Water. desalination + reuse

TECHNOLOGY May-June 2014 | Desalination & Water Reuse | 31 | responsible for the fouling rate observed in practice. This was supported by the measurement of MFI with membranes of different pore sizes varying from 0.8 µm down to 0.05 µm for RO feedwater, which resulted in respective MFI values increasing from 4 to 4,500 s/L 2 . Consequently, the MFI-UF test with UF membranes was developed to capture these smaller particles. Brackish water measurements with the MFI-UF test using 13 kDa molecular weight cut off (MWCO) UF membranes (see Figure 5) demonstrated that the cake/gel formed on the membrane surface was quite compressible (see Figure 6). Due to this compressibility, accurate prediction of fouling in RO was not possible using the new MFI-UF test in constant pressure mode. Hence, the MFI-UF test was developed in constant flux mode, whereby pressure increase to maintain constant flux over time is recorded. The fouling potential I is derived, from the slope in equation 2, and converted into MFI (Equation 1). The MFI-UF constant flux equipment uses flat 25 mm diameter UF membranes (see Figure 7). Filtration flux can range between 10 L/m 2 h to 300 L/m 2 h. Water from the North Sea tested with 10 kDa and 100 kDa membranes showed rather high MFI and a remarkably strong dependency on flux. 10 kDa membranes gave MFI values 4-5 times higher than 100 kDa, which clearly indicate that small particles dominate the fouling potential (Figure 8). The dependency of MFI on flux, means that to accurately predict particulate fouling in RO systems, the MFI should be measured at a flux similar to a RO system (close to 20 L/m 2 h) or extrapolated from higher fluxes. PrEdiCTiNG PrEssurE iNCrEasE iN rO sYsTEms Generally, RO desalination plants operate at constant flux to meet production requirements. Changes in feedwater temperature are compensated for by adjusting feed pressure. Similarly, fouling resulting in an increase in membrane resistance is compensated for by increasing the feed pressure and hence net driving pressure (NDP). In this case, increase in the NDP can be predicted through equation 2. However, for accurate prediction a correction factor, deposition factor Ω has to be incorporated. Ω takes into account that not all particles passing the membrane surface (in cross flow) deposit and remain attached. Note: Osmotic pressure enhanced fouling is not accounted for in this equation. Consequently the pressure development might be under-predicted 14,15 . Based on equation 3, a theoretical "safe MFI" can be calculated, assuming, eg, an allowable increase in NDP of 1 bar in 6 months. Figure 9 illustrates MFI calculated as a function of the deposition factor Ω at a flux of 10 to 20 L/m 2 h, which is commonly applied in seawater RO. "Safe MFI" values are heavily dependent on the deposition factor, emphasizing the need to determine deposition factors in full scale and pilot plants. An indication of the deposition factor can be obtained by measuring the MFIfeed in feed water and MFIconc in the concentrate and applying equation 4, which is based on a balance for MFI: TECHNOLOGY Equa on 3 Equa on 4 Figure 11: Relation between MFI-UF (constant flux) and TEP 10 kDa 16 Figure 10: Effect of pre-treatment on MFI-UF in a seawater pilot plant using PES test membranes of 100, 50 and 10 kDa size 10 . MFI-UF s/L 2 24,000 20,000 16,000 12,000 8,000 4,000 0 2 2 1 1 TEP 1 0kDa (mg X eq /L) -2 0.0 0.4 0.8 1.2 1.6 y=12911x +1352.3 R 2 =0.669 (b)

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