INFORMATION

POTW “A” is a marine discharger that treats portions of ten communities. The service area for 

POTW “A” covers approximately 50 square miles and has approximately 360,000 residents. 

Fifty percent of the collection system is combined sewer area and the remaining is separate 

sewer area. The combined sewer area contains sixty-seven combined sewer overflows. 

Approximately ninety-one percent of the influent flow at POTW “A” is from residential sources 

and approximately six percent is due to commercial uses. The remaining three percent of the 

influent flow is from industrial users. While none have been identified as significant sources of 

mercury, the types of industrial discharges are listed in Table 3.1A. 

Table 3.1A: Types of Significant Industrial Users (SIUs) in POTW “A” Service Area 

TYPE OF SIU NUMBER

APPROXIMATE 

TOTAL DAILY 

FLOW, GALLONS 

PER DAY (GPD) 

Electroplaters / Metal Finishers 61 1,069,305 

Metal Molding & Casting 2 13,883 

Metal Formers 1 2,000 

Chemical Manufacturers 5 18,197 

Printers 1 1,000 

Specialty Paper Coaters 1 19,800 

Electric Companies 1 93,000 

Central Treatment Facilities 1 18,210 

During the study period, POTW “A” was conducting an interceptor sewer cleaning program that 

consisted of using a water jet to force grit contained in the larger sewer pipes of the collection 

system to manholes where it could be removed. This operation had the potential to suspend 

mercury that had accumulated in the sewers over many years and flush this mercury to the 

headworks of the POTW. This work was conducted at various points throughout the collection 

system. The amount of suspended mercury that reached the headworks of the POTW was a 

function of the distance from the cleaning operation to the POTW and the specific flow 

conditions in that interceptor. The cleaning activities, which at times also included releasing grit 

removed from the sewer lines directly into the headworks of the plant, may have occurred on 

days while sampling for this study was being conducted. 

In July 2003, POTW “A” began construction of a large underground storage tunnel to eliminate 

or reduce combined sewer overflows. This construction project generated a large amount of 
16 

mining/groundwater that was discharged to the headworks of the POTW. Flow rates from this 

project changed dramatically over the course of the project. Prior to May 2004, the flow rate 

was less than 0.5 MGD. Between May 2004 and December 2005, the flow rate increased to a 

steady maximum of just over 3.5 MGD. After December 2005, the flow rate from the tunnel 

project decreased to 2.25 MGD. The characteristics of the wastewater from this project also 

changed. When mining was occurring, the wastewater contained a significant amount of mineral 

grit. The mining ceased on December 1, 2005, and after this date the amount of TSS in the 

wastewater from the tunnel decreased. 

MERCURY CONTROL PROGRAM

In late 2003, POTW “A” began implementation of a best management practice program for 

dental facilities. The program incorporates two options for the dental facilities. A dental facility 

may opt to install and maintain an ISO Certified Amalgam Separator and follow best 

management practices or the facility may construct a sampling location that collects all amalgam 

waste, routinely sample this wastestream and maintain full compliance with a mercury limit of 

0.005 parts per million. There are 100 dental facilities/dentists located in the service area. At the 

beginning of the sampling project, none of the facilities/dentists had installed amalgam 

separators. However by the culmination of the project, all of facilities/dentists had installed them 

accordingly. The amalgam separator status at this POTW is shown in Table 3.1B. 

 

Table 3.1B: Amalgam Separator Status at POTW “A” 

DATE

PERCENT OF DENTAL 

FACILITIES/DENTISTS OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 0 

January 1, 2004 0 

July 1, 2004 0 

January 1, 2005 98 

July 1, 2005 100 

January 1, 2006 100 

July 1, 2006 100 

RESULTS

As Figures 3.1A and 3.1B indicate, effluent mercury concentrations decreased significantly (p 

<0.05) from July 2003 to July 2006. At the start of the study, the 6-month median effluent total 

mercury concentration was 10.7 ng/L and, by the culmination of the study, the 6-month median 

effluent total mercury concentration was 5.7 ng/L. This statistically significant decrease in 

effluent mercury concentration may be related to amalgam separator installation. The 

concentration of mercury in the biosolids, as shown in Figure 3.1A, appears to have remained 

constant throughout the course of the study, possibly indicating that the effects of amalgam 

separator installation are being masked by the effects of the interceptor cleaning activities and 

the tunneling project. Figure 3.1B indicates that, while the effluent mercury concentrations 

were decreasing, the effluent TSS was increasing as a result of the tunneling project and/or the 

interceptor cleaning activities. At the beginning of the study, the 6-month median effluent TSS 

concentration was 9 mg/L and, by the culmination of the project, the 6-month median effluent 

TSS concentration was 14 mg/L. The increase in effluent TSS concentration was not found to be 
17 

statistically significant at the α = 0.05 level, but it was so at the 0.10 level. The influent mercury 

loading was variable and the effluent loading remained relatively constant throughout the course 

of the study, as depicted in Figure 3.1C. The high variability in influent mercury loading is 

believed to be partly the result of drain line cleaning and drain trap replacements occurring at 

dental facilities as a required part of the installation of amalgam separators. It may further be 

due to the tunneling project and the ongoing interceptor sewer cleaning activities. 

POTW “A” also collected additional influent and effluent samples during the course of the threeyear NACWA Mercury Sampling Project. The results from these samples may demonstrate a 

decline in both the influent and effluent mercury loading over the course of the study. The 

amount of data collected by POTW “A” is substantial and would lend further support to the 

conclusion that the effluent mercury concentrations are decreasing at POTW “A.” However, 

these additional samples were not collected or analyzed using the same methods as in the 

NACWA Mercury Sampling Project and therefore are not presented here. 

0

4

8

12

16

20

0% ASUs Installed0% ASUs Installed 98% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L 

0

3

6

9

12

Biosolids Mercury Concentration, mg/kg (dry) 

Effluent Hg

Biosolids Hg

Figure 3.1A: Effluent and Biosolids Mercury Concentrations at POTW “A” 
18 

0

5

10

15

20

0% ASUs Installed0% ASUs Installed 98% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L 

0

20

40

60

Effluent TSS Concentration, mg/L 

Effluent Hg

Effluent TSS

Figure 3.1B: Effluent Mercury and TSS Concentrations at POTW “A” 

0

15

30

45

60

75

90

0% ASUs Installed0% ASUs Installed 98% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

5

10

15

Effluent Mercury Load, grams/day

Effluent Hg

Influent Hg

Figure 3.1C: Influent and Effluent Mercury Load at POTW “A” 
19 

3.2 POTW “B” 

PLANT PROCESSES 

The wastewater treatment at POTW “B” consists of preliminary, primary, and secondary 

treatment, including disinfection and phosphorus removal. The preliminary treatment consists of 

mechanical bar screens followed by grit removal in non-aerated detritus tanks. The grit is 

landfilled in an approved landfill. The primary treatment is made up of settling tanks where the 

surface scum is skimmed and the biosolids are settled out and pumped from the bottom. Ferric 

chloride or “pickle liquor” is added during primary treatment to aid in the removal of 

phosphorus. (POTW “B” began using ferric chloride exclusively in July 2005). The skimmings 

are incinerated in an on-site grease reactor. The scrubber water from the grease reactor is 

reintroduced into the treatment process immediately before the primary settling tanks. No 

biosolids processing occurs at POTW “B”; all biosolids are pumped via a force main to another 

facility for processing. Aeration basins and settling tanks make up the secondary treatment of 

the activated sludge process. POTW “B” produced 25,967 dry tons of biosolids in 2002. The 

final step in the treatment process at POTW “B” is chlorination by means of liquid sodium 

hypochlorite and dechlorination of the effluent with sodium bisulfite. The following treatment 

chemicals are utilized at POTW “B”: 

• Sodium hydroxide is used for pH control; 

• Sodium hypochlorite is used in the disinfection of the final effluent; 

• Sodium bisulfite is used for removal of chlorine residual; 

• Ferric chloride (or “pickle liquor” from local industry) is used in the reduction of 

phosphorus. 

SAMPLE COLLECTION

Influent grab samples were obtained after the screening and degritting process. The sample 

location was prior to the recycle stream return, and the samples were collected using clean 

procedures. The effluent sample was a monthly grab sample that was collected after 

dechlorination, also using clean procedures. The sludge samples were collected prior to transfer 

of the sludge to another facility for further processing. 

COLLECTION SYSTEM INFORMATION

POTW “B” is owned and operated by a regional utility that owns and operates two other 

POTWs. The average flow at POTW “B” is approximately 100 MGD; however, the plant has a 

design flow of 155 MGD and the capability to provide secondary treatment of up to 330 MGD 

during wet weather. POTW “B” serves a population of over 334,000 people. This POTW does 

not accept hauled-in wastes for treatment. The service area for POTW “B” is approximately 76 

square miles. In the entire service area operated by the regional utility, there are 75 square miles 

of combined sewer service area and 280 square miles of separate sewer service area. Forty 

percent of the service area at POTW “B” is combined and sixty percent is separate. The major 

classifications of industrial users for the entire regional facility are provided in Table 3.2A. 
20 

Table 3.2A: TYPES OF SIUs IN POTW “Regional Utility” SERVICE AREA 

TYPE OF SIU NUMBER

APPROXIMATE 

TOTAL DAILY 

FLOW, MGD 

Industry Subject to Categorical 

Pretreatment Standards 137 5.63 MGD 

Industrial Users Discharging>25,000 gpd 

of Process Wastewater 26 3.24 MGD 

>5% of the organic load 0 0 

>5% of the hydraulic load 0 0 

Others Designated by Control Authority 20 0.15 MGD 

MERCURY CONTROL PROGRAM

In 2002, a narrative local limit for mercury was developed and incorporated into the local sewer 

use code. This local limit requires industrial sources of mercury to implement best management 

practices (BMPs) to minimize discharges of mercury to the system. All significant industrial 

users were required to conduct mercury analysis to identify significant sources of mercury. 

Certain industrial users that were identified as significant sources of mercury, which include 

dental facilities, must submit and implement BMP plans for mercury discharge minimization. 

Implementation of the mercury BMP Program for dental facilities began in April of 2002. This 

program required all facilities that provide dental care to design, submit, and implement BMP 

plans by December 31, 2002. Certain BMPs were mandated while others were recommended. 

The mandatory BMPs did not include installation of amalgam separators in dental clinics; 

however, some dental clinics have voluntarily installed amalgam separators. The regional 

utility’s service area has 546 known dental facilities; 219 of them are in the POTW “B” service 

area. The amalgam separator status at this POTW is shown in Table 3.2B. 

 

Table 3.2B: Amalgam Separator Status at POTW "B" 

DATE

PERCENT OF DENTAL FACILITIES 

OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 5 

January 1, 2004 6 

July 1, 2004 7 

January 1, 2005 8 

July 1, 2005 9 

January 1, 2006 9 

July 1, 2006 9 

RESULTS

As indicated by Figures 3.2A and 3.2B, POTW “B” exhibited a slight decrease in median 

effluent total mercury concentrations. At the start of the study, the 6-month median effluent total 

mercury concentration was 3.21 ng/L but, by the culmination of the study, the 6-month median 

effluent total mercury concentration had decreased to 2.64 ng/L. The decrease in effluent 

mercury concentration, though, was not found to be statistically significant (p >0.10). However, 
21 

the biosolids mercury concentrations exhibited a statistically significant increase (p <0.05) from 

July 2003 to July 2006. A study was conducted by the regional entity during the late summer 

and fall of 2005 to determine the cause of these elevated biosolids mercury concentrations. It 

was concluded that the elevated biosolids concentrations were the result of cleaning operations 

that took place from June 2003 through November 2005 in two large interceptor sewers. The 

removal of grit and debris from the interceptors during cleaning may have caused settled 

mercury and other heavy substances within the interceptors to be re-suspended and transported to 

POTW “B.” Throughout the course of the study, the effluent TSS concentrations and effluent 

mercury loading remained relatively constant (Figure 3.2B and 3.2C). As shown in Figure 3.2C, 

the influent mercury loading was highly variable. 

0

1

2

3

4

6% ASUs

Installed

7% ASUs

Installed

8% ASUs

Installed

9% ASUs

Installed

9% ASUs

Installed

9% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Hg Concentration, ng/L

0

2

4

6

Biosolids Hg Concentration, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.2A: Effluent and Biosolids Mercury Concentrations at POTW “B” 
22 

0

1

2

3

4

5

6% ASUs Installed7% ASUs Installed8% ASUs Installed9% ASUs Installed9% ASUs Installed9% ASUs Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concetration, ng/L

0

5

10

15

20

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.2B: Effluent Mercury and TSS Concentrations at POTW “B” 

0

25

50

75

100

6% ASUs

Installed

7% ASUs

Installed

8% ASUs

Installed

9% ASUs

Installed

9% ASUs

Installed

9% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

2

4

6

8

10

12

14

16

Effluent Mercury Load, grams/day 

Effluent Hg

Influent Hg

Figure 3.2C: Influent and Effluent Mercury Load at POTW “B” 
23 

3.3 POTW “C” 

PLANT PROCESSES

POTW “C” uses a trickling filter/solids-contact biological process. The preliminary treatment 

consists of climber bar screens and aerated grit tanks. The grit is landfilled at an approved 

landfill. The primary treatment occurs in primary settling tanks. The skimmings generated are 

incinerated off-site. Secondary treatment consists of trickling filters, solids contact tanks, 

aeration, and final settling tanks. Biosolids are treated via gravity thickening and centrifugal 

dewatering with cationic polymer addition prior to onsite incineration. The resulting dry ash is 

transported to an approved landfill. The scrubber water from the incinerator is reintroduced into 

the plant’s influent prior to the grit channels. This is followed by disinfection using sodium 

hypochlorite and dechlorination using sodium bisulfite. Hauled-in wastes are not accepted for 

treatment. The following treatment chemicals are utilized at POTW “C”: 

• Sodium hypochlorite is used in the disinfection of the final effluent; 

• Sodium bisulfite is used for residual chlorine removal; 

• Ferric chloride is used for phosphorus removal; 

• Sodium hydroxide is used for pH control; 

• Cationic polymer is used in dewatering. 

SAMPLE COLLECTION

Influent grab samples were initially obtained from an influent diversion chamber before the flow 

actually reached the plant, and therefore, prior to grit removal. In early 2004, the influent 

sampling location was relocated to a location within the plant; however the sampling location 

remained prior to grit removal. This sampling location was also prior to recycle stream return. 

The influent samples were collected using clean procedures. The effluent grab samples were 

collected after chlorination, but prior to dechlorination, adhering to clean procedures. The sludge 

samples were collected from the sludge handling facility after the addition of polymer. 

COLLECTION SYSTEM INFORMATION

POTW “C” is owned and operated by a regional utility that also owns and operates two other 

POTWs. POTW “C” has a design flow of 70 MGD and treats an average 26.3 MGD that 

includes wastewater from over 103,000 residents. The service area for POTW “C” is 

approximately 10 square miles. In the entire service area operated by the regional utility, there 

are 75 square miles of combined sewer service area and 280 square miles of separate sewer 

service area. Eighty-five percent of the service area at POTW “C” is combined and fifteen 

percent is separate. The major classifications of industrial users in the entire regional service 

area are identified in Table 3.3A. 
24 

Table 3.3A: Types of SIUs in POTW “Regional Utility” Service area 

TYPE OF SIU NUMBER

APPROXIMATE 

TOTAL DAILY 

FLOW, MGD 

Industry Subject to Categorical 

Pretreatment Standards 137 5.63 MGD 

Industrial Users Discharging>25,000 gpd 

of Process Wastewater 26 3.24 MGD 

>5% of the organic load 0 0 

>5% of the hydraulic load 0 0 

Others Designated by Control Authority 20 0.15 MGD 

 

MERCURY CONTROL PROGRAM

In 2002, a narrative local limit was developed and incorporated into the local sewer use code. 

This local limit requires industrial sources of mercury to implement best management practices 

(BMPs) to minimize discharges of mercury to the system. All significant industrial users were 

required to conduct mercury analysis to identify significant sources of mercury. Certain 

industrial users that were identified as significant sources of mercury, which include dental 

facilities, must submit and implement BMP plans for mercury discharge minimization. 

Implementation of the mercury BMP Program began in April of 2002. This program required all 

facilities that provide dental care to design, submit, and implement BMP plans by December 31, 

2002. Certain BMPs were mandated while others were recommended. These mandatory BMPs 

did not include installation of amalgam separators in dental clinics; however some dental clinics 

have voluntarily installed amalgam separators. The regional utility’s service area has 546 known 

dental facilities; 18 of them are in the POTW “C” service area. Table 3.3B indicates the percent 

of the dental facilities that have installed amalgam separators. 

 

Table 3.3B: Amalgam Separator Status at POTW “C” 

DATE

PERCENT OF DENTAL FACILITIES 

OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 6 

January 1, 2004 6 

July 1, 2004 6 

January 1, 2005 6 

July 1, 2005 6 

January 1, 2006 6 

July 1, 2006 6 

RESULTS

Throughout the course of the study, there was no statistically significant trend (p >0.10) found in 

the effluent mercury concentrations at POTW “C” (as shown in Figure 3.3A). The same was 

found for the biosolids mercury and effluent TSS concentrations (Figures 3.3A and 3.3B). 
25 

0

1

2

3

4

6% ASUs Installed6% ASUs Installed6% ASUs Installed6% ASUs Installed6% ASUs Installed6% ASUs Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L 

0

2

4

6

8

Biosolids Mercury Concentration, mg/kg (dry) 

Effluent Hg

Biosolids Hg

Figure 3.3A: Effluent and Biosolids Mercury Concentrations at POTW “C” 

0

1

2

3

4

6% ASUs Installed6% ASUs Installed6% ASUs Installed6% ASUs Installed6% ASUs Installed6% ASUs Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

4

8

12

16

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.3B: Effluent Mercury and TSS Concentrations at POTW “C” 
26 

0

4

8

12

6% ASUs Installed 6% ASUs Installed 6% ASUs Installed 6% ASUs Installed 6% ASUs Installed 6% ASUs Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0.0

0.5

1.0

1.5

Effluent Mercury Load, grams/day

Effluent Hg

Influent Hg

Figure 3.3C: Influent and Effluent Mercury Load at POTW “C” 

3.4 POTWs “D” and E” 

PLANT PROCESSES

POTWs “D” and “E” are operated by a regional agency. POTW “D” has a design flow of 300 

MGD and an actual average flow of 90 MGD. POTW “E” has design and actual average flows 

of 390 and 83 MGD, respectively. These two POTWs are connected to each other in two ways. 

At several locations in the collection system, flows may be diverted to either plant. The plants 

are also directly connected by pipeline. This allows transfer of sewage, biosolids, or both. 

Hauled-in waste that includes leachate from a landfill and airplane de-icing fluid are accepted for 

treatment at POTW “D,” however no hauled-in wastes are accepted at POTW “E.” 

The two plants provide traditional secondary treatment. Wastewater is subjected to coarse 

screening, grit removal, primary settling, aeration, secondary settling, and hypochlorite and 

bisulfite addition prior to being discharged to a freshwater environment. Iron salts are also added 

to aid in treatment at both facilities. For POTW “D,” this occurs following grit removal, while 

for POTW “E,” it takes place after primary settling. 

Biosolids processing at POTW “D” consists of anaerobic digestion, gravity belt thickening, and 

occasional plate-and-frame dewatering. Processing of biosolids at POTW “E” involves gravity 

belt thickening followed by heat drying. In addition, a biosolids pipeline connects the two 

treatment plants. It may operate in either direction to maximize production of the preferred 
27 

product. In most cases, biosolids flow from POTW “D” to “E.” The following chemicals are 

used at the two treatment plants: 

• Sodium hypochlorite is used for chlorination; 

• Sodium bisulfite is used for dechlorination; and 

• Ferric chloride is used for phosphorus removal. 

SAMPLE COLLECTION

Flow-proportionate composite influent samples were collected prior to the coarse screens and 

grit removal. The effluent samples were collected as grab samples taken after chlorination and 

dechlorination. Biosolid samples collected for analysis were weekly composites of the final 

product. 

COLLECTION SYSTEM INFORMATION

The collection system for POTWs “D” and “E” encompasses 420 square miles and serves a 

population of 1.2 million people in twenty-eight communities. The system for POTW “D” 

consists of only separated sewers, while that for POTW “E” is mostly combined sewers. 

MERCURY CONTROL PROGRAM

An amalgam program has been established by the agency that operates POTWs “D” and “E” as 

part of a mercury minimization program. A rule was adopted in 2004 that requires best 

management practices (BMPs) and amalgam separators in dental facilities. BMPs were 

mandatory as of the date of adoption of the rule. A certification from all dental offices with 

amalgam was due in 2005. All facilities with amalgam must install amalgam separators by 

February 1, 2008. As shown in Table 3.4A, by the conclusion of the study in July 2006, 

approximately 38% of dental facilities in the total service area of POTWs “D” and “E” had done 

so. The number of dental facilities that have installed amalgam separators in the individual 

service areas is not known. 

Table 3.4A: Amalgam Separator Status at POTWs “D” and “E” 

DATE

PERCENT OF DENTAL FACILITIES 

OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 0.6 

January 1, 2004 1.5 

July 1, 2004 6 

January 1, 2005 16 

July 1, 2005 26 

January 1, 2006 32 

July 1, 2006 38 

RESULTS

At POTW “D,” there was an apparent decrease, not found to be statistically significant (p >0.10), 

in effluent mercury concentration over the course of the study. This is depicted in Figure 3.4A. 

