1

EVALUATION OF AMALGAM REMOVAL EQUIPMENT

AND DENTAL CLINIC LOADINGS

TO THE SANITARY SEWER

December 21, 2001

Peter A. Berglund, MCES & Richard W. Diercks, MDA

I. EXECUTIVE SUMMARY

Metropolitan Council Environmental Services (St. Paul, Minnesota) and the Minnesota Dental 

Association conducted a study to evaluate amalgam removal equipment and to quantify 

mercury loadings in wastewater discharged from general practice dental clinics to the sanitary 

sewer. Amalgam removal equipment was installed within the vacuum systems of dental 

clinics to treat wastewater for amalgam. The wastewater is generated within the operatories as 

a dentist cares for their patients and by the routine flushing of a clinic vacuum system.

Dental clinic wastewater contains mercury primarily in the form of particles of amalgam. 

There is also some mercury present in the wastewater that is in a soluble form. Due to the 

range in size of the amalgam particles and the uneven movement of solids within a clinic 

vacuum system, the discharge of amalgam from a clinic is extremely variable.

The amalgam removal equipment was evaluated in terms of general operation, effectiveness 

at treating wastewater, and cost. Equipment from five vendors was installed at dental clinics. 

One vendor provided two models. The total number of models tested was six. Some models 

were installed in clinics with liquid-ring vacuum pumps, and some models were installed in 

clinics with turbine vacuum pumps. In total, seven general practice clinics were used to 

conduct wastewater monitoring and equipment testing. The models tested were designed 

primarily to remove mercury in the form of amalgam particles. Some models of amalgam 

removal equipment are also designed to treat clinic wastewater for soluble mercury (none 

were tested as a part of this study). This study was not conducted to rank the various types of 

amalgam removal equipment.

The period during which wastewater monitoring and/or amalgam removal equipment was in 

place within any one of the seven clinics ranged from 7 to 18 weeks, for a cumulative total of 

87 clinic-weeks. All wastewater monitoring and equipment installations took place within 

each clinic’s vacuum system at a centralized location near the vacuum pump. There was no 

monitoring of any other dental clinic wastewater, such as process wastes including x-ray 

chemicals or wastewater from clean-up of tools. Tap water at the clinics was not analyzed for 

mercury to check background levels; however, tap water used to process samples was 

checked at MCES facilities and was found to have insignificant levels of mercury 

(< 0.2 g/L).

The amalgam removal equipment performed well mechanically and did not cause problems 

within the clinics. The equipment removed substantial quantities of mercury from dental 

clinic wastewater, achieving 91% - 99% removal of amalgam (measured as mercury) not 

already captured in basic chairside traps (i.e., removal from wastewater after the chairside 

traps). All seven clinics that were part of this study used basic chairside traps in their 

operatories. 
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For clinics with a vacuum filter already in place (located at the vacuum pump for those clinics 

with a liquid-ring pump), a vacuum filter may be expected to remove 42% of the amalgam 

remaining in the wastewater after the chairside traps. Therefore, if a clinic currently operates 

with a vacuum filter and installs amalgam removal equipment, more than twice as much 

amalgam may be removed from the wastewater. If a clinic currently operates without a 

vacuum filter (such as if using a turbine vacuum system), and installs amalgam removal 

equipment, a reduction of over 90% of the amalgam in the wastewater may be realized.

The cost to purchase and install the amalgam removal equipment models tested varies 

significantly, from approximately $160 (not installed) to $3500 (not installed). Unless there 

are significant plumbing or space considerations at a particular clinic, installation should be 

fairly inexpensive, taking approximately 1⁄2 to 2 hours of a plumber’s time. Some models fall 

within this price range and include installation. One model tested is installed and operated 

under a lease arrangement. 

Annual operational costs are less variable, in the range of approximately $300 - $500. It is 

possible that some systems may have lower operational costs. There has not been an in-depth 

evaluation of the costs for all of the models tested. 

There are other models of amalgam removal equipment that were not tested as part of this 

study. (Appendix B lists some of the other models, along with the models tested.) No cost 

information was collected for the models not tested. Although the cost to purchase and 

operate equipment may vary, the range of costs presented in this report likely mirror the 

general range of costs found in the marketplace.

The baseline amount of mercury discharged from the seven dental clinics studied, without 

amalgam removal equipment in place, was quite variable. The range of the individual overall 

averages for each of the clinics was 28 to 540 mg/dentist/day (standard deviation for this 

range was 199 mg/dentist/day). Within any one of the clinics and considering shorter time 

periods, the range of discharge would be expected to be even greater. This was found to be the 

case, with data showing a range of 8 to 1293 mg/dentist/day. The overall weighted average 

discharge rate was 234 mg/dentist/day. 

It is important to point out that because: (1) the number of dental clinics studied was small, 

(2) the selection of the clinics tested was not completely random, (3) the day-to-day 

variability within each clinic was great, and (4) the variability from clinic to clinic was great, 

any use of the weighted average should be done with caution and with reference to these 

limitations of the number.

It should also be noted that the majority of mercury present in dental clinic wastewater is in 

the form of amalgam. Given the high density of amalgam, some of the amalgam waste 

particles generated from a dental clinic may settle out in the sanitary sewer collection system. 

Therefore, the weighted average and the range of values reported in this study represent the 

amount of mercury in amalgam that may be present in wastewater discharged from a dental 

clinic. It may not be appropriate to assume that all of the amalgam would show up directly in 

a wastewater treatment plant, especially since the study was not meant to determine the fate of 

amalgam waste in a collection system or in a wastewater treatment plant.

