Membrane filtration constant flux mode

Membrane fouling may result in a significant increase in filtration resistance, leading to unstable filtration behavior. The pressure-driven membrane processes can be operated either with constant feed pressure or in constant flux mode. For constant pressure operation where the transmembrane pressure (TMP) is maintained at a constant value during the filtration, the flux will decline with time due to the... 

Feed pumps transfer water through 250 pm strainers to the membrane units at 1550 m /h and 5.5 bar g, as shown in Figure 4.14. The pumps maintain a constant feed water flow rate. The operation is in constant flux mode. The feed water enters the shell side of the hollow fibre and flows through the membrane pores into the fibre bore or lumen as filtrate. The membranes reject particles and colloids larger than the nominal pore size. The filtrate flows to filtered water storage tank. Both feed water and filtered water are monitored for turbidity. 

Patel et al (1994) employed a combined process of coagulation and MF to avoid a disinfection posttreatment. The coagulation step was used to eliminate phosphorus, arsenic, and viruses, to avoid fouling, decrease particle accumulation on the membrane surface, and improve backflush characteristics. MF pilot plant studies in constant permeate flux mode showed that turbidity, particles, and faecal coliforms could be removed, but TOC removal was unreliable. Crossflow MF showed no difference to dead-end filtration, and both methods were similar to or better than sand filtration. Results with coagulation and MF improved phosphorous and turbidity removal, but the process was not optimised. The treatment lead to a reduction of chlorine demand in the product water. 

The concept of the helical membrane module has been tested in a submerged membrane filtration mode with bubbling used for the membrane fouling control. Liu et al. [30] showed that the helical membrane with a twisted angle of 180° could achieve a 1.46-1.69 flux enhancement, compared to the membrane modules with 0° twisted angle, in the filtration of 500 mg/L kaolin suspension under a constant TMP of 2.8 and 3.2 kPa. The particle image velocimetry (PIV) analysis [31] showed that the tortured membrane surface of the helical membrane could generate rotational flow near the membrane surface and increase the wall shear rate. 

In concentrating WPs, DF is employed, in which water or buffer is continually added to the retentate while lactose and minerals are simultaneously removed in the filtrate, to increase WP purity [4,52,61]. This is commonly done in constant-volume mode where water or buffer is added to the retentate at the same rate as permeation. There is an optimum protein concentration in the retentate at which DF can be commenced where the trade-off between permeate flux and the number of diavolumes is balanced and only the minimum membrane area or process time is necessary [62,63]. Using 20 kDa MWCO polysulfone membrane sheets, Nilsson [18] found that, in the UF of reconstituted whey protein concentrate (WPC)-80, the relative flux reduction (RFR) increased with protein concentration and then plateaued up to about 3.2% protein concentration in the retentate. Beyond this concentration, the RFR increased sharply. Cheryan and Kuo [22] showed that at 335 kPa TMP and 50°C, the flux approached a minimum when the retentate reached about 3% protein concentration using polysulfone spiral wound membrane while the flux in the polysulfone hollow fiber is four times higher (Figure 19.1). This suggests that DF may be effectively carried out until 3% protein concentration in the retentate is reached. 

For relatively low solids content feeds, it is feasible to operate without continuous cross flow or surface shear, and this can reduce energy costs. Such applications include water treatment and pretreatment for RO desalination and reclamation. This mode of operation is called dead-end filtration or frontal filtration, and the key feature is that the deposition of retained species is allowed to grow. A t5q)ical cycle commences with a clean membrane (after backwash), and at constant flux the TMP rises according to Eq. (10.4). After a specified period (Q, or at a predetermined AP iax. the flux is stopped and the deposit is removed by backwashing and (usually) vigorous aeration. The contents of the membrane... 

The filtration unit operated consistently and reliably over a brief testing period. The unit was easy to operate and maintain. The filtration unit operated in a batch mode for six hours each day, for six days, and processed approximately KXX) gallcxis of feed per day. Over the six day test period, permeate flux was relatively constant. Based on a total membrane area of 3(X) ft for the system, the permeate flow rate for the four module filtraticxi unit averaged 2.6 gpm. Excessive fouling of the membrane, necessitating frequent cleaning or regeneration, was not encountered. However, the... 

---