However, the effluent mercury loading remained about the same throughout the study (Figure 

3.4C). The biosolids mercury and effluent TSS concentrations also exhibited no changes during 

the study (Figures 3.4A and 3.4B). 
28 

Similar to POTW “D,” the effluent mercury concentrations for POTW “E” also showed an 

apparent, but not statistically significant (p >0.10), decrease over the course of the study (Figure 

3.4D). The 6-month median effluent concentration at the start of the study was 2.6 ng/L, 

compared to 1.8 ng/L at the end of the study. The biosolids mercury concentration and effluent 

loading showed no apparent trends at POTW “E” (Figures 3.4D and 3.4F). Contrary to POTW 

“D,” though, the TSS appeared to increase throughout the study as seen in Figure 3.4E. It should 

be noted, however, that this apparent trend was not found to be statistically significant (p >0.10). 

0

1

2

3

4

1.5% ASUs

Installed

6.1% ASUs

Installed

16% ASUs

Installed

26% ASUs

Installed

32% ASUs

Installed

38% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

2

4

6

8

Biosolids Mercury Concentration, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.4A: Effluent and Biosolids Mercury Concentrations at POTW “D” 
29 

0

1

2

3

4

1.5% ASUs

Installed

6.1% ASUs

Installed

16% ASUs

Installed

26% ASUs

Installed

32% ASUs

Installed

38% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

5

10

15

20

25

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.4B: Effluent Mercury and TSS Concentrations at POTW “D” 

0

25

50

75

100

1.5% ASUs

Installed

6.1% ASUs

Installed

16% ASUs

Installed

26% ASUs

Installed

32% ASUs

Installed

38% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

2

4

6

Effluent Mercury Load, grams/day

Effluent Hg

Influent Hg

Figure 3.4C: Influent and Effluent Mercury Load at POTW “D” 
30 

0

1

2

3

4

1.5% ASUs

Installed

6.1% ASUs

Installed

16% ASUs

Installed

26% ASUs

Installed

32% ASUs

Installed

38% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

0.5

1

1.5

2

Biosolids Mercury Concentration, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.4D: Effluent and Biosolids Mercury Concentrations at POTW “E” 

0

1

2

3

4

1.5% ASUs

Installed

6.1% ASUs

Installed

16% ASUs

Installed

26% ASUs

Installed

32% ASUs

Installed

38% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

5

10

15

20

25

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.4E: Effluent Mercury and TSS Concentrations at POTW “E” 
31 

0

25

50

1.5% ASUs

Installed

6.1% ASUs

Installed

16% ASUs

Installed

26% ASUs

Installed

32% ASUs

Installed

38% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

1

2

3

4

5

6

7

8

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.4F: Influent and Effluent Mercury Load at POTW “E” 

3.5 POTW “F” 

PLANT PROCESSES

Treatment at POTW “F” consists of solids removal, settling, aeration, clarification, sludge 

processing, and ultraviolet disinfection. Grit chambers are used to remove heavy solids. The grit 

is disposed of in a sanitary landfill. Primary and secondary treatment processes consist of 

primary settling, aeration, secondary nitrification, biological phosphorus removal, and final 

clarification. The effluent from the secondary treatment process is disinfected in ultraviolet light 

chambers. The primary sludge is treated in gravity sludge thickeners, and waste activated sludge 

is treated in dissolved air flotation thickeners. After thickening, the biosolids are digested in 

anaerobic digesters and thickened on a gravity belt thickener and then stored prior to hauling to 

local farm land for application as fertilizer. The following chemicals are used to aid in treatment 

processes at POTW “F”: 

• Ferric chloride for biosolids processing; 

• Polymer for biosolids gravity belt thickening; 

• Sodium hypochlorite to disinfect effluent for internal reuse. 

SAMPLE COLLECTION

Composite influent samples were collected prior to grit removal, using four automatic samplers 

that contribute to a single daily composite sample, and analyzed for mercury one day per month. 

Effluent samples were collected as grab samples one day per month, adhering to clean sampling 

protocols. Grab samples of gravity belt thickened sludge were collected approximately four 
32 

times per week. A monthly composite was prepared from these grab samples and analyzed for 

mercury. 

COLLECTION SYSTEM INFORMATION

POTW “F” is a freshwater discharger that serves a population of more than 320,000. The service 

area occupies 176 square miles. This POTW treats approximately 40 million gallons of 

wastewater each day. The collection system is separate from the stormwater system. Over 130 

pumping stations are required to pump wastewater to the POTW. The percentage of each type of 

flow entering the plant is shown in Table 3.5A. 

Table 3.5A: Percentage of Flow at POTW “F” 

 FLOW (MGD) PERCENTAGE OF TOTAL

Residential 23.1 58.9 

Commercial 11.3 28.7 

Industrial 3.0 7.5 

Infiltration 1.9 4.8 

MERCURY CONTROL PROGRAM

Since 1997, POTW “F” has been voluntarily working to minimize mercury use in the community 

through outreach and education, focusing on dental facilities, hospitals, schools, and industry. 

Dental facilities within the service area have been encouraged to voluntarily install amalgam 

separators. The amalgam separator status at this POTW is shown in Table 3.5B. 

Table 3.5B: Amalgam Separator Status at POTW "F" 

DATE

PERCENT OF DENTAL FACILITIES 

OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 0 

January 1, 2004 0 

July 1, 2004 0 

January 1, 2005 0 

July 1, 2005 2 

January 1, 2006 12 

July 1, 2006 20 

RESULTS

POTW “F” joined the sampling project after the project had already started, and therefore results 

are not available for the first six-month period. The results from POTW “F” indicate relatively 

constant mercury concentrations in the effluent, but a statistically significant decrease (p <0.05) 

in biosolids mercury concentrations as depicted in Figure 3.5A. The effluent TSS concentrations 

remained about the same throughout the study and can be seen in Figure 3.5B. The influent and 

effluent loadings are shown in Figure 3.5C. Amalgam separators may be a factor in the decrease 

seen in the biosolids mercury concentrations; at the start of the study there were no separators 

installed within POTW “F’s” service area, but by the conclusion of the study, 20% of dental 

facilities had installed amalgam separators. 
33 

0

1

2

3

4

0% ASUs Installed 0% ASUs Installed 0% ASUs Installed 2% ASUs Installed 12% ASUs

Installed

20% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Hg Concentration, ng/L

0

2

4

6

Biosolids Hg Concentration, mg/kg (dry) 

Effluent Hg

Biosolids Hg

Figure 3.5A: Effluent and Biosolids Mercury Concentrations at POTW “F” 

0

1

2

3

4

0% ASUs Installed 0% ASUs Installed 0% ASUs Installed 2% ASUs Installed 12% ASUs Installed20% ASUs Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

5

10

15

20

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.5B: Effluent Mercury and TSS Concentrations at POTW “F” 
34 

0

10

20

30

40

50

0% ASUs Installed 0% ASUs Installed 0% ASUs Installed 2% ASUs Installed 12% ASUs Installed20% ASUs Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day 

0

0.5

1

1.5

2

Effluent Mercury Load, grams/day 

Influent Hg

Effluent Hg

Figure 3.5C: Influent and Effluent Mercury Load at POTW “F” 

3.6 POTW “G” 

PLANT PROCESSES

POTW “G” has a design flow of 48.4 MGD, with an actual average flow of 38.2 MGD. 

Treatment consists of preliminary, secondary, and tertiary processes. Hauled-in wastes are 

accepted at the plant. Preliminary treatment at POTW “G” includes bar screens followed by two 

detritor-type grit tanks. Due to the significant industrial flow, primary treatment is not 

necessary. Secondary treatment includes biological treatment using high-purity oxygenactivated sludge aeration tanks and secondary clarifiers. This is followed by flocculation tanks 

and dual media filters. Secondary sludge is thickened using dissolved air flotation thickeners 

followed by thermophilic and mesophilic anaerobic digesters. Digested sludge tanks hold the 

digested sludge until it is dewatered for land application by one of two centrifuges. The 

following treatment chemicals are utilized at POTW “G”: 

• Polymer is used in sludge treatment; 

• Ferric Chloride is used for nutrient removal; 

• Anti-foam solution is used when needed. 

SAMPLE COLLECTION

Influent samples were collected prior to screening and grit removal and composited every 15 

minutes over 24 hours using an automatic sampler. Effluent grab samples were collected twice 
35 

per month at a sampling location following all plant processes. Biosolids samples were collected 

after the digested sludge was dewatered by compositing three grab samples in a 24-hour period. 

COLLECTION SYSTEM INFORMATION

The population served by POTW “G” is 110,000. The service area for POTW “G” is 

approximately 80 square miles and is served by a completely separate collection system. 

Approximately 7 percent of the influent flow can be attributed to infiltration and inflow and 52 

percent of the influent at POTW “G” is attributed to industrial flow. The major classifications of 

industrial users in the POTW “G” service area are provided in Table 3.6A. 

Table 3.6A: Types of SIUs in POTW "G" Service Area 

TYPE OF SIU NUMBER

APPROXIMATE 

TOTAL DAILY 

FLOW, GPD 

Pulp & Paper 2 18,820,000 

Metal Finishing 3 4100 

Organic Chemical 1 3900 

Other 11 1,010,000 

MERCURY CONTROL PROGRAM

POTW “G” has worked with local and state dental associations and the American Dental 

Association (ADA) to develop best management practices for waste management for the 100 

dentists in the service area, which included the voluntary installation of amalgam separators. A 

majority of the amalgam separator installations were funded either directly by POTW “G” or by 

a grant received by POTW “G” and the local dental society prior to the start of the NACWA 

Mercury Sampling Project. Waste amalgam disposal infrastructure is also offered through a 

small business/household hazardous waste program. Broad outreach programs to the general 

public, dentists, colleges, and high schools have been conducted. These have included fever 

thermometer and high school and college laboratory mercury exchanges. The amalgam 

separator status at this POTW is shown in Table 3.6B. 

Table 3.6B: Amalgam Separator Status at POTW "G" 

DATE

PERCENT OF DENTISTS OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 44 

January 1, 2004 86 

July 1, 2004 94 

January 1, 2005 98 

July 1, 2005 100 

January 1, 2006 100 

July 1, 2006 100 

RESULTS

At the start of the study, POTW “G” had a relatively high percentage of dentists with amalgam 

separators installed and low mercury concentrations in the effluent and biosolids. There were no 

statistically significant changes (p >0.10) in these mercury concentrations or in effluent TSS 
36 

concentrations over the course of the study, as depicted in Figures 3.6A and 3.6B, respectively. 

The influent and effluent loadings at POTW “G” are shown in Figure 3.6C. 

0

2

4

6

8

86% ASUs Installed94% ASUs Installed98% ASUs Installed 100% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Hg Concentration, ng/L 

0

2

4

6

8

Biosolids Hg Concentration, mg/kg (dry) 

Effluent Hg

Biosolids Hg

Figure 3.6A: Effluent and Biosolids Mercury Concentrations at POTW “G” 
37 

0

2

4

6

8

86% ASUs

Installed

94% ASUs

Installed

98% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

10

20

30

40

50

60

70

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.6B: Effluent Mercury and TSS Concentrations at POTW “G” 

0

5

10

15

20

25

30

35

86% ASUs

Installed

94% ASUs

Installed

98% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

100% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

1

2

3

4

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.6C: Influent and Effluent Mercury Load at POTW “G” 
38 

3.7 POTW “H” 

PLANT PROCESSES

POTW “H” is one of four POTWs operated by a large regional agency. The plant has a design 

flow of 54.4 MGD, but an actual average flow of approximately 35 MGD. Hauled-in wastes are 

accepted for treatment at the plant, with 90% of the hauled waste being domestic sewage. 

POTW “H” consists of two separate influent streams that combine at secondary treatment. The 

wastewater treatment at POTW “H” consists of preliminary, primary, secondary and advanced 

wastewater treatment, including disinfection prior to discharge. Preliminary treatment consists 

of trash removal at bar screens, followed by cyclone grit removal at the headworks. From the 

grit chamber, wastewater passes into primary clarifiers, where gravity settling separates the solid 

materials from the water. The primary sludge is screened and pumped to a dissolved air 

floatation thickener, where it is blended and then pumped into anaerobic digesters. Digested 

biosolids are then pumped to belt filter presses where polymer is added and excess water is 

pressed out. The dewatered biosolids are then trucked to a storage facility where they are stored 

prior to being applied to agricultural land as a soil supplement. Secondary treatment is completed 

using trickling filters followed by clarifiers. Solids from the secondary clarifiers are returned to 

the beginning of the treatment process. The secondary effluent is then pumped to activated 

sludge basins. Following final clarification, the treated wastewater is disinfected using ultraviolet 

light disinfection. Treated effluent from POTW “H” is discharged to a river. The following 

treatment chemicals are utilized at POTW “H”: 

• Ferrous sulfate is used for odor control; and 

• Polymer is used for sludge thickening. 

SAMPLE COLLECTION

Influent samples were collected and flow-composited at the wet-well at the headworks of each 

influent stream, prior to grit removal. Effluent samples were collected and time-composited after 

the oxychargers prior to discharging to the river. Biosolids were collected after the belt filter 

press prior to being trucked to the biosolids storage facility. 

COLLECTION SYSTEM INFORMATION

The population served by the POTW is 350,000, encompassing a service area of 165 square 

miles. The sanitary sewer system for the entire service area is completely separate from the 

storm drain system. Approximately 10% of the total flow to POTW “H” is from industrial 

dischargers. The major classifications of industrial users in the POTW “H” service area are 

provided in Table 3.7A. 

Table 3.7A: Types of SIUs in POTW "H" Service Area 

TYPE OF SIU NUMBER

APPROXIMATE 

TOTAL DAILY 

FLOW, GPD 

Electroplaters 6 225,000 

Metal Finishers 32 5,629,300 

Centralized Waste Treatment 2 25,000 

Transportation Equipment and Cleaning 1 8,000 

Petroleum Refining 1 102,000 
39 

MERCURY CONTROL PROGRAM

In 2000, a mercury code of management practices program was implemented by POTW “H.” 

This is a voluntary program designed to decrease mercury levels entering the POTW through 

cooperation with mercury dischargers. Among other actions, the program phased in guidelines 

for the 200 dentists in the service area to comply, in Phase 1, by installing treatment systems to 

remove at least 50% of the mercury discharged from their facilities. In Phase 2, which began in 

2002, dental offices installed ISO-certified amalgam separators for which removal efficiencies of 

at least 95% had been indicated. The dentists are also required to perform annual wastewater 

monitoring and periodically submit compliance documentation. Approximately 98% of the 

dentists in the service area have complied with the program by installing amalgam separators, 

particle traps or other technologies. Inspections and maintenance of the dental offices were not 

mandated, resulting in POTW staff periodically conducting non-regulatory inspections. Annual 

submittal of self-monitoring reports was requested by the POTW. These reports included preand post-treatment mercury analyses and best management practices performed by the dental 

office. The amalgam separator status at this POTW is shown in Table 3.7B. 

Table 3.7B: Amalgam Separator Status at POTW "H" 

DATE

PERCENT OF DENTISTS OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 unknown 

January 1, 2004 60 

July 1, 2004 89 

January 1, 2005 95 

July 1, 2005 95 

January 1, 2006 98 

July 1, 2006 98 

RESULTS

There was no apparent trend in either the effluent mercury or TSS concentrations at POTW “H” 

during the course of the study, as depicted in both Figures 3.7A and 3.7B. There was, however, 

a statistically significant increase (p <0.05) in biosolids mercury concentrations as shown in 

Figure 3.7A. The influent and effluent mercury loadings are shown in Figure 3.7C. In mid2005, the POTW lost key staff members who had been responsible for the mercury source 

control program. This resulted in limited staff and resources available to oversee the dental 

office program. The absence of POTW staff reviewing the program performance resulted in 

several of the dental offices failing to maintain their amalgam separators, potentially increasing 

the mercury loadings. Also, between June 2005 and April 2006, POTW “H” underwent a 

construction upgrade that resulted in combining the two influent streams into one. This major 

construction project resulted in changing the entire primary treatment phase, and relocating 

influent transmission lines. This construction activity may have resulted in resuspension of 

mercury particles that may have been residing in various stages of the treatment plant, thereby 

potentially contributing to the increase in biosolids mercury concentrations. 
40 

0

2

4

6

8

60% ASUs

Installed

89% ASUs

Installed

95% ASUs

Installed

95% ASUs

Installed

98% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L 

0

2

4

6

8

10

12

Biosolids Mercury Concentration, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.7A: Effluent and Biosolids Mercury Concentrations at POTW “H” 

0

2

4

6

8

60% ASUs

Installed

89% ASUs

Installed

95% ASUs

Installed

95% ASUs

Installed

98% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

10

20

30

40

50

60

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.7B: Effluent Mercury and TSS Concentrations at POTW “H” 
41 

0

5

10

15

20

25

30

60% ASUs

Installed

89% ASUs

Installed

95% ASUs

Installed

95% ASUs

Installed

98% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

5

10

15

20

25

30

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.7C: Influent and Effluent Mercury Load at POTW “H” 

3.8 POTW “I” 

PLANT PROCESSES

POTW “I” treats an average of 2.38 MGD but is designed for 4.79 MGD. No hauled-in waste is 

accepted at the treatment plant. POTW “I” is an activated sludge secondary treatment plant. The 

grit is removed at gravity grit removal stations located in the trunk sewers and hauled to a 

sanitary landfill. At the headworks of the plant, the sewage is screened through six-millimeter 

traveling screens. The liquid is then processed in primary clarifiers, secondary aeration tanks, 

and finally, in secondary clarifiers. Solids are removed from both clarifiers. The secondary 

sludge is thickened in a gravity belt thickener and added to the primary sludge. The combined 

sludge is then de-watered in a rotary press, pasteurized and stabilized with heat and lime in an 

RDP thermo-blender. Lime-stabilized, non-heat treated sludge is also produced for use in 

composting. The sludge produced during the three-year study period was applied to farmland, 

used for top cover at the sanitary landfill, composted or buried. The landfilled material is usually 

de-watered sludge that has not been treated with lime or heat. The following treatment 

chemicals are utilized at POTW “I”: 

• Bioxide is used for odor control; 

• Polymer (Hydrofloc 1620) is used for thickening; and 

• Lime is used for pasteurization and stabilization. 
42 

SAMPLE COLLECTION

Influent samples were collected from the combined influent line after the trunk sewer grit 

removal stations, but before six-millimeter traveling screens. Effluent samples were collected 

upstream of a Parshall Flume on the plant’s final effluent, which is discharged to the 

environment through a marine outfall. The lime-stabilized sludge samples were composites 

comprised of daily grabs from the sludge bin collected over a 3-4 week period. 

COLLECTION SYSTEM INFORMATION

POTW “I” serves a population of approximately 30,000 people and covers approximately 12 

square miles. The entire service area is comprised of separate sanitary sewers. Approximately 

4% of daily flow to POTW “I” is from industrial users. 

MERCURY CONTROL PROGRAM

There are 15 dental clinics that handle mercury amalgam and discharge wastewater to the sewer 

located within the service area for POTW “I.” These account for approximately 50% of the total 

mercury load to the POTW, based on estimates made in a 1997 study. Eleven regulatory codes 

of practice have been developed and implemented within the service area since 1999, including 

codes for dental offices and laboratories. The dental code of practice (mandating separator 

installation) became effective throughout the service area in July 2001. All but one of the dental 

facilities in the collection area had amalgam separators installed throughout the study period. 

There have been no significant industrial mercury sources identified in the collection area. 

RESULTS

POTW “I” had amalgam separators installed for 93% of the dental facilities at the start of the 

study. The effluent mercury and TSS concentrations indicated no apparent trends throughout the 

study, as shown in both Figures 3.8A and 3.8B. The influent and effluent loadings for POTW 

“I” are depicted in Figure 3.8C. The biosolids mercury concentration exhibited a statistically 

significant decrease (p <0.05) throughout the study. At the start of the study, the 6-month 

median biosolids mercury concentration was 1.0 mg/kg, but had decreased to 0.1 mg/kg by the 

end of the study. These results indicate that the use of amalgam separators may have more of an 

effect in reducing the amount of mercury that ends up in the biosolids as opposed to the effluent. 

