Enhancement of methane production by anaerobic co-digestion of IWK sludge and food wastes

Indah Water Konsortium (IWK) sludge, a wastewater treatment by-product in Malaysia, contains substantial organic matter and presents promising potential for bioenergy recovery through anaerobic digestion. However, mono-digestion of sewage sludge often yields low methane production due to limited biodegradability, low solids content, and microbial imbalance. This study investigates the enhancement of methane production via anaerobic co-digestion (AcoD) of IWK sludge with food wastes, specifically non-dairy creamer waste (NDCW) and instant coffee waste (CW), using IWK thickened sludge as inoculum. The study objectives were to optimize the inoculum-to-substrate (I:S) ratio and hydraulic retention time (HRT), evaluate the effect of chemical pre-treatment on methane yield, and assess system performance at pilot scale. Physicochemical characterization of substrates and inoculum included pH, total solids (TS), volatile solids (VS), chemical oxygen demand (COD), volatile fatty acids (VFA), alkalinity, total suspended solids (TSS), and volatile suspended solids (VSS). Batch experiments were conducted using biochemical methane potential (BMP) tests in 125 mL serum bottles, followed by scale-up in a 40 L continuous stirred tank reactor (CSTR) operated under mesophilic conditions (35–37 °C) and pH 6.8–7.0. The optimal I:S ratio of 2:1 (IWK sludge: CW) for the batch experiments produced the highest methane yield of 17.31 mL CH₄/gVS. Acid pre-treatment using 2 M H₂SO₄ further enhanced methane production to 18.49 mL CH₄/gVS, representing a 45% improvement. In contrast, the control treatments—Control-CW (0.013 ± 0.0007 mL CH₄/g VS) and Control-NDCW (0.005 ± 0.0003 mL CH₄/g VS)—produced negligible methane, emphasizing the necessity of an active inoculum and balanced substrate interactions. At pilot scale, the pre-treated 2:1 mixture achieved a methane yield of 40,936.7 mL CH₄/gVS and COD removal efficiency of 61.6%. Methane composition remained stable at 50–70%, confirming effective digestion performance. Overall, the findings demonstrate that optimizing operational parameters and applying chemical pre-treatment significantly improve anaerobic digestion efficiency. The successful scale-up in 40L continuous stirred tank reactor confirms the feasibility of co-digesting sewage sludge with food waste for sustainable biogas production.