Despite this range in numbers, and assuming a similarly broad range in the dental population 

as a whole, this study demonstrates potential for significant reduction in amalgam waste. As a 
3

result, along with the incorporation of sound internal waste management practices, dental 

clinics should be encouraged, where feasible, to incorporate amalgam removal equipment 

technology.

In summary, it is felt that the goals of this study were met. Several types of amalgam removal 

equipment were shown to effectively capture amalgam from dental clinic wastewater. 

Wastewater mercury loadings, although widely variable, were quantified for the dental clinics 

used in this study. Additional knowledge was gained regarding dental clinic operating 

procedures, vacuum system types and configurations, and wastewater monitoring and sample 

processing methods.

Excerpts Of Tables and Figure From Full Report:

Table 5

Amalgam Removal Equipment - Summary of Performance and Ease of Use *

Amalgam 

Removal Equip.

Mfger & Model

Day-to-Day Use

of Equipment

Management of Used

Unit or Waste Solids,

other Considerations

Avprox, Inc.

Asdex Filter

in Liquid-Ring

Vacuum System

Filters were easy to 

change.

Filters loaded sooner than expected. Filters lasted up to 

12 operating days (with vacuum filter in place). 

Assume less time without vacuum filter in place. 

Cap filters for disposal or reprocessing.

R & D Services

The Amalgam

Collector

Easy & quick to 

decant. Requires (up 

to) daily decanting.

Sludge in settling tank to be managed by clinic staff. 

Tank and lid need improving regarding how they 

connect together and seal. Central inlet fitting should 

be improved. (Use “union” fitting possibly?)

Air Techniques

for Durr Dental

7800/7801

in Turbine and 

Liquid-Ring

Vacuum System

Automatic operation.

No day-to-day 

involvement.

Centrifuge cassettes easy to change and seal.

Rebec Solutions

2000 EL

Automatic operation.

No day-to-day 

involvement.

Settling/Filtration container easy to change. May be 

done by service contractor. Verify during installation 

that checkvalve “seats” when vacuum is on.

Rebec Solutions

2000 E

Automatic operation.

No day-to-day 

involvement.

Settling/Filtration container easy to change. May be 

done by service contractor.

PWDMETASYS

ECO II

Automatic operation.

No day-to-day 

involvement.

Settling container easy to change. Done by service 

contractor. Plumbing connections need improving, 

possibly by using “union” fittings, sized for American 

pipe dimensions. 

* Note that manufacturers have made improvements to their systems since this study began.
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Table 6 Amalgam Removal Equipment – Purchase and Operational Costs

Amalgam 

Removal Equip.

Mfger & Model

Initial

Cost

(Dollars)

Installation

Included

Yes or No

Operating

Cost

(Dollars)

Waste Processing

Avprox, Inc.

Asdex Filter

$160

which includes 

first filter

No $50-60 per filter

See comment below 

on life of filter.

Dentist is responsible for disposal. 

Avprox, Inc. may help with 

making arrangements with a 

disposal company.

R & D Services

The Amalgam

Collector

$625*

No Mfg: “Expect tank to 

operate for 2 years 

between clean-outs.”

Dentist is responsible for 

management of sludge. (Tank is 

reusable after sludge removal.)

Air Techniques

for Durr Dental

7800/7801

Centrifuge

$3500 No $75 per cassette

Air Techniques:

“Expect cassette to 

last for 1 year.”

Dentist is responsible for 

management of waste cassettes.

Rebec Solutions

2000 E

& 2000 EL

$1700 –

$1900

(depending 

on dealer)

which includes 

first settlingfiltration unit

Yes $300 – $400

per settling-filtration 

unit.

Mfg: “Expect unit to 

last 1 year.”

Cost of settling-filtration unit 

includes waste processing and 

change-out.

Settling-filtration unit changed by 

dealer.

PWD-METASYS 

ECO II

$260 Yes Lease Arrangement

$30 per month plus 

possible cost if using 

recommended 

PWD-METASYS 

brand vacuum system 

cleaner

Monthly fee includes waste 

processing and change-out.

(cleaner may be additional cost)

ECO II changed by 

PWD-METASYS

contractor.

* $625 for model tested, which was centrally located near the vacuum pump. Less ($425 

each), if a smaller system is purchased for each operatory.
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Figure 1 diagrams two types of current vacuum systems, showing basic chairside traps and the 

placement of a vacuum filter. If amalgam removal equipment is installed, it is generally 

installed at a central location, near the vacuum pump. (Other installation options are possible.) 
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AMALGAM REMOVAL EQUIPMENT VENDOR LIST

FOR THE TREATMENT OF 

DENTAL CLINIC VACUUM SYSTEM WASTEWATER:

Centrifuge......................................Durr Dental GmbH & Co. KG - System 7800/7801

Air Techniques, Inc.; New York,

1-800-247-8324 ext. 5610 (Mr. Frederick Fischer)

Filter (Vacuum).............................Asdex Filter

Avprox, Inc.; St. Petersburg, Florida, 1-800-300-1249

Settling/Filtration...........................Sweden Recycling International AB - SRAB 99

and BullfroHg models

Distributor: Dental Recycling North America; New Jersey

1-800-360-1001

Settling/Filtration.......................... Models 2000 E & 2000 EL (also known as: CatchHg)

Rebec Solutions; Seattle, Washington

1-800-569-1088

Settling..........................................The Amalgam Collector - Design I

R & D Services; Seattle, Washington,

1-206-525-4994

Settling....................................ECO II (Economy System Type 2)

Pure Water Development LLC – METASYS; Miami, Florida

1-877-638-2797

Electrochemical Batch Reactor..........E-Clean

Faraday Technology, Inc.; Clayton, Ohio

1-937-836-7749

Settling/Filtration/Ion Exchange.......Model Hg5

SolmeteX, Inc.; Northborough, Massachusetts

1-800-216-5505

Settling.....................................Model 2000

Maximum Separation Systems, Inc.; Victoria, BC, Canada

1-250-652-5279

Note: This list has been compiled by Metropolitan Council Environmental Services (MCES), the 

POTW in the Minneapolis-St. Paul Minnesota metropolitan area. The distributors listed above may 

carry other models of amalgam removal equipment. MCES is evaluating some of the models listed. 

