Research On Methane Production Performance Of Zucchini Stem During Anaerobic Digestion

Recent years,the demand for fresh vegetables is rapidly increasing with the growth of the economy and people's consumption level in China.Zucchini is one of the most widely cultivated vegetables in China with annual production of 8 million tons.Each year,about 2.4 million tons of zucchini stem(ZS)were produced during the process of zucchini cultivation.Due to the high moisture and organic contents,the large-scale discard of ZS not only increases the pressure of garbage disposal and meanwhile causes environmental pollution,but also leads to a tremendous waste of natural resource.Anaerobic digestion(AD)is a widely applied technology for converting organic wastes into bio-methane,which not only disposes wastes in large quantities,but also generates clean energy economically and efficiently.Therefore,this study systematically investigated the methane production performance of ZS during AD process.Firstly,response surface methodology was applied to optimize the feed to inoculum ratio(F/I)and organic loading(OL)during AD of ZS.Results showed that both F/I and OL had significant influences on cumulative methane yield(CMY)of ZS.Optimal F/I and OL were determined to be 0.75 and 17.78 gvs.L-1,respectively,with maximum CMY of 239.6 mL·gvs-1 and biodegradability(Bd)of 56.93%.Secondly,for further improving the methane production efficiency,effects of various pretreatments,including KOH,Ca(OH)2,NaOH,alkali hydrogen peroxide(AHP),HCl and microwave(MW)pretreatment,on methane production performance of ZS were investigated.Among all the pretreatments,4%NaOH,4%AHP,and 2%KOH pretreatment significantly improved the CMY of ZS(p?0.05).CMY of 4%NaOH,4%AHP,and 2%KOH pretreated ZS were up to 338.3 mL·gvs-1,277.6 mL·gvs-1,and 266.1 mL·gvs-1,with the Bd of 80.39%,65.96%,and 63.21%,which were 74.92%,43.54%,and 37.59%higher than raw ZS(193.4 mL·gvs-1),respectively.Moreover,lignocellulosic compositions and structural analyses showed that 4%NaOH,4%AHP,and 2%KOH pretreatment reduced the lignin and hemicellulose contents in ZS and effectively destroyed the complex lignocellulosic structure,which increased the accessibility of microorganism to cellulose and hemicellulose contents,while HCl pretreatment had no significant impact on the lignocellulosic structure.Finally,the methane production performance and AD system stability of ZS at different organic loading rates(OLRs)in continuous AD mode were investigated.And then,effects of 2%KOH,4%NaOH,and 4%AHP pretreatment on continuous AD were compared.Results showed that raw ZS performed a stable AD system at OLRs of 1.0-3.0 gvs·L-1·d-1 and obtained the highest volumetric methane yield(VMY)of 0.546 L·L-1·d-1 at an OLR of 3.0 gvs·L-1·d-1.Besides,AD system after 2%KOH pretreatment was also stable at OLRs of 1.0-3.0 gvs·L-1·d-1.At an OLR of 3.0 gvs·L-1·d-1,2%KOH pretreatment significantly enhanced the VMY to 0.708 L.L-1·d-1,which was 29.7%higher than that of raw ZS.However,AD system of 4%NaOH and 4%AHP pretreatment became unstable at an OLR of 2.0 gvs·L-1·d-1,and the extreme accumulation of acetic and propionic acids accounted for the instability of AD system.This study systematically investigated the efficient conversion method of ZS which not only provides reference for utilization of ZS,but also forms a foundation for future industrial applications of ZS.