How Much Degradation Occurs in Sewer Systems?
Exposure modeling plays an important role in national-scale risk assessment of chemicals that are disposed of down-the-drain (for example, home and personal care product ingredients). Exposure models commonly account for chemical removal that occurs during wastewater treatment processes at treatment facilities. However, for many chemicals, a significant amount of removal (biodegradation) can also occur in the sewer system. Combining chemical-specific biodegradation data from laboratory studies with estimates of typical “sewer residence times” provides a way for exposure modeling to represent this aspect of environmental fate and transport. However, given the thousands of municipal sewer systems across the U.S., how can we estimate the typical range for sewer residence time?
Waterborne scientists collaborated with scientists from Procter & Gamble to address this question in a recent study by developing a geographic information systems (GIS) approach to estimate the distribution of sewer residence times for the U.S. using road networks as a spatial proxy for sewer networks. While available data for sewer networks is limited, we evaluated the similar spatial distributions of case study sewer networks and road networks. Building upon that analysis, our experts analyzed the spatial distribution of population density and over 3,400 facility locations across the U.S. to estimate sewer residence times using existing national datasets and sewer system design standards.
Our analysis estimated a median sewer residence time of 3.3 hours for the U.S, which is comparable to values reported in literature. The distribution of residence time values generated from our analysis enables this parameter to be represented probabilistically (instead of just as a single point value) which adds robustness to risk assessments. Using our analysis results, we estimated in-sewer removal across a range of hypothetical, but realistic, chemical biodegradation rates to illustrate that a significant amount of removal is likely to occur in the sewer for many chemicals. We also specifically evaluated a group of readily biodegradable surfactants used in home and personal care products for which biodegradation data was available, and estimated removals of 62% to 99% during sewer transit (based on a median residence time of 3.3 hours). Significant in-sewer removal estimated for many down-the-drain chemicals has implications for estimation of influent concentrations at wastewater treatment facilities, and ultimately for predicted environmental concentrations in receiving waters.
This study is an example of how best-available data resources can be paired with advanced GIS capabilities to address important data gaps in exposure modeling and add value to the environmental risk assessment process. The work was recently published in Science of the Total Environment.
Kapo, KE, Paschka, M, Vamshi, R, Sebasky, M, McDonough, K. 2017. Estimation of U.S. sewer residence time distributions for national-scale risk assessment of down-the-drain chemicals. Science of the Total Environment 603-604:445-452. https://doi.org/10.1016/j.scitotenv.2017.06.075
Contact Katherine Kapo, Senior Scientist, Environmental Risk Assessment, at email@example.com with questions about our down-the-drain expertise.