MCES does not endorse these models or manufacturers. If there are other distributors or 

manufacturers of equipment available, please contact MCES. 

MCES contact person: Peter Berglund, Industrial Waste Section, 651-602-4708. 

Mailing address: MCES; Industrial Waste Section; 230 East 5th Street; St. Paul, MN; 55101

e-mail: peter.berglund@metc.state.mn.us (list date: 12/21/01) 

word file: Hg-Dental Report 2001-3

rd Conf. Summ & Sampling Info
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THE FOLLOWING IS AN EXCERPT FROM THE MCES/MDA 

2001 DENTAL LOADING AND AMALGAM SEPARATOR 

EVALUATION:

III. C. Planning and Preparation for Monitoring and Installing The Amalgam

 Removal Equipment

Monitoring dental clinic wastewater and evaluating amalgam removal equipment is difficult 

and potentially confusing, given the special conditions encountered. These conditions relate 

to:

Collecting and processing wastewater samples that contain dense amalgam particles.

(The dense amalgam particles are of primary interest.)

Collecting wastewater samples under vacuum.

Clinics with turbine (“dry”) vacuum systems.

Clinics with liquid-ring (“wet”) vacuum systems (with a vacuum filter and pump water).

Determining the necessity of baseline monitoring.

Installing amalgam removal equipment after the vacuum filter, or in place of the vacuum 

filter.

Understanding and reporting data on amalgam removal equipment, if the amalgam 

removal equipment was installed after a vacuum filter or in place of the vacuum filter.

Installing an extra vacuum filter.

Hiring a contract plumber with experience in dental clinic vacuum system plumbing and 

dental vacuum pumps.

Preparations for emergency vacuum pump service and repair.

The first and second special conditions noted here are discussed in more detail in 

Section III. D. The other special conditions are discussed below.

III. C. [1] Monitoring at clinics with turbine vacuum systems

Monitoring at clinics with turbine vacuum systems is generally easier and more 

straightforward. Usually a baseline monitoring period is not necessary because there is no 

vacuum filter that could be replaced by amalgam removal equipment. The amalgam removal 

equipment and monitoring equipment are installed, and monitoring begins. When the 

monitoring is completed, all of the equipment is removed.

The solids in the amalgam removal equipment are analyzed for mercury (see Sections 

III. E. 2-3), and the wastewater collected during the monitoring period is also analyzed for 

mercury. These two quantities of mercury are summed together and they should equal the 

amount that would have been discharged from the vacuum system had no amalgam removal 

equipment been installed (i.e., the baseline level). Removal efficiencies can then be 

determined.
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III. C. [2] Monitoring at clinics with liquid-ring vacuum systems

Monitoring at clinics with liquid-ring vacuum systems is potentially more elaborate since 

there is usually a vacuum filter present and city supply water is routed through the vacuum 

pump. Questions to resolve first are:

Does the volume of city, supply water routed through the vacuum pump preclude the 

collection of wastewater samples after the vacuum pump?

If wastewater samples are collected upstream of the vacuum pump, will the samples have 

to be collected under vacuum?

Is the amount of mercury collected in the vacuum filter to be determined?, 

Is the amalgam removal equipment going to replace the vacuum filter?, 

If the amalgam removal equipment is installed after the vacuum filter, can an assumption 

be made that the amalgam removal equipment would have captured the amalgam caught 

in the vacuum filter?, and

Can the mercury be quantified from the waste solids captured in the amalgam removal 

equipment? (Note that this is also a consideration for amalgam removal equipment 

installed in a turbine vacuum system.)

III. C. [3] Is baseline monitoring necessary?

The following situations necessitated baseline monitoring:

The vendor of the Rebec 2000 E preferred that the 2000 E be installed in place of the 

vacuum filter. Therefore, baseline monitoring data were needed with the vacuum filter in 

place to determine how much mercury was retained in the filter and how much was 

present in the wastewater.

The Durr Centrifuge had to be installed at the outlet of the A/W separator of the liquidring vacuum system. Since the volume of wastewater passing through the centrifuge at 

this location was too large to sample, a baseline monitoring period was necessary.
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III. C. [4] Installing amalgam removal equipment after the vacuum filter, or in 

 place of the vacuum filter?

MCES installed the amalgam removal equipment after the vacuum filter in most cases during 

this study. This was done for the following reasons:

More monitoring experience would be gained regarding baseline discharges from clinics.

More data would be obtained on the amount of mercury captured in vacuum filters.

If the vacuum filter was removed, and amalgam removal equipment was installed, it 

would be unclear as to how much mercury captured in the amalgam removal equipment 

would have been captured in a vacuum filter.

