Open field burning and tilling the rice straw (RS) back into the fields causes environmental threats by contributing to the increased greenhouse gas emissions. Energy and nutrient recovery from RS through anaerobic digestion (AD) is an effective solution for its utilization. Although RS has good methane potential, its characteristics make it a difficult substrate for AD. This paper reviews the characteristics of RS, mass balance, and distribution of nutrients into liquid and solid digestate in the AD. The present review also discusses the effect of temperature, co-digestion, mixing, inoculum, organic loading rate, recycling liquid digestate, the addition of trace elements, and their bioavailability on the enhancement of biogas/methane yield in the AD of RS. In addition, the digestion of RS at various scales is also covered in the review.

Rice straw is an agricultural residue produced in abundant quantities. Open burning and plowing back the straw to the fields are common practices for its disposal. In-situ incorporation and burning cause emissions of greenhouse gas and particulate matter. Additionally, the energy potential of rice straw is lost. Anaerobic digestion is a technology that can be potentially used to utilize the surplus rice straw, provide renewable energy, circulate nutrients available in the digestate, and reduce greenhouse gas emissions from rice paddies. An innovative temperature phased anaerobic digestion technology was developed and carried out in a continuous circulating mode of mesophilic and hyperthermophilic conditions in a loop digester (F1). The performance of the newly developed digester was compared with the reference digester (F2) working at mesophilic conditions. Co-digestion of rice straw was carried out with cow manure to optimize the carbon to nitrogen ratio and to provide the essential trace elements required by microorganisms in the biochemistry of methane formation. F1 produced a higher specific methane yield (189 ± 37 L/kg volatile solids) from rice straw compared to F2 (148 ± 36 L/kg volatile solids). Anaerobic digestion efficiency was about 90 ± 20% in F1 and 70 ± 20% in F2. Mass fractions of Fe, Ni, Co, Mo, Cu, and Zn were analyzed over time. The mass fractions of Co, Mo, Cu, and Zn were stable in both digesters. While mass fractions of Fe and Ni were reduced at the end of the digestion period. However, no direct relationship between specific methane yield and reduced mass fraction of Fe and Ni was found. Co-digestion of rice straw with cow manure seems to be a good approach to provide trace elements except for Se.

Anaerobic co-digestion technology (AcoD) can be used to process rice straw (RS) and cow manure (CoM) to produce energy and a digestate rich in nutrients, while the improper disposal of RS and CoM causes environmental problems. The overall effectiveness of the anaerobic digestion technology can be improved by utilizing the nutrients available in the digestate. It is also a way to reduce the usage of mineral fertilizer by recycling the nutrients available in the digestate. The co-digestion of RS with CoM was performed in a newly developed digester (F1) and in a mesophilic digester (F2) used as a reference. The mass balance of C, macronutrients (N, P, K, Ca, Mg, and S), and their distribution into a liquid digestate (LD) and a solid digestate (SD) was investigated in both digesters. The mass balance was used to evaluate the carbon available in the biogas and in the digestate. It was also used to investigate the recovery potential of the macronutrients after the AD process. Moreover, the assessment of the resulting digestate was carried out to suggest its potential use in agriculture. The amount of C measured in the biogas was the same in both digesters (41.0% and 38.0% of the initial C). Moreover, the conversion efficiency of C from the substrate into methane was 23.4% for F1 and 21.0% for F2. The Ca, Mg, K, and P were conserved in the digestate because their recovery rates (RR) were close to 100%. However, a relatively low RR was observed for N (84.1% in F1 and 86.8% in F2) and S (87.1% in F1 and 86.5% in F2) in both the digesters. After separation n of the SD, from 79.1 to 83.4% (in F1) and 75.0 to 82.4% (in F2) of the final nutrients were available in the LD. The assessment of the SD suggested its use in agriculture not only for soil amendment but also as a K-providing organic fertilizer.