Anaerobic Co-digestion of Neem (Azadirachta indica A. Juss) Pulp with Cattle Manure: Effect of Mixing Ratio and Dry Matter Content on Biogas Production

Neem (Azadirachta indica A. Juss) pulp, a by-product of biopesticide production, constitutes a phytosanitary waste whose management poses environmental challenges. Its energy valorization through anaerobic digestion could transform this waste into a renewable resource. This study aimed to evaluate the effect of neem pulp/cattle manure co-digestion ratio and dry matter content on biogas production. Batch trials (200 L digesters) were conducted over 45 days under tropical ambient temperature (29-37°C). Four co-digestion ratios (P100: 100 % pulp; P75B25: 75 % pulp + 25 % manure; P50B50: 50 % pulp + 50 % manure; B100: 100 % manure) and two dry matter contents (TS10: 10 % dry matter; TS15: 15 % dry matter) were tested in triplicate. Results showed that pulp alone (P100) completely inhibited methanogenesis (12.2 L in 45 days, final pH 4.10-4.14) due to its bioactive compound content (azadirachtin, salanine, nimbin). Co-digestion with cattle manure progressively alleviated this inhibition through its buffering capacity and inhibitor dilution. The optimal ratio was P50B50 at TS15 (332.4 L in 45 days). A high significant interaction (p < 0.001) between co-digestion ratio and dry matter content was demonstrated. This study shows that controlled co-digestion enables energy valorization of neem based biopesticide waste under tropical climate, thus paving the way toward a circular economy in agricultural systems.