The amount of amalgam captured in one of the types of amalgam removal equipment plus 

the amount of amalgam passing the amalgam removal equipment (i.e., in the wastewater) 

could be added together (both measured as mercury) to determine the amount passing 

through a vacuum filter.

If it seemed likely that amalgam caught in a vacuum filter would be caught in the 

amalgam removal equipment, then the two quantities could be measured separately and 

then added together mathematically.

Waste amalgam caught in a vacuum filter should also be caught in either the Asdex filter 

or in The Amalgam Collector (if The Amalgam Collector was operated properly).

The Durr Centrifuge had to be installed at the outlet of the A/W separator of the liquidring vacuum system, which meant the vacuum filter had to be left in place.

There were some questions initially as to the ability to determine the amount of mercury 

caught in the Asdex filters. (If the Asdex filter was installed in place of the vacuum filter, 

and then could not be analyzed, then only wastewater data may have been obtained. There 

would have been no other information as to the amount of mercury passing through the 

chairside traps at the clinic.)

The Asdex filter, and some of the other types of amalgam removal equipment, could be 

used in place of a vacuum filter or in addition to a vacuum filter with no, or minimal, 

change in wastewater treatment performance.

III. C. [5] Installing an extra vacuum filter

In discussions above, it was stated that in some clinics the sampling or amalgam removal 

equipment was installed after the vacuum filter. It should be noted, however, that in some 

clinics it was not physically possible to install equipment after the vacuum filter (i.e., between 

the filter and the pump). Therefore, when the vacuum filter was to be left in, MCES installed 

an identical vacuum filter upstream of either MCES sampling equipment or the amalgam 

removal equipment. This vacuum filter would then simulate what was already in the clinic. 

(The MCES filter included the manifold with plumbing fittings, the filter bowl & gasket, and 

the filter screen itself.)

One exception to this was the MCES vacuum filter installed at 3-PLR. The MCES filter was 

not identical to the filter already in place at the clinic. The MCES vacuum filter had the same 

mesh size, however the filter “bowl” was slightly smaller than that of the clinic’s original 

filter bowl. An exact duplicate was not available from the vacuum pump manufacturer.

A side benefit of MCES installing a vacuum filter was that by leaving the original filter in 

place at the clinic, the original filter would serve as a backup for the vacuum pump, in case 
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any waste solids made it past either the MCES sampling equipment or the amalgam removal 

equipment.

III. C. [6] Hiring a contract plumber with experience in dental clinic vacuum 

 system plumbing and dental vacuum pumps

All vacuum system plumbing work related to installing sampling equipment and the new 

amalgam removal equipment was performed by professionals experienced in such work. The 

plumbing contractor also understood the plumbing within clinic operatories and vacuum 

pumps. The same plumbing contractor performing the main services was also available if any 

emergency plumbing was needed for work associated with the study. 

Upon the completion of the study, the vacuum system plumbing was returned to its original 

condition (with the exception of new sections of piping and fittings) by the same plumbing 

contractor.

III. C. [7] Preparations for emergency vacuum pump service and repair

Contractual arrangements were set up with three dental supply companies to provide 

emergency vacuum pump service to the dental clinics volunteering to host the amalgam 

removal equipment. If there was a problem with the clinic pump, associated with the 

wastewater sampling equipment or the amalgam removal equipment, the clinic would choose 

one of these supply companies for emergency services. 
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III. D. Sampling and Sample Processing

III. D. [1] Overview of types of samples and sample processing

The samples collected in this study were of two basic types:

(1) Wastewater samples from either baseline monitoring or treated wastewater collected 

downstream of the amalgam removal equipment. Wastewater samples were generated on an 

ongoing basis (e.g., every 1-5 days) over a particular sampling time period (e.g., several 

weeks). Wastewater samples were each separated into a wastewater component and a solids 

component.

(2) Solids samples were the solids collected in the various types of amalgam removal 

equipment, chairside traps and vacuum filters that were in place for an extended period of 

time (days – weeks). The solids separated out from wastewater samples were processed and 

analyzed similarly as the other solids samples.

Tap water was used to clean and extract the solids from sampling containers, sample settling 

tanks, amalgam removal equipment, chairside traps, and vacuum filters. Originally, reagent 

water (such as: Milli-Q) was to be used for this purpose. Since using tap water simplified the 

process of preparing samples, it was tested for mercury and shown to be an acceptable 

alternative. Test data for twelve samples of tap water showed non-detect levels of mercury (all 

< 0.0002 mg/L). The volume of the tap water used was accounted for as part of the data 

analysis.

III. D. [2] Wastewater sample collection methods

Dental clinic vacuum system wastewater contains dense amalgam particles of varying size, 

making it difficult to take a representative subsample. Therefore, all of the wastewater 

generated during the sampling time periods was collected and analyzed. This made it 

impossible to collect both “upstream” and “downstream” samples at the same time while an 

amalgam removal system was in operation. 

The only exception when not all of the wastewater was collected was for some of the samples 

taken during the testing of The Amalgam Collector. These samples were collected after 

overnight settling had occurred. It was expected that the settled wastewater was homogeneous 

at this time. See discussion in Section VI. B. 

Samples were taken from the vacuum system in the general location of the vacuum pump. 

Some dental clinics use vacuum pumps that are liquid-ring pumps. City, or fresh supply water 

is plumbed to a typical liquid-ring vacuum pump at the rate of approximately 3⁄4 - 1 gallon per 

minute, which then joins with the wastewater from the operatories, inside the pump. This 

quantity of wastewater was too much to collect.

