Comparative study of biogas production from chemically-treated powdered and un-powdered rice husks

Abstract

Biogas production was carried out using fresh powdered rice husk (PRH), un-powdered rice husk (U-RH) and a blend of both (CRH) in the ratio 1:1. They were left to degrade for 73 days before pre-decaying in water. The pre-decayed wastes were found to be acidic. Consequently, a concentrated solution of potassium hydroxide (KOH, 50%w/v) in acetic acid (CH3COOH, 99% v/v) was used to treat the wastes to achieve waste stabilization and neutrality. The pre-decayed, chemically treated rice husks were charged separately as; powdered rice husk (P-RH), un-powdered rice husk (U-RH) and combined rice husks (C-RH) (which is a combination of P-RH and U-RH 1:1) into metal prototype biodigesters of 50 L capacity in the ratio of approx. 2:1 of water to waste. They were all subjected to anaerobic digestion under a 31 day retention period and mesophilic temperature range of 24 to 42°C. Results of volume of gas yield from the different digesters indicated that the P-RH had a cumulative gas yield of 140.60 ± 2.12 L/ total mass of slurry (TMS) while the U-RH system had a cumulative gas yield of 146.3 ± 2.07 L/TMS. When blended and treated the cumulative biogas yield of the C-RH significantly increased (p≤ 0.05) to 168.3 ± 2.26L/ TMS. The different digester systems became flammable at different periods. The biogas from the C-RH system ignited on the 4th day of the digestion period while U-RH and P-RH systems produced flammable gas on the 5th and 6th days, respectively. Generally, the results showed that chemical treatment enhanced the biogas production of the rice husks in terms of cumulative gas yield and onset of gas flammability. The overall results indicate that combining the two types of rice husk gives the best results in terms of those two parameters. Key words: Powdered rice husk, unpowdered rice husk, biogas production, flammable biogas, cumulative gas yield.