Strategies for enhancing methane production from coffee pulp via co-digestion, urea supplementation, and biochar-assisted process stabilisation.

Coffee pulp (CP), a lignocellulosic agro-industrial residue rich in organic matter, contains phenolic compounds and other constituents that can limit anaerobic digestion (AD) performance. This study evaluated semi-continuous AD of CP in continuously stirred tank reactors under mesophilic conditions (37 °C), comparing co-digestion with coffee wastewater (CWW) and sisal wastewater (SWW) against CP mono-digestion. Co-digestion achieved stable reactor performance, with methane production rates of 564 ± 89 and 624 ± 92 mL CH4/L/day and methane contents of 56-59%. This improved performance was associated with enhanced nutrient balance (C/N = 23.8), dilution of potential inhibitory compounds, and low VFA concentrations (<1 g/L). In contrast, CP mono-digestion resulted in process instability, characterised by reduced methane production, VFA accumulation (>1.5 g/L), pH decline, and nitrogen limitation (TAN = 56.0-64.8 mg/L). Urea supplementation restored nutrient availability and reactor stability, resulting in methane production of 343 ± 18 mL CH4/L/day. Although combined urea and biochar supplementation supported recovery (326 ± 26 mL CH4/L/day), the independent effect of biochar could not be isolated, and biochar did not prevent instability at increased organic loading rates. Batch assays further showed that inoculum adaptation enhanced initial methane production kinetics, whereas cattle manure inoculum achieved the highest final methane yield (328 ± 8 mL CH4/g VS). Overall, co-digestion with agro-industrial wastewaters represents an effective strategy for improving CP digestion, while maintaining adequate nitrogen availability is critical for stable CP mono-digestion.