Intermittent Effluent Recirculation for the Efficient Treatment of Low Strength Wastewater by an EGSB Reactor

In order to establish the appropriate methane fermentation technology for low strength wastewater, a 2.0 L EGSB reactor was operated at 20°C with 0.3 - 0.4 g COD/L of sucrose-based synthetic wastewater for 500 days. At the start up period, the reactor was operated in EGSB mode with a 5 m/h up flow velocity by continuous effluent recirculation. However, the EGSB reactor exhibited insufficient COD removal (50-60%) at COD loading of 7.2 - 9.6 kg COD/m3⋅day due to the low COD concentration in the sludge bed. Therefore, we proposed the new operation mode by switching to UASB mode (without recirculation, 0.7 m/h up flow velocity) for 30 minutes and EGSB mode for 10 minutes in 40 minutes cycle. Moreover we added sodium sulfide to make the low ORP condition. In this operation, COD removal increased dramatically, from 65% to 91%. Additionally, physical properties of the retained sludge were well maintained in this operation mode. Furthermore, the retained sludge possessed appropriate levels of methanogenic activity (0.2-0.4 g COD/g VSS/day) at 20°C.