The data also indicate that mercury reductions following separator installation may take several 

years, as the dental facilities had installed separators two years before the start of the study. 
43 

0

2

4

6

8

10

12

14

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

1

2

3

4

5

Biosolids Mercury Concentration, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.8A: Effluent and Biosolids Mercury Concentrations at POTW “I” 

0

2

4

6

8

10

12

14

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

-5

5

15

25

35

45

55

65

75

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.8B: Effluent Mercury and TSS Concentrations at POTW “I” 
44 

0.0

0.1

0.2

0.3

0.4

0.5

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

93% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.8C: Influent and Effluent Mercury Load at POTW “I” 

3.9 POTWS “J” AND “K” 

PLANT PROCESSES

POTWs “J” and “K” are two of three treatment plants operated by one agency that discharge to a 

saltwater estuary. They serve a population of approximately 1.4 million people. POTW “J” 

treats an average of 81.3 MGD but is designed for an average of 115 MGD. POTW “K” treats 

an average of 119.3 MGD, with a design flow of 133 MGD. Treatment at both POTWs includes 

preliminary, primary, secondary, and disinfection processes. Preliminary treatment consists of 

bar screens followed by aerated grit chambers. The debris and grit removed during this process 

are trucked to a landfill. After preliminary treatment, the wastewater settles in primary 

sedimentation tanks. The skimmings and sludge from this process are then sent to the solids 

handling process, which includes blending and thickening in a dissolved air flotation tank 

followed by anaerobic digestion. Secondary treatment involves the use of aeration tanks and 

secondary clarifiers. The secondary effluent is then disinfected using chlorine. Some of the 

secondary effluent also undergoes advanced treatment (coagulation, filtration, disinfection) and 

is then reused on-site in plant processes. 

SAMPLE COLLECTION

For both POTWs, influent samples were collected between the bar screens and grit removal at a 

point after chlorine had been added for odor control. Samples were not collected if recycled 

streams from maintenance of process tanks had been introduced into the system. Effluent grab 
45 

samples were collected downstream of chlorine addition. Biosolids samples were 24-hour 

composites collected after dewatering. 

COLLECTION SYSTEM INFORMATION

The treatment plants serve a total area of 64.7 square miles. The service area for POTW “J” 

consists of only separate sewers. In the POTW “K” service area, there are mostly combined 

sewers, with a total of 176 combined sewer overflows. For POTW “J,” 43% of incoming flow to 

the plant during the winter is residential, 17% is commercial, and 3% is industrial. The 

remaining flow comes from stormwater inflow and groundwater infiltration. At POTW “K,” 

36% of incoming flow during the winter is residential, 19% is commercial, 1% is industrial, and 

44% is from stormwater inflow and groundwater infiltration. POTW “K” participates in a “flow 

swap” program from May through October. During this time period the POTW receives 

approximately 2.0 MGD from an area over which it does not have pretreatment control. 

MERCURY CONTROL PROGRAM

In July 2001, POTWs “J” and “K” informed dental facilities that they needed to comply with 

local discharge limits for mercury. This limit was set at 0.2 mg/L. The dentists were given until 

July 2003 to be in compliance. A letter and a fact sheet were sent to each dental practice with 

instructions on how to comply with the discharge limits. Inspectors then visited dental offices in 

the service area to explain the regulations and assist dentists in getting their practices into 

compliance. The POTWs completed their informational visits in August 2003 and then started a 

program to inspect a portion of dental practices each year for compliance with the regulations. 

From 2004 through 2006, approximately 10 % of the offices were inspected each year. A dental 

facility in the service area can demonstrate that it is in compliance with sewer limits if it (a) 

follows best management practices (BMPs) for amalgam wastes (detailed in a fact sheet), (b) 

properly handles used X-ray fixer, and (c) installs an approved amalgam separator—or has 

demonstrated that it meets the local limits through sampling and has obtained a permit to 

discharge. Currently, only one dental office has elected to receive a permit and perform routine 

sampling rather than install an approved separator. The percentage of dental facilities that have 

installed amalgam separators in the total service area is shown in Table 3.9A. The number of 

dental facilities that have installed amalgam separators in the individual service areas is not 

known. 

Table 3.9A: Amalgam Separator Status at POTWs "J" and “K” 

DATE

PERCENT OF DENTAL FACILITIES 

OPERATING

DENTAL AMALGAM SEPARATORS

July 1, 2003 94 

January 1, 2004 97 

July 1, 2004 97 

January 1, 2005 97 

July 1, 2005 97 

January 1, 2006 97 

July 1, 2006 98 
46 

These POTW’s have specifically exempted certain specialties from the requirement of installing 

amalgam separators or obtaining a permit. These specialties are: periodontics, orthodontics, oral 

pathology/oral medicine; oral and maxillofacial surgery; radiology; prosthodontists and 

endodontists that do not place and remove amalgam as a courtesy for their clients. Dentists, such 

as some pediatric dentists, that place or remove amalgam on three days or fewer per year are also 

exempt from these requirements. If any of these exemptions applies to the dental office, it does 

not need to install separator or apply for a permit. However, it still needs to follow BMPs for 

waste amalgam and may be inspected for compliance. 

In May 2005, POTW “J” stopped accepting septage waste from dental offices without prior 

screening and evaluation of mercury concentration levels. 

RESULTS

For both treatment plants, there was a statistically significant decrease (p <0.05) in effluent and 

biosolids mercury concentrations throughout the study (Figures 3.9A and 3.9D). POTW “J” also 

had a statistically significant (p <0.05) effluent TSS decrease, while POTW “K” did not (Figures 

3.9B and 3.9E). Both plants showed an apparent decrease in effluent mercury loadings, as seen 

in Figures 3.9C and 3.9F. The effluent mercury concentrations showed a greater decrease than 

the biosolids mercury concentrations. This may be due to the relatively high mercury 

concentrations present in the effluent at the beginning of the study and the decreases in effluent 

TSS throughout the study period. Since a large percentage of dental facilities had amalgam 

separators installed for the duration of the study (97% at beginning and 98% at end), it could be 

that reductions in mercury gained through the installation of separators may not occur 

immediately. It is also possible that the apparent reductions in mercury may also be the result of 

improvements to treatment processes suggested by the effluent TSS decreases or due to other 

activities occurring within the service area, such as implementation of BMPs recommended by 

the mercury control program. 
47 

0

2

4

6

8

10

12

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Hg Concentration, ng/L 

0

2

4

6

8

Biosolids Hg Concentration, mg/kg (dry) 

Effluent Hg

Biosolids Hg

Figure 3.9A: Effluent and Biosolids Mercury Concentrations at POTW “J” 

0

2

4

6

8

10

12

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

10

20

30

40

50

60

70

Effluent TSS Concentration, mg/L 

Effluent Hg

Effluent TSS

Figure 3.9B: Effluent Mercury and TSS Concentrations at POTW “J” 
48 

0

25

50

75

100

125

150

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

2

4

6

8

10

12

14

16

18

20

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.9C: Influent and Effluent Mercury Load at POTW “J” 

0

2

4

6

8

10

12

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentrations, ng/L

0

2

4

6

8

Biosolids Mercury Concentrations, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.9D: Effluent and Biosolids Mercury Concentrations at POTW “K” 
49 

0

3

6

9

12

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

10

20

30

40

Effluent TSS Concentration, mg/L

Effluent Hg

Effluent TSS

Figure 3.9E: Effluent Mercury and TSS Concentrations at POTW “K” 

0

10

20

30

40

50

60

70

80

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

97% ASUs

Installed

98% ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

2

4

6

8

10

12

14

16

18

20

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.9F: Influent and Effluent Mercury Load at POTW “K" 
50 

3.10 POTW “L”

PLANT PROCESSES

POTW “L” is designed to treat 55 MGD, and has an actual average flow of 46 MGD. The 

treatment process at POTW “L” includes grit removal, primary and secondary treatment, and UV 

disinfection. Hauled-in wastes are accepted for treatment at this POTW. Preliminary treatment 

includes bar screens and preaeration tanks to remove grit. Primary sludge is blended with 

Dissolved Air Flotation Unit-thickened waste activated sludge and centrifuged. The cake is 

incinerated and the centrate and wet scrubber water are returned as influent to the secondary 

process. Following secondary treatment, the wastewater is disinfected using ultraviolet 

lightbulbs. A small portion of final effluent flow is sent to the dual-media filter plant and 

chlorinated for recycled water use. The backwash water from dual-media filter plant returns as 

influent to the headworks. Hauled-in wastes, which include residential septage, portable toilet 

waste, domestic septage from non-residential sites, and restaurant grease, are introduced into the 

plant at the headworks. The following chemicals are used in treatment processes at POTW “L”: 

• Hydrogen peroxide – odor control; 

• Sodium hypochlorite; 

• Carbide lime; and 

• Polymer. 

SAMPLE COLLECTION

Influent grab samples were taken from the headworks, prior to grit removal. These included 

some recycled flows from both the treatment plant and the filter plant. Effluent composite 

samples were taken at a point after UV disinfection. Biosolids samples were sludge cake, taken 

prior to incineration. Certain sidestreams carrying non-negligible concentrations of mercury are 

returned as influent to the headworks. These sometimes include wet scrubber water and filter 

backwash water. On the days on which influent grab samples for this study were taken, none of 

these flows were being recycled. However, as a result of the customary recycling to the 

secondary process influent of incinerator wet scrubber water, it is believed that effluent mercury 

concentrations on a day-to-day basis are affected. 

COLLECTION SYSTEM INFORMATION

POTW “L” is an estuarine discharger that serves a population of 450,000. The service area 

encompasses approximately 143 square miles. 

MERCURY CONTROL PROGRAM

As part of the mercury control program at POTW “L”, about 23 industrial users are monitored up 

to four times per year for mercury: once or twice by the POTW and the rest through selfmonitoring. Dentists are encouraged to install amalgam separators, but there is no mandatory 

program requiring separators. In the future, POTW “L” will be implementing a requirement for 

amalgam separators. Influent mercury concentrations are anticipated to decline; the effect on the 

effluent concentrations will be evaluated. Household hazardous wastes and small quantity 

generator mercury waste is accepted at the Household Hazardous Waste Collection Facility, 

where they are sequestered and hauled away for proper disposal. In addition, best management 

practices information has been distributed to all hospitals in the service area. 
51 

RESULTS

At POTW “L”, the effluent mercury concentration ranged from 10.4 ng/L to 205 ng/L, effluent 

mercury loading ranged from 1.9 grams/day to 33.8 grams/day, and effluent TSS concentrations 

ranged from 3 mg/L to 29 mg/L, as shown in Figures 3.10A and 3.10B. No statisticallysignificant trends (p >0.10) are evident for any of these variables. The biosolids mercury 

concentration remained relatively constant throughout the sampling (Figure 3.10A). The influent 

and effluent loadings are depicted in Figure 3.10C. By the end of the study, an estimated 11% of 

the dental facilities/dentists in the service area of POTW “L” had installed separators. 

 

0

10

20

30

40

50

60

70

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

11 % ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentration, ng/L

0

3

6

9

Biosolids Mercury Concentration, mg/kg (dry)

Effluent Hg

Biosolids Hg

Figure 3.10A: Effluent and Biosolids Mercury Concentrations at POTW “L” 
52 

0

10

20

30

40

50

60

70

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

11 % ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Effluent Mercury Concentrations, ng/L

0

5

10

15

20

25

30

35

40

45

Effluent TSS Concentrations, mg/L

Effluent Hg

Effluent TSS

Figure 3.10B: Effluent Mercury and TSS Concentrations at POTW “L” 

0

20

40

60

80

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

Unknown ASUs

Installed

11 % ASUs

Installed

Jul 03 - Dec 03 Jan 04 - Jun 04 July 04 - Dec 04 Jan 05 - Jun 05 Jul 05 - Dec 05 Jan 06 - Jul 06

Influent Mercury Load, grams/day

0

10

20

30

40

Effluent Mercury Load, grams/day

Influent Hg

Effluent Hg

Figure 3.10C: Influent and Effluent Mercury Load at POTW “L” 
53 

4. COMBINED RESULTS AND DISCUSSION 

In addition to those for individual plants, trends were also examined by combining the results 

from all of the POTWs. A variety of potential factors that may be influencing mercury 

concentrations at POTWs were examined. Excluded from this analysis were those factors in 

which there were too many variables to effectively establish a relatively conclusive relationship. 

4.1 INFLUENT, EFFLUENT, AND BIOSOLIDS

Figure 4.1A depicts, with a logarithmic y-axis, the influent total mercury concentrations, for each 

facility, in six-month increments. There is clearly a high degree of variability in the influent 

mercury concentrations within each facility as well as between the facilities. POTWs B, C, I, J, 

K and L collected influent samples as grab samples while POTWs A, D, E, F, G, and H collected 

influent composite samples. Using composite sampling techniques to monitor influent mercury 

concentrations potentially reduces variability to some degree. The high variability in influent 

concentrations is most likely due to the inconsistent nature of the influent and the fact that, 

because the samples have varying amounts of solid particles, a truly representative influent 

sample is virtually impossible to collect. Much of the mercury entering a POTW is thought to be 

in the particulate form and would be contained within these solids. In light of this inherent 

variability, the ability to use influent mercury concentrations to determine impacts from potential 

mercury reduction techniques in the collection system is limited. 

10

100

1000

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06 Jul-06Jan-04

Jul-04 Jan-05Jul-05 Jan-06Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04 Jul-04Jan-05

Jul-05 Jan-06Jul-06

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Influent Mercury, ng/L 

Figure 4.1A: Influent Mercury Concentrations 
54 

POTWs “B” and “I” collected influent samples after grit removal while the remaining POTWs 

collected influent samples prior to grit removal. Influent that contains a high amount of 

particulate matter is suspected to also contain a high amount of particulate mercury. However, 

as shown in Figure 4.1B, no difference is clearly evident in influent mercury concentrations 

between those facilities where influent samples were collected prior to grit removal and those 

facilities where influent samples were collected after grit removal processes. 

10

100

1000

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"A"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"J"

POTW

"K"

POTW

"B"

POTW "I"

Influent Mercury, ng/L 

Sample Collected 

Prior to Grit Removal

Sample Collected 

After Grit Removal

 Figure 4.1B: Influent Mercury Concentrations and Sample Collection Location 
55 

0

5

10

15

20

25

30

35

40

45

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Effluent Mercury Concentration, ng/L 

Figure 4.1C: Effluent Mercury Concentrations 

Figure 4.1C depicts the effluent mercury concentrations, in six-month increments, at each facility 

during the course of the sampling project. Similar to the influent mercury concentrations, there 

is a high degree of variability in effluent mercury concentrations between different POTWs. 

Effluent mercury concentrations are generally higher at POTWs A, I, J, K, and L than they are at 

POTWs B, C, D, E, F, G, and H. Speculations as to the potential causes of these differences are 

examined in later sections of this report. However, unlike the influent concentrations, the 

effluent mercury concentrations are more consistent at individual facilities. Some facilities, 

particularly POTW “E”, exhibited very little variability in effluent mercury concentrations. The 

decreased variability within individual POTWs indicates that, generally speaking, each 

individual POTW is consistently performing to the same level. 
56 

0

2

4

6

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Biosolids Mercury Concentration, mg/kg (dry)

Figure 4.1D: Biosolids Mercury Concentrations 

Similar to both the influent and the effluent mercury concentrations, the effluent biosolids 

concentrations exhibit considerable variability from POTW to POTW, as depicted in Figure 

4.1D. It is also clear from this figure that some facilities have higher variability within their 

biosolids mercury concentrations than do other facilities. Potential causes of this are examined 

further in later sections of this report. POTW “E” demonstrates a very small degree of 

variability, while POTWs “B” and “L” each display a very high degree of variability. At POTW 

“B” the high variability is attributable to the interceptor cleaning activities that were discussed in 

Section 3.2 of this report. 

4.2 REMOVAL EFFICIENCY

The mercury removal efficiency calculated from the median of each facility’s influent 

concentrations and the median of each facility’s effluent concentrations measured during the 

study was examined to determine if there were differences in the abilities of individual POTWs 

to remove mercury from the wet stream. As shown by Figure 4.2A, the treatment plants had 

median removal efficiencies ranging from 89% at POTW “L” to 99% at POTW “D.” It should 

be noted that POTW “D” is connected to POTW “E” via pipeline and it is likely that 

concentrations at POTW “D” are strongly affected by concentrations at POTW “E.” However, at 

POTW “F”, which has the second highest median removal efficiency and where results are not 

affected by those of another treatment facility, the median removal efficiency of 98.75% is only 

slightly lower that that of POTW “D”. 
57 

0

10

20

30

40

50

60

70

80

90

100

POTW

"L"

POTW

"K"

POTW

"A"

POTW

"J"

POTW

"C"

POTW

"I"

POTW

"G"

POTW

"B"

POTW

"H"

POTW

"E"

POTW

"F"

POTW

"D"

Removal Efficiency, %

Figure 4.2A: Median Mercury Removal Efficiency 

4.3 DISSOLVED MERCURY 

A key factor often thought to affect the ability of a POTW to remove mercury and the resulting 

effluent mercury concentration is the extent to which the mercury is in the dissolved or 

particulate form. Most of the particulates are expected to settle in the collection system or be 

removed during treatment processes. The dissolved form may pass through the treatment 

system, impacting effluent concentrations. A limited amount of sampling was performed in 

order to attempt to quantify the amount of dissolved mercury in the effluent. Dissolved mercury 

analysis was performed at all POTWs except POTWs “D,” “E,” and “F” during the first quarter 

of the study. POTWs “D,” “E,” and “F” volunteered to submit their data for consideration in the 

study well after the study had begun and had not been aware of the plan to collect dissolved 

mercury data. 

Some POTWs provided additional dissolved mercury data as well. These data were used to 

observe the ratio of dissolved mercury to total mercury in the effluent of the participating 

POTWs. Figure 4.3A shows the median percent of the effluent total mercury that is dissolved 

for each facility with available data. The percent of effluent total mercury that was dissolved 

was found to vary based on the POTW. The highest median dissolved mercury occurred at 

POTW “G” at 59%, while the lowest median was at POTW “A” at 22%. 

It has been hypothesized that recycled incinerator scrubber water flow within a POTW can 

increase the amount of dissolved mercury in the POTW effluent. Figure 4.3B indicates that, for 

the POTWs in the study, there does not appear to be a relationship between the presence of 
58 

recycle scrubber water flows and the percentage of the effluent mercury concentration that is 

dissolved. 

0

25

50

75

100

POTW "G" POTW "C" POTW "I" POTW "B" POTW "K" POTW "L" POTW "J" POTW "H" POTW "A"

Percent of Effluent Total Mercury that is Dissolved

Figure 4.3A: Median Percent of Effluent Total Mercury that is Dissolved 
59 

0

25

50

75

100

POTW "A" POTW "B" POTW "C" POTW "L" POTW "G" POTW "H" POTW "I" POTW "J" POTW "K"

Percent of Effluent Total Mercury that is Dissolved

Have recycle flows Do not have recycle flows

Figure 4.3B: Presence of Recycle flows and the Median Percent of Effluent Total Mercury that is Dissolved 

*Scrubber water flow at POTW “A” did not occur during the final seven months of the sampling project

4.4 TOTAL SUSPENDED SOLIDS

It has been hypothesized that the quantity of TSS in POTW effluent can provide an indication of 

the amount of mercury that is found in the effluent. Figure 4.4A shows that, when the effluent 

mercury concentration is plotted versus effluent TSS concentration for all POTWs, there is a 

positive linear relationship. However, the R2 value indicates the ability to predict the mercury 

concentration using TSS concentrations is very low. The relationship between effluent mercury 

concentration and TSS concentrations were also examined at the individual POTWs. Table 4.4A 

provides the equation of the linear trendlines and the associated R2

 values obtained from this 

analysis. For many of the POTWs, there was a strong positive linear relationship between 

effluent mercury concentrations and effluent TSS concentrations, and the R2

 values were higher. 

At POTWs “C”, “E”, “K”, and “L”, however, the predictive ability of the equation was low. 

The use of various chemicals, especially iron salts, may have an impact on the relationship 

between TSS and mercury concentrations. Because of this, POTWs that use iron salts were 

compared to those that do not. It was found that there was no clear relationship between the use 

of iron salts and the ability to predict effluent mercury concentrations from effluent TSS 

concentrations. Therefore, it is unknown why, at some POTWs, effluent TSS concentration 

appears to be an adequate predictor of effluent mercury concentration and, at others, it is not. 
60 

y = 0.5221x + 2.8435

R2

 = 0.0605

0

5

10

15

20

25

30

0 5 10 15 20 25 30

Effluent TSS, mg/L

Effluent TR Hg, ng/L

Figure 4.4A Effluent Mercury Concentrations versus Effluent TSS Concentrations 

Table 4.4A. Trendline Equations and R2 Values for Effluent Mercury Concentrations versus Effluent TSS 

Concentrations 

POTW Equation R2 Value 

POTW “A” y = 0.2105x + 7.2324 0.3004 

POTW “B” y = 0.2774x + 1.9742 0.3281 

POTW “C” y = 0.1084x + 2.0188 0.0434 

POTW “D” y = 0.1155x + 1.3448 0.2837 

POTW “E” y = 0.089x + 1.5362 0.0711 

POTW “F” y = 0.5491x - 0.4202 0.4194 

POTW “G” y = 0.2649x + 1.5544 0.2964 

POTW “H” y = 0.3133x + 2.6572 0.4879 

POTW “I” y = 0.6743x + 1.937 0.3848 

POTW “J” y = 0.2407x + 4.1588 0.2821 

POTW “K” y = 0.2199x + 4.8325 0.0998 

POTW “L” y = 1.0615x + 21.401 0.0181 

4.5 DENTISTS PER FLOW

In order to establish whether the density of dental facilities has an impact on mercury levels in 

treatment plants, the numbers of dentists per plant flow were examined in comparison to influent, 

effluent, and biosolids mercury concentrations. POTWs “D”, “E”,”J”, and “K” were excluded 

because the number of dentists for their individual service areas was not known. Most POTWs 

track the number of dentist facilities within the service area, however some count the number of 
61 

dentists, and it is not known, for every POTW, whether they counted the total number of dentists 

or excluded facilities that do not use amalgam. A further complication is that many dentists who 

do not place amalgam restorations nonetheless remove amalgam-containing restorations, and this 

is a significant contributor to dental mercury discharges. Although limitations of doing so are 

recognized, for the purposes of this analysis, the number of facilities and the number of dentists 

are used interchangeably. As seen in Figures 4.5A and 4.5B, a decrease in the number of 

dentists per million gallons per day (MGD) was related to a general decrease in both influent and 

effluent mercury concentrations. Both of these relationships were found to be statistically 

significant (p <0.05) despite the limitations noted above. The median influent mercury 

concentration and the number of dentists per MGD resulted in a p-value of 0.00007, indicating 

that the median influent mercury concentration at a POTW is very highly correlated to the 

number of dentists per MGD. The median effluent mercury concentration was also highly 

correlated to the number of dentists per MGD with a p-value of 0.002. 

