Current research projects:
Understanding light-mediated inactivation of pathogens in surface waters
Bacteria and viruses in water may be inactivated by exposure to sunlight, yet little is known about the dominant mechanisms. The goal of our research is to quantify the rates of inactivation of indicator organisms and waterborne pathogens due to three different sunlight-mediated mechanisms. The first mechanism is direct damage to cell components caused by absorption of ultraviolet (UV) light. The other two mechanisms are indirect, resulting in cell damage by reactive oxygen species (ROS) that are produced by the interaction of sunlight with sensitizer molecules in the presence of oxygen, and are initiated either inside the cell (endogenous) or outside the cell (exogenous). Many researchers are currently working on several of the key pieces of the puzzle we are trying to solve, for example aquatic chemists are studying the photochemistry of ROS, biochemists are studying oxidative stress in bacteria, and engineers are studying the inactivation rates of pathogens in surface waters. However, my students and I are the first to integrate all of these topics and develop mechanistic explanations for pathogen inactivation in the presence of sunlight and the various types of organic matter that are present in surface waters. For example, we recently demonstrated that MS2 can be inactivated by singlet oxygen produced when sunlight is absorbed by sensitizers such as natural organic matter (NOM) and algal components (Kohn and Nelson 2007). Furthermore, because singlet oxygen is produced within the sensitizer, inactivation is dominated by the NOM that is associated with MS2 (Kohn et al. 2007). These results provide insight into the fate of pathogens in rivers, lakes, and the coastal ocean, as well as in engineered systems for treating wastewater, such as WSP, and the solar disinfection of drinking water.
Tertiary filtration of wastewater for agricultural reuse
California is a global leader in the practice of reclaiming and reusing wastewater; the largest reuse project for irrigation of food crops is in the Salinas Valley. The wastewater from the cities of Salinas, Castroville, and Monterey is collected and treated at the Monterey Regional Water Pollution Control Agency's tertiary filtration plant and then distributed to the valley for the irrigation of 12,000 acres of high value crops such as lettuce, artichokes, broccoli and strawberries. However, the demand for recycled water exceeds the current supply, and although more wastewater is available the existing tertiary filtration plant can only treat about 75% of the incoming flow because the loading rate is limited by the California Department of Health Services (DHS) to 5 gal/ft2/min. The goal of this project is to determine whether the existing tertiary filters can treat a higher flowrate and still produce water that is safe for irrigation. We have completed the first phase of our research, which was performed at a pilot facility we built at the Monterey treatment plant. The results are the first to confirm that particle removal in wastewater filtration is consistent with clean bed filtration theory, and also suggest that loading rates up to 7.5 gal/ft2/min can produce acceptable effluent quality (Williams et al. 2007). The second phase of the research will be conducted at five full-scale treatment plants across California. Throughout the project, we are working closely with DHS because our results may form a basis for changing the state law.
"Easy Water", Article in Forefront Magazine, Spring 2005.
Inactivation of Ascaris eggs by temperature, pH, and ammonia
An estimated 1.47 billion persons are infected with the intestinal parasitic worm Ascaris lumbricoides, making it the third most common human infection. Adult Ascaris worms, measuring up to 12 in. in length, parasitize the intestinal tract of their host, competing for nutrition and potentially causing a range of other complications. Currently, an estimated 1.5 million children suffer from Ascaris infections that will result in permanent growth retardation, even if the infection is cured. Ascariasis is transmitted by eggs, which are excreted by infected individuals. During most types of wastewater treatment, Ascaris eggs are concentrated in the sludge by sedimentation, and this sludge must be treated prior to disposal or land application. The highest incidence of ascariasis occurs in regions with warm climates and poor sanitation, but even in the United States the concentration of eggs in wastewater sludge is high enough to drive the development of improved sludge treatment technologies. Alkaline treatment is one of the most common types of sludge treatment, and, unlike many types of sludge treatment (such as mesophilic sludge digestion), is known to inactivate Ascaris eggs under some conditions. We have conducted experiments under controlled laboratory conditions as well as using actual sludges in Mexico to understand the effects of temperature, pH, and ammonia on Ascaris eggs, and to quantify the degree of inactivation achieved by varying these three factors (Pecson et al., 2007; Pecson and Nelson, 2005). This research provides critical information separating the effects of these different parameters, which have often been confused by previous researchers. Our results have major implications for the design and regulation of alkaline treatment methods, especially for meeting US EPA Class A biosolids standards. The results are also useful for developing low-cost treatment methods for fecal wastes from other sources, such as urine-separating dry latrines.
Pathogen detection using quantitative PCR
Existing culture-based detection methods present many limitations to studying and controlling waterborne pathogens. There is currently much interest in the development and application of molecular detection methods. We are developing several innovative methods for detecting pathogens and pathogen indicator organisms using real-time, quantitative polymerase chain reaction (qPCR). Our qPCR method for detecting Ascaris eggs is a major advance because we successfully developed a quantitative approach despite the fact that Ascaris is a eukaryotic, multi-cellular organism (Pecson et al., 2006). Furthermore, the method can distinguish infectious from non-infectious eggs. We are currently conducting follow-up research with embryonated eggs and actual biosolids samples to assess whether this method could become a standard assay for assessing biosolids. We have also developed a qPCR method for E.coli that overcomes the limitations of previously published methods by eliminating contaminant E.coli DNA that is present in polymerase enzyme preparations, thus lowering the detection limit of the assay (manuscript is in preparation). We are currently using this assay along with several others to provide quantitative information on the prevalence of traditional water quality indicator organisms as well as host-specific targets in different types of waste. This information will contribute to approaches for identifying the specific sources (e.g., human vs. various types of animal) of fecal pollution in a water body.
Low-cost technologies for disinfection of drinking water at the point-of-use
Technologies appropriate to the economic and cultural environment in developing countries are urgently needed to provide services to the 1.1 billion people that do not have access to safe drinking water and 2.6 billion that do not have access to adequate sanitation. We are evaluating several technologies for low-cost treatment of drinking water at the point-of-use (POU), including:
More information on these technologies, as well as our collaborative efforts to provide rigorous field evaluation and marketing studies on POU technologies, is available on the website describing our Blum Pilot Initiative on Safe Water and Sanitation.