If a clinic used such a liquid-ring vacuum pump, the pump water could not become part of the 

wastewater sample due to the volume. This meant that the samples had to be collected 
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upstream of the pump, which also meant that samples from such a clinic were taken “under 

vacuum”. 

To sample under vacuum, the vacuum system plumbing was opened at an existing juncture or 

fitting, or it was cut open. Sampling tanks, performing the same function as an air/water 

separator, were installed to collect the wastewater samples. Vacuum systems were resealed 

upon completion of the study.

Wastewater samples were collected at each of the seven clinics in either of two types of 

containers. The two types of containers were:

(1) Samples from 1-PLR and 2-PLR were collected in custom-made stainless steel containers 

built to withstand a vacuum. The tops of the tanks were open, with a thick, clear plastic cover 

bolted to a flange on the sidewalls that allowed viewing into the tank. The inside of each steel 

tank was painted with an epoxy paint made for the interior of tanks holding aqueous chemical 

or petroleum products (paint by: Sherwin-Williams Co. “Shelcote ® II”). 

The welded seams of the tanks appeared rough and possibly slightly rusted when first 

delivered to MCES. The tanks were sandblasted and painted by MCES. Painting provided a 

clean, smooth, light colored surface. Another reason for painting the interior of the tanks, was 

that of a possible concern that mercury may amalgamate with the metals in the tank or in the 

welded seams.

The container for 1-PLR held 14.5 liters. The container for 2-PLR held 27 liters. Samples 

were drained from the stainless steel tanks and put into multiple 9 liter high-density 

polyethylene Nalgene ® containers. The stainless steel tanks were then rinsed with tap water, 

using a pressure sprayer, and the rinsewater was also put into the same set of polyethylene 

containers for each particular sample. The volume of rinsewater was recorded to account for 

dilution. The containers were transferred to MCES. (This tank rinsewater volume is noted in 

the tables of Appendix C data as “Sample Tank Rinse”.)

(2) The second type of sampling container was either a vacuum rated or non-vacuum 20 liter 

polypropylene container that was customized with plumbing fittings (“bulkhead” & “union” 

fittings) to be interchangeable at the clinics. These were used to collect samples at the other 

five clinics. Each clinic had its own set of three dedicated sample containers, also called 

carboys. One was left at each clinic as a spare. Each sample container itself also served as the 

transport container to bring the samples back to MCES.

The transport containers (a set of 9 liter Nalgene ® containers or a 20 liter container) were 

taken to MCES and the wastewater samples were poured into settling tanks. These transport 

containers were rinsed out with tap water. The volume of tap water was recorded to account 

for dilution. (This rinsewater volume is noted in the tables of Appendix C data by different 

names, such as: “Nalgene Jug Rinse”, “Carboy Rinse”, and “Sample Tank Rinse”.)
13

III. D. [3] Wastewater sample processing 

Dental clinic wastewater samples are a heterogeneous mixture of water and solids. Therefore, 

mixing and subsampling did not seem feasible. Given the relatively large volume of water 

present in each sample, and the small volume of solids present, MCES needed a method to 

separate the water from the solids to facilitate analysis.

The methods considered initially for completing the sample separation were: simple settling, 

filtration, and centrifuging. The method chosen was simple settling, for a minimum of 24 

hours. MCES tried vacuum filtering some samples using a 0.45 micron filter. The wastewater 

did not flow quickly enough through the filter to process large, multi-liter volume samples.

Settling was chosen due to its simplicity. Filtering or centrifuging samples would have 

required the use of more glassware, tubing, and hardware. This would have meant much more 

cleaning or disposal of this sample handling equipment. Settling also allowed for the 

preparation of solids samples without filter paper. If samples were filtered, the solids would 

have been submitted to the laboratory on filter paper, which then would have become part of 

the analytical digestion process.

Choosing 24 hours as a minimum settling time was based primarily on conversations with 

R & D Services. Their treatment system (The Amalgam Collector, one of the systems 

evaluated in this study) made use of simple settling. The minimum settling time period 

employed by their system is generally from 5:00 pm to 8:00 am (when a clinic would close at 

the end of the day to when the clinic would reopen the next morning). R & D Services 

believed that the vast majority of solids would settle out overnight, over approximately 15 

hours. The data in Appendix C show that the vast amount of mercury in wastewater is 

associated with the solids, for wastewater not treated with amalgam removal equipment. 

While MCES did not scrutinize the data for this purpose, no significant difference was noticed 

in wastewater samples that were settled for longer time periods, such as samples collected on 

a Friday and settled over a weekend.

Processing of the samples was accomplished by settling to separate out the solids, leaving a 

water layer and a layer of wet solids. There was no attempt to split or sub-sample the overall 

sample prior to settling. The settling of the samples was performed at MCES facilities.

For purposes of this study, the liquid fraction was assumed to be relatively homogeneous after 

settling. However, in case there was some stratification, three sub-samples (50 milliliters 

each) were taken after settling. These three equal volume sub-samples were taken from the 

top, middle, and bottom of the sample as each settling tank was decanted. The three subsamples were mixed together (composited) and put into a glass jar for submittal to MCES’ 

laboratory. A 50 milliliter, polyethylene ladle was used to collect the sub-samples. A 

peristaltic pump was used to decant the wastewater from the settling tank.

Duplicate composites of three sub-samples were also collected during the decanting of many 

of the settling tank wastewater samples. These composites were filtered with a 0.45 micron 

filter by the MCES laboratory to determine the concentration of soluble mercury, as 

commonly accepted by the US EPA.

