Characteristics of biogas producing by anaerobic co-digestion of agricultural straw and swine wastewater based on improved straw-bed bioreactor

Abstract: Agricultural straw and livestock wastewater were the main source of agricultural contaminant, how to reduce, reuse and recycle these wastes has become more and more important. Anaerobic digestion is an attractive technique for bio-waste treatment which can produce biogas and bio-fertilizer. Co-digested two and more wastes is beneficial for anaerobic digestion. By this way, we can not only get more biogas, but also reduce the investment of biogas plant. Two or more agricultural wastes were converted to biogas in one anaerobic bioreactor. Although much work has been done on anaerobic co-digestion of two or more bio-wastes, but most work was done with batch model and there was no report on anaerobic co-digestion of agricultural straw and livestock wastewater. In pig farming, swine wastewater is the main contaminant and is generated every day, so swine wastewater must be added to the anaerobic digester daily, which means that we must use semi-continuous feeding bioreactor in a pig farm biogas plant. In this study, the feasibility of straw-bed anaerobic bioreactor (SAB) was investigated. In the straw-bed bioreactor, baling straw was used as the stationary phase and swine wastewater was used as the mobile phase, and a wastewater advanced anaerobic bioreactor (USR) was connected to the posterior of the straw-bed bioreactor. The results showed that the straw-bed anaerobic digestion system (SADS) was feasible and there was no significant difference between co-digestion and mono-digestion of straw and swine wastewater. The total solid biogas yield of straw of SAB and mono-digestion were 394.96 and 382.11 mL/g, respectively. Compared to mono-digestion, the stability of daily biogas yield and average methane content of co-digestion was improved significantly. The average methane content of SADS, baling straw and swine wastewater digested alone were 57.40%, 47.32% and 60.37%, respectively. The volumetric biogas production was also improved significantly and the volumetric biogas production of treatment of baling straw and swine wastewater digested alone were 69.42% and 66.94% to that of SADS. After 35 days reaction, air permeability of baling straw was decreased which leading to more serious floating of baling straw and short flow of influent. After that, COD content of effluent increased sharply and then maintained at higher level. Meanwhile daily biogas yield of wastewater advanced anaerobic bioreactor increased sharply. Therefore, it is necessary to connect a wastewater advanced anaerobic bioreactor to the posterior of straw-bed bioreactor in order to deep purification of effluent of SAB. The above results indicated that co-digested baling straw and swine wastewater in straw-bed anaerobic digestion system is feasible and is beneficial for biogas production.