Effects Of Sludge Concentration On Membrane Fouling During Cross-flow Ultrafiltration

Ultrafiltration membranes have been widely employed in the utilization of unconventional water resources due to its excellent contaminants’retention capacity.However,membrane fouling is always a major problem encountered during the application and operation of ultrafiltration membrane.Sludge concentration is an important operating parameter,which can influence membrane fouling,via affecting sludge mixture properties,cake layer formation and permeability coefficient etc.The in-depth study of sludge concentration on the membrane fouling is of great significance for controlling membrane fouling and promoting ultrafiltration membrane technology.In this study,the effects of activated sludge on membrane fouling characteristics were systematically investigated under different transmembrane pressures(TMP)and concentrations,using a standard cross-flow ultrafiltration membrane bioreactor.A constant permeation flux of 20 L/(m~2×h)was maintained through automatic adjustment of peristaltic pump controlled by a programmable logic controller(PLC).(1)Under a fixed sludge concentration of 1000 mg/L,the effects of activated sludge on ultrafiltration membrane fouling were investigated at different TMP values of 20,40,60 and 80 kPa.Results showed that the membrane fouling rate(d TMP/dt)of activated sludge operated at 20 kPa was the lowest among the analyzed samples.Meanwhile,the d TMP/dt,the rising rates of cake layer filtration resistance(ΔRc/Δt)and the pore blocking filtration resistance(ΔRp/Δt)at 60 kPa were obviously lower than those under 40 and80 kPa.The cake layer filtration resistance(Rc)accounted more than 92%of the total filtration resistance(Rt)for all of the samples,and was positively correlated with d TMP/dt(R~2=0.9907,p<0.05).Loosely bound extracellular polymer substance(LB-EPS)was the main contaminant influenced Rc,and its concentration was both positively associated with d TMP/dt andΔRc/Δt.The recovery rates of TMP by chemical cleaning with Na Cl O(0.05%)at 20,40,60and 80 kPa were 92.4%,107.0%,113.0%and 79.7%,respectively.Results of these indicated that membrane fouling potential of activated sludge at the TMP value of 60 kPa was relatively low.(2)Under a fixed transmembrane pressure of 60 kPa,the effects of activated sludge on ultrafiltration membrane fouling were studied at the sludge concentrations of 2000,3000,4000,5000 and 6000 mg/L.Results showed that4000 mg/L was the critical sludge concentration,evidenced by the highest d TMP/dt among the investigated concentrations.The values of d TMP/dt rapidly decreased when the sludge concentration increased to 5000 and 6000mg/L,which were obviously lower than other samples.The cake layer formation was the major factor induced membrane fouling,with Rc/Rt higher than 96%for all the samples.Besides LB-EPS,dissolved extracellular polymers substances(S-EPS)were also positively correlated with membrane fouling rate.The recovery rates of TMP by chemical cleaning with Na Cl O(0.05%)at sludge concentrations of 2000,3000,4000,5000 and 6000 mg/L were 102.5%,84.50%,99.20%,110.0%and 97.7%,respectively.Results of these suggested that membrane fouling potential of activated sludge at 5000mg/L was relatively low.