There was no relationship found between dentists per MGD and median biosolids mercury 

concentrations (Figures 4.5C). This was unexpected since a major portion of the mercury 

released by dentists is thought to be removed during treatment in the POTW and, therefore, 

would end up in the biosolids. One possible reason for these results may be that the number of 

dentists with amalgam separators installed changed throughout the study. Therefore, reductions 

in mercury gained in areas where more separators were being installed may have offset the 

general trend of increasing mercury resulting from an increase in the number of dentists. This 

finding may also reflect that biosolids mercury is more affected by separator installation than is 

effluent mercury. 

0

100

200

300

400

500

600

C - Jul 04C - Jul 05

C - Jan 04C - Jan 05C - Jan 06

C - Jul 06

A - Jul 06A - Jul 05

B - Jul 05B - Jul 04

A - Jan 06

B - Jan 04

A - Jul 04

A - Jan 05A - Jan 04

B - Jan 05B - Jan 06

G - Jan

B - Jul 06

G - Jan

G - Jul 06

G - Jan

G - Jul 04G - Jul 05

F - Jan 05

F - Jul 04F - Jul 06F - Jul 05

F - Jan 06

H - Jul 05

I - Jul 04I - Jul 06

H - Jan 04

H - Jul 04

H - Jan 05H - Jan 06

H - Jul 06

I - Jan 04I - Jan 05

I - Jul 05

I - Jan 06

L - Jul 06L - Jul 05L - Jul 04

L - Jan 04L - Jan 05L - Jan 06

POTW and Time Period

Influent Mercury Concentration, ng/L 

0

2

4

6

8

10

12

14

16

18

Number of Dentists per MGD 

Influent Hg

Dentists 

per Flow

Figure 4.5A: Influent Mercury Concentration and Dentists per Flow 
62 

0

10

20

30

40

50

60

70

C - Jul 04C - Jul 05

C - Jan 04C - Jan 05C - Jan 06

C - Jul 06

A - Jul 06A - Jul 05

B - Jul 05B - Jul 04

A - Jan 06

B - Jan 04

A - Jul 04

A - Jan 05A - Jan 04

B - Jan 05B - Jan 06

G - Jan

B - Jul 06

G - Jan

G - Jul 06

G - Jan

G - Jul 04G - Jul 05

F - Jan 05

F - Jul 04F - Jul 06F - Jul 05

F - Jan 06

H - Jul 05

I - Jul 04I - Jul 06

H - Jan 04

H - Jul 04

H - Jan 05H - Jan 06

H - Jul 06

I - Jan 04I - Jan 05

I - Jul 05

I - Jan 06

L - Jul 06L - Jul 05L - Jul 04

L - Jan 04L - Jan 05L - Jan 06

POTW and Time Period

Effluent Mercury Concentration, ng/L 

0

2

4

6

8

10

12

14

16

18

Number of Dentists per MGD 

Effluent Hg

Dentists per Flow

Figure 4.5B: Effluent Mercury Concentration and Dentists per Flow 

0

1

2

3

4

5

C - Jul 04C - Jul 05

C - Jan 04C - Jan 05C - Jan 06

C - Jul 06

A - Jul 06A - Jul 05

B - Jul 05B - Jul 04

A - Jan 06

B - Jan 04

A - Jul 04

A - Jan 05A - Jan 04

B - Jan 05B - Jan 06

G - Jan

B - Jul 06

G - Jan

G - Jul 06

G - Jan

G - Jul 04G - Jul 05

F - Jan 05

F - Jul 04F - Jul 06F - Jul 05

F - Jan 06

H - Jul 05

I - Jul 04I - Jul 06

H - Jan 04

H - Jul 04

H - Jan 05H - Jan 06

H - Jul 06

I - Jan 04I - Jan 05

I - Jul 05

I - Jan 06

L - Jul 06L - Jul 05L - Jul 04

L - Jan 04L - Jan 05L - Jan 06

POTW and Time Period

Biosolids Mercury Concentration, mg/kg (dry)

0

3

6

9

12

15

18

Number of Dentists per MGD 

Biosolids Hg

Dentists per Flow

Figure 4.5C: Biosolids Mercury Concentration and Dentists per Flow 
63 

4.6 DENTISTS WITHOUT SEPARATORS PER FLOW

Once it was determined that a potential positive relationship between the numbers of dentists and 

mercury levels at POTWs exists, whether the use of amalgam separators by dentists also impacts 

mercury levels was examined. The number of dentists at each treatment plant that did not have 

separators installed was first determined. This number was then divided by total flow to the 

plant. In doing so, it was found that there is no statistically significant relationship between this 

value and the median influent mercury concentration as shown in Figure 4.6A. 

The median effluent mercury concentration is negatively correlated to the number of dentists 

without separators per flow (Figure 4.6B). This negative correlation was highly significant with 

a p-value of 0.004. This clearly indicates that increasing amalgam separator installations did not 

necessarily lead to a decrease in effluent mercury concentrations at the POTWs in the project. 

This result could be due, in part, to the limited number of amalgam separator installations that 

occurred at the POTWs with generally lower effluent mercury concentrations at the study’s 

outset. 

For biosolids, there was a statistically significant increase in the median mercury concentration 

with an increase in the number of dentists per flow that do not have separators installed (Figure 

4.6C). With a p-value of 0.0002, this correlation is highly statistically significant. In fact, of the 

relationships examined in the analysis of the data collected during the sampling project, the 

strength of this relationship is second only to that of the relationship between influent total 

mercury concentrations and the number of dentists per MGD. Since the use of separators is 

expected to reduce particulate mercury entering wastewater treatment plants, this relationship 

strongly supports the hypothesis that amalgam separators can reduce the quantity of mercury 

entering treatment plants. 
64 

0

100

200

300

400

500

600

A - Jul 05

A - Jan 06

A - Jul 06

G - Jul 05

G - Jan 06

G - Jul 06

I - Jan 04

I - Jul 04

I - Jan 05

I - Jul 05

I - Jan 06

I - Jul 06

A - Jan 05

G - Jan 05

G - Jul 04

G - Jan 04

C - Jul 04C - Jul 05

C - Jan 04C - Jan 05C - Jan 06

C - Jul 06B - Jul 05B - Jul 04

B - Jan 04

A - Jul 04

B - Jan 05B - Jan 06

A - Jan 04

B - Jul 06

F - Jul 06

F - Jan 06F - Jan 05

F - Jul 04F - Jul 05

POTW and Time Period

Influent Mercury Concentration, ng/L 

0

1

2

3

4

5

Number of Dentists Without Separators per MGD 

Influent Hg

Dentists without 

Separators per 

Flow

Figure 4.6A: Influent Mercury Concentration and Dentists without Separators per Flow 

0

4

8

12

16

20

A - Jul 05

A - Jan 06

A - Jul 06

G - Jul 05

G - Jan 06

G - Jul 06

I - Jan 04

I - Jul 04

I - Jan 05

I - Jul 05

I - Jan 06

I - Jul 06

A - Jan 05

G - Jan 05

G - Jul 04

G - Jan 04

C - Jul 04C - Jul 05

C - Jan 04C - Jan 05C - Jan 06

C - Jul 06B - Jul 05B - Jul 04

B - Jan 04

A - Jul 04

B - Jan 05B - Jan 06

A - Jan 04

B - Jul 06

F - Jul 06

F - Jan 06F - Jan 05

F - Jul 04F - Jul 05

POTW and Time Period

Effluent Mercury Concentration, ng/L 

0

1

2

3

4

5

Number of Dentists Without Separators per MGD 

Dentists without 

Separators per 

Flow

Effluent Hg

Figure 4.6B: Effluent Mercury Concentration and Dentists without Separators per Flow 
65 

0

1

2

3

4

A - Jul 05

A - Jan 06

A - Jul 06

G - Jul 05

G - Jan 06

G - Jul 06

I - Jan 04

I - Jul 04

I - Jan 05

I - Jul 05

I - Jan 06

I - Jul 06

A - Jan 05

G - Jan 05

G - Jul 04

G - Jan 04

C - Jul 04C - Jul 05

C - Jan 04C - Jan 05C - Jan 06

C - Jul 06B - Jul 05B - Jul 04

B - Jan 04

A - Jul 04

B - Jan 05B - Jan 06

A - Jan 04

B - Jul 06

F - Jul 06

F - Jan 06

F - Jul 04F - Jul 05

POTW and Time Period

Biosolids Mercury Concentration, mg/kg (dry) 

0

1

2

3

4

5

Number of Dentists Without Separators per MGD 

Dentists without 

Separators per 

Flow

Biosolids Hg

Figure 4.6C: Biosolids Mercury Concentration and Dentists without Separators per Flow 

4.7 PLANT FLOW

An examination of plant flow as it is related to median mercury concentrations in POTW 

influent, effluent and biosolids revealed no statistically significant (p >0.10) correlations. These 

relationships are shown in Figures 4.7A through 4.7C. 
66 

0

200

400

600

POTW

"K"

POTW

"D"

POTW

"B"

POTW

"E"

POTW

"J"

POTW

"A"

POTW

"L"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"C"

POTW

"I"

Influent Mercury Concentration, ng/L

0

50

100

150

Average Plant Flow, MGD

Influent Hg

Plant Flow

Figure 4.7A: Influent Mercury Concentration and Average Plant Flow 

0

1

2

3

4

POTW

"K"

POTW

"D"

POTW

"B"

POTW

"E"

POTW

"J"

POTW

"A"

POTW

"L"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"C"

POTW

"I"

Biosoilds Mercury Concentration, mg/kg (dry)

0

50

100

150

200

Average Plant Flow, MGD 

Biosolids Hg

Plant Flow

Figure 4.7B: Biosolids Mercury Concentration and Average Plant Flow 
67 

0

10

20

30

40

POTW

"K"

POTW

"D"

POTW

"B"

POTW

"E"

POTW

"J"

POTW

"A"

POTW

"L"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"C"

POTW

"I"

Effluent Mercury Concentration, ng/L

0

20

40

60

80

100

120

Average Plant Flow, MGD

Effluent Hg 

Plant Flow

Figure 4.7C: Effluent Mercury Concentration and Average Plant Flow 

4.8 NON-INDUSTRIAL FLOW

Another factor that was examined to determine the effects on mercury concentrations was the 

percentage of industrial flow. POTWs “D” and “E” were excluded because the percent of the 

influent from industrial sources is not known. POTW “B” was excluded because the effects of 

the interceptor cleaning project could skew the results. The data collectively indicated that there 

is an increase in influent, effluent, and biosolids mercury concentrations with an increase in 

relative non-industrial flow (Figures 4.8A through 4.8C). Although influent and biosolids 

mercury concentrations appear to be more strongly related to non-industrial flow than effluent 

concentrations do, the correlations for median influent and median biosolids are not statistically 

significant (p >0.05), while that for median effluent is, with a p-value of 0.036. 

However, included within these results are those for both combined and separate sewer systems. 

If the stormwater included in the combined systems had a lower mercury concentration than 

other non-industrial flows did, the strength of these relationships may have been influenced. To 

eliminate this variable, only the results from POTWs with solely separate sewer systems were 

examined (Figures 4.8D through 4.8F). The correlations between influent, effluent, and biosolids 

median mercury concentrations and percentage of non-industrial flow were all found to be 

statistically non-significant at the α = 0.05 level. However, these correlations for median influent 

and median effluent, but not median biosolids, mercury concentrations were significant at the α = 

0.10 level. Considering all of these results, it is possible that industrial flow generally contains 

less mercury than other wastewaters, thereby providing dilution when it is present. This is 

consistent with the findings of a 2000 NACWA (formerly Association of Metropolitan Sewerage 
68 

Agencies) report titled “Evaluation of Domestic Sources of Mercury” that demonstrated that 

domestic wastewater is a significant source of mercury at POTWs. 

0

100

200

300

400

500

600

POTW "G" POTW "C" POTW "H" POTW "F" POTW "I" POTW "A" POTW "J" POTW "K" POTW "L"

Influent Mercury Concentration, ng/L

0

20

40

60

80

100

% of Flow due to Non-Industrial Sources

Influent Hg

Non-Industrial Flow

Figure 4.8A: Influent Mercury Concentration and Non-Industrial Flow 
69 

0

10

20

30

40

POTW "G" POTW "C" POTW "H" POTW "F" POTW "I" POTW "A" POTW "J" POTW "K" POTW "L"

Effluent Mercury Concentration, ng/L 

0

25

50

75

100

% of Flow due to Non-Industrial Sources 

Non-Industrial Flow

Effluent Hg

Figure 4.8B: Effluent Mercury Concentration and Non-Industrial Flow 

0

1

2

3

4

5

POTW "G" POTW "C" POTW "H" POTW "F" POTW "I" POTW "A" POTW "J" POTW "K" POTW "L"

Biosolids Mercury Concentration, mg/kg (dry) 

0

25

50

75

100

% of Flow due to Non-Industrial Sources 

Non-Industrial Flow

Biosolids Hg

Figure 4.8C: Biosolids Mercury Concentration and Non-Industrial Flow 
70 

0

100

200

300

400

500

600

POTW "G" POTW "H" POTW "F" POTW "I" POTW "J" POTW "L"

Influent Mercury Concentration, ng/L 

0

20

40

60

80

100

% of Flow due to Non-Industrial Sources (Separate Sewer Systems)

Influent Hg

Non-Industrial Flow

Figure 4.8D: Influent Mercury Concentration and Non-Industrial Flow in Separate Sewer Systems 

0

10

20

30

40

POTW "G" POTW "H" POTW "F" POTW "I" POTW "J" POTW "L"

Effluent Mercury Concentration, ng/L 

0

25

50

75

100

% of Flow due to Non-Industrial Sources (Separate Sewer Systems)

Non-Industrial Flow

Effluent Hg

Figure 4.8E: Effluent Mercury Concentration and Non-Industrial Flow in Separate Sewer Systems 
71 

0

1

2

3

4

5

POTW "G" POTW "H" POTW "F" POTW "I" POTW "J" POTW "L"

Biosolids Mercury Concentration, mg/kg (dry) 

0

25

50

75

100

% of Flow due to Non-Industrial Sources (Separate Sewer Systems)

Non-Industrial Flow

Biosolids Hg

Figure 4.8F: Biosolids Mercury Concentration and Non-Industrial Flow in Separate Sewer Systems 

4.9 FRESHWATER VS. MARINE/ESTUARINE DISCHARGERS

A comparison was made between the POTWs that have freshwater discharges and those with 

marine or estuarine discharges. There are no clear differences between these two categories of 

treatment plants for both influent and biosolids mercury concentrations (Figures 4.9A and 4.9C). 

It was found, however, that POTWs with discharges to freshwater generally have lower effluent 

mercury concentrations than did their saltwater counterparts (Figure 4.9B). The reason for this 

difference is unclear and should be further examined. It is possible that POTWs that discharge to 

freshwater are providing different levels of treatment than POTWs that discharge to 

marine/estuarine waters for the removal of wastewater constituents with which effluent mercury 

concentrations are incidentally lowered. 
72 

0

200

400

600

800

1000

1200

1400

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"A"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Influent Mercury Concentration, ng/L 

Freshwater Discharges Marine/Estuarine Discharges

Figure 4.9A: Influent Mercury Concentration and Receiving Water Type 

0

10

20

30

40

50

60

Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"A"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Effluent Hg Concentration, ng/L

Freshwater Discharges Marine/Estuarine Discharges

Figure 4.9B: Effluent Mercury Concentration and Receiving Water Type 
73 

0

1

2

3

4

5

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"A"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Biosolids Mercury Concentration, mg/kg (dry)

Freshwater Discharges Marine/Estuarine Discharges

Figure 4.9C: Biosolids Mercury Concentration and Receiving Water Type 

4.10 IRON SALTS

The results from the sampling indicate that those POTWs that do not use iron salts (e.g., ferric 

chloride, ferrous sulfate) have generally higher effluent mercury concentrations (Figure 4.10A). 

Since iron salts are used as flocculants, especially for metals removal, this trend is not 

unexpected. The POTWs that use iron salts are the same as the POTWs with freshwater 

discharges. This may help to explain the difference in effluent mercury concentrations that was 

observed when freshwater discharges were compared to saltwater/estuarine discharges. There 

was no relationship found between use of iron salts and biosolids mercury concentrations (Figure 

4.10B). This finding suggests that effluent mercury levels may be affected by iron salt addition, 

while its impact on biosolids mercury levels is not significant. This would be consistent with the 

relative magnitude of effluent versus biosolids mercury quantities. 
74 

0

10

20

30

40

50

60

Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06 Jan-04 Jul-04 Jan-05 Jul-05 Jan-06 Jul-06

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"A"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Effluent Hg Concentration, ng/L

Use Iron Salts Do Not Use Iron Salts

Figure 4.10A: Effluent Mercury Concentration and Use of Iron Salts 

0

1

2

3

4

5

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"A"

POTW

"I"

POTW

"J"

POTW

"K"

POTW

"L"

Biosolids Mercury Concentration, mg/kg (dry)

Use Iron Salts Do Not Use Iron Salts

Figure 4.10B: Biosolids Mercury Concentration and Use of Iron Salts 
75 

4.11 HAULED-IN WASTES

POTWs that accept hauled-in wastes were examined to determine if they have higher influent, 

effluent, and biosolids mercury concentrations. No differences were found in these three 

parameters between plants that do and do not accept hauled-in waste (Figures 4.11A through 

4.11C). 

0

100

200

300

400

500

600

700

800

900

1000

POTW

"D"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"L"

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"E"

POTW

"I"

POTW

"J"

POTW

"K"

Influent Mercury Concentration, ng/L

Accept Hauled-in 

Waste

Do Not Accept Hauled-in Waste

Figure 4.11A: Influent Mercury Concentrations and Hauled-in Waste 
76 

0

5

10

15

20

25

30

35

POTW

"D"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"L"

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"E"

POTW

"I"

POTW

"J"

POTW

"K"

Effluent Mercury Concentration, ng/L

Accept Hauled-in Waste Do Not Accept Hauled-in Waste

Figure 4.11B: Effluent Mercury Concentrations and Hauled-in Waste 

0

1

2

3

4

5

POTW

"D"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"L"

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"E"

POTW

"I"

POTW

"J"

POTW

"K"

Biosolids Mercury Concentration, mg/kg (dry)

Accept Hauled-in Waste Do Not Accept Hauled-in Waste

Figure 4.11C: Biosolids Mercury Concentrations and Hauled-in Waste 
77 

4.12 POPULATION DENSITY

Population density within each POTW’s service area was also examined to determine if there 

was a correlation with mercury concentrations. It was found that there is no clear trend between 

an increasing number of persons per square mile and the influent, effluent, or biosolids mercury 

concentrations (Figures 4.12A through 4.12C). A more in-depth examination of influent trends, 

such as decreasing flow due to water conservation activities, could provide more insight. 

0

100

200

300

400

500

600

POTW

"C"

POTW

"B"

POTW

"A"

POTW

"J"

POTW

"K"

POTW

"L"

POTW

"D"

POTW

"E"

POTW

"I"

POTW

"H"

POTW

"F"

POTW

"G"

Influent Mercury Concentration, ng/L

0

1000

2000

3000

4000

5000

6000

7000

8000

Persons per Square Mile

Influent Hg

Population 

Density

Figure 4.12A: Influent Mercury Concentration and Population Density 
78 

0

5

10

15

20

25

30

35

40

45

50

POTW

"C"

POTW

"B"

POTW

"A"

POTW

"J"

POTW

"K"

POTW

"L"

POTW

"D"

POTW

"E"

POTW

"I"

POTW

"H"

POTW

"F"

POTW

"G"

Effluent Mercury Concentration, ng/L

0

1000

2000

3000

4000

5000

6000

7000

8000

Persons per Square Mile

Population Density

Effluent Hg

Figure 4.12B: Effluent Mercury Concentration and Population Density 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

POTW

"C"

POTW

"B"

POTW

"A"

POTW

"J"

POTW

"K"

POTW

"L"

POTW

"D"

POTW

"E"

POTW

"I"

POTW

"H"

POTW

"F"

POTW

"G"

Biosolids Mercury Concentration, mg/kg (dry)

0

1000

2000

3000

4000

5000

6000

7000

8000

Persons per Square Mile

Biosolids Hg

Population Density

Figure 4.12C: Biosolids Mercury Concentration and Population Density 
79 

4.13 COMBINED OR SEPARATE COLLECTION SYSTEMS

Mercury concentrations at POTWs with combined collection systems were compared to those 

with separate collection systems. As shown in Figures 4.13A through 4.13C, there was not a 

clear difference between those areas with combined sewers versus those with separate sewers. 

