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Rice straw is an abundant lignocellulosic biomass used to produce biogas. However, its resistant structure hinders biogas production. In this study, three pretreatments, microwave, ammoniation, and microwave-assisted ammoniation (MAA), were evaluated for their use in improving biogas production from rice straw. After an 18-day digestion, methane production by rice straw pretreated with MAA was 281.56 mL/g (volatile solids of substrate), which was 25.43%, 7.96%, and 18.18% more than that produced following the control, microwave, and ammoniation pretreatments, respectively. These increases were mainly attributed to the change in the resistant structure of rice straw following the MAA pretreatment and a more stable anaerobic digestion system. The degradation rate for the volatile solids of the rice straw pretreated with MAA during anaerobic digestion was 71.20%, which was 4.71%, and 4.86% higher than the rates observed using microwave and ammoniation pretreatments, respectively. In addition, the MAA method had a T90 fermentation cycle of 6 days, which was 33.3%, 25.0%, and 33.3% shorter than those for the control, microwave, and ammoniation methods, respectively. This technology shows the potential to enhance biogas production from the anaerobic digestion of rice straw. [ABSTRACT FROM AUTHOR]

Biogas production by anaerobic digestion is an attractive approach to producing clean fuel biomethane and reducing environmental pollution. However, many fluctuations in production occur during a batch digestion period. This study investigated the dynamics of microbial communities involved in biogas production during a 20-day digestion of rice straw (RS) at 35 °C. The results demonstrated roughly consistent trends in biogas and methane production and revealed distinct stages of production. The lowest and peak methane yields were 82 ± 19.9 mL/(L•day) at day 3 and 328.5 ± 0.9 mL/(L•day) at day 8, respectively, whereas most (97.1%) of the methane was produced in the first 14 days of digestion. Ultimately, the biogas potential of RS reached 393.2 ± 13.6 mL/g added volatile solids (CH4% = 57.1%), and the logistic model best fitted actual methane production compared to the modified Gompertz, first-order kinetic, and Cone models. The microbial communities at different critical stages are a clear reflection of the biochemical dynamics in digesters. The bacterial genera Bacteroides and Clostridium_sensu_stricto_1 were the dominant microorganisms at the early stage of digestion, while the archaeal genus Methanosarcina was significantly more abundant at the peak stage of methane production than at other times during the digestion. This study demonstrates the detailed micro- and macro-conditions that exist during the fluctuating and constant digestion of RS, which will benefit industrial applications during the anaerobic digestion of RS. [ABSTRACT FROM AUTHOR]

To simplify the procedure of biogas production using agricultural straw as raw materials, the biogas performance of both dry and fresh rice straw (RS) was assessed during batch anaerobic digestion (AD) at 35 °C. The results showed that biogas production from fresh RS was 400.5 mL/g volatile solids (VS), which was 40.9% larger than that from dry RS substrate. Meanwhile, the methane content in biogas from fresh RS was 56.05% (approximately 2.5% larger than for dry RS), and the VS removal rate from feedstock was 72% (which was 7.2% greater than for the dry RS substrate). Moreover, the scum volume resulting from digestion of dry RS was 3.16 times the volume from fresh RS at the end of AD, indicating that RS feedstock in a fresh state was more readily biodegradable and more easily used as a substrate by microorganisms. Therefore, compared with dry RS, fresh RS as AD feedstock has practical application value and should be included in further research on the AD of biomass resources. [ABSTRACT FROM AUTHOR]

• A novel hydraulic digester with the ability to control scum formation was developed. • Improved biogas yield and feedstock degradation were achieved in conical digesters. • The conical shapes increased the diversity and evenness of the archaea community. • The digester was operated in batch and continuous modes. Scum formation is a widespread phenomenon and causes serious damage in straw biogas digesters. A 10-L novel hydraulic conical digester for controlling scum was developed in this work and compared with a hydraulic cylindrical digester that simulated the conventional digester. After 30 d of batch and 120 d of semi-continuous fermentations using banana stems, the scum volumes of in cylindrical digesters were 4.12 and 2.12 times that in the conical digesters, respectively. The conical digesters increased biogas production by 5.7% and 11.6% in batch and semi-continuous tests, respectively. The VS removal of feedstock in conical digesters were 5.6 and 7.2% greater than for the batch and semi-continuous cylindrical, respectively. The microbial diversity and evenness were higher in conical than cylindrical digesters. The results demonstrated that conical shape was an effective structure for controlling scum formation and improving biogas production. [ABSTRACT FROM AUTHOR]