Improving the methane production from zucchini stem by response surface methodology and different pretreatments

Each year, about 2.4 million tons of zucchini stem (ZS) are discarded after harvesting of zucchini and result in serious environmental pollutions. This study adopted anaerobic digestion (AD) technology to convert ZS to clean energy and systematically investigated the methane production performance of ZS. For improving AD efficiency, organic loading (OL) and feed to inoculum ratio (F/I ratio) were optimized by response surface methodology (RSM), and various pretreatment techniques including KOH, Ca(OH)(2), NaOH, alkaline hydrogen peroxide (AHP), HCl and microwave (MW) pretreatment were studied. RSM results showed that OL and F/I ratio had obvious influences on cumulative methane yield (CMY), and the highest CMY of 239.6 mL.g(vs)(-1) was determined at OL of 17.8 g(vs).L-1 and F/I ratio of 0.75. Among all the pretreatments, ZS pretreated with 4% NaOH, 4% AHP, and 2% KOH significantly promoted CMY to 338.3 mL.g(vs)(-1), 277.6 mL.g(vs)(-1), and 266.1 mL.g(vs)(-1), with the biodegradability of 80.39%, 65.96%, and 63.21%, which were 74.91%, 43.52%, and 37.54% higher than raw ZS, respectively. Furthermore, SEM, XRD, and FTIR analyses showed that alkaline pretreated ZS had preferable structural changes. This study not only explores a novel and sustainable AD feedstock and develops an efficient utilization method for ZS in future application but also converts ZS into bio-methane, which can be used instead of fossil fuels for improving energy structure.