The release of dissolved methane in effluent from anaerobic wastewater treatment processes into the atmosphere has been of little concern so far, but it strongly contributes to global warming. We investigated whether dissolved methane can be recovered as a useful gas in order to reduce methane discharge, using a closed down-flow hanging sponge (DHS) unit. A continuous methane recovery experiment was carried out for almost two years using effluent of actual municipal sewage anaerobic treatment, which contains 70.3 mgCOD-CH4·L-1 of dissolved methane on average when the ambient temperature is over 20°C. The results showed that by controlling the air supply rate, burnable off-gas containing over 30 % methane could be recovered. However, there is a trade-off between the recovered methane content and methane recovery efficiency. We found that both methane production and oxidation occurred in the closed DHS unit and that methane production was more predominant at higher temperatures.
