Characterization of food waste and sewage sludge mesophilic anaerobic co-digestion under different mixing ratios of primary sludge, secondary sludge and food waste

Abstract

The main aim of the present work was to characterize the mesophilic anaerobic co-digestion of food waste (FW) and sewage sludge (SS) by considering different mixing ratios of primary sludge (Pr), secondary sludge (Sc) and food waste. The experiments were carried out using batch reactors (R1-R7). The applied ratios included VSPr:VSSc (1:1 and 2:1) and VSFW:VSSS (0:4, 1:3, 2:2 and 4:0) with VSx (g) as the volatile solids content of the substance x within the total feed of each reactor and VSSS as the sum of VSPr and VSSc. According to the obtained results, ratio of carbon to nitrogen (C:N) was much more effective than VSPr:VSSc ratio considering VS reduction and biogas production efficiencies. A second-order polynomial curve fitted well the data of VS reduction vs. C:N, representing an optimum C:N around 16 to achieve the highest VS removal. A high R-squared linear correlation was found between data of gas production and C:N ratio according to which, with increasing the C:N within the test range (8–19.7), the biogas production increased steadily. Doubling VSPr:VSSc under lower ratios of VSFW:VSSS (0:4 and 1:3) positively affected both VS reduction and biogas production, while under higher VSFW:VSSS ratio (2:2) it influenced adversely. Doubling VSPr:VSSc also improved the digestate dewaterability under all applied ratios of VSFW:VSSS. A synergistic gas production was observed for the reactors with 50% share of VSFW (VSFW:VSSS of 2:2) between which, R5 with VSPr:VSSc of 1:1 showed the highest synergistic gas production. The highest methane content (70.3%) also belonged to R5.