Norihisa MATSUURA
Technology to prevent of dissolved methane gas emission from effluent of anaerobic wastewater.
Takashi YAMAGUCHI
Anaerobic wastewater treatment has been focused on its eco-friendly nature of improved energy conservation and
reduction of carbon dioxide emission. However, the anaerobic process discharges non-recovered methane. Methane is one of
strong greenhouse gases, and has a global warming potential of 21 times larger than that of carbon dioxide. One souse of methane
emissions from anaerobic process is dissolved methane in process effluent. It was reported that the mass of dissolved methane in
the process effluent was as large as the recovered methane in case of low strength wastewaters such as municipal sewage.
Therefore, anaerobic treatments need a post-treatment process for effluent polish-up as well as dissolved methane treatment. But
in common post-treatment process following anaerobic process such as activated sludge and aerated lagoon, dissolved methane is
not paid attention. To date, little knowledge was obtained about dissolved methane recovery or treatment. In this study, we
proposed a novel post-treatment system for recovery and treatment of dissolved methane using down-flow hanging sponge (DHS)
reactor. DHS reactors of two stages placed in closed boxes were applied to a post-treatment of UASB treating an actual municipal
sewage.
In the first DHS, recovery of dissolved methane was carried out by the way of physical gasification based on gas-liquid
equilibrium. The air was supplied from the lower part of the DHS column in a rage of flow rate 250 to 375 L-3・day-1. The off air
with methane gas was recovered from the upper portion of the DHS. The residual dissolved methane was introduced to the second
DHS reactor and biologically oxidized in the column into which the air was fed at a rate of 2500 L・m-3・day-1 from the upper part.
The off air was released from the lower part of the second DHS reactor. During the experimental periods, the dissolved methane
concentration of UASB effluent was 75 mg COD・L-1. In the first DHS reactor, the dissolved methane could successfully be
recovered as burnable gas containing methane concentration over 30 %. The results showed an average recovery efficiency of
73% for dissolved methane during summer (more thane 25℃), while it dropped to 60% during wintertime (about 10℃). In the
second stage of DHS reactor, the influent resident dissolved methane was mostly removed to about 0.11 mg COD・L-1. The
dissolved methane from UASB reactor was removed totally more than 99 % by the post treatment of the two stages of closed DHS
reactors. Cloning of the 16S rRNA genes as well as phylogenetic analysis revealed that methane-oxidizing bacteria were detected
in closed DHS reactor during the experiment period.
In addition, the two stages closed DHS system had an ability to remove BOD, becoming to the final effluent with 15
mg・L-1, respectively. In conclusion, we accomplished the development of dissolved-methane recovery and oxidation system for
post-treatment of UASB treating municipal sewage.
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