Anaerobic co-digestion of sewage sludge and bio-based glycerol: Optimisation of process variables using one-factor-at-a-time (OFAT) and Box-Behnken Design (BBD) techniques

Abstract

The use of glycerol as a co-substrate in anaerobic processes is a promising method of reducing the amounts of contaminants in sewage sludge. Although anaerobic co-digestion has been in existence for more than a century, its operating conditions have not yet been fully optimised. This paper focuses on the optimisation of anaerobic variables so as to optimise biogas yield using both the one-factor-at-a-time (OFAT) method and the Box-Behnken Design (BBD) method. The experimental work was carried out using batch digesters that were operated between 25 and 50∘C, 0.5 and 1.5 g VSfed / (g VSinoculum), and 1 and 9 g glycerol / L for the optimisation of temperature, substrate to inoculum ratio (S:I) and glycerol concentration, respectively. Sewage sludge was used as a primary substrate, bio-based glycerol was used as the secondary substrate (co-substrate) and the digestate from a local wastewater treatment system was utilised as an inoculum. In both the OFAT method and the BBD method, the highest biogas yield of 0.94 L / (g VSfed) was attained at a temperature of 50∘C, S:I ratio of 1 g VSfed / (g VSinoculum) and glycerol concentration of 1 g glycerol / L. Biogas yield decreased below 50∘C, and the process experienced overloading above 1 g VSfed / (g VSinoculum) and 1 g glycerol / L. Using the BBD method, the effect of variables, which measures a change in response over a change in variable, showed that glycerol concentration had the greatest impact on the biogas yield, then temperature, and lastly S:I ratio. The model with the best fit was a quadratic model with a standard deviation of 0.09, coefficient of variation of 18.54%, and PRESS of 0.66. In conclusion, optimisation of anaerobic process variables to optimise biogas yield was successfully carried out using the OFAT method and the BBD method.