0

250

500

750

1000

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"K"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"I"

POTW

"J"

POTW

"L"

Influent Mercury Concentration, ng/L 

Combined Sewer Systems Separate Sewer Systems

Figure 4.13A: Influent Mercury Concentrations and Sewer System Type 
80 

0

10

20

30

40

50

60

70

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"K"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"I"

POTW

"J"

POTW

"L"

Effluent Mercury Concentration, ng/L 

Combined Sewer Systems Separate Sewer Systems

Figure 4.13B: Effluent Mercury Concentrations and Sewer System Type 

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

Jan-04

Jul-04

Jan-05

Jul-05

Jan-06

Jul-06

POTW

"A"

POTW

"B"

POTW

"C"

POTW

"D"

POTW

"E"

POTW

"K"

POTW

"F"

POTW

"G"

POTW

"H"

POTW

"I"

POTW

"J"

POTW

"L"

Biosolids Mercury Concentration, mg/kg (dry)

Combined Sewer Systems Separate Sewer Systems

Figure 4.13C: Biosolids Mercury Concentrations and Sewer System Type 
81 

5. SUMMARY AND CONCLUSIONS

An objective of this study was to examine the effect of practical and reasonable measures of 

controlling significant sources of mercury to the POTW collection system, such as installing 

dental amalgam separators, on the ability of POTWs to comply with anticipated effluent 

limitations. Mercury levels at various POTWs were monitored for a three-year period. These 

levels were then compared to amalgam separator use in the treatment plant’s service area and 

various other factors to determine which were most strongly related. 

Collectively, the influent mercury concentrations at the POTWs in the study were found to be 

highly variable, ranging from 14 ng/L to 3080 ng/L. When considering facilities individually, 

some exhibited decreases in their influent mercury concentrations over the course of the 

sampling project, while others had apparent increases. Effluent mercury concentrations also 

exhibited variability between plants but less variability within plants. For six-month periods, 

median effluent mercury concentrations were as low as 1.4 ng/L and as high as 45.15 ng/L. For 

the biosolids, there was variability both within and among the plants. For six-month periods, 

median biosolids concentrations were as low as 0.1 mg/kg and as high as 2.5 mg/kg. 

For the most part, the mercury removal efficiency from influent to effluent was relatively the 

same at each of the treatment plants and generally greater than 90%. While the POTWs were 

similar in this aspect, in other ways they were not. For instance, the percent of effluent mercury 

that was dissolved was found to vary based on the POTW. The highest median effluent 

dissolved mercury percentage occurred at POTW “G” (59%), while the lowest was at POTW 

“A” (22%). Additionally, at eight of the POTWs, there appeared to be a direct relationship 

between effluent mercury concentration and effluent TSS concentration while, at the remaining 

four POTWs, this relationship did not appear to exist. It is unclear as to why effluent TSS 

concentrations appear to be a good predictor of effluent mercury concentrations at some POTWs 

but not at others. 

Since a primary an objective for this study was to determine whether the installation of amalgam 

separators in dentists’ offices impacts mercury concentrations at treatment plants, it was first 

demonstrated that the mercury discharged by dentists impacts the amounts of mercury observed 

at the plants. Collectively, the study results indicate that the presence of dentists in a POTW’s 

collection system influences mercury concentrations at the plant. An increase in dentists per 

flow was significantly related to higher influent and effluent mercury concentrations. There was 

no relationship, however, between numbers of dentists and biosolids mercury concentrations. 

This may be due to amalgam separators being installed by dentists in many of the POTWs’ 

service areas. 

It was found that, collectively, an increase in the number of dentists without amalgam separators 

installed resulted in higher biosolids mercury, indicating that amalgam separators could be 

reducing biosolids mercury concentrations. However, at individual plants, the installation of 

separators did not always lead to a decrease in biosolids mercury concentrations. It may be that 

the amalgam separator installation alone is not enough to result in permanent mercury 

reductions. Some of the plants that showed decreases in mercury also have programs in place to 

ensure that the separators are well operated and maintained. It is also possible that, on an 
82 

individual-facility basis, apparent reductions or the lack thereof could be the result of other 

impacting factors such as previous elevated loadings related to sewer cleaning, construction 

activities, or historic depositions of mercury in the collection system. 

Effluent mercury concentrations were found to decrease at several POTWs as the percentage of 

dentists with amalgam separators increased. The POTWs that exhibited reductions all had 

relatively high effluent concentrations at the start of the study. It is possible that there were also 

reductions at the other POTWs, but these reductions were too small to be distinguished due to 

plant variability. Collectively, there was a negative correlation between the number of dentists 

without separators per flow and effluent mercury concentration. This may partly be a result of 

the limited number of amalgam separator installations that occurred at the POTWs that had 

generally lower effluent mercury concentrations at the study’s outset. 

In addition, based mostly on the results from POTW “I,” where 93% of the dental facilities had 

dental amalgam separators installed up to two years prior to the start of the study, it may be 

hypothesized that mercury reductions following separator installations can take several years or 

even longer to demonstrate significant trends. If so, this is likely due to the amount of time that 

is necessary to flush accumulated amalgam particles from the collection system. The timeframe 

of this study may not have been long enough to determine significant trends for POTWs whose 

dental facilities were in the process of installing amalgam separators during the course of the 

study. 

It should also be noted that some of the treatment plants exhibited reductions even without an 

increase in amalgam separator installation. These reductions may have come from changes in 

treatment processes or the implementation of BMPs. Therefore, on a facility-specific basis, the 

effectiveness of amalgam separators in reducing mercury should be examined in light of other 

activities that are occurring within the POTW and its service area to determine which factors are 

actually contributing to an observed reduction. 

In addition to the presence of dentists within a POTW’s collection area, several other factors 

were found to exhibit relationships with mercury levels at treatment plants. Those POTWs that 

use iron salts and those that discharge to freshwater systems were generally found to have lower 

effluent mercury concentrations than those that did not. Iron salts such as ferric chloride are 

typically used at wastewater treatment facilities as flocculants to increase settling; therefore, it is 

possible that iron salts lead to increased amounts of mercury settling out of the wastewater. 

Based on individual results found at POTW “A” and POTW “B”, sewer cleaning projects can 

have significant effects on biosolids mercury levels and potentially on influent and effluent 

mercury levels. This is because sewer cleaning likely resuspends particles that have settled in 

the sewer collection system, enabling them to migrate to the POTWs. 

Effluent and biosolids mercury concentrations at a treatment plant were also found to be related 

to industrial flow, although the statistical significance of this relationship was less than for other 

factors. An increase in the percent of non-industrial flow to a plant mostly resulted in increasing 

mercury levels. This likely reflects the fact that industrial flow generally has lower mercury 
83 

concentrations than other types of collection system flow and therefore, when it is present, tends 

to dilute wastewater mercury from other sources. 

Other factors that were examined but not found to be related to mercury levels were plant flow, 

population density, whether the POTW accepts hauled-in wastes, and whether the collection 

system is comprised of combined or separate sewers. 

Overall, the results of this study indicate that many factors, not just amalgam separators, 

influence mercury concentrations. Therefore, a facility cannot predict with certainty that 

amalgam separators will decrease mercury concentrations without also exploring the other 

potential contributors to current mercury levels. Without this, the benefits resulting from dental 

amalgam separator installation cannot be reliably predicted. Each facility must look at its own 

individual conditions to determine the potential benefit of amalgam separator installation. 

Additionally, it should be noted that, even in those POTWs with demonstrably successful 

amalgam separator programs, effluent mercury concentrations were not low enough to 

consistently meet current and imminent effluent limits faced by some POTWs (e.g., 1.3 ng/L in 

the Great Lakes). 

While installing amalgam separators does not generally appear, at least within the timeframe of 

this study, to significantly reduce effluent mercury concentrations, a resulting reduction in 

biosolids mercury concentrations does appear to be likely according to the results. The data 

collected during the study do support a conclusion that the use of separators can decrease the 

amount of particulate mercury entering POTWs, thereby decreasing the amount of mercury that 

would be removed by plant processes and deposited in the biosolids. 

6. IMPLICATIONS FOR FURTHER RESEARCH

Evaluations of the data resulting from this study have revealed several implications for further 

research. The apparent gaps, if filled, may lead to a better understanding of mercury at POTWs 

and the effects of installing amalgam separators. Additional work could be conducted by using 

the data that was collected during the sampling project and supplementing it with additional data 

and information collection. 

Only limited dissolved mercury analysis was conducted during the sampling project. Further 

quantification of dissolved mercury levels would lead to a better understanding of the relative 

levels of particulate mercury verses dissolved mercury. This would be helpful since, at this 

point, it appears that amalgam separators mostly influence particulate mercury. 

This sampling project indicates that the results of influent sampling for mercury concentrations 

at POTWs is highly variable within each facility as well as between facilities. It has been 

suggested that influent loading, enabling an analysis of the overall quantity of mercury entering a 

POTW regardless of flow, would be a more useful indicator of the effectiveness of amalgam 

separator installation. However, the amount of data collected during the sampling project was 

too limited to address the influent variability and therefore believed to be not robust enough for 
84 

influent loading calculations to provide insight. More influent data could be collected that would 

enable in-depth examinations of influent loadings, including day-to-day and seasonal variability 

and the potential effects of water conservation activities. 

In addition, biosolids loadings could not be determined with the information collected from the 

POTWs during the study. Further information collection that includes biosolids generation 

and/or disposal quantities would allow for the calculation of these loadings. While amalgam 

separators may not enable facilities to meet part-per-trillion-level effluent limits, they are likely 

reducing the overall loading to the POTW as reflected in biosolids mercury concentrations. 

Considering the interfacility differences in biosolids characteristics, calculations of biosolids 

loadings would be a better measure than concentrations and could confirm this. It is 

hypothesized that the observed significance of the relationship between dental amalgam 

separator installation and biosolids mercury levels could be improved even further if loadings 

were evaluated instead of concentrations. Such further evaluation may be possible with existing 

data and is therefore highly recommended as an addendum to this study. 

This sampling project indicates a clear difference in effluent mercury concentrations between 

facilities that discharge to freshwaters and facilities that discharge to marine environments. The 

possible reason(s) for this, including the use of iron salts, should be further explored. Looking at 

NPDES permit limits for parameters could provide some insight. However, often POTWs 

provide a higher level of treatment than what is required by NPDES permits; therefore, it is 

likely to be more useful to look at actual discharge concentrations. 

Each POTW in the study provided a list of chemicals used for treatment of wastewater. 

However, only limited information is currently available regarding the mercury concentrations in 

the chemicals used at these particular facilities. Previous unpublished work by some POTWs in 

the study has indicated that the mercury concentrations in such chemicals typically constitute 

negligible contributions to POTW effluent and biosolids mercury concentrations, but further 

research in this area is warranted. If, for example, chemical addition associated with 

chlorination/dechlorination at a particular treatment plant is found to potentially impact effluent 

mercury concentrations, a comparison between effluent samples collected before dechlorination 

and those collected after dechlorination could be necessary. 

Each POTW in the study collected its own influent, effluent, and biosolids samples. Some 

facilities collected manual samples, while other facilities used automatic samplers. Samples 

were also either grab samples or composite samples (both time- and flow-paced). The potential 

effect of these varying sampling techniques on the data is uncertain. A comparison of the 

different techniques may be warranted in the future, or further research may require all facilities 

to use the same sampling method. 

It is also possible that temporal differences exist within the data. Further data collection and 

analysis could be conducted to establish whether seasonal, daily or hourly differences exist in the 

mercury concentrations in POTW influent, effluent, and biosolids. 

 
85 

No comparisons were made between the data collected from this study and other projects. By 

doing so, a more complete picture of mercury flux at POTWs could be seen. It may also clarify 

whether the variability that was seen in this project is common to mercury sampling in general. 

Finally, this sampling project clearly warrants follow-up sampling and data analysis. It would be 

useful to examine mercury levels at each of the studied POTWs in the future to determine 

whether trends seen in the current study have continued or whether trends not previously 

apparent have developed. In addition, the results from future sampling could also be compared 

with how well amalgam separators are operated and maintained to establish the extent to which 

this may be a key factor for continuing mercury reductions. 
A 

APPENDIX A: DATA
Appendix A: POTW “A”

A 

87 

Biosolids

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS 

(mg/L)

Hg 

(mg/kg, dry)

7/15/2003 42 232 157 8.6 4.8 n/a 8 1.2

8/29/2003 39 126 137 9.5 2 8 3.1

9/24/2003 39 73.7 137 11.9 2.6 8 1.1

10/8/2003 35 63.2 102 9.4 9 1.1

11/18/2003 36.44 257 219 16.7 14 1.6

12/16/2003 56.73 137 106 15.6 15 1.1

1/13/2004 44.05 297 183 10.1 10 1.0

2/12/2004 39.81 247 161 12.6 14 1.0

3/10/2004 37.96 3080 641 19.3 44 2.3

4/21/2004 54.19 77.7 173 11.7 7 1.3

5/12/2004 46.91 117 260 9.3 10 0.9

6/16/2004 38.32 130 265 8.6 7 0.9

7/21/2004 37.16 146 214 13.6 16 0.7

8/10/2004 36.38 103 291 8.9 17 1.8

9/15/2004 40.57 103 211 10.9 16 0.6

10/13/2004 38.9 107 185 7.8 5 1.0

11/16/2004 43.97 220 135 15 24 1.1

12/15/2004 51.83 85.5 138 17.6 35 0.99

1/27/2005 46.78 82.3 140 7.4 18 0.808

2/22/2005 54.97 211 119 7.1 10 1.168

3/17/2005 49.69 61.1 182 14 0.765

4/19/2005 50.12 119 139 7.9 <1.0 14 0.8035

5/25/2005 73.09 146 128 15.2 2.9 15 1.1165

6/22/2005 43.35 231 6.8 2.2 13 1.0895

7/26/2005 38.88 229 257 7.8 12 0.828

8/30/2005 72.51 369 134 10.7 13 0.784

9/28/2005 37.88 125 263 17 0.9165

10/5/2005 37.29 663 221 11.0 25.0 0.984

11/9/2005 81.96 264 194 14 45 0.657

12/13/2005 47.5 71.9 127 8.4 21 1.116

1/10/2006 49.64 81.4 137 5.7 21 0.519

2/7/2006 63.77 203 94 7.5 12 0.935

3/28/2006 38.95 153 167 12.2 14 2.327

4/18/2006 38.52 189 153 1.4 9 2.067

5/23/2006 54.09 88.9 139 5.5 1.1 14 1.002

6/27/2006 78.79 68.6 71 4.9 1.2 11

Influent Effluent

Percent of Dental Facilities/Dentists 

Operating Dental Amalgam 

Separators

July 1, 2003 0

January 1, 2004 0

July 1, 2004 0

January 1, 2005 98

July 1, 2005 100

January 1, 2006 100

July 1, 2006 100
Appendix A: POTW “A”

88 

Influent Mercury EPA 245.7

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity n/a

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
Appendix A: POTW “B” 

89 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg 

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity 

(NTU)

TSS 

(mg/L)

07/09/03 181 46.4 166 3.50 1.06 12

08/07/03 88 26.3 126 1.56 0.92 1

09/04/03 87 415 130 1.83 0.89 2

10/02/03 96 218 69 3.86 1.05 2.95 2

11/06/03 82.4 193 136 3.75 1.16 9.02 3

12/03/03 99 286 83 2.91 0.515 2.5 3

01/08/04 104 24.3 87 3.34 0.927 5.88 4.1

02/05/04 104 15.9 59 2.48 1.05 3.4 7

03/04/04 133 50.2 110 1.84 1 2 2.5

04/06/04 114 64.2 140 4.73 1.06 3.01 4.3

05/06/04 106 47.8 174 3.18 1.07 2.07 4

06/03/04 95 69.8 100 2.33 0.916 1.99 2

07/07/04 106 79.6 110 2.42 0.978 2.24 1.7

08/05/04 104 238 165 2.55 1.02 2.24 2.6

09/13/04 72 82.7 159 7.11 1.23 7.03 8

10/07/04 66 75.4 135 3.47 1.07 11.1 4

11/10/04 77 106 112 2.06 0.805 1.4 3

12/08/04 84 54.8 119 2.11 0.82 1.28 1

01/05/05 206 49 60 3.23 0.95 1.99 3

02/02/05 95 33.8 83 2.56 0.933 2.01 7

03/03/05 104 80.3 185 5.79 0.93 3.52 8

04/06/05 120 116 80 1.68 0.98 1.55 1

05/03/05 113 91.5 116 3.50 1.06 4.86 3

06/13/05 98 365 123 1.40 0.777 0.95 1

07/07/05 77 59.5 99 1.64 0.971 1 2

08/03/05 89 184 110 2.24 0.97 1.26 1

09/13/05 78 236 172 2.47 0.9 0.91 1

10/05/05 75 448 150 2.88 1 1.37 2

11/02/05 75 51 108 3.75 1 2.1 4

12/08/05 83 90.5 121 2.90 1.09 1.87 4

01/12/06 86 21.8 58 2.51 84 1.28 2

02/09/06 100 37.35 55 2.49 0.96 2.8 4

03/06/06 69 225 503 3.34 1.78 2.56 3

4/6/2006 69 91.15 93 2.77 0.93 2.2 3

5/3/2006 64 45.15 91 2.08 1.08 2.14 5

6/7/2006 67 277 102 3.26 1.24 2.65 3

Influent Effluent
Appendix A: POTW “B” 

90 

Sample 

Date

Hg 

(mg/kg, dry) Sample Date

Hg 

(mg/kg, dry) Sample Date

Hg 

(mg/kg, dry) Sample Date

Hg 

(mg/kg, dry)

07/01/03 1.7 8/3/2004 0.57 8/2/2005 0.88 11/1/2005 1.11

07/08/03 2.1 8/10/2004 1.11 8/9/2005 4.96 11/8/2005 2.30

07/15/03 0.9 8/17/2004 2.09 8/16/2005 1.93 11/15/2005 1.07

07/22/03 1.6 8/24/2004 1.25 8/21/2005 3.30 11/22/2005 0.46

08/05/03 0.5 8/31/2004 0.91 8/22/2005 3.09 11/29/2005 2.27

08/12/03 1.0 9/7/2004 0.36 8/23/2005 2.22 12/6/2005 0.36

08/19/03 0.2 9/14/2004 0.49 8/24/2005 2.60 12/11/2005 0.35

08/26/03 0.9 9/21/2004 0.88 8/25/2005 2.05 12/12/2005 0.31

09/02/03 1.9 9/28/2004 1.55 8/26/2005 2.69 12/13/2005 0.58

09/09/03 1.6 10/5/2004 0.56 8/27/2005 5.10 12/14/2005 0.38

09/16/03 1.3 10/12/2004 0.55 8/28/2005 5.22 12/15/2005 0.36

09/23/03 2.5 10/19/2004 1.74 8/29/2005 4.51 12/20/2005 0.53

09/30/03 0.8 10/26/2004 0.42 8/30/2005 1.87 12/27/2005 0.30

10/07/03 0.9 11/2/2004 0.47 9/1/2005 4.78 1/3/2006 0.56

10/14/03 0.1 11/9/2004 1.07 9/2/2005 3.00 1/24/2006 0.14

10/21/03 0.5 11/16/2004 0.42 9/3/2005 2.00 2/7/2006 0.61

10/28/03 0.7 11/23/2004 0.56 9/4/2005 1.75 2/14/2006 0.91

11/04/03 0.74 11/30/2004 1.00 9/5/2005 2.60 2/21/2006 38

11/11/03 0.70 12/7/2004 0.46 9/6/2005 5.02 2/28/2006 25.25

11/18/03 0.70 12/14/2004 0.51 9/7/2005 3.45 3/7/2006 1.76

11/25/03 0.70 12/21/2004 0.44 9/8/2005 14.02 3/14/2006 1.39

12/02/03 0.40 12/28/2004 0.70 9/9/2005 2.60 3/21/2006 0.56

12/09/03 0.37 1/11/2005 0.65 9/10/2005 10.68 3/28/2006 0.59

12/16/03 0.56 1/18/2005 0.73 9/11/2005 1.62 04/02/06 4.39

12/23/03 1.49 1/25/2005 0.30 9/12/2005 2.73 04/03/06 0.57

12/30/03 0.45 2/1/2005 0.36 9/13/2005 3.17 04/04/06 0.43

01/06/04 0.42 2/8/2005 1.32 9/14/2005 2.67 04/05/06 1.25

01/13/04 0.28 2/15/2005 1.12 9/15/2005 3.58 04/06/06 0.42

01/20/04 0.60 2/22/2005 0.44 9/16/2005 1.24 04/11/06 0.66

01/27/04 0.27 3/1/2005 0.97 9/17/2005 1.13 04/18/06 1.46

02/03/04 0.40 3/8/2005 0.77 9/18/2005 1.19 04/23/06 0.12

02/10/04 0.30 3/15/2005 0.65 9/19/2005 0.88 04/24/06 0.46

02/17/04 0.42 3/22/2005 0.36 9/20/2005 1.43 04/25/06 0.37

02/24/04 0.59 3/29/2005 0.72 9/21/2005 5.21 04/26/06 0.11

3/2/2004 0.36 4/5/2005 0.42 9/22/2005 1.61 04/27/06 0.30

3/9/2004 0.28 4/12/2005 0.81 9/23/2005 1.69 04/28/06 0.18

3/16/2004 0.29 4/19/2005 1.54 9/24/2005 2.15 04/29/06 0.26

3/23/2004 0.18 4/26/2005 0.50 9/25/2005 1.48 5/2/2006 0.46

3/30/2004 0.45 5/3/2005 0.88 9/26/2005 1.83 5/9/2006 0.38

4/6/2004 0.33 5/10/2005 1.45 9/27/2005 1.96 5/16/2006 0.25

4/13/2004 0.71 5/17/2005 0.48 9/28/2005 2.49 5/21/2006 1.64

4/20/2004 0.62 5/24/2005 0.64 9/29/2005 5.27 5/22/2006 0.46

4/27/2004 0.55 5/30/2005 0.54 9/30/2005 4.00 5/23/2006 0.43

5/4/2004 0.30 6/7/2005 1.44 10/1/2005 2.83 5/24/2006 0.73

5/11/2004 0.29 6/14/2005 5.54 10/2/2005 2.32 5/25/2006 0.41

5/18/2004 0.63 6/14/2005 3.56 10/3/2005 1.32 5/26/2006 0.67

5/25/2004 0.67 6/14/2005 3.76 10/4/2005 1.81 5/27/2006 0.57

6/1/2004 0.38 6/15/2005 1.51 10/5/2005 2.61 5/30/2006 0.81

6/8/2004 0.86 6/16/2005 2.50 10/6/2005 1.74 6/6/2006 1.07

6/15/2004 0.73 6/17/2005 18.49 10/7/2005 1.88 6/13/2006 0.80

6/22/2004 0.63 6/18/2005 0.45 10/8/2005 2.68 6/18/2006 0.59

6/25/2004 0.5 6/19/2005 5.61 10/9/2005 2.70 6/19/2006 0.54

6/26/2004 0.99 6/21/2005 0.90 10/10/2005 2.24 6/20/2006 0.34

6/27/2004 0.84 6/28/2005 2.27 10/11/2005 2.64 6/21/2006 0.73

6/29/2004 0.40 7/5/2005 1.17 10/12/2005 5.32 6/22/2006 0.66

7/14/2004 0.60 7/13/2005 0.88 10/13/2005 2.94 6/23/2006 0.68

7/21/2004 0.31 7/20/2005 4.96 10/18/2005 3.72 6/24/2006 6.11

7/28/2004 2.27 7/27/2005 1.93 10/25/2005 3.11 6/27/2006 0.57

Biosolids Biosolids Biosolids Biosolids
Appendix A: POTW “B” 

