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Session 04A: Resource Recovery
10:30am - 11:15am
Beyond Net Zero – Reaching the Next Level of Renewable Energy through Beneficial Use of Food Waste
The City of Gresham (City) Wastewater Treatment Plant (WWTP) is a 15 MGD facility located east of Portland, Oregon. The plant accepts and co-digests fats, oil, and grease and beneficially uses their biogas to fuel a combined heat and power system to achieve energy neutral status and they are evaluating how to go beyond net zero. The existing two mesophilic anaerobic digesters at the facility are currently at capacity and in need of an expansion to accommodate projected future loadings for city growth as well as to provide redundancy.
A recently adopted policy by Metro (regional entity) requires mandatory segregation, collection, and alternate processing (not landfilling) of food waste that is generated by businesses within the Portland metropolitan area. The City conducted a study to explore the financial payback options for a digestion expansion project that could fulfill the City’s capacity needs, accept food slurry from Metro’s food waste program (or other liquid organic waste sources), and beneficial use of the additional biogas produced.
The study assessed potential cost and non-cost benefits and impacts associated with additional liquid organic waste loading on the WWTP, digestion alternatives, sidestream recycle loading to the liquids treatment, solids dewaterability, biogas production, and biosolids end use. The study included a business case evaluation considering economic, environmental, social, and operational impacts to assess the favorability of pursuing the selected alternative. Finally, the study included a conceptual design for the selected alternative.
The study has shown that the City could spend 16 million dollars and construct a third mesophilic digester for capacity needs and not accept additional feedstocks nor receive a payback on the investment. Alternatively, the City could spend approximately 30 million dollars to convert two existing digesters to thermophilic technology, construct a third thermophilic digester, expand feedstock receiving, expand cogeneration, and obtain a payback on investment under 10 years by utilizing State incentive programs. The City is planning to conduct a predesign to further refine liquid organic waste availability, revenues and costs, and funding sources for the project.
11:15am - 12:00pm
Linking Anaerobic Digester Microbiomes With Resistance To Organic Overloads
Oregon State University, United States of America; firstname.lastname@example.org
Anaerobic co-digestion has become a popular option to increase biogas production, thus increasing recapture potential, with grease trap waste, or FOG (fats, oils, and greases), showing the highest methane production potential. In order to ensure reactor stability and optimal performance, the correct microbiome composition is essential. However, it is currently unknown what microbiome compositions are optimal for co-digestion nor what operational parameters are most effective at creating these optimal structures. This work monitored nineteen full-scale anaerobic digesters, at six separate facilities, monthly for one year to link operational characteristics with microbiome composition. Of the nineteen digesters studied, three perform FOG co-digestion, four perform co-digestion with biodiesel wash water, one is fed only TWAS (thickened waste activated sludge), and the remaining eleven are fed primary sludge and TWAS. Microbiome composition was analyzed using 16S rRNA amplicon sequencing. Operational data from each of the full-scale facilities (including pH, alkalinity, volatile fatty acids, detention time, temperature, total and volatile solids, free ammonia, organic N, dissolved P, organic P, chemical oxygen demand, and gas production) was examined to determine their influence on microbiome compositions.
Batch resistance assays were created to link microbiome compositions with digester functionality in response to organic over loadings. For each full-scale anaerobic digester tested, twenty-five 100 mL batch anaerobic digesters were used for batch resistance assays. Five batch anaerobic digesters were ran as digestate-only controls, and the remaining twenty were fed 1 mL, 5 mL, 10 mL, and 20 mL of canola oil (in quintuplet). The rates of methane production and methane content of the biogas were used to calculate the functional resistance of each full-scale anaerobic digester. The batch resistance assays were performed in winter and summer to account for seasonal variations in full-scale plant operation and microbiome compositions.