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In rural areas of China, household digesters play a significant role to improve the rural production and living conditions, leading to energy conservation and reduction of emissions. However, the cold temperature of digesters results in relatively low digestion efficiency and reduction of biogas yield. The present study aimed to investigate the potential of integrated solar heating techniques to raise the slurry temperature within a low-cost tubular digester and its impact on the biogas yield. Two similar digesters were used, the first one (D1) was heated by the solar greenhouse integrated with a solar water heating system and a capillary heat exchanger, while the second (D2) was heated by only solar greenhouse, and both digesters were above ground and were fed with cattle manure. The results showed average slurry temperature of 9.5 and 4.9 °C above the mean ambient temperature for D1 and D2, respectively. Furthermore, the mean specific biogas production of D1 and D2 were 247 and 181 L/kg VS, respectively, with no significant variations in the methane content (≈62.7%). The study indicated that using of integrated solar energy is efficient to achieve the optimum temperature for the process of biogas production roughly the most of the year. • We propose a new solar heating system for household biogas digesters in cold winter. • The solar heating system has increased the slurry temperature of about 4.9–9.5 °C. • The biogas production was increased by 27% when the solar water heating system is added to the digester. [ABSTRACT FROM AUTHOR]

• The different combination H 2 SO 4 with SE and SE with grinding pretreatment methods were examined. • SEG180 and DASE190 pretreatment were found the most effective for methane enhancement. • Rape straw (RS) pretreated with SEG180 was co-digested with cattle manure (CM). • A 60:40 ratio of CM with SEG180-treated RS produced the highest methane yield. The present study investigated the possibility of valorizing rape straw through anaerobic digestion and the possibility of improving biomethane yield by pretreatment with H 2 SO 4 , combined H 2 SO 4 with steam explosion (SE) and SE combined with superfine grinding (SFG). To evaluate the pretreatment method efficiency, several analytical techniques were applied. Additionally, the performance of co-digesting of cattle manure (CM) with pretreated rape straw (PRS) at different ratios was evaluated. The results showed that combined pretreatment could dissolve the lignocellulosic fiber structure, which positively stimulated methane yield. The highest cumulative CH 4 yield (CMY) of 305.7 mLg−1VS was achieved by combined SE at 180 °C for 5 min with SFG, which was 77.84% higher than the untreated. The CMY was further improved by 11.4–59% higher than the control (CM) using co-digestion. This study confirmed that, under optimal parameters of AD, pretreatment with SEG180 could significantly boost the CMY from co-digestion of CM and PRS. [ABSTRACT FROM AUTHOR]