Decanted volumes were noted so that the mass of mercury could be determined based on the 

concentration data reported by the laboratory. 
14

A layer of solids, or sludge, remained at the bottom of the settling tank after decanting most of 

the liquid off. These solids had to be recovered and processed since it was expected that the 

solids would contain significant quantities of mercury from amalgam. Processing of these 

solids is discussed in more detail in Section III. D. [4].

Settling tanks used for settling the wastewater samples were made of high-density 

polyethylene. Two sizes were used, both of which were held at an angle on customized 

wooden racks, such that the solids would settle down into a corner of the tank. 114 liter 

settling tanks were used initially, when some of the samples came from the 27 liter sample 

tank. Later, 42 liter settling tanks were used to settle the samples from the 20 liter sample 

containers. Both sizes of the settling tanks were made of high-density polyethylene. Each 

clinic had its own dedicated settling tank.

To summarize, a common, single wastewater sample collected from a clinic and processed 

would become:

1. a liquid sample (from decanting the settling tank)

2. another liquid sample (from decanting the water off of the beakers containing the solids)

3. a set of beakers of solids (usually multiple beakers per sample)

4. occasionally, a duplicate liquid sample (same as No. 1) to be filtered for soluble mercury

III. D. [4] Solids processing: Solids from wastewater samples and solids from 

 amalgam removal equipment, chairside traps, and vacuum filters

Solids were present at the bottom of the settling tanks used to settle wastewater samples, after 

decanting. Solids were also present in the amalgam removal equipment, chairside traps, and 

vacuum filters when this equipment was taken out of service. 

These various types of solids had to be processed. The MCES laboratory needed the solids 

delivered according to the following criteria:

In single beakers or multiple beakers (250 or 400 mL glass beakers),

With a relatively thin layer of solids at the bottom of each beaker 

(up to approximately 5 mm deep),

To be as free of water as was possible, via the decanting of the beakers.

The solids present at the bottom of a settling tank were washed into multiple beakers.

Solids, such as from chairside traps and vacuum filters, were also washed into multiple 

beakers.

During the processing of some of the types of amalgam removal equipment, more water was 

usually needed (e.g., multiple liters). In such a case, the water and solids were put into a 

settling tank. This overall mixture was processed the same way as a wastewater sample.

The mixture was settled in a settling tank, and the solids were transferred to beakers.
15

Processing the solids also involved using water for rinsing tools such as plastic spatulas, 

spoons, and brushes, which became part of the solids samples. The number of beakers used 

for each solids sample varied according to the amount of solids. (The number of beakers used 

per sample is given in Appendix C. Each chairside trap sample usually required only one 

beaker.)

The beakers included water and the solids. The beakers were settled for approximately 24 

hours. In the vast majority of samples, the water in the beakers was subsampled using a glass 

pipette. The remaining water was then decanted down close to the solids layer, such that the 

solids were not resuspended. The beakers were covered with Parafilm®, and delivered to the 

MCES laboratory. (The volume of the decanted water was recorded.) 

Generally, if the samples were solids from chairside traps, the water decanted from the 

beakers was not subsampled. Given that these solids tended to be “flushed” at the chairside, it 

was felt that the traps would not contain significant amounts of mercury that would show up 

in the water used by MCES to rinse the solids out of the traps. MCES data showed that this 

was the case. MCES also needed to minimize liquid samples submitted to the MCES 

laboratory. Further, MCES did not need exact quantities of mercury for the chairside traps 

since trap contents could not be discharged to the sanitary sewer. 

The approximate volume of the solids present in the samples was recorded. This volume was 

recorded to evaluate the holding capacity of various types of amalgam removal equipment and 

filters, and for general knowledge.

To summarize, a single solids sample from either a single type of amalgam removal 

equipment or a filter, after being taken out of service, was processed and became:

1. a liquid sample (from decanting the water off of the beakers containing the solids)

2. a set of beakers of solids (occasionally one beaker, e.g., for a chairside trap)

3. If a large amount of water was used to initially process the solids, and a settling tank was 

necessary, there would be an additional liquid sample from the decanting of the settling 

tank.

4. Some solids samples required multiple stages of washing of the solids into beakers or 

settling tanks. In these cases, there may have been additional liquid samples submitted due 

to the extra settling and decanting.
16

III. D. [5] Asdex Filter processing

The Asdex filter is made up of polypropylene fibers wound into a cartridge filter, and placed 

into a sealed polypropylene housing. The filter works under vacuum. The only openings in the 

housing are two ports that fit into the manifold during use. When the filter is saturated and no 

longer usable, it is removed from the manifold and caps are screwed down onto the two ports. 

Since the solids could not be removed easily from the filter, another method had to be used to 

quantify the amount of mercury in the solids retained by the filter. The basic method 

employed was to digest the filter with nitric acid to extract mercury.

The filters for 1-PLR and 2-PLR were processed by draining liquid and free solids out of each 

of the filters. The whole filters, that is, all of the media, were then digested in nitric acid for at 

least 24 hours. This was the first, and main digestion of the filters. The volume and mercury 

concentration of the acid determined the mass of mercury.

Three subsamples of the media were then cut out of the filter, from areas of the filter that 

appeared to be loaded lightly, moderately, and heavily. These subsamples were put together in 

a beaker and submitted to the MCES laboratory for analysis. The results of the three 

subsamples were scaled up (by weight, accounting for moisture) to estimate the remaining 

mercury present in the filter. This second digestion of the three subsamples was done to 

remove mercury not digested and quantified as part of the first extraction.

