Page 120 - ro membanes

5,000
5.3 MICROSCREENS 103
Intake Screen Costs
Drum Screens Band Screens
4,000
Construc􏰀on Cost (in US$1,000 )
2,000
1,000
0
0 40,000 80,000 120,000 160,000 200,000 240,000 280,000 320,000 360,000 400,000
Desalina􏰀on Plant Intake Flow Rate (m3/day)
FIGURE 5.7
Construction costs for drum- and band screens.
on this graph in general, band screens are less costly than drum screens for the same size proj- ect. Fig. 5.8 presents graph indicating order-of-magnitude costs of wedge-wire screens as a function of the plant source water flow.
5.3 MICROSCREENS 5.3.1 Types and Configurations
If the filtration pretreatment system selected for the RO desalination plant is of membrane type, the types of fine screens described in the previous sections do not provide sufficiently effective removal of source water particles to protect the integrity of the membrane pretreat- ment system. Typically, microscreensdmicrostrainers (Fig. 5.9) or disk filters (Fig. 5.10)d could be used for this application.
Most self-cleaning microstrainers (Fig. 5.9) consist of screen with small openings located in filtration chamber. The source water enters on the inner side of the strainers, moves radially out through the screens, and exits through the outlet. The gradual buildup of solids on the inner surface of the screens creates a filter cake, which increases the differential pressure be- tween intake and filtered water over time. When the differential pressure reaches certain pre- set value, the deposited solids are removed by jet of backwash water. The self-cleaning process typically takes 30e40 s.
The disk filters (Fig. 5.10) are equipped with polypropylene discs, which are diagonally grooved on both sides to a specific micron size. A series of these discs are stacked and com- pressed on a specially designed spine. The groove on the top of the disks runs opposite to the
3,000