Membrane fouling during ultrafiltration (UF) of surface water: Effects of sludge discharge interval (SDI)

• Effect of sludge discharge interval (SDI) on UF membrane fouling was studied.
• Foulants including particles and organic matter accumulate in retentate solution.
• Sludge discharge can alleviate both reversible fouling and irreversible fouling.
• All fouling resistances involved can be reduced by decreasing SDI.
• Both membrane fouling and production rate should be considered for choosing SDI.

The impact of sludge discharge on membrane fouling during ultrafiltration of surface water was investigated in this study. Discharge sludge and without discharge during UF were compared by measuring the increase of transmembrane pressure (TMP) and the accumulation of foulants with running time was studied. It was found that turbidity and dissolved organic carbon of retentate solution apparently increased as UF proceeded. Three-dimensional excitation/emission matrix fluorescence spectroscopy analyses of raw water and retentate solution demonstrated that UF could lead to the accumulation of humic and protein-like substances. Filtration experiments showed that membrane fouling could be alleviated by discharging the retentate solution after physical cleaning due to the reduced cake layer formation and concentration polarization. In addition, effects of sludge discharge intervals on the TMP and fouling characteristics were further investigated. Results indicated that short SDI would benefit control of both reversible and irreversible fouling. However, frequent discharge of retentate solution may increase the self-use water and decrease the water production rate of the UF system, leading to an expansion of running cost. In general, SDI was a critically important factor for both membrane fouling and water production rate.