Enhanced granulation and methane recovery at low load by downflow sludge circulation in anaerobic treatment of domestic wastewater.

The effects of downflow sludge circulation on granulation and methane recovery at low load were investigated for domestic wastewater treatment in a modified anaerobic reactor. Compared with conventional upflow anaerobic reactors, enhanced granulation with shortened start-up time was achieved and stable granules were successfully cultivated only after 58 days operations. The introduction of downflow sludge circulation resulted in reverse wastewater-sludge flow and uniform sludge distribution in the reaction zone, which contributed to enhanced wastewater-sludge mass transfer and satisfactory performance with a high soluble chemical oxygen demand (SCOD) removal efficiency of 94.8% at hydraulic retention time (HRT) 6 h. Besides, enhanced liquid-to-gas mass transfer caused a lower dissolved CH4 saturation index of 1.11 and a higher CH4 recovery efficiency of 79.48% at HRT 6 h. High throughput sequencing revealed a distinct shift of microbial community during start-up period from Proteobacteria to Bacteroidetes and Chloroflexi in the existence of downflow sludge circulation.