EXECUTIVE COMMITTEE 

PRESIDENT 

Suzanne E. Goss 

Government Relations Specialist 

JEA (Electric, Water & Sewer) 

Jacksonville, FL 

VICE PRESIDENT 

Julius Ciaccia, Jr. 

Executive Director 

Northeast Ohio Regional 

Sewer District 

Cleveland, OH 

TREASURER 

Karen L. Pallansch

Chief Executive Officer 

Alexandria Renew Enterprises 

Alexandria, VA 

SECRETARY 

Adel H. Hagekhalil

Assistant Director 

Bureau of Sanitation 

City of Los Angeles 

Los Angeles, CA 

PAST PRESIDENT 

David R. Williams 

Director of Wastewater 

East Bay Municipal 

Utility District 

Oakland, CA 

EXECUTIVE DIRECTOR 

Ken Kirk

September 18, 2012 

Robert Jones 

Chemical Control Division 

Office of Pollution Prevention and Toxics 

U.S. Environmental Protection Agency 

1200 Pennsylvania Ave, NW 

Washington, DC 20460 

Via Email: jones.robert@epa.gov 

Re: Enforceable Consent Agreement Negotiations for Two Cyclic Siloxanes 

 

Dear Mr. Jones: 

The National Association of Clean Water Agencies (NACWA) is providing these 

comments as an interested party in the enforceable consent agreement (ECA) 

negotiations to collect environmental monitoring data for 

octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). 

NACWA has been participating in the ECA discussions about these two siloxanes 

that began on June 27 and has reviewed the proposals that have been made for the 

ECA by the Silicones Environmental, Health and Safety Council of North America 

(SEHSC) and EPA, including EPA’s September 6 proposal. Our comments address 

aspects of the proposals’ recommended sampling efforts for publicly owned 

treatment works (POTWs), as well as other questions that EPA has asked about the 

effects of siloxanes at POTWs. 

The ECA negotiations on POTW sampling have focused on the number of POTWs 

that should be included in the data collection effort and the factors that should be 

used in determining which POTWs should be sampled. In its September 6 

proposal, EPA recommended 16 POTWs for the study, while SEHSC recommended 

five in its original proposal. NACWA recommends sampling at seven or eight 

POTWs, to potentially include two POTWs that use lagoon treatment and five or six 

POTWs that use activated sludge treatment. NACWA believes that data collected 

from seven or eight POTWs, when combined with other existing data sources, 

should provide EPA with sufficient data for an environmental risk assessment of 

D4 and D5. 

 

NACWA’s publicly owned wastewater treatment agency members are interested in 

this data collection effort because of the detrimental effects of siloxanes on the 

exhaust stages of combustion equipment when biogas is used as a renewable fuel 
NACWA Comments on Siloxane ECA 

September 18, 2012 

Page 2 of 5 

source, as well as the potentially harmful effects of siloxanes that may be released into the environment from 

wastewater effluent or biosolids. While NACWA recognizes that the problems associated with combustion 

equipment are not part of EPA’s environmental risk assessment, the Association believes that the data 

collection for the risk assessment will provide useful information for addressing this issue in the future. 

Proposals for the Enforceable Consent Agreement 

The proposals made by SEHSC and EPA for sampling at POTWs include measurements of D4 and D5 from the 

POTW influent, effluent, and biosolids, as well as surface water, sediment, and biota in the receiving waters. Air 

monitoring was also proposed for one POTW. The negotiations to date have focused on the factors that should 

be considered for selecting POTWs and determining how many POTWs that should be sampled. NACWA has 

the following comments on these issues. 

 

Population Size Served by the POTW 

By sampling POTWs, EPA would like to determine the contribution of siloxanes to POTW loading from 

domestic sources and has suggested that population size could provide information about these domestic 

contributions. NACWA agrees that quantifying domestic sources is important, since D4 and D5 are used in a 

wide range of consumer products and POTWs have no ability to regulate chemical discharges from domestic 

sources. However, NACWA disagrees that population size would provide the appropriate information. There is 

no basis for believing that use of products containing D4 and D5 will be dependent on the population served by 

the POTW. A better measure of the domestic source contribution of D4 and D5 would be the proportion of 

influent to a POTW that comes from domestic sources. In addition, the type of industrial discharges to the 

POTW must be considered. If a POTW has significant influent from industrial sources that do not discharge 

siloxanes, the domestic sources will be diluted. Conversely, the influent concentrations of D4 and D5 will be 

increased over what is expected from domestic sources if there is a significant contribution from industrial 

sources. 

EPA’s September 6 proposal states that “All monitored POTWs should have a low industrial contribution to 

influent (<10%).” NACWA agrees that the industrial contribution should be as low as possible, and the 

maximum 10 percent value for industrial contribution is a realistic goal for selecting POTWs. 

