Anaerobic Co-digestion for Enhanced Renewable Energy and Green House Gas Emission Reduction

ANAEROBIC CO-DIGESTION FOR ENHANCED RENEWABLE ENERGY AND GREEN HOUSE GAS EMISSION REDUCTION Navaneethan Navaratnam Marquette University, 2012 The need to develop renewable energy is important for replacing fossil fuel, which is limited in quantity and also tends to increase in price over time. The addition of high strength organic wastes in municipal anaerobic digesters is grow ing and tends to increase renewable energy production. In addition, conversion of wastes to energy significantly reduces uncontrolled greenhouse gas emissions. Co-digestion of municipal sludge with any combination of wastes can result in synergistic, antagonistic or neutral outcomes. The objectives of this study were to identify potential co-digestat , determine synergistic, antagonistic and neutral effects, determine economic benefits, quanti fy performance of bench scale co-digesters, identify influence of co-digestion on microbial communities and implement appropriate co-digestion, if warranted, after full-sca e testing. A market study was used to identify promising co-digestates. Most promising wast es were determined by biochemical methane potential (BMP) and other testing followed by a simple economic analysis. Performance was investigated using bench-scale diges ters receiving synthetic primary sludge with and without co-digestates. Denaturing gradie nt g l electrophoresis (DGGE) and quantitative polymerase chain reaction (qPCR) analyses were per fo med on the gene encoding the α subunit of methyl coenzyme M reductase ( mcrA) to compare methanogen communities among the digesters. One significant band contri buting to the greatest difference in banding patterns was excised, cloned, am plified and sequenced. Fullscale co-digestion was conducted using the most promising co-digestate at S outh Shore Wastewater Reclamation Facility (Oak Creek, WI). Over 80 wa stes were identified from 54 facilities within 160 km of an existing municipal digester. A sim ple economic comparison identified the greatest benefits for seven co-digestates. Methane production rates of two codigester systems increased by 105% and 66% in comparison to a cont rol sys em. These increases were great than anticipated based on theoretical metha ne production from the additional chemical oxygen demand (COD) of the co-digestates. Co -digestion of the most promising wastes with primary sludge was estimated to gener ate nough electricity to power more than 2500 houses. Synergistic outcomes of co-digestion may be ca used by chances in microbial community resulting in more rapid methane production rate and higher specific methanogenic activities of the biomass against acetate, propionate a nd H2 s substrates. The presence of Methanospirillum hungatei correlated to higher SMAs in the Co-Digester 1 system. In subsequent full-scale testing, acid whey in addition t o primary sludge increased methane production by 16 %, biogas methane content by 5%, methane yield per VS destroyed by 9% ( from 650 to 710 L CH 4 / kg VSdestroyed ) and volatile solids removal by 20%. Co-digestion is a promising technology to increase renewable energy production and convert municipal digesters into regional renewable energy facilities.