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Combined pretreatment using CaO and the liquid fraction of digestate (LFD) was proposed as an eco-friendly pretreatment method for high-quality anaerobic digestion (AD). In this study, rice straw was pretreated by CaO–LFD at three pretreatment temperatures (5, 20, and 35 °C) and four pretreatment times (1, 2, 3, and 4 days). The results showed that the optimum pretreatment condition for rice straw was 35 °C for 2 days. In this case, the maximum biomethane yield reached 308 mL g–1of VS, 77.4% higher than that of the control. The Fitzhugh and Cone models were demonstrated to be more suitable for CaO–LFD pretreated rice straw in the AD process. AD of pretreated rice straw had a shorter effective biomethane production period and lag phase time, exhibiting high AD efficiency. This study enhances the knowledge about CaO–LFD pretreatment from multiple angles, in addition to demonstrating promising possibilities for practical application.

The anaerobic digestion (AD) technology has been widely used to convert biowaste to bioenergy. In most cases in China, AD facilities need to use multiple feedstocks because of the availability of biowastes in certain areas. However, different biowastes have different biodegradabilities, leading to different AD conversion rates and lower conversion efficiency when two or more different biowastes are directly co-digested. This study used potassium hydroxide (KOH) and liquid fraction of digestate (LFD) to modify the refractory corn stover (CS) to improve its biodegradability and then be co-digested with readily biodegradable chicken manure (CM). The co-digestion efficiency was expected to increase because of the improved biodegradability of CS by the modifications. The results showed that as compared to untreated CS, 40.83 and 39.38% more biomethane were produced for the co-digestion of CM and CS with KOH and LFD modification, respectively. In addition, the digestion times were reduced by 20 and 30% than the CS + CM group, respectively. The function and response of the microbial community varied with the modifications in population and diversity under mesophilic conditions. Methanosaeta, Methanosarcina, and Methanobacteriumwere the dominant biomethane-producing archaeal in mono- and co-digestion. Both KOH and LFD modifications could improve the synergistic effect of co-digestion of CS and CM, but LFD modification is recommended because there is no need for extra cost input.