For POTWs with combined sewers, which convey both wastewater and stormwater, the influent will also be 

affected after precipitation events, and potentially much longer if the POTW stores stormwater to treat over 

time. This should be taken into account for any sampling that occurs at POTWs with combined sewer systems. 

Also, even separate sewer systems, which are not meant to collect stormwater, can have infiltration of 

groundwater into the sewer pipes and inflow of stormwater from illegal connections to the sewer. This 

infiltration and inflow from precipitation events dilutes the wastewater before it reaches the treatment plant. 

These factors must also be considered when determining the amount of influent from domestic sources. 

Treatment Method Used by the POTW 

NACWA agrees with EPA that the focus of the sampling should be on POTWs that use secondary treatment. 

Furthermore, NACWA recommends that the proportion of treatment processes used by the POTWs sampled 

for the ECA be similar to the proportion of the amount of wastewater treated with these processes in the U.S. 

SEHSC’s original ECA proposal included sampling at five POTWs: two with activated sludge treatment, one 

with oxidation ditch treatment, one with trickling filter treatment, and one with lagoon treatment. This 

sampling would not reflect the proportion of wastewater treated with these methods in the U.S. EPA’s 
NACWA Comments on Siloxane ECA 

September 18, 2012 

Page 3 of 5 

September 6 proposal, with sampling at 12 POTWs using activated sludge and four POTWs using lagoons, is a 

more realistic proportion of the treatment types used in the U.S. 

Flow Rate of the Receiving Waterbody 

Predicting the receiving body flow rate for a POTW may be difficult in some cases, since the flow rate may 

depend on recent events and the time of the year that the sampling occurs. Receiving body flow may depend on 

the amount of precipitation, any runoff from snowfall melt, and use of the water for other purposes, such as 

irrigation. Based on the chemical properties of siloxanes and previous studies of siloxanes at POTWs (e.g., 

Wang et al., 2012), the removal rates of siloxanes at POTWs are expected to be very high, with the majority of 

influent siloxanes expected to partition to the biosolids. The concentrations of siloxanes in the effluent are 

expected to be very low. While the flow rate of the receiving waterbody should be considered in the analysis of 

the sampling results, its unpredictability and the likely low discharges of siloxanes to the receiving water body 

mean that it should not be used as a deciding factor in choosing POTWs for sampling. Evaluating the impacts 

of this factor would be best accomplished by conducting sampling at the same POTW at different times of the 

year, as EPA has recommended in its September 6 proposal. 

Additional Factors of Interest 

EPA states in the September 6 proposal that “there would be value in sampling at several POTWs that discharge 

to a receiving water body with high organic content,” but “it would be difficult to identify POTWs for potential 

monitoring based on this factor.” NACWA agrees that organic content of the receiving water body should be 

used in the analysis of sampling results, but should not be used as a basis for choosing POTWs for sampling. 

NACWA also agrees with EPA that “environmental conditions in different parts of the country could affect the 

partitioning and sedimentation of siloxanes.” The POTWs chosen for sampling should be geographically 

diverse, but it is not necessary to determine a specific number of POTWs in different regions of the country. 

Sampling Details 

NACWA agrees that two sampling events at each site are sufficient for this study. In addition to sampling of 

POTW influent, effluent, biosolids, sediment at outfall, and surface water, biogas produced at the POTW 

should also be sampled. 

EPA has proposed sampling at 16 POTWs, while SEHSC has proposed sampling at five POTWs. EPA’s 

sampling plan matrix includes population served and dilution level (based on flow rate of the receiving water 

body). However, as explained above, NACWA does not believe that these factors are relevant. Furthermore, 

there is no justification given in the EPA proposal for the four-by-three size of this matrix. Sampling at only 

five POTWs, as proposed by SEHSC, would not provide sufficient data to evaluate unusual or outlying results, 

especially with only two of the five sites using activated sludge treatment. 

NACWA recommends using available data when determining the number and type of POTWs that should be 

used for sampling. Sampling at four POTWs with lagoon treatment, as recommended by EPA, is excessive due 

the small proportion of U.S. population served by POTWs with this treatment type and the available data in the 

Wang et al. (2012) study of Canadian POTWs. In this study, seven POTWs with lagoon treatment were 

sampled. EPA states in its September 6 proposal that “it is likely that the Canadian sites are not representative 

of U.S. POTWs.” NACWA disagrees with this assessment and believes that the Canadian data for lagoons are 
NACWA Comments on Siloxane ECA 

September 18, 2012 

Page 4 of 5 

consistent with other studies of siloxane behavior at POTWs. The number of lagoons sampled for EPA’s risk 

assessment should therefore be reduced. 

NACWA recommends sampling at seven or eight POTWs to provide enough data to evaluate the behavior of 

siloxanes at POTWs and the factors influencing this behavior. This number could include two POTWs that use 

lagoon treatment if EPA believes that it is necessary to verify the Canadian data. Five or six POTWs that use 

activated sludge treatment would allow evaluation of the variability of the data. 