The results of the nitric acid digestion and extraction for the following clinics are:

1-PLR filter (In place 7/23/99 to 8/12/99):

Mercury in free liquid drained from filter: 418 mg

Mercury in free solids drained from filter: 119 mg

Mercury in first acid extraction of whole filter: 10,800 mg

Mercury in second extraction (based on 3 subsamples of media): 2,120 mg

1-PLR Total mercury measured: 13,457 mg 

2-PLR filter (In place 8/4-19/99):

Mercury in first acid extraction of whole filter: 7,790 mg

Mercury in second extraction (based on 3 subsamples of media): 984 mg

(No free liquid or solids were present in filter.)

2-PLR Total mercury measured: 8,774 mg

The totals shown here for 1-PLR and 2-PLR are also shown in Appendix C. These totals were 

used to calculate daily averages for the amount of mercury retained in the Asdex filters.

The two Asdex filters used at 3-PLR were processed differently, but using the same basic 

principal of nitric acid digestion. Each of the two filters underwent a three-stage digestion:
17

1. New nitric acid was poured into the filter housing,

2. Caps were loosely put on filter, 

3. The filter sat for 24 hours (at room temperature),

4. The acid was poured out,

5. The volume of acid was measured,

6. A subsample of the acid was collected and submitted to the MCES laboratory.

7. The above steps were repeated a second and third time.

The results of these digestions are shown in Appendix C for 3-PLR. As was the case for the 

Asdex filters used at 1-PLR and 2-PLR, the first extraction yielded the majority of the 

mercury from the filters. The results for the filters used at 3-PLR showed a quick decline in 

the amount of mercury extracted from the filters over the course of the three digestions. 

Therefore, the filters were not subjected to a fourth digestion.

The acid used to digest the filters for the three clinics listed here was 10 Normal nitric acid. It 

was made up by mixing 2/3 parts concentrated nitric acid (approximately 70 %) with 1/3 part 

Milli-Q water.

A new Asdex filter (whole filter) was digested with nitric acid to check for the presence of 

mercury in an unused filter. The amount of mercury was measured at 0.00062 mg.
18

III. D. [6] Special solids processing involving physical removal of amalgam

particles prior to analysis for some chairside traps and 

one vacuum filter 

It was observed that some chairside traps had significant pieces of amalgam present among 

the other waste solids in the trap. It was decided that if these large pieces of amalgam were 

removed from the traps, there would be less amalgam to be chemically digested as part of the 

analytical procedures. This would minimize having high concentrations of mercury in the 

predigestion process, and thus minimize the subsequent analytical work, such as lab dilutions 

and “reruns”. The amalgam particles were put into separate, labeled containers for future 

reevaluation if necessary.

MCES believed that this was an adequate method to quantify amalgam and mercury for 

chairside traps. An exact determination was not necessary to fulfill the stated purposes of this 

study. (Trap solids must be managed in an approved manner other than sewering.) 

Therefore, while processing some chairside traps, particles of amalgam was separated out of 

the other waste solids. Based on appearance, the particles were removed, rinsed, dried (at 

room temperature), and weighed. The rinsewater and the remaining solids were processed and 

analyzed in the same way as the other solids samples described above. (See Sections: 

III. D. [4] and III. E.) If there was any question as to whether a piece of waste solids was 

amalgam, it was left with the other solids as the chairside trap was cleaned out and the 

contents transferred to a beaker.

The weight of the amalgam was used to estimate the amount of mercury. The weight of the 

particles was multiplied by 40% (0.40) to estimate the mercury content of the amalgam. This 

calculated amount was added to the amount of mercury determined by the laboratory for each 

chairside trap that had amalgam picked out. The data are in Appendix C. If no amalgam 

weight is given for a particular chairside trap, then no amalgam would have been picked out 

of the trap.
19

While processing one vacuum filter, MCES believed that there was a substantial amount of

amalgam present among the solids in the filter. The vacuum filter was in use from July 5, 

2000 to August 14, 2000, at 5-LR, during baseline monitoring.

The solids were rinsed with water to carry lighter, less dense material out of the beakers. The 

solids were swirled around underwater, and rinsed over and over again. All of the rinsewater 

and the light solids carried out of the beakers were put into a settling tank and settled. This 

material was processed in the same manner as solids from other filters. Analytical results for 

the material washed out of the beakers is presented in Appendix C. 

The cleaned material, that is believed to be primarily amalgam, was dried (at room 

temperature) and weighed. These particles are small, since they passed through a chairside 

traps. It was not as easy to determine that the particles were amalgam, as it was with the larger 

particles picked from the chairside traps. However, the particles are dense, and appear to be 

metallic.

At the time of writing this report, these vacuum filter particles have not been processed 

beyond what has been outlined here. It was not critical to know the exact amount of mercury 

present in the amalgam solids within the vacuum filter for this portion of the baseline 

monitoring period. The most important information was the amount of mercury discharged 

from the vacuum system after the vacuum filter, which was measured and reported.

No estimation has been made to quantify the mercury present in the solids separated out of the 

vacuum filter (such as multiplying by 40%, as was done with many chairside traps). These 

solids may contain some material other than amalgam. Therefore, data presented in this report 

for this specific vacuum filter represents only a partial determination of the amount of 

mercury.

As part of the evaluation of the amalgam removal equipment installed at this clinic (Durr 

centrifuge), the amount of mercury present in the vacuum filters during the baseline 

monitoring period was compared to the amount present in the vacuum filters during the time 

that the centrifuge was in place. 

