Masahiro OKAWARA
Development of Methane Energy Recovery Technology from Anaerobic Bioreactor for Treating Municipal Sewage
Takashi YAMAGUCHI
Formation and sustainment of anaerobic microbial biofilms are difficult treating low strength wastewater such as municipal sewage result in good effluent quality cannot achieve. On the other hand, the Expanded Granular Sludge Bed (EGSB) reactor fluidizes the biofilms by giving high up-flow velocity to the sludge bed, and achieves the improvement of the contact efficiency with the substrate. So a feasibility test of Expanded Granular Sludge Bed (EGSB) reactor was carried out by continuous feeding of municipal wastewater. The objective of this study is applicability evaluation of anaerobic sewage treatment. Expanded Granular Sludge Bed (EGSB) reactor, inoculated with mesophilic granular sludge, was applied to the treatment of municipal sewage, in order to investigate the process reliability. The EGSB reactor was operated at ambient temperature (9úæ~27úæ) for over 500 days with HRT of 2 hours, average OLR at 4.1 kgCODcr ¡¦ m-3 ¡¦ day-1 and 6 m ¡¦ h-1 of up-flow velocity. Suspended solid (SS) - COD removal efficiency showed an fluctuation due to the high upflow velocity by effluent recirculation in EGSB reactor, and influent sewage was not treated by primary-sedimentation. Consequently, the preliminary sedimentation tank had been installed just prior to inlet of the EGSB reactor and able to obtain the stability of influent SS concentration. As the result, the CODcr removal rate at HRT of 2 hours was maintained at 44% without any effects of inconstant ambient temperature. The decline of the temperature of sewage caused the decrease of the activity and amount of retained sludge. However, it is possible to retain the one-third of granular sludge from initial amounts after 500 days operation. Additionally, for the most part of produced methane by EGSB reactor was dissolved in effluent. So, This is the crucial problem. Dissolved methane in effluent of anaerobic wastewater treatment processes is not concerned about its release to the atmosphere so far, which contributes the global warming. We investigated whether dissolved methane can be recovered as a useful gas to prevent the methane discharge by a two closed Down-flow Hanging Sponge unit. A continuous methane recovery experiment was carried out for almost two years with an effluent of actual municipal sewage anaerobic treatment, containing dissolved methane of 70.3 mgCOD-CH4 ¡¦ L-1 in average when ambient temperature is over 20úæ. The results showed that by controlling air supply rate for 1st DHS, the recovered off-gas containing over 30% of methane was made, which is auto-burnable, meaning a useful energy resource. However, a trade-off was observed in a relationship between recovered methane content and methane recovery efficiency, suggesting that the efficiency decreases with increasing the methane concentration. Consequently, applied for the 2nd DHS, amount of dissolved methane that release to atmosphere intrinsically can be reduced over 97%.
Please use "previous botton" to return the previous page