Anaerobic Co-Digestion of Mechanically Pretreated Rice Husk and Cow Dung for Biogas Production

Abstract

In recent years, the fast-depleting nature of fossils has necessitated an upsurge in the use of biogas. This rise has made the utilisation of organic lignocellulosic biomass a critical substrate for biogas production. There is ample research on biogas production in Ghana; however, few studies have used a contemporary biomethane potential analyser, and even fewer have looked at using multiple validated kinetic models to determine how different particle sizes affect biogas production. Hence, the main aim of this research was to co-digest mechanically pretreated rice husks and cow dung for biogas production. It was hypothesised that finer particle sizes would enhance methane yield due to increased surface area for microbial action. This study set up a batch biomethane potential experiment under mesophilic conditions using a 3:1 mixing ratio and inoculum-to-substrate ratio of 3 for Cow dung and Rice Husk. A combination of mechanically pretreated rice husk and cow dung was digested for 126 days in a continuously stirred 5L container (glass jar) under anaerobic conditions. The co-digested substrates were; treatment 1 (0μm to ≤125μm), treatment 2 (>125μm to ≤250μm), treatment 3 (>250μm to ≤ 500 μm), treatment 4 (>500μm to 1180μm) and a control sample made up of raw cow dung and glucose. Proximate, structural and elemental analyses were conducted on the raw and co-digested substrate. After the analyses, the results obtained were used to compute the chemical formulas and the theoretical methane yields using the extended Buswell equation of Boyle. The experimental values recorded were: 230 ± 4.3mLCH4 /gVs for the control sample, 311 ± 2.7 mLCH4 /gVs, 289 ± 2.5 mLCH4 /gVs, 134 ± 0.3 mLCH4 /gVs and 262 ± 9.08 mLCH4 /gVs for treatments 1 to 4, respectively. The cumulative methane recorded increased with decreasing particle size (P<0.05) for all treatments except for treatment 3. Out of the seven models used in this study, 5 of the models (Cone, Modified Gompertz, Logistics Function, Fitzhugh, and Richards) fitted the experimental data. The R2 values for the 5 models were within the range of 0.983 – 0.996 with Root Mean Square Error (RMSE) 0.33–13.20. The Monod model was the only model that performed poorly with the all-data set. Its R2 value was 0.595–0.910, and its RMSE value was 33.2–39.12. After validating the model performance, using scatter index (SI), Nash-Sutcliffe efficiency (NSE), index of agreement (IOA), RMSE, and relative absolute error, only the first-order kinetic model performed poorly due to its low NSF value of -0.233. The remaining models were all 0.999 in conformity with the validating requirements. The biodegradability index of the Co-digested substrate varied from 89.50 to 97.31%, with only treatment 3 having 48.90%. Additionally, the analysis of the digestate showed a high VS (84.63 – 99.75%) and TS (55 – 97.33%) removal, while pH ranged from (7.39 –7.62) indicating a stable anaerobic digestion process. These research findings demonstrate the potential of co-digesting rice husk and cow dung for biogas production, highlighting its significance for waste management, bioenergy generation and sustainable agriculture.