MBR System Combines With Anaerobic Offsite Ferrous Ion Release And P-Fe Compounds Transformation Mechanism Study

The membrane bioreactor（MBR）and anaerobic membrane bioreactor（An MBR）have the advantages of good effluent quality,less excess sludge and small space requirement,which have been widely studied.However,membrane fouling is an important restriction for their further development.The introduction of electrocoagulation（EC）into MBR can effectively remove soluble phosphorus and slow down membrane fouling.However,for the conventional EC,the plate passivation can not be ignored.At the same time,a large amount of the P-Fe containing sludge accumulated in the whole EC-MBR process needs to be disposed.The research shows that the excess sludge is a promising raw material for phosphorus recovery.Among them,vivianite as a new product of phosphorus recovery,has relatively simple formation conditions and a very wide range of applications.It is a very stable iron phosphate crystal.In order to solve the problems of pollutants removal,membrane fouling,plate passivation and vivianite recovery in membrane bioreactor（MBR）.In this study,a new anaerobic baffled membrane bioreactor system（ABMBR,4 anaerobic units and 1membrane separation unit）was proposed.The iron plates were placed in the anaerobic unit,and they were electrolyzed in the low p H environment to release Fe²⁺,which was converted into Fe³⁺in the subsequent aerobic MBR to play the role of flocculation.In order to further explore the conversion mechanism of P-Fe containing compounds in the system and the formation mechanism of vivianite under aerobic and anaerobic conditions.The dissimilatory iron reduction under low oxidation-reduction potential（ORP）was realized by returning the P-Fe containing aerobic sludge to the anaerobic unit.The research indicates:（1）EC could reduce membrane fouling by forming sludge flocs,increasing sludge particle size and forming loose cake layer on membrane surface.When the ABMBR was used,protein and polysaccharide contents of soluble microbial products（SMP）and extracellular polymeric substances（EPS）from sludge and filter cake layer were decreased obviously;the sludge particle size was increased from 400 nm to 80μm.The filtration time of MBR was extended from 5-8 days to 63 days,and membrane fouling was obviously controlled.（2）The removal efficiencies of total organic carbon（TOC）and ammonia nitrogen（NH₄⁺-N）were 98%and 100%respectively,and the removal efficiency of total phosphorus（TP）was 87%by flocculation under the condition of no excess sludge discharge from the ABMBR system.（3）By placing the iron plate in the anaerobic unit of ABMBR,because of its low p H environment and no anaerobic suspended sludge,there was no Fe³⁺accumulation on anode surface when releasing Fe²⁺and the anode would not be covered by biofilm.After 194 days of operation,the plate passivation was still relatively slight,which effectively solved the problem of plate passivation in EC.（4）After anaerobic treatment,the specific component of phosphorus in dry sludge（DS）was mainly Fe-P,which accounted for 93%～97%of total phosphorus.Through further analysis of X-ray photoelectron spectroscopy（XPS）,the sludge was mainly composed of O,Fe and P elements,among which Fe₃（PO₄）₂ accounted for 30%～45%,and the P element was mainly in the form of PO₄^3-.X-ray polycrystalline diffraction（XRD）and Field emission scanning electron microscopy（FE-SEM）analysis showed that the final product of the sludge after anaerobic condition was mainly vivianite.During the whole process,Fe²⁺produced by electrolysis in anaerobic unit was oxidized to Fe³⁺in aerobic unit,which was hydrolyzed and flocculated to form P-Fe（Ⅲ）sludge.After that,the P-Fe containing sludge would be returned to the anaerobic section.During the anaerobic conditions of 124 days,the ORP and p H maintained at-350m V and 7.5 under anaerobic condition,which exactly met the conditions of vivianite formation.Iron-reducing bacteria（IRB）were found before and after anaerobic process,including Dechloromonas,Desulfovibrio,Aeromonas and Methanobacterium.Fe³⁺was reduced by IRB at low ORP to generate Fe²⁺,which finally formed vivianite with PO₄^3-.This study successfully realized the transformation of amorphous ferro-phosphorus compounds into vivianite under suitable dissolved oxygen conditions.