Low temperature thermal pretreatments for the improvement of waste activated sludge anaerobic digestion

Activated sludge is the main waste from municipal wastewater treatment plants (WWTPs) with a specific production of about 22-30 g VS/inhabitant•day (VS, volatile solids) (Bonomo, 2008). Anaerobic digestion (AD) is of particular interest in the treatment of activated sludge because of its capacity to reduce the total amount of biosolids while producing high methane content (50-60% v/v) biogas. However, when referring to activated sludge, it is well known that the complexity of its structure limits the biological process. So, in order to improve the efficiency of the AD process, low temperature thermal pre-treatments have been tested and the results are presented in this paper. Activated sludge samples from a large WWTP (2,500,000 e.i.) were collected and thickened to a total solid (TS) content of 4-5% w/w, characterized in terms of total volatile solid (TVS) content and COD (on the whole sludge sample and on the liquid phase only) and subjected to thermal treatments in a thermostatic bath settled on temperature values of 70°C, 80°C and 90°C. The efficiency of the thermal treatments was quantified using the disintegration rate (DR) parameter, that is the ratio between the COD transferred in the liquid phase and the COD of the whole sludge sample. For the considered treatment times (1-15 hours), an increase in the DR parameter from 15 to 25% for the temperature value of 70°C, from 15 to 28% for the temperature value of 80% and from 19 to 30% for the temperature value of 90°C was observed. The increase in the specific biogas production (biogas volume/TVS content) was verified on the four most promising systems using 6L-mesophilic digesters in batch modality. The results obtained in the digeribility test showed that the treated samples had increased their biogas production by amounts ranging from 15 to 26% compared to the untreated samples.