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Major Factors Impacting Environmental Levels of Siloxanes (D4/D5) at POTWs 

without Industrial Siloxane Sources and Selection of Sites for Monitoring

Charge: Build on the SEHSC proposal to develop a science-based, rational proposal for monitoring of 

Publicly Owned Treatment Works (POTW) sites which provides sufficient information and data on the 

presence and concentrations of D4 and D5 in the environment near POTWs, to support a national risk 

assessment. Monitoring that establishes the contribution of siloxanes to POTW loadings from 

manufacturing, formulating and processing facilities will be addressed separately.

Monitoring Objective: A sampling strategy should collect monitoring data that scans a variety of 

POTWs that have attributes that could be used to characterize the likely range of siloxane 

environmental concentrations around POTWs. With these data, an understanding of how population 

size, receiving water body flow and other factors contribute to the environmental loading of siloxanes 

near POTWs would be developed. 

Approach

The major factors in site selection likely to affect the measured levels of siloxanes in the sediment at 

outfalls from POTWs outside of SEHSC control would be established. These factors would then be used 

as input to an optimization process to develop a monitoring program design. This optimization method, 

D-Optimal design, coupled with the relevant major factors established, would provide a scientific basis 

for a monitoring program design to understand how the various factors contribute to siloxane measured 

results without testing the entire set of possible permutations. Additional sampling considerations such 

as air monitoring and seasonal variability would be considered in the future, but do not impact site 

selection and are not included here. 

Major Factors Relevant to the Environmental Presence of Siloxanes

Below is a list of major factors related to site selection that EPA believes most strongly impact the 

siloxane levels in the sediment and water at POTW outfalls. 

Influent sources

1. Population size served by the POTW (High/Medium/Low)

This factor could provide information on how consumer or household use of siloxanes 

contributes to POTW siloxane loadings. 

POTW characteristics

2. Treatment types (activated sludge, other secondary treatment types). Other secondary treatment 

types could include trickling filter, oxidation ditch or lagoons.
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A focus would be placed on POTWs that use secondary treatment because this would represent 

the vast majority of POTWs across the nation 

Tertiary treatment is not included because it is assumed there would be very low siloxane 

concentrations in effluent.

3. High Total Suspended Solids (TSS) in effluent.

Indications of high TSS in effluent from the POTW could be important as we know that siloxanes 

partition to solids. 

Receiving body characteristics

4. Geography (North, South)

Geography could be important as colder temperatures likely decrease the amount of siloxanes

that partitions to the air and will increase the amount that remains in the water and that 

reaches the sediment.

5. Receiving body flow (lake/ low flow stream/high flow river)

Receiving body flow directly impacts the degree of dilution and probably the turbulence in the 

water. Both could impact the partitioning behavior of siloxanes. 

6. Organic content of receiving body (high/low) 

High organic content in the receiving body could increase the amount of siloxanes that partition 

to the sediment.