Anaerobic co-digestion of mixed sewage sludge and fruits and vegetable wholesale market waste: Composition and seasonality effect

Abstract

Fruit and vegetable wastes (FVW) generated in a wholesale market, and mixed sewage sludge (MSS) from a municipal wastewater treatment plant (WWTP) are heterogeneous residues that may suffer important seasonal changes. We study the influence of those possible changes on the co-digestion of both residues. All the experiments were conducted under mesophilic conditions for samples corresponding to four different seasons, with a total feeding volume of 0.250 L d−1 and a constant hydraulic retention time (20 d), with six different FVW : MSS percentage volumetric relations (PVR): 0:100, 20:80, 40:60, 60:40, 80:20 and 100:0. The biogas and methane production rates increased with the volumetric percentage of FVW in the feed. For instance, as the PVR changed from 0:100 to 100:0 the increase of the biogas production was 10-fold in the summer season and 4-fold for the winter season. Regarding the biogas and methane specific production rates slight improvements can be observed with respect to the use of MSS alone, but remains almost constant for the PVRs between 40:60 and 100:0. This indicates that the main reason for the biogas production improvement is the larger concentration of VS in the FVW. The digester remained under stable conditions, even for the largest organic loading rate (OLR) studied (5.8 g L−1 d−1), regardless the variations between seasons. Therefore, management of both residues together at the facilities available at the WWTP would be a very good option.