– Opportunities Fuel From Food Waste Diane Nelsen, Jiaru Wu, Dave Stensel, John Ferguson, Heidi Gough, and Sally Brown University of Washington Anaerobic Reactor Tests The City of Tacoma Central Treatment Plant (CTP) currently has excess capacity in its sludge digestion system. This capacity would allow the plant to co-digest various wastes, which represents an opportunity to increase revenue through tipping fees. By increasing the volume of waste treated, the City can also increase production of biogas. Currently, biogas in excess of that needed for process heat is flared, but an increase in production would allow the plant to install a cogeneration unit to produce both electricity and process heat, which could further increase revenue. Objectives Methanosaeta vs. Methanosarcina Methanosarcina Methanosaeta Excessive organic loading of anaerobic digesters can result in excessive acetate production, which inhibits methanogens and destabilizes reactor operation. Furthermore, changes to the waste feed may affect the quality of the biosolids, which are currently class A and form the backbone of Tagro mixes. Challenges Bottle Tests These tests will be carried out in anaerobic, sealed serum vials, using a modified Hungate technique. The characteristics of both the co-digested wastes and the digester sludge will determine optimum loading rates for the anaerobic reactors. The wastes will be tested for their Biochemical Methane Potential (BMP) and their Chemical Oxygen Demand (COD). Bottle tests will also be used to measure the maximum organic loading rate and the maximum methane production rate of the reactor sludge. Courtesy of Anne Conklin, University of Washington This project will determine the feasibility of adding food wastes to the CTP sludge digestion system. Process stability, optimum loading rates, methane production rates, and effects on sludge characteristics will all be examined. This study will focus on three waste streams: solid foodwaste; liquid foodwaste; and fats, oils, and grease. The effect of foodwaste addition on the microbial community will also be studied. Bench-scale, two-stage anaerobic reactors will be used to simulate the process used at the CTP. The first stage will be maintained at 55˚C and the second stage at 35˚C, to reflect the temperature-phasing used at the CTP. Each stage will be fully mixed, with daily feeding and a retention time of five to ten days. One set of reactors will receive feed sludge mixed with food waste; the other set will receive only feed sludge and serve as a control. The reactors will be monitored for process stability, sludge characteristics, effluent quality, methane production rates, and microbial community composition. Increased loading can destabilize anaerobic digestion; conversely, higher loads can select for a microbial community that produces methane faster and better withstands disturbances. Digester stability and methane production rates depend on the relative populations of Methanosaeta and Methanosarcina. Microbial Community Effects Images Courtesy of Anne Conklin, University of Washington Both Methanosaeta and Methanosarcina are methane-producing, acetate-utilizing archaea, but they have differing growth rates and substrate utilization rates. Digesters are often dominated by Methanosaeta, which have slower growth rates but can thrive at lower acetate concentrations. When feeding rates are high, Methanosaeta cannot metabolize COD quickly enough and acetate accumulates to toxic levels, leading to digester instability. In contrast, Methanosarcina can metabolize COD more quickly, and Methanosarcina- dominated reactors can be more resilient when feed rates fluctuate. We hypothesize that the addition of food wastes will select for a greater proportion of Methanosarcina, and we will test this by comparing the microbial communities the experimental and control reactors Effect of community dominance on acetate accumulation during periods of high COD loading. Figure courtesy of Anne Conklin, University of Washington