Effect of primary sludge to waste activated sludge mixing ratio on anaerobic digester performance

Abstract

In this research, the effect of mixing ratio between primary sludge (PS) and waste activated sludge (WAS) on anaerobic digester performance was investigated. PS to WAS ratios of 65% PS/35% WAS v/v, 50% PS/50% WAS v/v, 35% PS/65% WAS v/v were assessed for a sludge retention time of 23 d at mesophilic temperature of 36.5°C. The sludge with the mixing ratio of 65/35 v/v produced the high - est amount of methane for 500 mL digester volume considered in the study. This particular mixing ratio is similar to the large scale operational condition in Beenyup Wastewater Treatment Plant, Western Australia. The kinetics of the digestion process was faster for this mixing ratio. In addi - tion, methane to carbon dioxide ratio for this mixing ratio of 65/35 v/v was found to be the highest (2.5–3.1). Sludge biodegradability in terms of reduction of total chemical oxygen demand was 46.6%, 53.7% and 72.3% and volatile solid removal was 32.6%, 25.8% and 34% for mixing ratios of (65/35, 50/50, 35/65)v/v, respectively. The sludge sample with greater proportion of WAS showed better dewaterability. In general, mixed sludge with higher proportion of PS has better effects on overall digester performance.