Employing a fundamental understanding of organic chemical reaction pathways, our research explores links between public health, engineering and sustainability. A key focus of our current research is Engineering for the Sustainable Use of Impaired Waters
The shortage of clean water represents a critical challenge for the next century, and has necessitated the use of impaired waters as potable water supplies. We are part of the NSF-funded Engineering Research Center for RE-Inventing the Nation's Urban Water Infrastructure (ReNUWit), a collaboration between Stanford, Berkeley, the Colorado School of Mines and New Mexico State University. Recent droughts in Texas and California have exacerbated routine water shortages in the arid southwest. While there has been some interest among the public in seawater desalination, utilities are far more focused on the recycling of municipal wastewaters, the reclamation of brackish groundwaters, and capture of urban stormwater, because these options employ far less energy than seawater desalination. Current projects include:
Health Risks from Byproducts of Food Disinfection
PhD Candidate: Adam Simpson
Chlorine disinfection of produce and meats in food processing facilities serves as a critical control point to reduce the risk of foodborne illnesses. Following the discovery that chlorine disinfectant reactions with organic matter in drinking water supplies produces carcinogenic byproducts (e.g., haloacetic acids), drinking water disinfectant exposures have been moderated to balance the risks posed by pathogens and chemical byproducts. The goal of this research is to develop a similar balance for food disinfection. Byproduct formation may be far more significant in food than in drinking water given that orders of magnitude higher chlorine doses (~100 mg/L vs. 2 mg/L) and precursor concentrations (solid produce vs. 1 mg/L) occur in food. The project is characterizing new byproducts from chlorine reactions with food biomolecules that are relevant to consumer exposure because they remain in the food (unlike haloacetic acids). Initial results indicate that the toxicity associated with consumption of chlorotyrosine byproducts retained in protein within a spinach salad is roughly two-fold higher than the risks posed by regulated byproducts in chlorinated drinking waters.
Read more: Komaki, Y.; Simpson, A.M.-A.; Choe, J.K.; Plewa, M.J.; Mitch, W.A. Chlorotyrosines versus volatile byproducts from chlorine disinfection during washing of spinach and lettuce. Environ. Sci. Technol., 2018, 52, 9361-9369. (Full Text)
Linking Anaerobic Secondary Treatment to Water Reuse
PhD Candidate: Alex Szczuka
Anaerobic biological secondary treatment can lower the energy cost of wastewater treatment by more than 75%, and reduce the costs associated with secondary solids disposal by more than 90%. However, demonstrating that anaerobically-treated wastewater can be reused is essential to both the adoption of anaerobic treatment technologies and increasing the sustainability of our water supply. For potable applications, treated wastewater is purified by membrane-based treatment technologies, such as reverse osmosis, which removes contaminants by size exclusion, and advanced oxidation processes, which remove contaminants by chemical oxidation. Prior to these treatments, anaerobic effluent must be disinfected, and the method of disinfection can effect the performance of downstream treatment units, and the formation disinfection by-products. The goal of this project is to evaluate the suitability of anaerobically-treated wastewater as a source water for potable water production. Compared to traditional effluents, we found that the organics in anaerobically-treated effluents have a lower propensity to foul membranes, and the disinfection byproduct formation from anaerobic effluents can be ten-fold lower. However, sulfide present in anaerobically-treated wastewater can render disinfection difficult, and developing optimal strategies for disinfection of sulfide-rich effluents is ongoing.
Read more: Szczuka, A.; Berglund-Brown, J.P.; Chen, H.K.; Quay, A.N.; Mitch, W.A. Evaluation of a Pilot Anaerobic Secondary Effluent for Potable Reuse: Impact of Different Disinfection Schemes on Organic Fouling of RO Membranes and DBP Formation. Environ. Sci. Technol. 2019, 53, 3166-3176 (Full Text)
Contributions of Disinfection Byproducts (DBPs) to the Toxicity of Drinking Water and Potable Reuse Water
Postdoc: Stephanie Lau
Disinfection of drinking water is the greatest public health achievement of the 20th century, but an unintended consequence of water disinfection is the formation of disinfection byproducts (DBPs) that could harm human health. Current research tends to focus on the concentrations of DBPs formed from different source water and water treatment processes. However, concentration alone doesn’t tell the whole story as DBPs vary a lot in their toxic potency. Furthermore, the majority of DBPs formed in disinfected water are unknown, and the toxic potency of unknown DBPs is unclear. In my research, I use toxic potency-weighting calculations to estimate the toxicity of a disinfected water due to known DBPs. To examine the toxicity of unknown DBPs, I extract disinfected water using XAD resins and perform a cell-free toxicity assay on the extract to predict its toxicity. Integrating the results from calculations and extractions/toxicity assays leads to a comprehensive picture of the DBP-associated toxicity of disinfected water. I am applying this integrative approach to examine the effectiveness of granular activated carbon (GAC) filtration in reducing the formation of the most toxic DBPs in chlorinated drinking water. I am also applying this approach to compare the DBP-associated toxicity of potable reuse water and conventional drinking water.