Additional Comments 

Neither the EPA nor SEHSC proposals consider septic tanks. Since approximately 20 percent of the U.S. 

population uses septic tanks for their wastewater treatment, and this method of treatment is vastly different 

from treatment at POTWs, EPA’s risk assessment should consider environmental exposure to siloxanes from 

septic system treatment. 

Costs Associated with Siloxane Removal at POTWs 

EPA asked NACWA to provide more information about the costs incurred by POTWs that use biogas as a 

renewable fuel source and must remove siloxanes from the biogas to prevent damage to combustion 

equipment. The following information on costs has been provided by NACWA members. 

• The Great Neck Water Pollution Control District in New York is installing two 65 kW microturbines 

that will convert methane gas from the anaerobic digesters to electricity with the by-product of hot 

water. As part of the microturbine system, the utility must install a siloxane stripping unit which is 

approximately 30 percent of the cost ($285,000) of the microturbine project and will require the 

majority of the maintenance costs. 

• Metro Wastewater Reclamation District in Denver struggles with heavy siloxane deposition on the heat 

exchangers that are located in the exhaust stacks of its combustion turbines. Rather than assume the 

expense of gas treatment to remove siloxane, the utility shuts down its cogeneration facility about once 

every three to four months to physically remove the siloxane from the heat exchanger coils using a highpressure water spray. The shutdown takes about 6-7 hours. The shutdowns are inconvenient, but 

currently cost less than gas treatment. 

• The Hampton Roads Sanitation District (HRSD) in Virginia has designed a project with construction 

completion expected in 2013. The Atlantic CHP (combined heat and power) project has two 1100 kW 

Cummins gas fired engines that will burn treated digester gas and includes Applied Filter Technology, 

Inc.’s SAG Pack System for siloxane removal. Moisture and hydrogen sulfide (H2S) removal is also done 

to protect both the engine and the siloxane removal media. The entire CHP project construction cost is 

$7.6 million. The construction cost of the siloxane removal system itself is $300,000 (4% of total). 

HRSD is removing H2S and moisture in order to protect the siloxane removal media and estimates that 

in total, $800K of the construction costs can be attributed to costs associated with siloxane removal. 

• In early 2004, the City of Eugene, Oregon, installed the SAG Pack biogas filter system by Applied Filter 

Technology, Inc., which consists of two filters. The total project cost was $343,000. Eugene produces an 

average of 350,000 ft3

/day of digester gas, requiring the siloxane filter media to be changed every 18 

months at a cost of $3,500. 
NACWA Comments on Siloxane ECA 

September 18, 2012 

Page 5 of 5 

• A NACWA member installed siloxane removal in 2011 as part of a larger project which included the 

addition of cogeneration equipment to utilize onsite biogas for power generation. Removal is achieved 

by passing the gas through a carbon media engineered specifically to remove siloxanes. It is part of a gas 

cleaning system which also includes H2S removal as well as moisture removal. The gas cleaning system 

is installed ahead of two 1059 kW engines. The cost of the siloxane removal system alone was about 

$310,000 and about $580,000 for the entire biogas cleaning system. These are roughly 12% and 23% of 

the cost of the cogeneration portion of the project, respectively. Since the project has been operating for 

less than a year, the utility does not have optimized operation and maintenance costs yet. In particular, 

the siloxane media has not been changed yet, and this is expected to be a large maintenance cost. This 

year, the H2S media was replaced for about $115,000, and the siloxane media is expected to cost more 

than the H2S media. Labor, power, and maintenance were estimated at $120,000/year for the entire gas 

cleaning system. 

• A NACWA member conducted a full-scale pilot of a reciprocating internal combustion engine exhaust 

treatment system consisting of digester gas clean-up and two post-combustion catalysts in series to 

reduce carbon monoxide (CO), oxides of nitrogen (NOx), and volatile organic compound (VOC) 

emissions. The pilot study was conducted to determine whether the challenges of working with biogas 

(namely, the presence of contaminants such as siloxanes and hydrogen sulfide) could be overcome in 

applying existing catalyst technology to biogas-fired engines. The pilot project’s biogas cleanup system 

utilized an existing gas dryer (heat exchanger) plus a new fixed-bed gas cleaning vessel with a blend of 

activated carbon. Only initial cost estimates are available for this system at this time. It is estimated 

that the digester gas cleaning vessels capital cost is approximately 20 percent of the capital cost of the 

two post-combustion catalysts and building improvements to house the catalysts. 

Thank you for your consideration of these comments. Please contact me at 202-533-1836 or cfinley@nacwa.org if 

you have any questions. 

Sincerely, 

 

Cynthia A. Finley, Ph.D. 

Director, Regulatory Affairs