Application of pulsed UV-irradiation and pre-coagulation to control ultrafiltration membrane fouling in the treatment of micro-polluted surface water

1 A major cause of ultrafiltration (UF) membrane fouling is the accumulation of 2 microorganisms and their associated soluble products. To mitigate fouling the 3 application of pulsed short-wavelength ultraviolet (UVC) light (around 254 nm) within 4 the membrane tank together with pre-coagulation was investigated. In mini-pilot-scale 5 tests carried out in parallel with conventional pre-treatment (CUF), the impact of pulsed 6 UV (CUF-UV) at different UV irradiances and fluxes on the increase of trans- 7 membrane pressure (TMP) was evaluated and explained in terms of the quantity and 8 nature of membrane deposits in the membrane cake layer and pores. 9 The results indicated that at a flux of 20 L.m -2 .h -1 , the pulsed UV (1 min within 31 10 min cycle) at 3.17×10 -2 W/cm 2 prevented any measureable increase in TMP over a 11 period of 32 days, while there was a fourfold increase in TMP for the conventional pre- 12 treatment. For the CUF-UV system the concentration of bacteria and soluble microbial 13 products was much less than the conventional CUF system, and the cake layer was 14 thinner and contained less biopolymers (proteins and polysaccharides). In addition, the 15 pores of the CUF-UV membrane appeared to have less organic deposits, and 16 particularly fractions with a high molecular weight (>10 kDa). 17 At a lower UV irradiance (1.08×10 -2 W/cm 2 ), or higher flux (40 L.m -2 .h -1 ) with the 18 same UV irradiance, there was a measurable increase in TMP, indicating some fouling 19 of the CUF-UV membrane, but the rate of TMP development was significantly lower 20 (~50%) than the conventional CUF membrane system. Overall, the results show the 21 potential advantages of applying intermittent (pulsed) UVC irradiation with 22 coagulation to control UF membrane fouling.

was fed into a constant-level tank to maintain the water head for the membrane tanks. 115 An optimal coagulant dose of 0.15 mM Al2(SO4)3 (calculated as Al), corresponding to 116 a near zero zeta potential of the resulting flocs, was continuously added into the rapid For the CUF-UV tank, the UVC lamp with a quartz sleeve was suspended at the 134 bottom of the membrane module. Two lamps were used at different times in the 135 experiments (details below), with a nominal power rating of 10 W and 5 W (Jeneca, 136 radiometry as 3.17×10 -2 W/cm 2 and 1.08×10 -2 W/cm 2 at 5 mm distance from the light 138 source. During the period of experimentation each lamp was operated on a cycle of 1 139 min on and 30 min off, corresponding to the lamp being on during the membrane 140 backwashing; this ensured that the water in the membrane tank was actively mixed 141 while being irradiated by UVC light during the backwash process.

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The experiments were carried out in 3 phases in which the flux and UV irradiance  , and to make sure that the EPS was not released from bacterial 156 cells. The method is described briefly as follows. The sludge suspension and cake layers 157 were first dewatered by centrifugation (Model 5417C, Eppendorf, Germany) in a 50-9 mL tube at 3000 g for 5 min. The sludge pellet in the tube was re-suspended in 20 mL 159 phosphate buffer saline (PBS) solution, sheared by a vortex mixer (Vortex-Genie® 2, 160 Mo Bio laboratories, Inc., USA) for 15 min, ultrasonically treated (Nusonics, USA) for 161 3 min, and heated to 80 o C in a water bath for 30 min. The mixture was centrifuged at 162 10000 g for 15 min. The supernatant solution was collected for EPS analysis.

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After the membrane surface was wiped with high pressure tap water and a sponge,      contrast to the lack of any measurable increase during the first phase (~ 0 kPa/day).

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After the initial, proportional rise in TMP owing to the change in flux through the clean 241 membrane (~4.5 kPa), the subsequent increase in TMP was more gradual, but less than 242 the rise for the CUF membrane; for the period between days 34 and 45 the rise in TMP 243 was approximately 3 kPa for the CUF-UV and 6 kPa for the CUF membranes. After 244 physical cleaning of the membranes at day 46, the TMP of the cleaned CUF membrane 245 was slightly higher (~4.9 kPa) than that after the first wash (~4.2 kPa), suggesting some 246 13 increase in deposits within the membrane pores. In contrast, the initial TMP for the 247 CUF-UV membrane at the beginning of phase 3 was not measurably different to that at 248 the beginning of phase 1 for the new membrane (2 kPa).

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For the third phase (days 46 to 63), the membrane flux was decreased back to 20 250 L/(m 2 .h), but the UV irradiance was reduced by replacing the 10 W lamp with the 5 W 251 lamp. For the CUF membrane the rate of change of TMP was the same as in phase 1, 252 as expected, while the TMP in the CUF-UV system was observed to steadily increase,      were sufficiently minor that they had no measurable impact on the initial TMP.