| Membrane bioreactor has its limitation from being widely adopted in engineering projects due to high initial cost, and easy membrane fouling. In recently years, studies on fabric membrane bioreactor have been published, which indidated that not only does fabric membrane bioreactors share numerous advantages with their traditional counterparts, it also possesses unique characteristics like low energy consumption, low intial cost and high strength. However, drawbacks like poor operation flux and easy membrane fouling still confine the development and implementation of fabric membrane bioreactors.Focusing on current drawbacks of fabric membrane bioreactors such as low operation flux and short operation period, this study conducted a feasibility research on selected four non-woven fabrics (PE5μm、PE25μm、PP5μm、PP25μm) and two nylon textile fabrics (500meshes and800meshes) and conluded with optimal membrane material for fabric membrane bioreactors. The study also developed two noval fabric membrane bioreactors:submerged fabric membrane bioreactors and open fabric membrane bioreactors. The optimal operation mechanism of the two novel bioreactors was and determined through comparatively analysis on their performance under three operation modes with the goal to prolong operation period and to slow down membrane. At last, a preliminary study was carried out on membrane foulding of fabric membrane bioreactors. The results from the preliminary study are as follows:Relative high hydrophilic was identified on the smooth side of PE5μm and PE25μm non-wovens and500meshes and800meshes nylon textile fabrics. Poor hydrophilic was found on the coarse side of PE5μm and PE25μm non-wovens and both sides of PP5μm and PP25μm non-wovens which led to easy membrane fouling. One the other, muddy water flux decreased quickly on smooth side of PE25μm non-woven and500meshes and800meshes nylon textile fabrics, which was prone to membrane blocking. As such, the study selected smooth side of PE5 μm non-woven as membrane material for follow-up study.In the effluent from studied submerged fabric membrane bioreactors, supernatant CODcr was removed by91.79%and89.83%, NH4+-N was removed by86.74%and85.89%. The average effluent turbidity was3.5NTU, indicating that submerged membrane bioreactors possess good retention capacity. In addition, membrane module flux decresed sharply under non current-limiting operation mode, which lasted fot only880min. When operating under current-limiting mode, however, the system could operate for11days.If initial effluent flux of studied open fabric membrane bioreactors were set as40L/m2·h,30L/m2·h,20L/m2·h、10L/m2·h respectively, the operation period of such bioreactors could reach1day,10days,23days and35days with corresponding total treated water volume at960L/m2,5928L/m2,7920L/m2and10104L/m2respectively. Taking into consideration on operation time and total treated volume,20L/m2·h was selected as the optimal intial flux. Sludge bulking occured during device operation, during which CODcr and turbidity removal rate increased by2.01%and61.75%respectively while NH4+-N removal decreased by17.25%.In the effluent from studied submerged fabric membrane bioreactors of modified UNITANK device, supernatant CODcr was removed by92.05%and90.15%, NH4+-N was removed by92.86%and91.96%. In the modified device, supernatant flowing through member was directly discharged. As such, flushes of sludge particles on the membrane surface was reduced, which slowed membrane fouling rate and achived better retention effect. The average effluent turbidity from the device was only1.705NTU. Under the operation flux at30L/m2·h, the total treated water volume could reach12048L/m2.The transmembrane pressure of submerged membrane bioreactors was approximately8500Pa while that of open membrane bioreactors was nearly zero since several membrane modules of the latter type were directly exposed to the air. When intial flux was set at20L/m2·h, submerged bioreactors could only operate for5days before membrane foulding while opem bioreactors could last for23days. Experiments proved that given the pressure for stable effluent, lower transmembrane pressure resuled in longer operation time and slower membrane fouling.MLSS could not be detected in the effluent from modified UNITANK device with fabric membrane bioreactor which direcly discharges supernatant flowing through membrane. However, MLSS was detected at4000mg/L in effluent from open fabric membrane bioreactor. When intial flux was set at30L/m2·h, open bioreactor could operate for10days while modified UNITANK could operate for19days. The above result indicates that when MLSS concentration increases, the filtering performance of sluge becomes worse which, at last, leads to membrane fouling. As such, membrane foulding can be slowed down when no MLSS is detected in device effluent. Backwash only washed away the sludge layer on membrane surface while mechanical wash, on the other hand, effectively removes filtering cake layer on membrane surface. However, both the two measures have poor removal effect on gel layer, whose running recovery rates are only17.39%and43.48%respectively. Chemical cleaning is enough to wash off microbial fibers siltation, removing the deep pollution on non-wovens. The running recovery rate by chemical cleaning can reach86.96%. |
PDF Full Text Request