Anaerobic Co-Digestion Planning and Research for Green Bay, WI
The aspiration to find an alternative sustainable fuel source is an ever growing concern. Anaerobic digestion might hold the answer to finding an alternative sustainable energy source. Anaerobic digestion does not only provide a renewable resource in the form of biogas, but also stabilizes wastes, preventing large amounts from being landfilled or incinerated. Anaerobic digestion of municipal wastewater sludges can often be improved by the addition of high strength industrial wastes, in a process called co-digestion. High strength industrial wastes provide an additional carbon source for the microbes to utilize and convert into usable biogas. Co-digestion also offers the opportunity for the high strength wastes to be converted to renewable energy (biogas) rather than being landfilled of incinerated. During co-digestion, the three possible outcomes are synergistic, neutral or antagonistic with respect to gas production. This study was designed to test the effects of co-digesting various high strength industrial wastes in addition to actual municipal sludge from a wastewater treatment plant. Preliminary screening of 20 industrial wastes was completed to narrow the list down to four of the most promising wastes to be used during the bench-scale study. The most promising wastes were characterized by haul distance, chemical oxygen demand (COD), Volatile Solids (VS):Total Solids (TS) and biochemical methane potential (BMP) results. A long term bench-scale study was designed to test the effect of anaerobic digestion as well as co-digestion. The bench-scale digesters were run for 275 days over four different phases. The first phase tested the effect of digesting actual municipal sludge at an organic loading rate (OLR) range of 3 to 6 gCOD/L-day. The second, third and fourth phases tested co-digestion of a consistent mix of the four co-digestates with municipal wastewater sludge at increasing organic loading rates. The OLRs increased during each phase: Phase 2 OLR (4 to 7 gCOD/L-day), Phase 3 OLR (4 to 9 gCOD/L-day) and Phase 4 OLR (6 to 10 gCOD/L-day). Phase 3 and Phase 4 tested the effect of increasing the volume of co-digestate added in an attempt to achieve the maximum OLR of the bench-scale digesters. Co-digestion during Phase 2 and Phase 3 proved to be very beneficial. Phase 2 compared to Phase 1 resulted in an increase in CH4 production ranging from 18% to 31% as well as an increase in VSR of 6.7% to 13%. Phase 3 compared to Phase 1 saw an increase in CH4 production ranging from 34% to 45% as well as an increase in VSR ranging from 9.4% to 23%. Phase 4 digestion proved to be near or above the maximum OLR for the bench-scale digesters, resulting in operational issues and digester failure. Co-digestion could prove to be a solution to finding an improved renewable energy, but testing on the different high strength wastes that could be used to improve this technology.