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Contribution of Liquid/Gas Mass-Transfer Limitations to Dissolved Methane Oversaturation in Anaerobic Treatment of Dilute Wastewater
The
mechanisms controlling the accumulation of dissolved methane
in anaerobic membrane bioreactors (AnMBRs) treating a synthetic dilute
wastewater (a glucose medium) were assessed experimentally and theoretically.
The AnMBR was maintained at a temperature of 24–26 °C
as the organic loading rate increased from 0.39 to 1.1 kg COD/m<sup>3</sup>-d. The measured concentration of dissolved methane was consistently
2.2- to 2.5-fold larger than the concentration of dissolved methane
at thermodynamic equilibrium with the measured CH<sub>4</sub> partial
pressure, and the fraction of dissolved methane was as high as 76%
of the total methane produced. The low gas production rate in the
AnMBR significantly slowed the mass transport of dissolved methane
to the gas phase. Although the production rate of total methane increased
linearly with the COD loading rate, the concentration of dissolved
methane only slightly increased with an increasing organic loading
rate, because the mass-transfer rate increased by almost 5-fold as
the COD loading increased from 0.39 to 1.1 kg COD/m<sup>3</sup>-d.
Thus, slow mass transport kinetics exacerbated the situation in which
dissolved methane accounted for a substantial fraction of the total
methane generated from the AnMBR