Comparing the hydrolysis and biogas production performance of alkali and acid pretreatments of rice straw using two-stage anaerobic fermentation

•Moderate temperatures comparison of acid and alkali pretreatment of rice straw.•Comparison of CH4 production in two-stage anaerobic fermentation performance.•HCl pretreatment hydrolysate had greater CH4 production at similar loading.

To enhance rice straw degradation and improve biogas production in a two-stage anaerobic system, rice straw was treated with either dilute alkali or acid. Firstly, the effects of NaOH or HCl concentration, temperature and time on glucose conversion, degradation of cellulose and hemi-cellulose were investigated. It was found that glucose conversion rate and the organic matter content were greater with alkali rather than acid pretreatment. Suitable conditions for hydrolysis using NaOH pretreatment were 2% (w/w), 60 h and 60 °C. With these conditions the glucose conversion rate was 55.5%. Secondly, the hydrolysate was used as substrate for batch anaerobic fermentation tests at different organic loadings. The maximum methane yield of hydrolysate from NaOH pretreatment was 193.2 ml g−1 [COD]. For the HCl pretreatment, the maximum methane production rate was 287.0 ml g−1 [COD] which was 43.89% higher than with NaOH. Although the NaOH solution was more efficient in improving rice straw hydrolysis, the hydrolysate from the HCl pretreatment had greater methane production at a similar organic loading.