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.

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The nitrogen concentrations of piggery wastewater and its activated sludge treatment effluent were surveyed in nine farms with various facilities. The biological oxygen demand and suspended solids in the wastewater were sufficiently removed by activated sludge treatment because their effluent concentrations averaged 59 mg·L^-1 and 45 mg·L^-1, respectively; however, nitrogen was not sufficiently removed because its effluent concentration averaged 430 mg·L^-1. When the nitrogen concentration was higher than the general permissible limit under Japanese law, the nitrogen composition of the effluent tended to be mainly ammonium when the pH was 8 or higher, and a mixture of ammonium, nitrite and/or nitrate when the pH was below 8. For the post-treatment of activated sludge from the latter-case effluents, we assumed the application of anammox or denitrification treatment using piggery wastewater as the electron donor and estimated nitrogen removal potential based on the simplified stoichiometric reaction. It was estimated that denitrification and anammox treatments could respectively remove 19 % and 40 % of the total nitrogen from the activated sludge effluent on average.

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