Even with the partial determination, the data indicated that there was much more mercury 

present in the vacuum filters during the baseline monitoring period than during the time that 

the centrifuge was in place at the clinic. This topic is discussed in more detail in Section V, as 

part of the comments on the Durr centrifuge.
20

III. E. Analysis

Samples were delivered to the MCES laboratory as either liquid samples in a glass jars, or as 

solids in glass beakers.

III. E. [1] Analysis of liquid samples

Liquid samples were analyzed using EPA Method 245.1. Some duplicate liquid samples from 

the decanting of settling tanks containing wastewater were filtered with a 0.45 micron filter. 

This filtrate was also analyzed using EPA Method 245.1 to determine the concentration of 

soluble mercury. All of the volumes associated with the liquid samples were recorded, such as 

original sample volumes, rinsewater, and decanted volumes. The volume data were used with 

the laboratory data (given as a concentration) to compute a mass of mercury. 

III. E. [2] Analysis of solids samples - Overview

Due to the expected heterogeneous nature of the solids, the solids collected and processed as 

described above were predigested in the beakers to dissolve amalgam and mercury, such that 

the resulting “predigestate” was homogeneous. The predigestion of the solids was based on 

Method 3030 E, Standard Methods, 18th Ed., 1992. The predigestion method is given in 

Section III. E. [3].

The content of each beaker was predigested. The resulting liquid predigestate from each 

beaker was filtered, put into a volumetric flask (usually a 100 mL flask), and brought up to the 

mark. The contents of each volumetric flask were analyzed separately, and data are reported 

for each beaker. (Filtering the predigestate was done to remove coarse particles, to protect the 

analytical instrument. Coarse particles may include material such as dental hygiene polishing 

compound.)

As the work of the study progressed, the predigestates (from multiple beakers for each overall 

sample) were combined together prior to analysis to minimize the amount of analytical work.

For example, if there were four beakers for one solids sample, each beaker was digested and 

the four predigestates were put into one volumetric flask (e.g., a 500 mL flask).

Subsamples were taken from the volumetric flasks and analyzed using EPA Method 245.1. 

(Often, the subsamples needed to be diluted to be within range of the analytical method.)

This gave the mercury concentration of the predigestate(s) for each solids sample. By 

knowing the volume of predigestate(s), the mass of mercury in each solids sample was 

determined.

Data reported by the laboratory for the solids samples were given as a mass of mercury.

The “September 1998 Draft Procedure to Evaluate Amalgam Removal Equipment” developed 

for this study stated that the solids samples would be dried and weighed. It was determined 

that since all of the solids were being digested it was not necessary to dry and weigh the 

solids. This avoided the drying procedure and analyzing a portion of the solids and 

extrapolating the data to the remaining solids (which may possibly be inaccurate if the solids 

are not homogeneous). 
21

III. E. [3] Predigestion method to analyze solids samples

The following method was used as a predigestion step for the analysis of the waste solids 

collected and processed as outlined above. This method was used for both the solids from the 

wastewater samples (after separation) and the solids removed from filters and amalgam 

removal equipment. The following is taken from MCES laboratory procedures:

Predigestion Method for Dental Waste Solid Material

January 2001

Samples arrive to the Lab in 250 mL or 400 mL beakers and are refrigerated 

until analysis. At time of analysis, samples are removed from the cooler and 

allowed to warm to room temperature (approximately 20 – 30 minutes).

Prior to Predigestion

1. Turn hot plates to desired temperature

2. Arrange samples and label beakers, using a Sharpie, with the sample ID number 

or other code. Remove sample label and any remaining tape or parafilm from the 

sample beaker.

3. Include two clean beakers per batch and label as LRB (laboratory reagent blank). 

Be sure to use caution when handling these samples throughout the digestion to 

prevent cross contamination.

4. Include one amalgam QC sample in a clean beaker. Using the Solids balance, 

weigh approximately 12 mg’s of amalgam into a clean beaker. Be sure to use 

caution when handling these samples throughout the digestion to prevent cross 

contamination. Record weight of sample on predigestion sheet.

5. Record sample ID number or other code onto the predigestion sheet.

Predigestion

1. Add 10 mLs of milli-q water using an auto pipette

2. Add 20 mLs of concentrated nitric acid using the repipettor

3. Place appropriate size watch glass cover on top of beaker and swirl sample 

gently; place on the hot plate.

4. Allow samples to reflux for 4 hours. If volume appears low, remove sample from 

hot place immediately so as to prevent sample from going to dryness.

5. Remove samples from hot plate

6. Allow samples to cool

7. Slide watch glass cover slightly and add 10 mLs of concentrated nitric acid to the 

beaker; replace watch glass cover, swirl and return to the hot plate.

8. Allow samples to reflux for 1 hour. Again, if volume appears low, remove sample 

from hot place immediately so as to prevent sample from going to dryness.

9. Remove samples from hot plate

10.Allow samples to cool

Filtering
22

1. Label volumetrics with sample ID number or other code

2. Place filter paper (Schleicher and Schuell, size 12.5 cm, item #10319344, #588 

pleated) into plastic funnel and filter all samples into appropriate size volumetrics 

with caps.

3. Cap and invert several times.

4. Record volumetric volume on the predigestion sheet

Before Analysis

Samples in volumetrics may require serial dilutions before digestion, i.e., 

before analysis. We have discovered that the concentration of mercury in the 

sample correlates with the color; the deeper the yellow to green color, the more 

concentrated it is.

Analysis

Analyze samples using EPA Method 245.1.