Response to pre-treatments and digestion intensification of sewage sludge

Abstract

One of the aims of the European Project END-O-SLUDG is the reduction of the amount of sewage sludge produced in sewage plants. This goal is due to major restrictions to landfill disposal imposed by European regulations. As part of the project, the intensification of the anaerobic digestion of sewage sludge is investigated. The promotion of the anaerobic digestion firstly achieves an improved result in the energy yield (recovered as methane) and, simultaneously, a more stabilized digestate (a direct consequence of the carbon mineralization). As anaerobic digestion is a biological process, the degree of stabilization will depend on the biodegradability of the substrate. Hydrolysis represents the rate-limiting step for the degradation of sewage sludge. Consequently, any pre-treatment entailing an acceleration of this stage will have the mentioned benefits. Ultrasound disruption, endogenous enzymatic hydrolysis (42ºC, 48 hours) and combinations of these pre-treatments were applied. The hydrolysis/solubilisation of sludge and final conversion into methane and digestate was studied in the short term (72 hours), in batch reactors (one month) and in continuous regime. The results depend on the characteristics and type of sewage sludge (primary, secondary or mixed).