Sludge disintegration during heat treatment at low temperature: A better understanding of involved mechanisms with a multiparametric approach

Although many information is currently available about sludge minimization processes in wastewater treatment plant (WWTP), few data are available about their fundamental mechanisms especially microbial changes. In order to clarify the relationship between sludge reduction efficiency and both chemical and biological modifications, the effects of thermal treatment on activated sludge were investigated by combining the monitoring of cell lysis using flow cytometry (FCM), organic matter solubilization, floc structure and biodegradability. For the maximal temperature (95 °C) applied, COD, proteins, HLS and sugars solubilization degrees reached 12.4 (±1.3)%, 18.6 (±1.8)%, 9.6 (±1)% and 7.4 (±1.9)%, respectively, showing clearly the transfer of organic matter from the particulate to the soluble fraction of the sludge. The results from FCM analysis showed that thermal treatment induces a progressive cell lysis when increasing temperature from 50 to 95 °C. However, the impact on floc structure seemed to be limited as floc destructuration was limited to the temperature elevation at 50 °C. Above 50 °C floc size distribution remained almost constant. The results from biodegradability tests did not show any improvement of the intrinsic biodegradability after the thermal treatment. Only the increase of the specific digestion rate was observed. This study which provides a complete investigation of chemical, physical and biological effects of thermal treatment allows a better knowledge of fundamental mechanisms involved during heat treatment to improve sludge reduction processes.

► Chemical and biological characterizations of sludge thermal treatment (≤100 °C).
► Highlight the use of flow cytometry to investigate cell lysis during heat treatment.
► Low thermal treatment induces cell lysis and a partial floc destructuration.
► Such treatment conditions do not enhance total biogas production.
► Low thermal treatment (≤100 °C) is enough to improve global biogas production rate.