91 

Percent of Dental Facilities 

Operating Dental Amalgam 

Separators

July 1, 2003 5

January 1, 2004 6

July 1, 2004 7

January 1, 2005 8

July 1, 2005 9

January 1, 2006 9

July 1, 2006 9

Influent Mercury EPA 1631, Rev. E

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury EPA 245.1

Effluent Turbidity 2130B (Std. Methods 18th Ed.)

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
Appendix A: POTW “C” 

92 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg 

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity 

(NTU)

TSS

(mg/L)

07/09/03 40.8 18.5 102 2 0.86 6

08/07/03 23.4 54.6 116 2.01 1.33 4

09/04/03 23.7 44.9 123 1.97 0.9 4

10/02/03 33.2 58.5 75 2.92 0.894 3.99 3

11/06/03 20.6 185 189 3.47 1.04 3.33 4

12/03/03 25.5 46.6 83 3.72 1.27 3.96 5

01/08/04 27.8 50.0 77 3.16 1.04 5.46 3.9

02/05/04 27.4 29.6 84 3.24 1.12 3.2 5

03/04/04 36.1 29.7 111 2.68 0.967 3.18 4.8

04/06/04 31.2 62.4 146 3.45 1.26 3.35 4

05/06/04 28.9 23.6 110 0.51 0.415 2.47 4

06/03/04 25.4 37.2 85 2.61 1.06 3.14 3

07/07/04 24.3 34.75 80 1.94 0.973 1.58 2.9

08/05/04 23.3 80 117 2.70 1.16 1.99 1.8

09/13/04 22.4 57 88 1.80 0.97 2.14 2

10/07/04 19.9 115 119 2.15 1 2.66 3

11/10/04 19.1 78.7 129 2.85 1.01 1.9 2

12/08/04 23.4 42.7 67 2.98 0.94 3.3 5

01/05/05 77.8 39.8 54 4.52 0.86 3.44 5

02/02/05 24.1 62.1 90 1.88 0.892 3.86 5

03/03/05 27.5 42.6 110 2.60 0.78 4.23 8

04/06/05 28.9 49.5 68 2.02 0.96 2.89 4

05/03/05 28.3 23.7 71 2.24 0.86 3.43 5

06/13/05 27.9 137 117 1.65 0.851 2.1 4

07/07/05 20 177.5 291 2.12 1 1.26 3

08/03/05 20.6 481 70 1.36 0.78 1.31 4

09/13/05 20.8 51.5 250 1.61 0.98 1.73 2

10/05/05 19.5 72.2 146 2.29 1.07 2.06 4

11/02/05 21.1 62.3 123 1.98 0.85 2 4

12/08/05 20.1 38.1 119 2.45 1.3 3.09 5

01/12/06 22.1 60.9 82 2.43 0.95 2.68 6

02/09/06 24.5 37.3 60 3.22 0.92 3.3 8

03/06/06 19.4 27 90 2.34 0.82 4.05 6

04/06/06 19.9 32.8 68 2.74 1.05 2.3 4

5/3/2006 18.3 41.55 92 2.32 1.2 2.37 6

6/7/2006 19.2 79 103 3.48 1.38 2.38 4

Influent Effluent
Appendix A: POTW “C” 

93 

Sample 

Date

Hg 

(mg/kg, dry)

Sample 

Date

Hg 

(mg/kg, dry)

Sample 

Date

Hg 

(mg/kg, dry)

Sample 

Date

Hg 

(mg/kg, dry)

07/07/03 0.6 03/15/04 0.5 12/13/2004 0.5 9/12/2005 0.6

07/14/03 0.6 03/22/04 0.7 12/20/2004 0.4 9/19/2005 0.8

07/21/03 2.2 04/05/04 0.4 12/27/2004 0.8 9/26/2005 0.85

08/04/03 1.1 04/12/04 0.6 1/3/2005 0.5 10/10/2005 1.61

08/11/03 0.7 04/19/04 0.6 1/10/2005 0.5 10/17/2005 0.58

08/18/03 0.5 04/26/04 0.8 1/17/2005 0.4 10/24/2005 1.13

08/25/03 1.5 05/03/04 0.5 1/24/2005 0.4 11/7/2005 0.68

09/01/03 1.3 05/10/04 0.5 2/7/2005 0.8 11/14/2005 0.46

09/08/03 0.9 05/17/04 0.7 2/14/2005 0.4 11/21/2005 0.62

09/15/03 0.6 05/24/04 0.7 2/21/2005 0.4 11/29/2005 0.82

09/22/03 1.2 5/31/2004 0.8 3/7/2005 0.5 12/5/2005 0.75

09/29/03 0.9 6/7/2004 0.3 3/14/2005 0.4 12/12/2005 0.57

10/06/03 0.9 6/14/2004 0.5 3/21/2005 0.8 12/19/2005 0.47

10/13/03 0.6 6/22/2004 0.6 3/28/2005 0.4 12/26/2005 0.34

10/20/03 0.6 7/5/2004 0.6 4/11/2005 0.4 1/2/2006 0.49

10/27/03 0.6 7/19/2004 0.8 4/11/2005 0.4 1/16/2006 0.74

11/03/03 0.7 7/26/2004 0.6 4/18/2005 0.4 1/23/2006 0.59

11/10/03 0.8 8/2/2004 0.8 4/25/2005 0.3 2/6/2006 0.7

11/17/03 0.7 8/9/2004 0.7 5/2/2005 0.4 2/13/2006 0.48

11/24/03 1.1 8/16/2004 0.5 5/9/2005 0.4 2/20/2006 1.14

12/01/03 0.7 8/23/2004 1.1 5/16/2005 0.9 3/6/2006 0.43

12/08/03 0.4 9/6/2004 0.5 5/23/2005 0.4 3/13/2006 0.4

12/15/03 0.3 9/13/2004 0.8 6/6/2005 0.7 3/20/2006 0.56

12/22/03 0.9 9/20/2004 0.9 6/13/2005 0.73 3/27/2006 0.48

12/29/03 1.3 9/27/2004 0.6 6/20/2005 0.88 4/3/2006 0.85

01/05/04 0.8 10/4/2004 0.7 7/4/2005 0.5 4/10/2006 0.68

01/12/04 0.2 10/11/2004 1.0 7/11/2005 0.9 4/24/2006 0.56

01/19/04 0.7 10/18/2004 0.5 7/18/2005 0.8 5/1/2006 0.5

01/26/04 0.5 10/25/2004 0.9 7/25/2005 1.2 5/8/2006 0.79

02/02/04 0.4 11/1/2004 0.5 8/1/2005 1.1 5/15/2006 0.57

02/09/04 0.5 11/8/2004 0.5 8/8/2005 0.5 5/22/2006 0.52

02/23/04 0.5 11/15/2004 0.5 8/15/2005 0.8 6/19/2006 3.2

03/01/04 0.7 11/22/2004 0.6 8/22/2005 1.1 6/26/2006 1.01

03/08/04 0.2 12/6/2004 0.5 9/5/2005 1.0

Biosolids Biosolids Biosolids Biosolids
Appendix A: POTW “C” 

94 

Percent of Dental Facilities 

Operating Dental Amalgam 

Separators

July 1, 2003 6

January 1, 2004 6

July 1, 2004 6

January 1, 2005 6

July 1, 2005 6

January 1, 2006 6

July 1, 2006 6

Influent Mercury EPA 1631, Rev. E

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity 2130B (Std. Methods 18th Ed.)

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
Appendix A: POTW “D” 

95 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

9/17/2003 64 230 420 2.1 n/a n/a NA 9/8/2003 2

10/6/2003 71 160 310 2.0 7.0 10/6/2003 0.8

11/5/2003 193 322 260 3.1 7.5 11/3/2003 0.9

12/2/2003 77 97 360 1.8 5.5 12/8/2003 1.2

1/12/2004 83 230 230 2.2 4.0 1/7/2004 1.4

2/4/2004 85 220 310 2.1 12 2/2/2004 1.6

3/3/2004 158 100 160 3.2 6.5 3/1/2004 0.9

4/7/2004 114 210 300 5.2 8.4 4/6/2004 1.6

5/5/2004 94 240 270 1.8 3.2 5/3/2004 2.1

6/7/2004 115 480 240 1.1 2.0 6/7/2004 0.035

7/14/2004 116 100 380 1.3 2.6 7/7/2004 1.2

8/2/2004 83 110 240 1.5 2.8 8/2/2004 1.3

9/8/2004 74 250 280 2.0 5.8 9/7/2004 1.2

10/13/2004 70 180 340 2.8 5.6 10/4/2004 2.4

11/3/2004 86 380 NA 2.9 NA 11/1/2004 2.1

12/7/2004 139 620 250 2.4 22 12/6/2004 2

1/12/2005 122 250 240 1.8 15 1/3/2005 0.7

2/1/2005 73 220 280 1.5 3.8 2/8/2005 1.3

3/7/2005 180 220 88 2.0 5.6 3/7/2005 0.7

4/11/2005 118 100 200 2.5 6.6 4/4/2005 1.4

5/9/2005 72 84 210 1.6 6.2 5/2/2004 0.7

6/8/2005 78 110 390 1.7 5.0 6/6/2005 0.6

7/6/2005 73 89 170 1.5 3.0 7/5/2005 2.1

8/1/2005 70 57 320 1.6 4.6 8/1/2005 1.2

9/1/2005 78 130 590 1.9 7.4 9/6/2005 2

10/3/2005 74 190 340 1.5 6.3 10/3/2005 2.7

11/2/2005 64 150 650 2.7 10 11/7/2005 0.7

12/1/2005 88 51 350 1.7 5.6 12/5/2005 2.6

1/16/2006 90 84 250 1.5 7.0 1/3/2006 0.7

2/15/06 80 130 220 1.4 4.8 2/6/2006 1

3/8/06 97 100 190 1.7 6.2 3/6/2006 0.80

4/4/06 259 14 64 5.5 20 4/3/2006 1.20

5/4/06 100 160 120 2.5 5.8 5/1/2006 1.10

6/7/2006 85 110 300 1.7 5.2 6/5/2006 1.6

Influent Effluent Biosolids

Percent of Dental Facilities 

Operating Dental Amalgam 

Separators

July 1, 2003 0.6

January 1, 2004 1.5

July 1, 2004 6.1

January 1, 2005 16

July 1, 2005 26

January 1, 2006 32

July 1, 2006 38
Appendix A: POTW “D” 

96 

Influent Mercury SW-846 7470A

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity n/a

Influent TSS 160.2 (EPA 600/4-79-020)

Effluent TSS 160.2 (EPA 600/4-79-020)

Analytical Methods
Appendix A: POTW “E” 

97 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

9/17/2003 74 87 230 3.0 2.5 9/7/2003 0.43

10/6/2003 66 62 220 2.1 3.5 10/5/2003 0.33

11/5/2003 146 231 280 3.3 5.5 11/2/2003 0.41

12/2/2003 70 24 340 1.5 5.5 12/7/2003 0.29

1/12/2004 76 38 280 1.7 5.5 1/4/2004 0.37

2/4/2004 74 72 340 2.7 7.0 2/1/2004 0.43

3/3/2004 75 72 200 2.0 5.5 3/7/2004 0.36

4/7/2004 81 54 270 2.2 5.2 4/14/2004 0.4

5/5/2004 80 100 250 1.9 6.0 5/2/2004 0.27

6/7/2004 93 220 1.2 5.6 6/6/2004 0.5

7/14/2004 114 140 290 2.6 7.4 7/4/2004 0.31

8/2/2004 79 120 240 1.4 5.4 8/1/2004 0.31

9/8/2004 78 230 310 2.1 3.2 9/5/2004 0.35

10/13/2004 68 440 1.8 5.4 10/3/2004 0.25

11/3/2004 63 70 NA 2.0 NA 11/7/2004 0.45

12/7/2004 153 60 150 3.7 11 12/5/2004 0.23

1/12/2005 183 150 260 2.5 11 1/2/2005 0.17

2/1/2005 70 120 260 2.2 7.8 2/6/2005 0.3

3/7/2005 117 85 220 1.4 7.4 3/6/2005 0.13

4/11/2005 85 100 200 1.4 4.8 4/3/2005 0.6

5/9/2005 86 110 420 1.3 7.4 5/1/2005 0.33

6/8/2005 96 84 220 1.4 3.6 6/5/2005 0.23

7/6/2005 72 74 260 2.5 5.6 7/3/2005 0.35

8/1/2005 67 64 26 1.7 6.6 8/7/2005 0.24

9/1/2005 57 67 410 1.5 7.0 9/4/2005 0.29

10/3/2005 61 370 230 1.9 4.6 10/2/2005 0.21

11/2/2005 64 52 320 2.1 6.8 11/6/2005 0.26

12/1/2005 68 110 250 2.8 6.4 12/4/2005 0.39

1/16/2006 59 47 400 1.7 6.2 1/15/2006 0.31

2/15/06 76 66 250 1.9 5.6 2/19/2006 0.39

3/8/06 99 140 390 1.4 9.2 3/19/2006 0.38

4/4/06 241 76 170 2.8 9.4 4/16/2006 0.4

5/4/06 74 100 92 1.2 2.0 5/21/2006 0.25

6/7/2006 73 87 370 3.7 5.6 6/18/2006 0.31

Influent Effluent Biosolids

Percent of Dental Facilities 

Operating Dental Amalgam 

Separators

July 1, 2003 0.6

January 1, 2004 1.5

July 1, 2004 6.1

January 1, 2005 16

July 1, 2005 26

January 1, 2006 32

July 1, 2006 38
Appendix A: POTW “E” 

98 

Influent Mercury SW-846 7470A

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity n/a

Influent TSS 160.2 (EPA 600/4-79-020)

Effluent TSS 160.2 (EPA 600/4-79-020)

Analytical Methods
Appendix A: POTW “F” 

99 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

Sample 

Date

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

3/2/2004 41.18 253 287 3/2/2004 2.16 4.4 3/1/2004 1.014

4/6/2004 39.57 211 271 4/6/2004 1.58 4.3 4/1/2004 1.149

5/4/2004 40.24 314 255 5/4/2004 2.89 4.7 5/1/2004 1.225

6/1/2004 52.98 210 280 6/1/2004 1.55 3.8 6/1/2004 1.884

7/13/2004 46.43 213 247 7/7/2004 2.64 5.2 7/1/2004 1.652

8/4/2004 52.39 209 252 8/3/2004 1.72 4.7 8/1/2004 1.547

9/8/2004 42.95 164 251 9/7/2004 1.54 6.1 9/1/2004 1.467

10/5/2004 41.02 319 10/5/2004 2.33 6.4 10/1/2004 1.446

11/16/2004 39.62 176 350 11/2/2004 3.06 5.4 11/1/2004 1.62

12/7/2004 44.34 142 296 12/14/2004 2.8 7.6 12/1/2004 1.164

1/4/2005 39.07 227 235 1/11/2005 1.92 5.4 1/1/2005 1.526

2/1/2005 39.41 173 248 2/1/2005 1.95 4.5 2/1/2005 1.09

3/1/2005 40.54 149 231 3/8/2005 2.2 4.4 3/1/2005 1.148

4/5/2005 41.74 74.7 252 4/12/2005 2.04 4.8 4/1/2005 2.236

5/3/2005 40.33 246 251 5/10/2005 2.32 4 5/1/2005 1.058

6/7/2005 41.78 136 245 6/7/2005 1.97 4.1 6/1/2005 1.843

7/6/2005 39.73 161 244 7/6/2005 1.1 5.2 7/1/2005 2.483

8/3/2005 40.55 149 242 8/2/2005 2.17 5.5 8/1/2005 1.462

9/14/2005 42.34 236 236 9/7/2005 1.98 5 9/1/2005 1.452

10/4/2005 42.65 133 232 10/4/2005 1.89 2.8 10/1/2005 1.336

11/1/2005 38.3 160 265 11/2/2005 6.54 9.1 11/1/2005 1.529

12/7/2005 39.24 126 232 12/13/2005 2.68 6.8 12/1/2005 1.378

1/10/2006 38.81 110 265 1/11/2006 2.28 4.9 1/1/2006 1.04

2/7/2006 41.26 209 231 2/7/2006 4.96 5.5 2/1/2006 1.01

3/7/2006 41.06 136 257 3/7/2006 1.86 4.4 3/1/2006 1.01

4/4/2006 46.12 117 226 4/4/2006 1.86 3.4 4/1/2006 1.05

5/2/2006 44.95 97.9 241 5/2/2006 1.47 4.3 5/1/2006 0.86

6/13/2006 40.17 192 235 6/13/2006 2.42 4.7 6/1/2006 1.06

Influent Effluent Biosolids

Percent of Dental Facilities 

Operating Dental Amalgam 

July 1, 2003 0

January 1, 2004 0

July 1, 2004 0

January 1, 2005 0

July 1, 2005 2

January 1, 2006 12

July 1, 2006 20

Influent Mercury EPA 245.7

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity n/a

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
Appendix A: POTW “G” 

