Combining submerged membrane technology with anaerobic and ...

mgN·gMLVSSmgCH4·gMLVSSmgO2·gMLVSSChapter 6Volumetric mass transfer coefficient for oxygen was also calculated (0.90 d -1 ) in orderto determine the oxygen transferred from the environment to the first MBR chamber. Thisvalue was not negligible, representing 48, 25 and 12% of the oxygen transferred with therecirculation from the aerobic membrane filtration chamber at R of 0.5, 1.0 and 2.0respectively.Table 6.3. Average denitrification, methane and oxygen apparent specific consumptionrates and N-NO x:C-CH 4 and CH 4:O 2 molar ratio in the first (anoxic) chamberR-1·d-1-1·d-1-1·d-1mol NO3 - /mol CH4mol CH4/mol O20.5 14.3 40.1 4.9 0.5 16.41.0 22.5 43.7 10.9 0.6 8.02.0 16.8 15.9 13.0 1.2 2.4The experimental molar ratio between the oxidized methane and the oxygenconsumed was from 2.4 to 16.5, which is much higher than theoretical molar relationship1:1 according to the stoichiometry of the aerobic pathway (equation 6.1), suggesting acombination of both, aerobic and anaerobic oxidation of methane. It should be taken intoaccount the importance of anaerobic oxidation of methane, especially at lower recirculationrates. The increase in recirculation ratio from the aerated membrane filtration chamberincreased the amount of oxygen entering the first (anoxic) chamber and probably causedthe observed sharp decrease of methane oxidation rate. The oxidation of methane seemedto be inhibited. Other authors have reported the complex role of oxygen in the process,observing and increase of specific denitrification rate with oxygen up to a maximum, butdecreasing again after it (Thalasso et al., 1997). Moreover, as reported by Waki et al.(2009), the removal of nitrogen in the presence of methane and oxygen is a complexprocess that might occur through some different mechanisms such as aerobic andanaerobic methane oxidation coupled to denitrification or even anammox. This could be areason for the observed impact of recirculation, or oxygen, on methane oxidation rateduring the present study. In this sense, the high dissolved oxygen concentrations in the164