Continuous anaerobic treatment of swine wastewater by using self-agitation anaerobic baffled reactor

With the development of pig industry in China, environmental pollution from such development has become more and more serious. One of the most important problems of pig breeding is the wastewater treatment. Swine wastewater is characterized by the high concentrations of organic matters. Anaerobic digestion has been regarded as a promising technology for swine wastewater treatment to remove the high-concentration organic materials. At the same time, renewable energy in the form of biogas was produced through anaerobic degradation. Previous studies have reported that anaerobic baffled reactor (ABR) can achieve good performance in treating swine wastewater. However, due to the absence of agitation system, the mass transfer is an ABR is unsatisfactory. In order to improve the performance of an ABR, the self-agitation anaerobic baffled reactor (SaABR) was proposed. In the SaABR, a U tube was assembled inside the reactor. SaABR is previously studied in the anaerobic treatment. However, fewer its application in treating swine wastewater was found. The current study therefore investigated the performance of a long term operated SaABR in treating swine wastewater. The total and working volume of SaABR in this study were 16 and 10 L, respectively. Anaerobic treatment of swine wastewater for a period of 200-day was continuously conducted. Mesophilic condition was maintained at 37 ℃ while hydraulic retention time (HRT) was gradual shortening from 5, 3, 2, 1, to 0.5 days. The effects of HRT on the biomass retaining, biogas gas production, process stability, and specific methanogenic activity (SMA) was investigated. At the same time, in this study, we also conducted a 78-day continuous experiment by using a parallel continuous stirred tank reactor (CSTR) as control test. The results derived from this study indicated that SaABR significantly retained high biomass concentration. At the HRT 3 d period, the volatile solid (VS) concentration of the first to fourth sampling ports of SaABR was 10.2, 4.1, 44.2 and 2.5 g/L, respectively. The concentration of VS in the CSTR was as low as 2.6 g/L. The higher microbial biomass significantly increased the degradation efficiencies of organic and reduced the VFAs concentration of SaABR effluent. The degradation efficiency of SaABR also decreased with the shortening of HRT. The specific methane yield of the SaABR was 0.43 L/g at the HRT 1 d period. At the HRT 0.5d period stable biogas production can still be achieved. Through the long term experiment, an increase of volumetric gas production rate was observed: 0.85, 1.14, 1.62 and 1.71 L/(L•d) at HRTs 3, 2, 1 and 0.5 days, respectively. Specific methane gas yield decreased from 0.69 to 0.24 L/g when HRT was reduced from 3 to 0.5 d. These values were slightly higher than results of a previous study using ABR treating swine manure at 40 ℃, which reported specific methane yield was 0.59 L/g. The CSTR ceased to produce biogas at HRT 1 d due to the washout of biomass. At HRT 3 d period, the sludge SMA from the first to the fourth sampling port were 116, 97, 44 and 85 mL/(g•d), respectively. Conclusively, the results obtained in this study showed that SaABR had the advantages of retaining high biomass and would be a promising technology for the anaerobic treatment of swine wastewater.