Mechanism of Sunscreen Toxicity to Corals and Sea Anemones
PhD Student: Djordje Vuckovic
Sunscreen has been linked to coral bleaching and death at order of magnitude lower concentrations than found at coral reef sites. The sunscreens are introduced into the sites by tourists, often as part of eco-tourism groups. As the presence of sunscreens cannot be avoided, (the visits are crucial for coral reef conservation and sunscreen is necessary for protection against skin cancer), it is important to determine which sunscreen components are harming corals and why. In our research, we are using sea anemones as coral prototypes, to elucidate a toxicity mechanism common to several of the most frequently used organic sunscreens. Like corals, sea anemones have symbiotic relationships to algae. We are finding that a large amount of hydrophobic sunscreen accumulates in the sea anemones and is bio-metabolized into more toxic compounds. Understanding the chemical mechanism of their toxicity should aid development of coral-safe sunscreens.
Public Health Implications of Exposure to Wastewater-Associated Disinfection Byproducts
PhD Candidate: Kirin Emlet Furst
Communities around the world increasingly rely on sources of drinking water that are contaminated by municipal wastewater. Disinfection of drinking water is critical to protect public health from waterborne pathogens, particularly in wastewater-impacted source waters. Unfortunately, in addition to inactivating pathogens, disinfectants react with dissolved organic matter to form a wide variety of contaminants known as disinfection byproducts. Several classes of disinfection byproducts that are of particular toxicological concern have been associated with wastewater-derived precursors. The goal of this research is to examine the public health implications of wastewater-associated disinfection byproducts by estimating their contributions to toxicity in various source waters and treatment scenarios. These include potable reuse of municipal wastewater effluents in the high-income countries and use of sewage-impacted drinking water supplies in low-income countries. Initial results indicate that haloacetonitriles and haloacetamides may be a significant concern in wastewater-impacted drinking waters. However, little is known about the mechanisms of exposure to these disinfection byproduct classes. We are developing novel sampling methods to enable the translation of concentrations in water to exposure via ingestion and bathing.
Read more: Furst, K.E.; Pecson, B.M.; Webber, B.D.; Mitch, W.A. Tradeoffs between pathogen inactivation and disinfection byproduct formation during sequential chlorine and chloramine disinfection for wastewater reuse. Water Res. 2018, 143, 579-588. (Full Text)
Development of an Electrochemical Advanced Oxidation Process for Potable Reuse Facilities
PhD Student: Cindy Weng
Advanced treatment trains for the potable reuse of municipal wastewater rely on the multiple barrier approach to remove the wide array of chemical contaminants in wastewater. A typical treatment train combines broad-screen physical removal using reverse osmosis (RO) membranes with advanced oxidation processes (AOPs), which generate radicals to degrade a broad array of chemical contaminants that may pass through RO membranes. Currently, most potable reuse facilities employ the UV/hydrogen peroxide AOP, which generates hydroxyl radical (*OH) by the UV photolysis of hydrogen peroxide (HOOH). UV photolysis of hydrogen peroxide is inefficient, with only ~10% of the hydrogen peroxide used despite application of UV doses ~10-fold greater than needed for disinfection. Utilities are also interested in alternatives to RO-based treatment trains (e.g., ozone/biological activated carbon), but their effluents have lower UV transmittance (UVT), hindering the performance of subsequent UV-based AOPs. We are developing a novel AOP to generate radicals electrochemically. We are finding that this AOP is more efficient than the current UV/hydrogen peroxide AOP and can be applied to low UVT waters.
Treatment of Concentrates from Reverse Osmosis Processes at Potable Reuse Facilities
PhD Student: Jack King
Reverse osmosis (RO) treatment serves as an effective physical barrier to remove a wide range of pathogens, chemical contaminants and salts from municipal wastewater within potable reuse treatment trains, but RO generates a concentrate containing these contaminants. There are growing concerns about the impacts of the discharge of these concentrates to receiving waters. In marine waters such as poorly flushed estuaries, concerns are primarily from toxicity associated with chemical contaminants. We evaluated the efficacy of ozone treatment of RO concentrates to remove emerging contaminants including pesticides and metal chelates. Meanwhile, halogenated contaminants are a major class of chemical contaminants resistant to ozone and other oxidative treatments. To degrade halogenated contaminants in water, we are employing activated carbon cathodes for electrochemical reduction. Since most halogenated compounds are hydrophobic, they first adsorb to the electrodes; then, they can be destroyed by supplying electrons to break halogen bonds to form harmless halide ions (e.g., Cl-).