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Is Treated Sewage Safe for Water Reuse?

John Montgomery-Brown
Department of Civil & Environmental Engineering
Stanford University
December 2001


As the world's population increases, so does our need for drinking water. Increasingly however, it is becoming more difficult to find unpolluted water resources -- especially in arid climates. As part of more sustainable water management practices, wastewater treatment plant effluents (i.e., discharges) are being used to supplement both potable (i.e., drinking) and non-potable water resources. Since wastewater effluents contain thousands of chemicals, it is important to determine if these chemicals persist at low levels in the environment and whether they constitute a health hazard to humans or other animals. My research focuses on detecting these compounds and observing their degradation products in the environment. Specifically, I am interested in understanding whether, and how, trace levels (10-9 to 10-6 grams of contaminant per liter of water) of organic chemicals can be degraded, by biological or non-biological processes, in the environment.

Because arid environments, like those found in the southwestern United States, northern Africa, and Australia, tend to lack surface freshwater resources (i.e., lakes and rivers), most people living in these regions rely on groundwater aquifers to supply their domestic and agricultural water requirements. An aquifer is essentially a subsurface reservoir of water. Often water is removed from these aquifers more quickly than it is replaced; this imbalance can result in subsidence or, in coastal regions, salt-water intrusion. Subsidence, the sinking or settling of the ground, can be a big problem -- for example, the elevation of Mexico City has decreased by approximately 7.5 meters during the past 100 years. Salt-water intrusion, when salt-water from the sea mixes with local groundwater, can contaminate drinking water aquifers and render them undrinkable.

What can we do to increase natural recharge rates? Artificial recharge is one technique commonly used to increase the rate of movement of surface water into the subsurface. Recharged waters can then be used for agricultural or commercial irrigation or for consumption. But, as previously mentioned, arid regions typically lack substantial water sources, so where does the water necessary for artificial recharge come from? Potential sources include storm water runoff, floodwaters, and municipal wastewater treatment plant (WWTP) effluents.

Soil Aquifer Treatment (SAT) is an artificial recharge technique in which WWTP effluents are discharged into basins and allowed to seep into the subsurface aquifer. In essence, SAT mimics the natural recharge procedures, like riverbank infiltration (i.e., the flow of river water into the subsurface via the riverbank), observed in less arid environments like the eastern United States. During SAT, as with natural recharge procedures, the water quality of the recharged water is greatly improved. However, because the recharged water may eventually mix with drinking water supplies there is a justified concern about whether any chemicals present in wastewater effluents survive SAT.

Interestingly, many compounds that are inherently degradable seem to persist at very low concentrations (10-9 to 10-6 grams of pollutant per liter of water) in the environment; these chemicals are known as trace organic contaminants. Because many trace organic contaminants, such as some detergent residues, PCBs, and pesticides, are toxic to humans at very low levels, it is imperative that we detect, identify, and quantify these pollutants, and study the processes involved in their attenuation (i.e., removal) and techniques to enhance their attenuation.

To facilitate my research, I have chosen to focus on a single group of compounds, namely, alkylphenol ethoxylate (APEO) metabolites because, relative to other trace organic compounds, they are present at high concentrations in WWTP effluents and represent a potential health hazard to humans and other animals. APEOs are a group of trace organic compounds that are used in many household and industrial cleaning products, plastics manufacturing, and spermicidal preparations; worldwide, approximately 500 million kilograms are used annually. During wastewater treatment, these compounds are incompletely degraded and approximately 65% of their residues are released into the environment. Several APEO metabolites (i.e., breakdown products) are estrogenic (i.e., able to regulate or affect estrogen receptors in humans and other animals) and can persist for long periods (> 1 year) in groundwater environments. A few metabolites can even stimulate certain hormone-dependent cancers.

My research uses SAT sites because they are excellent locations from which to study the environmental fate and transport of pollutants. Furthermore, because of the similarity between SAT and natural recharge processes, many of the results from this research will be transferable to other locations and environments. In the future, my research on APEO metabolite degradation can be used to help chemists design more environmentally friendly (i.e., less toxic and more degradable) commercial and industrial chemicals.