The influence of coagulant type on the biological treatment of sewage sludge

Abstract

Anaerobic digestion (AD) and composting are commonly utilized sludge management methods however, the influence of different coagulant types on these biological processes and their stabilized biomass characteristics have not been fully explained. In this study, the effect of the coagulant used in municipal wastewater treatment on the biological stabilization of sludge was investigated. Fully controlled and monitored small-scale AD and composting bioreactors were utilized. The coagulants tested included an inorganic coagulant (IC), polyaluminium chloride (PAC), and organic coagulants, (OC) polyamine (pAmine) and chitosan (Chit). Overall, the coagulant applied showed a measurable influence on the biological stabilization of sludge. The presence of complex organics compounds from OC in the sludge biomass was found to decrease biomass biodegradability while increasing gas production. During AD, Chit-sludge achieved higher biogas production than pAmine- and PAC-sludges (13 % and 16 %, respectively, in Nm³ CH₄ t^-1VS). In composting, pAmine-sludge achieved the highest feedstock temperature (34-35 °C) and CO₂ gas emissions, followed by Chit- (33 °C) and PAC-sludges (32 °C). Generally, tot-P concentration in PAC-sludge was higher than in pAmine and Chit-sludges both before (20, 17 and 15 g/kg DM, consecutively) and after AD (23, 21 and 20.5 g/kg DM, consecutively), and during the composting (31, 29.5 and 26 g/kg DM, consecutively) process. Tot-N concentrations (g/kg DM) showed a substantial increase after AD (pAmine and PAC ca 50 % and Chit 81 %), while a decrease was observed after composting, specifically in PAC-sludge (PAC 28 %, pAmine and Chit ⁓5 %). The selection of the most suitable coagulant by wastewater treatment facilities depends on the objective of the biological stabilization process. In cases where AD is applied and biogas yield is selected as the target output, the semi-natural OC Chit was found to be the best option among the coagulants tested. Comparably, when the nutrient content of resulting biosolids (AD or composting) is more relevant, it was found that OC-produced sludge contained higher N concentrations, while IC-produced sludge contained slightly higher P concentrations.

Keywords: Biosolids; Composting; Digestion; Resource recovery; Wastewater.