Fruit and Vegetable Waste Management Through Co-Digestion with Local Market Wastewater: Operating Conditions and System Kinetics

Environmental pollution from fruit and vegetable waste (FVW) produced by local markets in South Africa is inevitable. Nevertheless, the current management strategy of disposing of FVWs in landfills contributes to the emission of greenhouse gases. Therefore, valorizing agricultural waste into bioenergy is critical for achieving zero waste and reducing the carbon footprint. In this study, parametric optimization of the co-digestion of FVWs with market wastewater (MW) was conducted using the Box–Behnken design (BBD) adapted from response surface methodology (RSM). The study identified optimal combinations of process variables, i.e., temperature, pH, hydraulic retention time (HRT), and organic loading rate (OLR), to produce biogas while reducing volatile solids (VS) and chemical oxygen demand (COD) from wastewater. At optimal operating conditions of 40°C, HRT of 10 days, pH of 7.2, and an OLR of 3.98 kg VS·m⁻³·day⁻¹, a desirability of 100% was achieved. A biogas production rate of 717 mL·day⁻¹ was reported, with VS and COD removals of 73.37% and 79.24%, respectively. The robustness of the predictive models developed using RSM was corroborated by R² values greater than 0.9 for all output variables. The Modified Gompertz model was well-fitted to the experimental data, yielding an R² of 0.995 and a lower root mean square error (RMSE) of 21.08. The findings of the present study suggest that the valorization of FVW through co-digestion with wastewater can be considered a promising, environmentally sustainable technology for agro-waste management and bioenergy production.