Biogas Production from Mechanically Pretreated Rice Husk and Cow Paunch Content – Part 1: Experimental Performance of Co-Digestion System

Mechanical pre-treatment can improve anaerobic digestion of lignocellulosic residues, but the effect of rice husk (RH) particle size during co-digestion with cow paunch content has received limited attention. This study evaluated whether smaller mechanically milled RH particles increase methane production during batch co-digestion with cow paunch content. Rice husk was milled and separated into four particle-size fractions: 0–125 µm (F1), >125–250 µm (F2), >250–500 µm (F3), and >500–1180 µm (F4). Batch biomethane potential assays were conducted under mesophilic conditions for 126 days using cow paunch content as inoculum and co-substrate. Particle-size distribution, substrate composition, cumulative methane yield, biodegradability index, pH, volatile solids, and total solids removal were assessed. F1 produced the highest cumulative methane yield (311 ± 2.7 mL CH 4 /g VS), followed by F2 (289 ± 2.5 mL CH 4 /g VS), F4 (262 ± 9.08 mL CH 4 /g VS), the control (230 ± 4.3 mL CH 4 /g VS) and F3 (134 ± 0.3 mL CH 4 /g VS). Relative to the control, F1, F2 and F4 increased methane yield by approximately 35%, 26% and 14%, respectively, whereas F3 showed lower conversion. The final digestate pH (7.39–7.62), volatile solids removal and biodegradability indices indicated stable digestion for most treatments. The findings support the hypothesis that RH particle-size reduction can enhance methane recovery during co-digestion with cow paunch content, although the F3 outcome indicates that particle size alone does not determine process performance. Thus, energy-use and cost assessments are needed before scale-up.