Alleviating Inhibition of the Methane Fermentation Process Receiving Dewatered Sludge Cake with Reduced Agitation

The anaerobic degradability of dewatered sludge cake collected from the municipal wastewater treatment plant was investigated in the continuous lab-scale digester under varied agitation intensities, where the system received the total volatile solids (TVS) of ca. 155 g-TVS kg-cake−1 and was operated in a partial liquor displacement using water to control ammonium concentration at about 2,200 mg-N L−1. When the agitation was strengthened to 2.0 m s−1, a soluble total organic carbon (sTOC) was accumulated up to 3,400 mg-C L−1 in association with a lowered methane production. The limited accumulation of acetate and propionate (11% of sTOC) during the event suggested the hydrolysis and/or acidogenesis were inhibited in the operation. The reduced agitation intensity ameliorated the inhibition and attained an almost comparative process performance to that of the control experiment fed with a diluted cake of 80 g-TVS kg-cake−1. The offline respirometry tests revealed the inhibitory substance generated in the digester was not present in the soluble portion of digestate but in the colloidal form. The tests also indicated the inhibitory substance led to a methanogen’s decay. The source of the inhibitory substance was speculated to be polyacrylamide used for the sludge dewatering process in the plant and/or high humic substances present in the cake.