Alkali pretreatment at ambient temperature: A promising method to enhance biomethanation of rice straw

•1% NaOH pretreatment at ambient temperature led to >34% increase in methane yield.•Ambient alkali treatment produced 514L biogas/kg VS (59% CH4) from rice straw.•Dimensional equation used for temporal prediction of efficacy of alkali treatment.•Optimization of multiple process parameters maximized biomethanation of rice straw. Anaerobic digestion is the most cost effective technology for sustainable biogas production from rice straw. Rice straw was subjected to ambient pretreatment with alkali and subsequently digested anaerobically. A dimensional equation was developed to predict the efficacy of alkali treatment in terms of soluble COD. Biomethanation process parameters like temperature, initial pH, particle size, substrate/inoculum ratio, trace element supplementation, C/N ratio and hydraulic retention time were optimized. The highest biogas production under optimized conditions was 514L/kg VS/day (∼59% CH4) from milled rice straw (1mm) pretreated with sodium hydroxide (1% w/v) at ambient temperature for 180min. The digester was operated at 15days HRT at 37°C and neutral pH. C/N ratio was optimized at 25 using urea. Higher biogas yield from rice straw treated with lower concentration of NaOH at ambient temperature may make this process more economical than the previous reports.