Comprehensive insight of the performances of excess sludge reduction by 90 °C thermal treatment coupled with activated sludge at pilot scale: COD and N removal, bacterial populations, fate of heavy metals

A continuous lab-scale activated sludge process coupling thermal (90 °C) sludge disintegration of a part of return sludge was investigated. The objective was a comprehensive characterization of the process, including both operational (sludge reduction efficiency, pollution removal, fate of micropollutants, cost analysis) and more fundamental aspects (microbial community evolutions). Thermal disintegration effectively decreased sludge production by 30% when comparing results with control reactor running in parallel. However, the solubilization of sludge compounds and the decrease of VSS concentration in the aeration tank led to a higher organic load in the reactors. This affected the overall treatment performances: COD and TSS removal efficiencies were decreased by 15%. The loss of nitrification efficiency was evidenced in the treated line and was linked with bacterial community modifications (evidenced by PCR-DGGE techniques). The similarity percentage between the control and the thermally treated sludge was equal to 59% after 9 weeks. Moreover, the fate of heavy metals (Cd, Cu) was also affected, due to sludge physico-chemical characteristics modifications: the release of copper in the treated effluent increased and the heavy metals sludge contents was increased. A simple energy balance showed that thermal treatment could be economically viable as soon as sludge global handling cost exceeds 72 €/wet ton.