Synergistic effect and supernatant nitrogen reduction from anaerobic co-digestion of sewage sludge and pig manure

The degradation kinetics of anaerobic co-digestion of pig manure (PM) and sewage sludge (SS) were investigated for a utility that has concerns on nitrogen in the digestate return stream. In this study, batch-scale anaerobic reactors were set up with SS/PM volatile solids (VS) ratios at 7:1, 14:1, 21:1, and mono-digestions of SS and PM. PM (239 mL/g VS_added) produced more methane than SS (182 mL/g VS_added). As a result, increasing PM in co-digestion also increased methane yield. Kinetic models indicated synergistic effects by co-digestion with a faster degradation rate, an increased cumulative methane yield, and a shortened digestion time. Despite total nitrogen increased in digestate, the total nitrogen in co-digestion supernatant was lowered, which interprets to a nominal impact to the mainstream treatment with the additional PM loading. Nitrogen content in dewatered biosolids is expected to increase between 39.5%–53.5% thus benefiting land application.