A Mass Balance-Based Method for the Anaerobic Digestion of Rice Straw

Current rice straw disposal practices have serious repercussions on the environment and, in addition, do not consider its energy potential. On the contrary, the anaerobic digestion of rice straw makes it possible to produce renewable energy and to reintroduce into the soil the nutrients present in the digestate, at the same time, reducing greenhouse gas emissions from paddies. For rice straw of different geographical origin, by applying a mass balance method to the digester, the minimum requirements in terms of conditioners (nitrogen, phosphorus and potassium) and water, which allow obtaining the maximum production of methane, were calculated. The results obtained show that after the first 30 days (hydraulic retention time) for each ton of rice straw digested, the daily water consumption varies considerably from one country to another, from a minimum value of 1.5 m3/d to a maximum of 4.3 m3/d. After the same time, the addition of nitrogen and phosphorus is only required for the optimal anaerobic digestion of Indian rice straw. The low presence of these elements in Indian straw requires an addition of 3 kg/d of urea and 1.5 kg/d of superphosphate to compensate for the lack of nitrogen and phosphorus, respectively. In all the examined cases, the concentration of potassium, even if higher than the optimal value, does not reach levels that can significantly affect the methane production.