Abstract: To solve the long existing obstacles of following season growth and other environmental risks caused by rice straw returning to the field, as well as the issue of local organic fertilizer supply deficit in water resource area, a series of batch tests on an integrated stainless-steel anaerobic digester (with total solids of 20%) were conducted. The characteristics of both biogas and residue production from the studied dry anaerobic digestion (AD) system with limited pig manure (PM) input (as nitrogen source) were investigated, to explore the biogas and digestate productivity from AD systems with high C/N ratio. Results show that the maximum gas yield was 4.7 L·d^-1 when C/N ratio reached 50∶1 (group C3). On the 12th day, the methane content in C3 group reached to 50% and biogas yield reached to 294 L·kg^-1, which were only 11.2% lower than the optimal group C6 (with C/N of 30∶1). It indicates that the upper limit of feedstock C/N ratio in dry AD system could be set at 50∶1. Moreover, the cellulose in C3~C6 (with C/N ratio of 50∶1~30∶1) could also be significantly degraded after digestion processes. The total nutrient in the dehydrated digestate of different treatments (C3~C6) was only around 2.25% to 3.71%, but other criteria such as organic matter content, pH value and heavy metal contents could all meet the organic fertilizer standards NY/T 525-2021 in agricultural industry, thus could be processed to high-quality organic fertilizer products. Based on engineering process simulation, when rice straw was processed at a rate of 12 t·d^-1, the annual profit of the studied system was 781 600 yuan, which is significantly higher than the profit of direct returning of straw to field. Therefore, it's considered both economically and environmentally feasible to apply biogas and fertilizer co-production from rice straw-based dry AD in water resource area.

Key words: water resource area, rice straw, co-digestion, dry anaerobic digestion, fertilizer potential