Combining submerged membrane technology with anaerobic and ...

Denitrification with dissolved methane in an MBR after a methanogenic pre-treatment at ambient temperatureto the MBR, the OLR applied was between 0.1 and 0.9 kgCOD·m -3·d -1 , depending on thesuspended solids washed-out from the UASB reactor.Regarding MLVSS, the concentrations in the UASB reactor, the first MBR chamberand the membrane filtration chamber ranged between 28-35 g·L -1 , 2-5 g·L -1 and 4-8 g·L -1 ,respectively. Biomass concentration in the biofilm was around 45 gMLVSS·m -2 , which wasequivalent to an MLVSS concentration of approximately 6 g·L -1 . Sludge retention time(SRT), referred to the MBR, was maintained between 15 and 30 d during the wholeoperation. Anaerobic biomass was not purged from the UASB reactor during the study.Food to microorganism (F/M) ratio applied to the MBR was around 0.03 gCOD·gMLVSS -1·d -1 , referred to non-methane soluble COD. Dissolved oxygen concentration in the anoxiccompartment ranged between 0.1 and 0.3 mg·L -1 .6.4.2. Influence of dissolved methane on denitrificationThe remaining non-methane biodegradable COD and dissolved methane in theeffluents from the UASB can be used as carbon source for denitrification. Soluble COD inthe UASB effluent during the experiments was very low, 57±34 mg·L -1 . Moreover, VFAs inthe UASB effluent were monitored during the six experimental periods, being itsconcentration below minimum detection limit of the method used (20 mg·L -1 ). Dissolvedmethane in the influent to the MBR was normally between 19 and 25 mg·L -1 , except onperiod IV, when methane was stripped off and its concentration decreased to valuesbetween 3 and 8 mg·L -1 .Most of the total nitrogen in the effluent of the UASB reactor was present as solubleammonia (35.7±7.9 mg·L -1 ). Ammonia was fully nitrified during period I, during which thefirst MBR chamber was maintained under aerobic conditions. Dissolved total nitrogen(DTN) in the permeate was similar to the ammonia concentration fed to the MBR system(figure 6.2). Therefore, no nitrogen removal took place during this period. Moreover, afterthe first experimental days of period I, most of the ammonia oxidation to nitrate took placein the first chamber (figure 6.2b). During the first operating days of period II, completedenitrification of nitrate was observed in the first (anoxic) chamber. Later on, theconcentration of this compound increased. Significant nitrogen removal was also observedduring periods III, IV, V and VI, during which the first MBR chamber was operated underanoxic conditions. This caused a remarkable diminution of DTN in the permeate (figure6.2a). Up to 60% nitrogen removal was observed during periods II, III and VI.159