Removal of residual dissolved methane gas in an upflow anaerobic sludge blanket reactor treating low-strength wastewater at low temperature with degassing membrane

摘要

In this study, we investigated the efficiency of dissolved methane (D-CH4)collection by degasification from the effluent of a bench-scale upflow anaerobicsludge blanket (UASB) reactor treating synthetic wastewater. A hollow-fiberdegassing membrane module was used for degasification. This module was connectedto the liquid outlet of the UASB reactor. After chemical oxygen demand (COD)removal efficiency of the UASB reactor became stable, D-CH4 discharged from theUASB reactor was collected. Under 35°C and a hydraulic retention time (HRT) of 10 h,average D-CH4 concentration could be reduced from 63 mg COD L–1 to 15 mg CODL–1; this, in turn, resulted in an increase in total methane (CH4) recovery efficiencyfrom 89% to 97%. Furthermore, we investigated the effects of temperature and HRT ofthe UASB reactor on degasification efficiency. Average D-CH4 concentration was ashigh as 104 mg COD L–1 at 15°C because of the higher solubility of CH4 gas in liquid;the average D-CH4 concentration was reduced to 14 mg COD L–1 by degasification.Accordingly, total CH4 recovery efficiency increased from 71% to 97% at 15°C as aresult of degasification. Moreover, degasification tended to cause an increase inparticulate COD removal efficiency. The UASB reactor was operated at the same CODloading rate, but different wastewater feed rates and HRTs. Although average D-CH4concentration in the UASB reactor was almost unchanged (ca. 70 mg COD L–1)regardless of the HRT value, the CH4 discharge rate from the UASB reactor increasedbecause of an increase in the wastewater feed rate. Because the D-CH4 concentrationcould be reduced down to 12 ± 1 mg COD L–1 by degasification at an HRT of 6.7 h, the3CH4 recovery rate was 1.5 times higher under degasification than under normaloperation.