Effect Of Fixing Biocarrier On Membrane Fouling Of Membrane Bioreactor Treating Municipal Wastewater

Membrane bioreactor(MBR) is an emerging wastewater treatment process that combines membrane separation technology with traditional activated sludge treatment technology.In the environment of increasingly strict wastewater discharge standards and increasing awareness of environmental protection,the MBR process has unique advantages with good effluent water quality,high activated sludge concentration,low residual sludge production,high resistance to shock loading,and small footprint.However,membrane fouling remains the main obstacle to the widespread application of MBR processes.In recent years,a large number of studies have shown that membrane fouling can be effectively alleviated by adding biocarriers to MBR.This study explores the effects of different fixed biocarriers in MBR on alleviating membrane fouling and improving wastewater treatment,especially nitrogen removal,under different carbon/nitrogen(C/N)ratios,expecting that the fixed biocarriers in MBR can alleviate membrane fouling and improving wastewater treatment.This study has a certain theoretical significance for exploring the use of fixed biocarriers to alleviate membrane fouling and improving wastewater treatment in MBR.It also provides experimental basis and reference for the practical application of fixed biocarriers in MBR.Polyvinyl chloride(PVC),polyurethane(PU)and polyvinyl formal(PVFM)biocarriers were added to different MBRs with influent at high C/N ratio(20.0)and low C/N ratio(6.7).This study investigated the mechanism of adding fixed biocarriers to alleviate membrane fouling and enhance the effect of nitrogen removal through the analyses of treatment performance,membrane fouling,sludge mixed liquid characteristics,cake layer characteristics and microbial community.The results showed that the characteristics of the bulk sludge(such as bigger sludge particle size,lower soluble microbial product(SMP)content,and lower SMP protein/polysaccharide(PN/PS)ratio)in MBR with biocarriers significantly changed.The membrane fouling cake layer properties were also significantly affected with lower polysaccharide and protein content.Besides,the addition of biocarriers with MBR reduced the abundance of membrane fouling related bacteria in the bulk sludge,such as Sphingobacterials?unclassified,Ohtaekwangia and Rhodocyclaceae?unclassified.Therefore,the addition of fixed biocarriers in MBR can significantly improve the bulk sludge characteristics,affect the diversity of microbial communities,and alleviate membrane fouling.This is more obvious under a low C/N ratio in the influent.which extended the membrane fouling cycle to 8.25 days(198h)with PVFM biocarriers compared with the blank group(R1)with 4.5 days(108h).The experimental results also showed that the MBR with biocarriers can maintain a good performance of wastewater treatment,the total nitrogen removal was also slightly improved,especially under the low C/N ratio in the influent.The effluent total nitrogen content of the blank group(R1),the R3(PU)and R4(PVFM)were 17.8 ± 3.1 mg L-1,14.4 ± 2.0 mg L-1 and 13.4 ± 1.7 mg L-1,respectively.That means the effluent total nitrogen content was reduced after adding PU and PVFM biocarriers.This may be due to the aerobic-anoxia microenvironment formed inside the biocarriers owing to the limitation of oxygen transfer,which enriched nitrifying and denitrifying bacteria including Thauera,Amaricoccus and Nitrosospira.Comparing among different fixed biocarriers,PVFM biocarriers are superior to PVC and PU biocarriers in terms of membrane fouling alleviation and wastewater treatment performance improvement,which might be due to the larger specific surface area.In summary,the MBR with fixed biocarriers can effectively alleviate membrane fouling,and improve the removal of total nitrogen under the effluent low C/N ratio;different fixed biocarriers have different effects on membrane fouling and total nitrogen removal efficiencies.Among the three fixed biocarriers,PVFM biocarrier was the best option in terms of membrane fouling mitigation and treatment performance especially nitrogen removal improvement.This study has certain guiding significance for the selection of high-efficiency biocarriers in practical applications,which helps to improve the economic benefits of sewage treatment and save the operating costs of MBR systems,and is conducive to promoting the widespread application of MBR processes.