Assessment of biomass functionalities in a biofilm membrane bioreactor (BF-MBR) targeting biological nutrient removal

Abstract

High concentrations of nutrients such as nitrogen, phosphorus and organicmatter can cause serious eutrophication in receiving water bodies. In biologicalwastewater treatment microorganisms remove nutrients from thewastewater and ensure that these components in the final effluent stay atacceptable levels.A biofilm membrane bioreactor (BF-MBR) removing nitrogen, phosphorusand carbon from municipal wastewater was operated for 85 days. Anassessment of the process focusing on the biomass in the various reactorswas carried out. The amount of ammonium oxidizing bacteria (AOB), nitriteoxidizing bacteria (NOB), phosphate accumulating organisms (PAO)and glycogen accumulating organisms (GAO) was quantified. Fluorescencein situ hybridization (FISH), confocal laser scanning microscopy (CLSM)and digital image processing with the software daime were used for thispurpose. Changes in the communities were compared to different operatingconditions. The effect of a decrease in the hydraulic retention time (HRT)and diluted concentrations of nutrients in the influent were evaluated. Thecorrelation between filamentous bacteria and the sludge volume index wasstudied. The viability of bacterial populations in a nitrifying biofilm wasalso investigated.The BF-MBR produced a high quality effluent in terms of chemical oxygendemand (COD), nitrogen and total suspended solids (TSS). The systemhandled well the reduction in the HRT. The removal of phosphorus was notsatisfactory.The microbial communities investigated by FISH analysis were presentin all samples, but their abundance varied. Substrate limitation due toreduced nutrient loading and the activity of denitrifying bacteria in theanaerobic reactor of the A/O bio-P process, affected the population of PAOsnegatively. Results and observations from this study suggest that for the enrichmentof PAOsin an A/O bio-P system it is crucial to have strictly anaerobicconditions in the reactor designated for this purpose and sufficient substrateavailable. When the abundance of PAOs increased, the phosphorusremoval improved. A higher decay rate was observed for PAOsthan GAOs.It might be assumed that the high decay rate of PAOs was not caused bythe substrate competition with GAOs, but rather with denitrifying bacteria.In this study, the abundance of GAOs and AOB seemed solely limitedby the available substrate being carbon and ammonium, respectively. Theabundance of NOB and the total nitrifying community increased despite ofperiods with ammonium limitations. A live/dead analysis of the nitrifyingbiofilm suggested limited diffusion rates in the deeper layers of the biofilm,leading to cell decay. The entire microbial community investigated handledthe reduction in the HRT well.The microbial communities of the biomass were found to be more protectedon the inside of a carrier, forming larger spherical clusters, than inthe activated sludge.The abundance of PAOs on a carrier was found to be superior of theamount of PAOs detected in the activated sludge surrounding the carrier inan IFAS system. A difference in the amount of PAOs and GAOs in the tworeactors of the A/O bio-P process was also found.Large amounts of filamentous bacteria were not observed in the A/Obio-P system despite the high sludge volume index.