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

In this study, we investigated the efficiency of dissolved methane (D-CH₄) collection by degasification from the effluent of a bench-scale upflow anaerobic sludge blanket (UASB) reactor treating synthetic wastewater. A hollow-fiber degassing membrane module was used for degasification. This module was connected to the liquid outlet of the UASB reactor. After chemical oxygen demand (COD) removal efficiency of the UASB reactor became stable, D-CH₄ discharged from the UASB 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 COD L⁻¹; this, in turn, resulted in an increase in total methane (CH₄) recovery efficiency from 89% to 97%. Furthermore, we investigated the effects of temperature and HRT of the UASB reactor on degasification efficiency. Average D-CH₄ concentration was as high 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 a result of degasification. Moreover, degasification tended to cause an increase in particulate COD removal efficiency. The UASB reactor was operated at the same COD loading rate, but different wastewater feed rates and HRTs. Although average D-CH₄ concentration 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 increased because of an increase in the wastewater feed rate. Because the D-CH₄ concentration could be reduced down to 12 ± 1 mg COD L⁻¹ by degasification at an HRT of 6.7 h, the CH₄ recovery rate was 1.5 times higher under degasification than under normal operation.