INCREASING ANAEROBIC DIGESTION ADOPTION THROUGH NOVEL INOCULUM PRESERVATION AND UNDERSTANDING THE EFFECT OF WASTE STREAM DYNAMICS
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This research aimed to decrease barriers to anaerobic digestion (AD) adoption through: 1) preservation of AD inoculum, 2) iron addition to improve biogas quality, 3) sulfur addition to increase potential waste streams treated, and 4) pretreatment methods for municipal solid waste (MSW) to increase methane (CH4) production from AD. Preservation of AD inoculum with 10% skim milk exhibited complete CH4 recovery, while 10% glycerol and 10% glycerol/skim milk mixture yielded 76% and 4% CH4 recovery, respectively. The inoculum growth phase before preservation (mid-exponential or stationary growth phase) did not significantly affect CH4 recovery. The study showed that inoculum can be preserved via lyophilization with a 10% skim milk cryoprotectant and reactivated for food waste digestion. Iron addition to dairy manure at 20 and 50 mM resulted in significant reductions of hydrogen sulfide (H2S) in the biogas (38 to 100% and 89 to 96%, respectively), with FeCl2 and FeSO4 additions yielding the highest H2S removal. However, FeCl2 and FeSO4 resulted in significant CH4 reduction, while 50 mM Fe2O3 addition did not reduce CH4 and decreased H2S concentration below the minimum requirement for biogas use in engine generation sets. Addition of more than 2 mM sulfate (SO42-) to dairy manure significantly decreased CH4 production by 21 to 65%, while elemental sulfur (S0) additions above 5 mM resulted in 26 to 63% reduction in CH4. K2SO4 and S0 addition greater than 5 mM resulted in significant increases in H2S, while FeSO4 reduced H2S by 44 to 96%. The SO42- additions were successfully treated, with a 48 to 95% decrease in SO42-, showing that dairy manure AD was able to treat high SO42- waste without fouling the AD system. MSW thermal pretreatment at 66, 77, and 99 °C resulted in no significant difference in CH4 production (241 to 277 mL CH4/g COD). Two pretreatments prior to AD did result in increased CH4 production: 1) washed and thermally treated MSW with 45 mM NaOH addition, and 2) pressing unwashed, thermally treated MSW. Use of AD to treat MSW wastewater with only thermal pretreatment of 66 °C would result in plant energy cost savings of $339,000.