Anaerobic co-digestion of edible and inedible food waste with sewage sludge
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Co-digestion processes of three specific mixing ratios of food waste (FW) with sewage sludge were tested in one-stage anaerobic digestion (AD)system induplicate to evaluate methane (CH4) and hydrogen sulfide(H2S) production. Two types of FW were considered in this study: a mixture of edible and inedible, termed "unsorted" and solely inedible FW. These two types of FW represent current observations of FW produced from the University of Missouri's Campus Dining Services (CDS) and an idealized scenario in which all edible FW is avoided. Over the whole period, all trials were under mesophilic condition (35.5+/-1 degrees C) with a stable hydraulic retention time (28 days). Under steady-state conditions, daily methane yield was 405L/kg VSadded in trial 2(25% unsorted FW). The increase of unsorted FW from 25 to 50% VS enhanced methane production in trial 1(50% unsorted FW), which was 460 L/kg VSadded. Trial 5 (50% unsorted FW) was also conducted to check the consistency of digestion process, showing that trial 1 and 5 produced similar methane yield in our study. Also, the daily methane yield in trial 4 (75% unsorted FW) was maintained at 438 L/kg VSadded, slightly lower than the one in trial 1. In addition, methane production was maintained at 349 and 386 L/kg VSadded in trials fed with 50% and 25% VS inedible FW, 31.81% and 4.92% lower than those in trial 1 and 2, respectively. Also, the production of H2S in trials with unsorted FW were approximately two times higher than those with inedible FW and further research is needed to explain this difference. In this study, energy potential output of unsorted and inedible FW from Columbia (MO) was calculated using an Anaerobic Digestion Development Iterative Tool (ADDIT model). The consideration of the cost of logistics, operations, maintenance and capital costs were also calculated in ADDIT model. It showed that this co-digestion of unsorted and inedible FW with sewage sludge could be able to produce 28,678,680 and 68,586 KWh/year total net energy. As a result, these co-digestion process will be able to take a desirable profit to Columbia city (MO) with an approximately $1,290,540 and $3086.4 annually income if all unsorted and inedible FW was sent to codigestion plant, assuming the average commercial electricity rate in MO is 4.50/kWh.