Co-digestion of kitchen waste and pig manure for enhanced biogas production

Abstract

Anaerobic co-digestion (ACoD) of kitchen waste (KW) and pig manure (PM) provides an effective means of treating organic waste and recovering biogas-based renewable energy. This study investigated the ACoD of KW and PM to enhance biogas production while recovering nutrients in a form suitable for agricultural reuse. Batch biochemical methane potential (BMP) tests were conducted at laboratory scale under mesophilic conditions (37 ± 2 °C) using six digesters configured with different KW:PM mixing ratios on a volatile solids (VS) basis: 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100. All substrates were subjected to a combined mechanical (4-5 mm size reduction) and enzymatic pre-treatment to improve hydrolysis and substrate accessibility, while a well-adapted inoculum was employed to minimize lag phases and ensure process stability. Key process indicators, including daily and cumulative biomethane yields, pH, and ammonia levels, were monitored throughout a 28-day digestion period, and the final digestates were characterized for proximate composition, thermogravimetric behaviour, elemental profiles (ICPMS), and agronomic quality. The findings indicated that co-digestion improved methane production relative to mono-digestion of either substrate. Digesters with higher KW proportions (100:0, 80:20, and 60:40) exhibited short lag phases, pronounced early peaks in daily biomethane yield, and sustained methane generation during the exponential phase, leading to the highest cumulative biomethane yields, with the 100:0 and 80:20 reactors achieving final values of approximately 767 and 719 mL CH₄ g⁻¹ VS added, respectively. In contrast, mixtures dominated by PM (20:80 and 0:100) produced lower cumulative yields (around 459 and 299 mL CH₄ g⁻¹ VS), confirming that excessive manure content reduced substrate biodegradability and conversion efficiency. Nevertheless, all treatments maintained near neutral pH, high biomethane fractions, and no inhibitory symptoms, indicating that the KW-PM combinations, supported by the buffering capacity of manure and the applied pre-treatment, provided stable digestion conditions across the tested ratios. Digestate analyses revealed significant stabilization of organic matter, with thermogravimetric profiles indicating reduced volatile fractions relative to the feedstocks, and CHN data showing significant carbon depletion associated with methane formation. At the same time, total nitrogen in the digestate was conserved on a mass-percentage basis, with a high proportion present as ammonium-N, thereby enhancing the immediate plant-available nitrogen. Major plant nutrients (N, P, K) were present at agronomically relevant levels, while potentially toxic elements such as Pb, Cd, Cr, Ni, As, and Hg occurred at very low concentrations, below analytical detection limits, and well within international guideline values for materials intended for land application. These findings demonstrate that the co-digestion of KW and PM, particularly at KW-rich mixing ratios of 80:20 and 60:40, results in high methane recovery and the production of a nutrient-rich, lowcontaminant digestate suitable for use asan organic fertilizer. Although the 100:0 reactor achieved the highest cumulative methane yield, the 80:20 and 60:40 KW:PM ratios offered superior performance by combining high methane recovery with improved process stability, nutrient balance, buffering capacity, and potential relevance for small-scale systems. The study, therefore, informs the design of of small-scale KW-PM co-digestion system as a decentralized waste-toenergy and nutrient-recycling option for rural and peri urban communities in South Africa and similar contexts.