Fermentation of Rice Straw and Its Hydrolysate with Sludge for Ethanol Production

Abstract

Rice straw hydrolysate produced in rice straw pretreatment, comprising a lot of fermentable sugars, is generally released into the environment. This not only causes environment pollution but also wastes fermentable sugars from rice straw. To alleviate environment impact, maximize ethanol production from rice straw, and reduce the cost of ethanol production, rice straw hydrolysate and NaOH-pretreated rice straw were converted to ethanol using ethanol-type fermentation and simultaneous saccharification and fermentation (SSF) by sludge. Meanwhile, microbial community in sludge was analyzed to find the relationship between ethanol production and microbial community succession during ethanol-type fermentation and SSF. Under the optimal condition of the COD, pH and oxidation–reduction potential (ORP) value of rice straw hydrolysate with 6280.56 mg/L, 6.7, and − 42 mV, ethanol-type fermentation with the sludge of 15 g obtained the highest ethanol concentration (8.34 g/L) and the highest COD removal rate (54.83%). For SSF, the maximum ethanol concentration (3.75 g/L) produced by pretreated rice straw and sludge from ethanol-type fermentation with the sludge of 15 g was higher than that (2.61 g/L) generated by pretreated rice straw and sludge from ethanol-type fermentation with the sludge of 22.5 g. This indicated that sludge from ethanol-type fermentation with the sludge of 15 g more efficiently converted rice straw to ethanol than sludge from ethanol-type fermentation with the sludge of 22.5 g. Microbial community analysis suggested that ethanol production had a negative correlation with the relative abundance changes of Bacteroidetes, when the relative abundance of Firmicutes constantly rose in ethanol-type fermentation and SSF. This study provides a scientific basis for maximizing ethanol production from rice straw by microbial regulation in sludge, which could further reduce the cost of ethanol production.