Combining submerged membrane technology with anaerobic and ...

Objectives and summaryconsidering the stoichiometry of denitrification with methane, either in microaerobic oranaerobic conditions. Recirculation ratio between the anoxic and aerobic chambers of theMBR system, and either the presence or absence of dissolved methane were shown asthe main important parameters governing the denitrification process. Nitrogen removaldecreased from 60 to 27% when dissolved methane was removed from the UASB effluent.At higher recirculation ratios the anaerobic oxidation pathway seemed to be inhibited,decreasing methane oxidation rate more than a 50%. This inhibition was associated to thehigher oxygen input to the anoxic chamber. This fact confirmed the results obtained inChapter 4, when the application of aerobic/anoxic cycles did not stimulate denitrificationprocess.The influence of denitrification with methane on membrane performance was alsostudied, showing a remarkable increase on biopolymer concentration when denitrificationactivity was affected by the removal of dissolved methane from the UASB effluent. Thiseffect is similar to that observed when nitrification is affected.In Chapter 7 a completely stirred tank anaerobic membrane bioreactor (AnMBR) wasoperated for the treatment of an herbal extraction wastewater. The complexity and lowbiodegradability of this industrial wastewater led to the operation of the bioreactor at highmixed liquor total solids (MLTS) concentrations. The exact relationship between MLTSconcentration and the steady-state permeate flux in an AnMBR has not been extensivelyinvestigated and the information regarding AnMBR operation at high MLTS concentrationis very limited. The fluxes achieved in the studied AnMBR ranged between 1 and 2.5 L·m -2·h -1 , working with MLTS between 38 and 61 g·L -1 . Although these values were similar tothose obtained in other AnMBR treating industrial wastewaters with submerged membranemodules at MLTS above 30 g·L -1 , the possibility of improving membrane performance byadding powdered activated carbon (PAC) was also evaluated. Batch and fed-batchexperiments with different activated carbons were performed and an optimum dosage of1.5 g·L -1 was determined.Stable operation of the system was maintained applying HRT below 4 d, at a feedconcentration of 8 g·L -1 resulting in an OLR of 2.0-3.0 kgCOD·m -3·d -1 without controllingalkalinity reaching COD removal efficiencies up to 60%. Nevertheless volatile fatty acid(VFA) concentration was extremely high during the operation, indicating some kind ofinhibition of the methanogenic process, probably related with the antibacterial activity ofrosemary extracts. This fact might have a harmful effect on anaerobic biological process,causing destabilization of the microbial populations leading to VFA accumulation that canacidify the reactor, and therefore inhibit methanogenic microorganisms. The control of27