Try Before You Buy – Bench-Scale Digestion Evaluation of Potential Co-Digestion Waste Streams

With limited digester capacity and the desire to increase digester gas production, Gwinnett County Department of Water Resources initiated a research project with Hazen and Sawyer and Georgia Institute of Technology to evaluate various fats, oils, and grease (FOG) and high strength waste (HSW) streams for co-digestion. The bench-scale evaluation included stream characterization, ultimate digestibility, continuous flow bench scale digester and biological modeling with BioWin. During waste stream characterization, the concentration, carbonaceous oxygen demand (COD), along with pH and nutrient (nitrogen phosphorus) concentration were measured. During the ultimate digestibility test, the COD concentration and methane production was determined after a 90-day residence time. The ultimate digestibility was used to compare the multiple waste streams to identify the streams that produced the most methane per volume of waste digested. Two waste streams, FOG and chewing gum wastes, was used in the continuous, anaerobic, bench scale reactor. The waste streams were co-digested with the plant's primary and waste activated sludges at similar organic loading to simulate the full-scale digestion process. Each waste stream was co-digested individually and then both waste streams were co-digester together. With each waste stream, the COD destruction and methane production exceeded the expected values of each individual waste stream from the ultimate digestibility tests. The continuous bench scale reactor was modeled using a calibrated BioWin model by EnviroSim that matched the theoretical and underestimated the observed COD degradation and methane production.