100 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

7/7/2003 78.27 50 128 1/7/2004 32.75 60 456 7/13/2004 40.68 50 146

7/9/2003 44.83 20 186 1/8/2004 31.74 60 328 7/14/2004 38.79 50 132

7/14/2003 44.91 20 172 1/12/2004 32.16 30 166 7/21/2004 38.74 50 318

7/15/2003 39.04 30 198 1/14/2004 31.77 20 174 7/22/2004 37.95 280 320

7/23/2003 38.12 40 240 1/20/2004 31.92 50 218 7/26/2004 36.28 50 244

7/24/2003 37.22 100 204 1/21/2004 32.93 70 224 7/27/2004 36.03 50 232

7/28/2003 37.29 210 328 1/28/2004 30.73 60 180 8/4/2004 35.83 70 220

7/29/2003 36.66 130 258 1/29/2004 31.69 1910 244 8/5/2004 34.64 60 206

8/6/2003 37.39 70 292 2/2/2004 31.36 90 300 8/10/2004 44.45 50 240

8/7/2003 37.24 80 260 2/5/2004 32.03 180 232 8/12/2004 41.58 40 144

8/11/2003 36.85 70 372 2/11/2004 30.92 230 200 8/16/2004 37.8 70 168

8/12/2003 36.97 30 324 2/12/2004 31.79 160 236 8/17/2004 38.37 40 110

8/19/2003 37.81 80 381 2/17/2004 29.87 100 296 8/23/2004 37.48 100 200

8/21/2003 36.30 20 358 2/18/2004 31.58 150 296 9/1/2004 37.77 80 178

8/26/2003 37.35 50 260 2/23/2004 34.22 90 242 9/7/2004 35.78 40 310

8/27/2003 36.00 50 264 2/24/2004 33.56 550 240 9/8/2004 41.21 190 310

9/3/2003 35.19 60 250 3/3/2004 36.35 80 210 9/14/2004 38.46 200 104

9/4/2003 34.75 200 296 3/4/2004 35.46 220 226 9/16/2004 37.8 150 168

9/9/2003 36.17 90 368 3/8/2004 34.92 50 300 9/20/2004 37.13 50 126

9/16/2003 35.92 170 302 3/10/2004 40.80 60 296 9/23/2004 37.48 50 200

9/17/2003 35.47 100 282 3/16/2004 34.74 70 230 9/27/2004 37.76 40 164

9/24/2003 36.99 450 366 3/17/2004 35.03 50 166 9/28/2004 35.79 60 184

9/25/2003 35.55 90 284 3/24/2004 38.73 90 126 10/5/2004 39.44 30 170

9/29/2003 35.26 90 372 3/25/2004 49.36 140 190 10/6/2004 40.11 80 268

9/30/2003 33.02 120 450 3/29/2004 68.81 50 130 10/12/2004 37.9 100 444

10/6/2003 35.75 100 754 3/30/2004 56.07 50 146 10/13/2004 35.12 140 438

10/7/2003 32.99 70 426 4/7/2004 39.75 80 218 10/18/2004 34.87 170 204

10/15/2003 34.08 90 238 4/8/2004 42.15 70 238 10/19/2004 35.74 90 202

10/16/2003 34.20 60 230 4/14/2004 37.00 60 224 10/25/2004 46.07 80 192

10/22/2003 33.31 50 298 4/15/2004 37.40 50 140 10/28/2004 41.18 90 258

10/23/2003 33.58 90 226 4/19/2004 49.79 220 116 11/3/2004 41.75 40 196

10/27/2003 32.53 90 196 4/20/2004 44.76 110 500 11/4/2004 40.53 50 208

10/28/2003 37.74 40 174 4/26/2004 37.16 100 266 11/8/2004 39.22 60 220

11/5/2003 33.77 30 156 4/27/2004 34.26 70 202 11/9/2004 38.47 90 196

11/12/2003 38.18 90 218 5/4/2004 34.38 80 262 11/15/2004 37.21 50 416

11/13/2003 35.50 90 188 5/10/2004 38.36 100 236 11/18/2004 35.71 70 200

11/17/2003 35.60 60 164 5/13/2004 37.15 90 158 11/22/2004 35.77 60 246

11/18/2003 35.94 50 204 5/19/2004 71.61 50 166 11/23/2004 34.04 60 290

11/24/2003 32.61 50 252 5/20/2004 40.52 50 118 12/1/2004 33.55 40 280

11/25/2003 32.47 50 262 5/25/2004 40.37 290 184 12/2/2004 33.74 30 300

12/1/2003 33.83 80 222 6/1/2004 56.57 50 104 12/8/2004 33.37 830 408

12/2/2003 33.30 150 232 6/2/2004 50.20 80 122 12/9/2004 33.4 130 314

12/9/2003 32.36 70 238 6/7/2004 43.33 100 188 12/13/2004 34.26 50 226

12/10/2003 31.24 210 144 6/8/2004 42.89 100 202 12/14/2004 32.83 60 224

12/17/2003 32.82 60 224 6/14/2004 39.16 190 256 12/20/2004 34.98 640 482

12/18/2003 32.53 40 234 6/17/2004 37.58 420 342 12/21/2004 33.61 30 178

12/22/2003 34.09 60 270 6/23/2004 37.17 60 194 12/27/2004 32.46 260 296

12/29/2003 26.83 50 222 6/24/2004 36.09 50 168 12/28/2004 30.98 70 586

12/30/2003 25.54 60 292 6/28/2004 34.81 70 260 1/3/2005 33.39 30 164

1/1/2004 28.94 60 484 6/29/2004 34.56 130 230 1/4/2005 33.68 50 188

Influent Influent Influent
Appendix A: POTW “G” 

101 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

1/11/2005 32.72 50 160 7/6/2005 38.97 50 164 1/10/2006 35.61 80 274

1/12/2005 32.16 30 160 7/7/2005 39.46 50 122 1/17/2006 32.49 330 298

1/19/2005 31.95 70 328 7/13/2005 38.53 130 116 1/18/2006 33.91 120 170

1/20/2005 29.97 50 226 7/14/2005 38.2 220 96 1/23/2006 35.73 140 306

1/24/2005 31.82 50 156 7/18/2005 39.78 100 302 1/26/2006 33.26 100 282

1/27/2005 32.38 50 202 7/19/2005 36.7 120 322 1/30/2006 36.05 110 296

2/1/2005 32.42 50 230 7/25/2005 35.5 160 194 2/2/2006 36.51 60 298

2/3/2005 35.72 30 238 7/26/2005 37.26 50 148 2/7/2006 34.72 100 236

2/7/2005 33.82 40 184 8/4/2005 37.76 100 378 2/8/2006 34.93 90 292

2/8/2005 33.87 20 202 8/9/2005 34.33 160 258 2/13/2006 34.28 1480 274

2/14/2005 34.26 70 156 8/10/2005 35.24 70 210 2/14/2006 33.25 60 328

2/16/2005 34.17 220 284 8/16/2005 30.32 60 240 2/22/2006 34.01 40 258

2/23/2005 32.66 70 274 8/18/2005 33.68 90 122 2/23/2006 33.00 50 334

2/24/2005 33.25 50 204 8/22/2005 35.52 120 546 3/1/2006 40 338

3/1/2005 31.59 90 210 8/29/2005 32.94 70 276 3/2/2006 120 190

3/3/2005 33.27 100 224 8/30/2005 33.81 50 262 3/9/2006 200 208

3/9/2005 35.37 240 236 9/7/2005 35.51 180 822 3/14/2006 60 294

3/10/2005 33 50 274 9/8/2005 35.38 140 298 3/15/2006 210 280

3/16/2005 31.57 50 378 9/14/2005 25.54 190 336 3/20/2006 420 292

3/17/2005 32.05 70 460 9/15/2005 25.64 250 484 3/21/2006 120 220

3/21/2005 34.37 70 326 9/19/2005 34.02 130 358 3/27/2006 60 218

3/22/2005 36.43 180 272 9/20/2005 30.95 170 220 3/29/2006 110 162

3/23/2005 37.87 50 230 9/26/2005 39.4 300 340 4/3/2006 56.94 50 316

3/24/2005 40.78 40 216 9/27/2005 37.75 220 272 4/4/2006 52.92 560 146

3/29/2005 63.98 30 462 10/3/2005 35.24 390 340 4/11/2006 42.6 40 190

4/6/2005 58.76 160 206 10/4/2005 103.97 80 126 4/12/2006 42.06 150 312

4/7/2005 46.88 40 208 10/5/2005 127.56 70 198 4/19/2006 38.49 110 212

4/12/2005 41.83 70 174 10/11/2005 39.5 110 180 4/20/2006 39.12 290 252

4/18/2005 33.66 50 252 10/12/2005 39.63 70 244 4/24/2006 32.3 90 324

4/21/2005 42.46 40 314 10/19/2005 36.69 170 202 4/25/2006 29.53 200 176

4/26/2005 39.61 90 350 10/20/2005 38.81 110 216 5/3/2006 41.99 90 184

4/27/2005 39.27 40 180 10/24/2005 37.87 90 152 5/4/2006 40.33 50 178

5/4/2005 36.67 30 194 10/25/2005 37.08 150 178 5/9/2006 47.38 350 198

5/5/2005 36.31 70 144 11/1/2005 36.47 1220 184 5/10/2006 46.93 60 170

5/10/2005 39.71 70 152 11/2/2005 36.22 570 190 5/15/2006 57.05 30 190

5/11/2005 39.5 40 148 11/8/2005 34.46 120 216 5/16/2006 49.12 50 228

5/16/2005 36.73 70 168 11/9/2005 36.36 100 166 5/22/2006 37.69 120 274

5/19/2005 66.41 30 116 11/14/2005 43.08 130 174 5/24/2006 36.71 50 350

5/23/2005 42.34 40 160 11/19/2005 40.01 60 280 6/5/2006 42.86 90 254

5/24/2005 39.78 100 224 11/21/2005 40.95 90 346 6/6/2006 42.81 130 328

6/1/2005 39.77 100 196 11/28/2005 63.00 250 232 6/7/2006 40.25 70 282

6/2/2005 39.22 90 204 12/1/2005 43.53 60 332 6/8/2006 37.83 70 258

6/7/2005 44.64 70 142 12/5/2005 39.56 70 240 6/13/2006 34.94 160 314

6/8/2005 54.61 40 129 12/6/2005 37.24 230 260 6/14/2006 34.81 70 228

6/13/2005 45.72 200 190 12/13/2005 36.01 60 188 6/20/2006 36.73 90 378

6/14/2005 55.58 210 134 12/15/2005 35.14 50 286 6/22/2006 38.84 60 208

6/20/2005 53.64 760 154 12/19/2005 34.96 180 300 6/26/2006 43.9 120 220

6/23/2005 44.25 50 198 1/4/2006 37.10 100 226 6/27/2006 47.65 100

6/27/2005 39.91 70 174 1/5/2006 37.34 80 260

6/28/2005 37.92 80 216 1/9/2006 36.23 120 258

Influent Influent Influent
Appendix A: POTW “G” 

102 

Sample 

Date

TR Hg 

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity 

(NTU)

TSS

(mg/L)

Sample 

Date

TR Hg 

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity 

(NTU)

TSS

(mg/L)

7/2/2003 1.8 2 2 7/9/2005 1.8 4.85 4.6

7/15/2003 1.8 3 3 7/19/2005 5.3 10.13 3

8/1/2003 1.9 4 4 8/9/2005 2.1 12.93 4.6

8/15/2003 2.3 17 6 8/16/2005 3.7 16.2 10.6

9/3/2003 1.4 5 4.4 9/1/2005 3.2 9.63 3.2

9/15/2003 3.4 42 7 9/15/2005 2.5 3.89 7

10/1/2003 3.0 50.34 17.8 9/16/2005 2.2 4.76 10

10/14/2003 3.0 2.3 18.3 10.6 10/6/2005 13.6 15.53 22.4

11/3/2003 2.8 19.55 6.8 11/2/2005 4.9 9.14 3.6

11/17/2003 3.1 9.26 3.6 11/17/2005 2.0 2.97 3.4

12/1/2003 3.0 9.83 2.6 12/8/2005 3.9 40.13 18.2

12/15/2004 2.8 11.37 1.8 12/20/2006 3.4 8.9 5.6

1/5/2004 2.2 11.4 2.4 1/5/2006 1.9 7 4.8

1/15/2004 2.4 11.6 3.0 1/31/2006 1.9 4.8 4.2

2/3/2004 2.8 11.1 4.0 2/7/2006 2.5 2.2 3.4 2.4

2/13/2004 3.3 11.2 2.2 2/23/2006 1.4 1 3.1 3.4

3/1/2004 2.5 10.0 1.4 3/9/2006 1.8 1.2 6.2 3.4

3/15/2004 2.2 10.6 2.6 3/27/2006 2.3 1.3 7.9 4.2

4/1/2004 3.1 2.7 1.4 4/7/2006 2.4 1.4 6.79 1.6

4/16/2004 2.0 1.4 10.8 3.4 4/17/2006 2.2 0.8 8.42 5.2

5/7/2004 1.8 5.9 2.4 4/26/2006 4.7 0.8 9.31 8.4

5/18/2004 2.0 3.8 2.6 5/3/2006 2.5 0.9 4.36 2.2

6/10/2004 4.3 6.4 0.8 5/23/2006 1.7 4.89 3.6

6/15/2004 5.3 7.2 4.8 6/1/2006 2.7 4.28 3.4

7/1/2004 2.6 5.92 5.6 6/5/2006 3.4 4.57 7.2

7/23/2004 1.8 1.2 3.98 5.2 6/7/2006 2.9 5.57 4.4

8/4/2004 2.5 3.82 7.4

8/17/2004 2.2 12.32 4.4

9/1/2004 1.7 3.1 4

9/16/2004 4.1 1.9 3.03 6.4

10/4/2004 1.8 3.17 3.4

10/18/2004 2.1 2.31 3

11/1/2004 2.0 2.51 4

11/15/2004 2.7 3.94 4.6

12/9/2004 2.8 6.94 3

12/20/2004 2.4 11.03 4.6

1/3/2005 2.0 7.14 10.8

1/19/2005 1.2 5.17 2

2/1/2005 2.2 8.73 5

2/18/2005 2.2 3.95 5.8

3/4/2005 3.1 5.5 2.6

3/15/2005 1.7 6.21 4

4/4/2005 11.3 16.7 9.4

4/18/2005 1.5 4.1 3.6

5/2/2005 3.8 8.57 7.2

5/16/2005 3.0 2.99 2.4

6/1/2005 2.6 3.85 3

6/16/2005 2.0 2.36 1

Effluent Effluent
Appendix A: POTW “G” 

103 

Sample 

Date

Hg 

(mg/kg, dry)

Sample 

Date

Hg 

(mg/kg, dry)

Sample 

Date

Hg 

(mg/kg, dry)

7/9/2003 0.71 7/1/2004 0.48 7/10/2005 0.49

7/14/2003 0.32 7/12/2004 0.6 7/19/2005 0.26

7/22/2003 0.33 7/19/2004 0.63 7/25/2005 0.38

7/28/2003 0.57 7/26/2004 0.55 8/1/2005 0.35

8/4/2003 0.51 8/4/2004 0.35 8/15/2005 0.44

8/11/2003 0.27 8/9/2004 0.36 9/1/2005 0.36

8/18/2003 0.41 8/16/2004 0.20 9/7/2005 0.31

8/26/2003 0.35 8/23/2004 0.29 9/12/2005 0.35

9/1/2003 0.36 8/31/2004 0.37 9/19/2005 0.52

9/8/2003 0.32 9/8/2004 0.55 9/27/2005 0.54

9/15/2003 0.31 9/13/2004 0.34 10/4/2005 1.08

9/22/2003 0.51 9/21/2004 0.44 10/9/2005 0.66

9/29/2003 0.36 9/28/2004 0.36 10/12/2005 0.37

10/6/2003 0.28 10/5/2004 0.22 10/23/2005 0.51

10/20/2003 0.20 10/12/2004 0.41 10/24/2005 0.43

10/27/2003 0.26 10/19/2004 0.34 11/1/2005 0.66

11/4/2003 0.24 11/1/2004 0.49 11/7/2005 1.11

11/12/2003 0.17 11/15/2004 0.3 11/14/2005 0.87

11/17/2003 0.28 11/22/2004 0.35 11/21/2005 0.42

11/21/2003 0.25 11/29/2004 0.25 11/28/2005 0.37

12/1/2003 0.24 12/6/2004 0.34 12/6/2005 0.42

12/8/2004 0.71 12/13/2004 0.31 12/12/2005 0.29

12/16/2003 0.22 12/20/2004 0.27 12/19/2005 0.24

12/23/2003 0.52 12/27/2004 0.16 12/26/2005 0.20

12/29/2003 0.19 1/3/2005 0.22 1/3/2006 0.28

1/5/2004 0.18 1/10/2005 0.26 1/6/2006 0.17

1/13/2004 0.26 1/17/2005 0.24 1/16/2006 0.19

1/19/2004 0.27 1/21/2005 0.23 1/24/2006 0.17

1/28/2004 0.21 1/31/2005 0.26 1/30/2006 0.27

2/3/2004 0.34 2/7/2005 0.19 2/6/2006 0.31

2/9/2004 0.44 2/14/2005 0.23 2/13/2006 0.22

2/16/2004 0.38 2/21/2005 0.23 2/20/2006 0.26

2/23/2004 0.37 2/28/2005 0.19 2/27/2006 0.24

3/1/2004 0.36 3/7/2005 0.34 3/13/2006 0.31

3/10/2004 0.33 3/14/2005 0.29 3/20/2006 0.22

3/15/2004 0.30 3/21/2005 0.38 3/27/2006 0.24

3/22/2004 0.28 3/29/2005 0.37 4/3/2006 0.29

3/29/2004 0.32 4/4/2005 0.33 4/10/2006 0.37

4/5/2004 0.40 4/12/2005 0.24 4/17/2006 0.30

4/12/2004 0.34 4/18/2005 0.35 4/24/2006 0.12

4/20/2004 0.44 4/25/2005 0.45 5/1/2006 0.35

4/26/2004 0.50 5/2/2005 0.45 5/9/2006 0.36

5/3/2004 0.32 5/8/2005 0.39 5/23/2006 0.30

5/10/2004 0.38 5/16/2005 0.21 5/31/2006 0.30

5/17/2004 0.49 5/24/2005 0.22 6/5/2006 0.54

5/24/2004 0.38 5/30/2005 0.14 6/13/2006 0.33

5/31/2004 0.5 6/6/2005 0.37 6/19/2006 0.44

6/7/2004 0.42 6/13/2005 0.28 6/26/2006 0.42

6/15/2004 0.74 6/19/2005 0.32

6/21/2004 0.44 6/27/2005 0.36

Biosolids Biosolids Biosolids
Appendix A: POTW “G” 

104 

Percent of Dentists Operating 

Dental Amalgam Separators

July 1, 2003 44

January 1, 2004 86

July 1, 2004 95

January 1, 2005 98

July 1, 2005 100

January 1, 2006 100

July 1, 2006 100

Influent Mercury EPA 245.1

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity 2130B (Std. Methods 18th Ed.)

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
Appendix A: POTW “H” 

105 

Sample 

Date

Total 

Daily Flow 

(MGD)

Flow Weighted 

Influent average 

TR Hg (ng/L)

Flow Weighted 

Influent average 

TSS (mg/L)

TR Hg 

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity 

(NTU)

TSS 

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

7/28/2003 34.7 72.59 257.13 2.30 1.1 0.81 1 7/28/2003 0.21

8/22/2003 35.86 112.57 283.50 3.30 1.2 2 4.4 8/22/2003 0.334

9/22/2003 36.74 251.25 335.42 4.00 1.4 3 3.19 9/22/2003 1.24

10/27/2003 36.58 146.18 303.58 2.60 2.2 <1.00 10/27/2003 0.925

11/21/2003 33.21 78.38 277.90 2.90 8 5.2 11/21/2003 0.963

12/17/2003 33.04 86.34 248.79 5.10 11.5 14.2 12/18/2003 0.746

1/26/2004 33.72 86.92 267.13 7.10 4.6 14.7 1/26/2004 0.615

2/23/2004 34.77 96.31 286.07 8.20 2.8 4 2/23/2004 0.84

3/18/2004 36.63 70.64 211.39 6.70 2 <1.00 3/18/2004 0.806

4/21/2004 35.06 186.05 351.76 2.30 2 4 4/22/2004 1.12

5/20/2004 37.39 79.99 308.49 3.90 1.2 <1.0 5/20/2004 0.951

6/16/2004 38.07 137.27 218.81 2 <1.0 6/16/2004 1.27

7/22/2004 40.42 79.67 240.78 1.90 na 3.2 Jul-04 1

8/24/2004 36.37 208.83 358.85 2.90 na 9.1 Aug-04 1.11

9/16/2004 33.14 140.27 301.84 4.00 2.3 <1.0 Sep-04 0.9

10/21/2004 33.72 108.98 321.23 3.80 2.96 3.2 Oct-04 1.3

11/30/2004 34.32 114.82 274.87 10.10 3.31 <1.0 Nov-04 1

12/21/2004 33.35 78.15 266.15 2.90 na 2 Dec-04 0.9

1/25/2005 35.54 248.27 282.88 3.10 2.6 4.7 Jan-05 1.2

3/23/2005 35.59 116.03 269.81 3.40 2.8 2.1 Feb-05 1.7

4/22/2005 34.35 127.30 351.61 2.40 <1.0 na 1.7 Mar-05 1.2

6/30/2005 38.78 234.29 1.80 <1.0 na <1.0 Apr-05 1.72

7/27/2005 33.38 85.31 168.48 2.00 1.4 <1.0 May-05 1.2

8/17/2005 38.65 320.15 1.90 2 <1.0 Jun-05 1.1

9/13/2005 34.53 186.49 232.10 2.50 1.5 <1.0 7/27/05 1.2

10/18/2005 32.84 74.50 464.83 2.10 1.6 <1.0 8/17/05 2.1

11/14/2005 33.37 102.56 169.93 3.50 1.5 1 9/13/05 0.9

12/12/2005 34.03 147.22 237.85 2.80 5 4.6 10/18/05 2

1/18/2006 33.76 106.12 255.66 7.00 7 11/14/05 1

2/15/2006 31.08 105.50 215.70 3.5 5.5 7.3 12/12/05 1.1

3/15/2006 31.64 181.69 214.39 11.2 12.5 14 1/18/06 0.7

4/12/2006 31.94 244.15 409.05 9.1 <1.0 15.5 24.9 2/15/06 1.2

5/18/2006 32.61 123.13 384.95 2.1 <1.0 3 <1.0 3/15/06 1.4

6/28/2006 32.68 78.85 189.20 2.2 <1.0 2 <1.0 4/12/06 2.1

5/18/06 2.5

6/7/06 1

Influent Biosolids Effluent

8/24/2004:

No associated 

field blank

Percent of Dentists Operating 

Dental Amalgam Control 

Technologies

July 1, 2003 n/a

January 1, 2004 60

July 1, 2004 89

January 1, 2005 95

July 1, 2005 95

January 1, 2006 98

July 1, 2006 98
Appendix A: POTW “H” 

106 

Influent Mercury EPA 245.7

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity ?

