Experimental design of mixture for the anaerobic co-digestion of sewage sludge

Batch experiments were conducted for assessing the biogas generation potential of two sets of mixtures (set A and set B). Mixtures consisting of sewage sludge with cow dung and garden waste were studied with set A, whereas mixtures consisting of sewage sludge with cow dung and fruit juice wastewater were studied with set B. Augmented simplex centroid design was used to design the mixture composition for the anaerobic co-digestion. The reactor performance was assessed using cumulative methane volume and percentage volatile solids destruction as the criteria. Synergetic effect was found to be 9.71% of garden waste (GW), 20.29% of cow dung (CD) and 70% of sewage sludge (SS) in set A and 75.5% of SS and 24.5% of CD in set B. Antagonism was observed with the mixes containing garden and fruit juice wastewater (FJW) with the SS. The reason may be due to the rapid acidification of FJW which leads to the soaring of the reactor. In case of GW, the substrate was limiting and the biogas production was negligible after 10th day. Response surface methods were used to find out optimum mixture combination for maximising the biogas production.

► ASCD was used to design the experiment for the co-digestion. ► RSM was used to interpret the interactions between the mixture components. ► Synergetic and antagonistic effects were found for different mixture combinations. ► Contour and 3D surface plots were drawn to find the optimum zones. ► The results of the model were validated using confirmatory experiments.