Effects of urea addition on anaerobic digestion characteristics of hulless barley straw pretreated with KOH

The objective of this study was to improve the comprehensive utilization of hulless barley straw during anaerobic digestion (AD) by exploring the effects of urea addition on anaerobic digestion characteristics. In the medium-temperature batch anaerobic fermentation experiments, KOH was used as the pretreatment reagent and urea was added to adjust carbon-nitrogen (C/N) ratio. Response surface methodology was used to optimize the treatment condition of barley straw. The results showed that a KOH concentration of 5.02%, a pretreatment time of 24.07 h, and a C/N ratio of 23.68 constitute the optimal treatment conditions for hulless barley straw. Under such conditions, cumulative methane production peaked at 280.55 mL/g VS, which was 6.8% and 69.7% higher than that with 7% KOH pretreatment and that without treatment, respectively. Moreover, the cost of pretreatment agents can be partially recovered by the increased digestate nutritional value of remnant K and N. Notably, KOH treatment with added urea increased the microbial community diversity and the relative abundances of functional taxa in AD. Specifically, the relative abundances of Firmicutes, Spirochaetes, and Fibrobacteres at phyla level, and Christensenellaceae_R-7_group, Treponema at genera level were markedly increased. Consistently, the enriched relative abundances of archaeal genera Methanosarcina, Methanocorpusculum, and Methanoculleus activated the metabolic pathways for methane generation in the AD systems. Our data jointly prove that KOH pretreatment with urea addition is an efficient and environmental-friendly method to enhance methane yield using hulless barley straw.

Automated summary

This study aims to improve methane production from hulless barley straw by pretreatment with KOH and addition of urea. The optimal treatment conditions were determined, resulting in significantly increased methane production and enhanced microbial diversity and functional taxa.