Influent TSS ?

Effluent TSS ?

Analytical Methods
Appendix A: POTW “I” 

107 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

7/8/2003 2.33 332 228 3.7 6 7/14/2003 3.35

8/12/2003 2.32 247 108 6.2 3.7 5 8/11/2003 0.99

9/9/2003 2.30 156 111 5.6 2.2 4.4 9/15/2003 0.86

10/8/2003 2.27 155 272 5.8

11/7/2003 2.25 82.6 234 5 5.2 11/19/2003 1.44

12/10/2003 3.00 264 4.6 2.4 12/19/2003 0.416

1/13/2004 2.92 169 186 4.6 3.4

2/10/2004 2.61 180 212 8.8 17.2 2/9/2004 0.668

3/9/2004 2.82 538 136 7.3 7

4/5/2004 2.45 144 160 27.0 19 4/13/2004 0.459

5/4/2004 2.28 634 188 6.6

6/8/2004 2.28 305 244 6.8 2.9 6/14/2004 0.62

7/8/2004 2.25 216 4.8 6 7/13/2004 0.616

8/4/2004 2.27 128 232 4.7 4

9/14/2004 1.98 172 194 5.3 6.8

9/17/2004 2.39 171 176 3.9 4.8 9/1/2004 0.823

10/15/2004 2.23 94.2 166 14.5 8.4 10/25/2004 0.554

11/8/2004 2.64 151 114 5.5 3

12/13/2004 3.35 522 114 4 2.8 12/13/2004 0.587

1/11/2005 2.62 861 186 4.4 5.4 1/13/2005 0.319

2/8/2005 2.98 134 126 12.7 20 1/31/2005 0.446

3/15/2005 2.32 154 218 29.4 15.6 2/28/2005 0.641

4/7/2005 2.85 151 162 6.2 2.6 8.4 3/31/2005 0.736

5/10/2005 2.32 117 208 14.1 3.2 17 4/24/2005 0.216

6/6/2005 2.36 148 206 8.9 2.7 21 5/31/2005 0.209

7/7/2005 2.35 136 204 13.4 12.6 6/30/2005 0.101

8/4/2005 2.28 123 200 6.9 13.6 7/30/2005 0.149

9/13/2005 2.25 102 234 4.6 10 8/31/2005 0.149

10/25/2005 2.29 115 242 7.2 14.7 9/30/2005 0.12

11/21/2005 2.36 164 166 7 8.2 10/31/2005 0.404

12/7/2005 2.37 96.6 176 5.4 8.8 11/30/2005 0.144

1/23/2006 2.88 242 4.5 9.8 12/28/2005 0.154

2/6/2006 3.18 56.2 134 5 12.4 1/31/2006 0.123

3/13/2006 2.59 95.3 160 6 5.4 2/28/2006 0.183

4/10/2006 2.52 193 210 3.5 3.4 3/30/2006 0.025

5/23/2006 2.76 156 180 5.4 2.2 6 4/28/2006 0.172

6/6/2006 2.31 123 230 3.8 2.1 4.8 5/31/2006 0.1

6/30/2006 0.122

Influent Effluent Biosolids

6/8/2004:

No associated field 

blank.

Percent of Dental Facilities 

Operating Dental Amalgam 

July 1, 2003 93

January 1, 2004 93

July 1, 2004 93

January 1, 2005 93

July 1, 2005 93

January 1, 2006 93

July 1, 2006 93
Appendix A: POTW “I” 

108 

Influent Mercury EPA 245.7

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 3050B or 3051, EPA 245.7

Effluent Turbidity n/a

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
Appendix A: POTW “J” 

109 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

7/15/2003 58.34 172 10.1 6.2 7/14/2003 1.80

8/15/2005 51.64 306 9.3 4.8 08/18/03 1.40

9/25/2003 57.78 89.2 308 10.8 3.3 15.9 25 09/15/03 1.40

10/15/2003 63 219 290 10.3 8 14 10/13/03 1.10

11/13/2003 65.93 95.2 308 9.4 13.4 24 11/17/03 1.20

12/22/2003 84.39 150 285 6.4 6.5 16 12/08/03 1.40

1/15/2004 113.2 225 254 12.5 14 20 01/19/04 0.84

2/5/2004 99.52 91.1 220 6.1 6.3 16 02/16/04 0.93

3/11/2004 78.51 162 328 8.7 8.3 20 03/15/04 1.40

4/16/2004 74.67 174 292 6.6 6.4 14 04/19/04 1.30

5/14/2004 74.95 224 213 7.5 6.7 11 05/17/04 1.20

6/11/2004 60.7 274 312 8.7 10.8 15 06/28/04 3.22

7/9/2004 61.48 783 410 9.9 20 23 07/20/04 2.26

8/19/2004 60.82 95.1 276 11.6 7.1 16 08/09/04 1.30

9/10/2004 52.74 362 304 8.7 11.5 11 09/20/04 1.40

10/21/2004 70.36 95.1 264 10.9 8.2 13 10/18/04 1.40

11/12/2004 75.18 133 319 6.9 11 18 11/16/04 0.90

12/17/2004 90.63 474 335 5.6 6.2 14 12/13/04 1.10

1/14/2005 74.93 130 156 9.4 6.5 12 01/24/05 1.10

2/25/2005 69.36 193 316 7.7 9 16 02/14/05 1.83

3/11/2005 70.94 193 280 5.9 8 10 03/14/05 1.94

4/15/2005 72.90 204 457 8 1.5 6 12 04/18/05 1.60

5/20/2005 77.10 112 236 6.2 1.8 4.1 16 05/16/05 1.20

6/10/2005 66.40 76.4 296 6 1.9 4.1 11 06/20/05 0.92

7/15/2005 63.30 106 238 6.7 10.9 16 07/18/05 0.91

8/5/2005 61.60 139 316 6.8 6.6 10 08/15/05 0.95

9/9/2005 61.40 154 328 6.8 7.2 10 09/19/05 1.10

10/14/2005 60.00 184 384 6.5 4 14 10/17/05 1.60

11/11/2005 80.30 90.8 260 8.8 9.4 19 11/14/05 1.20

12/14/2005 69.00 77.9 280 8 22 12/12/05 1.10

NA 01/23/06 0.96

2/17/2006 85.7 87.1 328 7 9.7 16 02/20/06 0.88

3/10/2006 84.4 273 248 6.4 6.5 13 03/13/06 1.00

4/7/2006 60.3 188 284 9.8 2 10 22 04/17/06 1.50

5/19/2006 58.1 145 358 5 1.4 8.3 12 05/15/06 0.95

6/2/2006 76.9 98.3 252 4.4 1.1 5 9 6/19/2006 0.9

Influent Effluent Biosolids

Percent of Dental Facilities 

Operating Dental Amalgam 

July 1, 2003 94

January 1, 2004 97

July 1, 2004 97

January 1, 2005 97

July 1, 2005 97

January 1, 2006 97

July 1, 2006 98
Appendix A: POTW “J” 

110 

Influent Mercury EPA 245.7

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury EPA 7471A (06-01-004-003)

Effluent Turbidity ?

Influent TSS ?

Effluent TSS ?

Analytical Methods
Appendix A: POTW “K” 

111 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

08/15/03 86.89 215 284 36.6 9.4 no data 08/18/03 4.44

09/24/03 84.21 134 276 9.2 3.5 5.8 11 09/15/03 2.48

10/15/03 181.85 161 258 8.9 11 10/13/03 1.72

11/13/03 110.28 109 203 6.8 3.32 6 11/17/03 2.57

12/22/03 89.34 43.1 119 7.3 5.47 8 12/15/03 1.4

01/15/04 136.68 90.3 143 3.9 3.01 6 01/19/04 1.4

02/05/04 108.39 53.4 118 4.3 2.66 9 02/23/04 1.82

03/11/04 87.80 58.2 163 13.0 6.41 9 03/15/04 1.30

04/16/04 76.52 102 189 5.7 4.38 9 04/19/04 1.20

05/14/04 74.35 360 295 7.6 4.99 3 05/17/04 1.30

06/11/04 105.04 107 188 9.0 9.68 13 06/21/04 1.30

07/09/04 84.04 85.6 212 9.9 4.11 8 07/20/04 2.10

08/19/04 84.78 128 241 4 2.94 8 08/16/04 1.20

09/10/04 143.32 82.7 206 8.5 6.39 13 09/20/04 1.20

10/21/04 82.77 103 173 11.0 5.69 14 10/18/04 1.81

11/12/04 70.54 62.8 9.6 5.71 11/16/04 1.30

12/17/04 93.62 89.7 180 5.3 4.82 10 12/13/04 1.20

01/14/05 84.36 106 218 7.6 4.64 7 01/24/05 1.60

02/25/05 71.8 138 179 8.8 5.12 12 02/14/05 1.75

03/11/05 71.71 77.2 202 7.1 3.58 7 03/14/05 1.20

04/15/05 152.55 71 192 4.7 3.6 3.02 6 04/18/05 1.40

5/20/2005 131.82 124 163 3.9 4.92 12 05/16/05 1.50

6/10/2005 80.70 84.3 194 7.8 2.9 6.54 9 06/20/05 1.30

7/15/2005 78.71 69.1 192 7.8 5.2 8 07/18/05 1.30

8/5/2005 73.41 443 208 6.8 4.94 7 08/15/05 1.00

9/9/2005 76.78 169 430 7.5 6.62 10 09/19/05 1.50

10/14/2005 124.72 94.5 209 6.7 4.44 14 10/17/05 1.40

11/11/2005 115.25 51.9 188 4.9 3.44 5 11/14/05 1.10

12/14/2005 70.63 75.1 234 3.4 2.95 5 12/12/05 1.50

01/30/06 1.30

2/17/2006 93.94 128 123 5 3.77 8 02/13/06 1.20

3/10/2006 110.01 35 132 5.3 3.74 4 03/13/06 1.10

4/7/2006 90.49 51.7 206 4.7 2.9 3.12 5 04/17/06 1.10

5/19/2006 91.12 156 198 5 2.2 4.1 15 05/15/06 3.15

6/2/2006 138.89 52.2 167 4.5 1.6 3.59 14 6/19/2006 1.81

Influent Effluent Biosolids

Percent of Dental Facilities 

Operating Dental Amalgam 

July 1, 2003 94

January 1, 2004 97

July 1, 2004 97

January 1, 2005 97

July 1, 2005 97

January 1, 2006 97

July 1, 2006 98
Appendix A: POTW “K” 

112 

Influent Mercury EPA 245.7

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury EPA 7471A (06-01-004-003)

Effluent Turbidity ?

Influent TSS ?

Effluent TSS ?

Analytical Methods
Appendix A: POTW “L” 

113 

Sample 

Date

Flow 

(MGD)

TR Hg 

(ng/L)

TSS 

(mg/L)

Sample 

Date

TR Hg

(ng/L)

Dissolved 

Hg (ng/L)

Turbidity

(NTU)

TSS

(mg/L)

Sample 

Date

Hg 

(mg/kg, dry)

7/29/2003 40.7 145 422 7/30/2003 17.8 6 3.1 5 7/29/2003 0.514

8/5/2003 42.3 137 189 8/6/2003 17.5 7.1 4.53 11 8/5/2003 0.629

8/18/2003 40.4 163 220 8/19/2003 28.6 10.6 5.6 9 8/19/2003 0.783

9/8/2003 41.8 676 190 9/9/2003 31.2 4.2 9 9/9/2003 0.157

10/6/2003 42.9 2040 203 10/7/2003 32.4 13.2 10/8/2003 1.86

11/3/2003 45.1 303 179 11/4/2003 17.9 6.4 10.6 11/4/2003 1.08

12/1/2003 41.28 200 189 12/2/2003 13.0 7.5 12/2/2003 0.777

1/6/2004 48.4 211 164 1/7/2004 20.2 4.41 8 1/7/2004 1.04

2/4/2004 41 77 164 2/5/2004 40.1 8.93 4.97 9 2/5/2004 1.52

3/3/2004 54.86 56 180 3/4/2004 26.6 5.49 9.95 3/4/2004 1.11

4/14/2004 41.35 269 246 4/15/2004 12.9 3.9 3.35 6.5 4/15/2004 1.01

5/3/2004 43.57 617 216 5/4/2004 205.0 4.2 6 5/4/2004 9.15

6/1/2004 42 229 246 6/2/041 22.5 5.88 10 6/1/2004 0.94

7/6/2004 42.7 226 259 7/7/2004 15.5 4.6 7 7/6/2004 0.908

8/12/2004 41.66 565 202 8/13/2004 17.1 4.32 3.36 3 8/12/2004 8.2

9/1/2004 40.7 120 196 9/2/2004 27.6 5.37 10 9/1/2004 1.33

10/7/2004 41.3 397 212 10/8/2004 15.8 3.8 4.15 8 10/7/2004 1.39

11/3/2004 41.1 210 178 11/4/2004 19.2 6.51 9 11/3/2004 1.45

12/1/2004 40 137 206 12/2/2004 20.3 2.1 8.23 7 12/1/2004 1.24

1/10/2005 72.8 103 158 1/11/2005 15.3 13.6 5.18 7 1/10/2005 0.82

2/1/2005 48.1 352 196 2/2/2005 47.7 7.99 5.2 2/1/2005 2.14

3/1/2005 56 120 165 3/2/2005 15.5 3.82 5.6 3/1/2005 0.91

4/5/2005 54.3 74 176 4/6/2005 21.5 3.1 6.4 4/5/2005 1.77

5/3/2005 45.3 317 192 5/4/2005 16.5 3.61 6.8 5/3/2005 0.79

6/1/2005 44.7 262 219 6/2/2005 20.6 6.31 3.58 7 6/1/2005 1.19

7/5/2005 41.1 264 222 7/6/2005 74 11.5 12.7 29 7/5/2005 0.82

8/1/2005 41.5 164 183 8/2/2005 34.2 4.27 6.6 8/1/2005 1.13

9/1/2005 40.8 404 230 9/2/2005 76.3 19.8 7.22 8.2 9/1/2005 1.39

10/3/2005 46.9 154 255 10/4/2005 56.1 8.09 9.6 10/3/2005 1.33

11/1/2005 39.6 180 238 11/2/2005 31.5 15.6 5.06 5.6 11/1/2005 1.86

12/1/2005 56.5 185 301 12/2/2005 23.9 6.75 12.4 12/1/2005 1.19

1/4/2006 89.2 176 89 1/5/2006 15 6.5 14.8 1/4/2006 0.72

2/6/2006 50.4 100 181 2/7/2006 19.7 3.6 6.4 2/6/2006 0.44

3/2/2006 60.4 413 118 3/3/2006 18.3 6.22 4.64 7 3/2/2006 1.03

4/2/2006 72.9 117 135 4/3/2006 14.7 5.78 8 4/2/2006 0.73

5/1/2006 53.7 133 158 5/2/2006 15.3 6 4.64 5/1/2006 0.84

6/5/2006 47.2 100 198 6/6/2006 10.4 5.97 7.4 6/5/2006 0.67

Influent Effluent Biosolids

Percent of Dental Facilities/Dentists 

Operating Dental Amalgam 

July 1, 2003 unknown

January 1, 2004 unknown

July 1, 2004 unknown

January 1, 2005 unknown

July 1, 2005 unknown

January 1, 2006 unknown

July 1, 2006 11

Influent Mercury EPA 245.2

Effluent Mercury EPA 1631, Rev. E

Biosolids Mercury SW-846 7471A

Effluent Turbidity 2130B (Std. Methods 18th Ed.)

Influent TSS 2540D (Std. Methods 18th Ed.)

Effluent TSS 2540D (Std. Methods 18th Ed.)

Analytical Methods
APPENDIX B: 

SUMMARY OF STATISTICAL ANALYSES FOR 

COMBINED DATA
Appendix B 

115 

The following are the results of correlation analyses conducted on data from all the 

POTWs. Kendall’s tau-b was used as the measure of correlation between variables. 

Alpha (α) was set at 0.05 for these statistical tests. 

Figure 4.5A: Median Influent Mercury Concentration versus Dentists per Flow 

Kendall’s tau-b = 0.405, p = 0.00007. Thus, this correlation is significant. Increasing 

number of dentists per MGD is associated with increasing influent mercury 

concentration. 

Figure 4.5B: Median Effluent Mercury Concentration versus Dentists per Flow 

Kendall’s tau-b = 0.316, p = 0.002. Thus, this correlation is significant. Increasing 

number of dentists per MGD is associated with increasing effluent mercury 

concentration. 

Figure 4.5C: Biosolids Mercury Concentration versus Dentists per Flow 

Kendall’s tau-b = 0.159, p = 0.121. Thus, this correlation is not significant. Number of 

dentists per MGD is not correlated with biosolids mercury concentration. 

Figure 4.6A: Influent Mercury Concentrations versus Number of Dentists without 

Separators per Flow 

Kendall’s tau-b = 0.032, p = 0.805. Thus, this correlation is not significant. Number of 

dentists without separators per MGD is not correlated with influent mercury 

concentration. 

Figure 4.6B: Effluent Mercury Concentrations versus Number of Dentists without 

Separators per Flow 

Kendall’s tau-b = -0.352, p = 0.004. Thus, this correlation is significant. Number of 

dentists without separators per MGD is negatively correlated with effluent mercury 

concentration. 

Figure 4.6C: Biosolids Mercury Concentrations versus Number of Dentists without 

Separators per Flow 

Kendall’s tau-b = 0.456, p = 0.0002. Thus, this correlation is significant. Increasing 

number of dentists without separators per MGD is associated with increasing biosolids 

mercury concentration. 

Figure 4.7A: Influent Mercury Concentration versus Average Plant Flow 

Kendall’s tau-b = 0.076, p = 0.783. Thus, this correlation is not significant. Average 

plant flow is not correlated with influent mercury concentration. 

Figure 4.7B: Biosolids Mercury Concentration versus Average Plant Flow 

Kendall’s tau-b = 0.308, p = 0.191. Thus, this correlation is not significant. Average 

plant flow is not correlated with biosolids mercury concentration. 
Appendix B 

116 

Figure 4.7C: Effluent Mercury Concentration versus Average Plant Flow 

Kendall’s tau-b = -0.107, p = 0.680. Thus, this correlation is not significant. Average 

plant flow is not correlated with effluent mercury concentration. 

Figure 4.8A: Influent Mercury Concentration versus Non-Industrial Flow 

Kendall’s Tau-B = 0.366, p = 0.208. Thus, this correlation is not significant. Percent 

non-industrial flow is not correlated with influent mercury concentration. 

Figure 4.8B: Effluent Total Mercury Concentration versus Non-Industrial Flow 

Kendall’s Tau-B = 0.592, p = 0.036. Thus, this correlation is significant. Increasing 

percent non-industrial flow is associated with increasing effluent mercury concentration. 

Figure 4.8C: Biosolids Total Mercury Concentration versus Non-Industrial Flow 

Kendall’s Tau-B = 0.479, p = 0.093. Thus, this correlation is not significant. Percent 

non-industrial flow is not correlated with biosolids mercury concentration. 

Figure 4.8D: Influent Total Mercury Concentration versus Non-Industrial Flow in 

Separate Sewer Systems 

Kendall’s Tau-B = 0.733, p = 0.060. Thus, this correlation is not significant. Percent 

non-industrial flow in separate sewer systems is not correlated with influent mercury 

concentration. Note that the p-value (0.06) is only slightly higher than α (0.05). 

Figure 4.8E: Effluent Total Mercury Concentration versus Non-Industrial Flow in 

Separate Sewer Systems 

Kendall’s Tau-B = 0.733, p = 0.060. Thus, this correlation is not significant. Percent 

non-industrial flow in separate sewer systems is not correlated with effluent mercury 

concentration. Note that the p-value (0.06) is only slightly higher than α (0.05). 

Figure 4.8F: Biosolids Total Mercury Concentration versus Non-Industrial Flow in 

Separate Sewer Systems

Kendall’s Tau-B = 0.2, p = 0.707. Thus, this correlation is not significant. Percent nonindustrial flow in separate sewer systems is not correlated with biosolids mercury 

concentration. 

Figure 4.12A: Influent Mercury Concentration versus Population Density 

Kendall’s tau-b = -0.215, p = 0.370. Thus, this correlation is not significant. Population 

density is not correlated with influent mercury concentration. 

Figure 4.12B: Effluent Mercury Concentration versus Population Density 

Kendall’s tau-b = 0.185, p = 0.449. Thus, this correlation is not significant. Population 

density is not correlated with effluent mercury concentration. 

Figure 4.12C: Biosolids Mercury Concentration versus Population Density 

Kendall’s tau-b = -0.047, p = 0.890. Thus, this correlation is not significant. Population 

density is not correlated with biosolids mercury concentration.