Optimizing Anaerobic Digestion of a Local South African Municipal Wastewater Using Box-Behnken Design of Response Surface Methodology

Abstract

in recent years, water and electricity consumption and demand have increased on both national and international levels. This has resulted in an increase in wastewater generation and greenhouse gas emissions due to inadequate waste management systems. In addressing these challenges, the application of anaerobic digestion (AD) presents a robust solution for the treatment of wastewater with the benefits of biogas production if operated efficiently. In this study, a lab-scale 1 L reactor operated under anaerobic conditions at 37.5 °C was employed for the treatment of local South African municipal wastewater. The influence and synergistic effect of influent pH (5-9), hydraulic retention time (HRT, 15-30 days), and magnetite load (0.125-0.875 g/L) were operating variables considered as a function of biogas produced and contaminants removal percentage (colour and COD). The experimental matrix was generated and conducted based on the Box-Behnken design (BBD) of the response surface methodology (RSM). The RSM analysis of variance (ANOVA) confirms that the quadratic models obtained are substantial, with a high coefficient of regression (R 2 ) above 0.98 and p-values below 0.05 for each variable. The optimum conditions of pH of 7.04, magnetite load of 0.42 g/L and HRT of 21 days were obtained. This resulted in desirability of 84% inferring a biogas yield of 93.72 mL/TDS, COD removal of 88.65% and colour removal of 97.55%. The post-analysis of the effluent showed 80 -98% reduction in the volatile solids, which suggests that the high organic content of wastewater was anaerobically converted into biogas.