Enhancement in methane yield from anaerobic co-digestion of walnut shells and cattle manure

In this study, anaerobic co-digestion was investigated for mixtures of walnut shells (WS) and cattle manure (CM), which are lignocellulosic wastes, in various different carbon: nitrogen (C/N) ratios. The best mixing ratio for WS and CM in anaerobic digestion (AD) was determined to be 1:3, offering a methane yield of 173.2 ml/g volatile solid (VS)(added). Effects of various alkaline pretreatments including calcium hydroxide (Ca(OH)(2)), sodium hydroxide (NaOH) and potassium hydroxide (KOH) were studied in the concentration range 1-5% wt/wt for the best mixing ratio of WS and CM. The optimal KOH and NaOH concentrations were each found to be 4%, giving methane yields of 312.6 +/- 5.1 and 342.5 +/- 9.6 ml/g VSadded, respectively, where these yields were found to increase by 80.5 and 97.2%, respectively, compared to the control reactor. By contrast, a 3% Ca(OH)(2)concentration was found to increase the methane yield by 67% compared to the untreated reactor. NaOH pretreatments resulted in relatively higher lignocellulosic solubilization and soluble chemical oxygen demand compared to KOH and Ca(OH)(2)pretreatments. This study contributes to our understanding of the alkaline pretreatment in taking advantage of CM and WS for methane productions in future applications.