Experimental Study On EPS Concentration By Ultrafiltration And Their Membrane Separation By Forward Osmosis

The activated sludge process currently is the most-widely applied technology for full-scale wastewater treatment.However,one of the annoying issues is the large amounts of resultant excess sludge which is difficult to handle/dispose.Indeed,extracellular polymeric substances?EPS?generally accounts for 10⁴0%of the excess sludge in dry weight and are highly associated with the properties of the sludge.Hereinto,the special functional groups of EPS offer a strong adsorption capacity for heavy metal ions in wastewater.Moreover,EPS affect the dewatering property of sludge.If they can be separated from sludge,the dewaterbility and volume of sludge can be improved and reduced significantly.Meanwhile,the recovered EPS can be used as metal adsorbents.The traditional method for extracting EPS is energy-intensive and always results in secondary pollution,Moreover,the recovered EPS still have a high water content.In contrast,membrane separation provides us an alternative to avoid the above misgivings.Thus,the present study was to apply ultrafiltration?UF?and forward osmosis?dead-end flow mode and cross flow mode?membrane filtration method to explore the membrane fouling mechanism with alginate and actual EPS as contaminants.In addition,the possibility of using this technique for EPS membrane separation and recovery coupling with heavy metal ions removal through adsorption was studied and proved.As the yield and composition of EPS varied with different extraction methods,EPS obtained by CER method was specifically used for the following experiments in the present study.Metal ions can be bound with carboxylic acid groups in EPS to mitigate the membrane foulingduringUFmembranefiltration.Themitigationdegreefollowedby Al³⁺>Fe³⁺>Ca²⁺>Mg²⁺,which implies that the trivalent ions are more effective in reducing membrane fouling.The maximum adsorption capacity of EPS to Pb²⁺was 645.2 mg/g in theory and the removal efficiencies of Pb²⁺,Cu²⁺,and Cd²⁺by EPS filter cake layer were all above 80%.The results showed that the removal effciency of heavy metal ions by EPS was associated with the EPS recovery efficiency with the UF membrane and the filtration pressure from the EPS filter cake filtration adsorption process.Ca²⁺and diatomite reduced the membrane filtration resistances of EPS recovery but had no adverse effect on the removal efficiency of heavy metal ions.Moreover,Ca²⁺effectively mitigated the membrane pollution only under low filtration pressure?p₁=20 kPa?.The orientation of forward osmosis membrane affected the water flux and the flux rate declined more quickly when the active layer faced to the driving liquid side in comparison with facing to the feed liquid side.Due to the cross-linking reaction between Ca²⁺and SA,the membrane fouling during the non-clapboard forward osmosis in cross flow and dead-end flow modes was alleviated.Since the dirt layer is dense during dead-end filtration,the water flux declining rate was higher than cross flowmode and the flux drop caused by the dilution of the driving liquid was not significant.Forward osmosis clapboard caused a higher specific fouling accumulation,which led to a sharp decline in flux,and the addition of Ca²⁺did not effectively reduce the membrane fouling.