Page 258 - ro membanes

11.2 PRETREATMENT SELECTION GUIDELINES 241
Such water quality already complies with the target pretreated water quality listed in Ta- ble 8.1 and, therefore, it is directly suitable for processing through the RO system without further pretreatment except for the addition of antiscalant. Therefore, the alternative pretreat- ment systems, which are most viable for this source water quality are:
• Cartridge filters only;
• Cartridge filters and single-stage dual media pressure granular media filters with a
loading rate of 16e25 m3/m2 h (7e10 gpm/ft2);
• Pressure-driven ultrafiltration/microfiltration (UF/MF) filters with a design flux of
100e120 Lmh (60e70 gfd).
In most well intakes collecting saline water, which is unaffected by fresh water runoff or fresh water aquifers, cartridge filtration is sufficient and no additional pretreatment (except for the addition of antiscalant) is needed (Voutchkov, 2010). In this case, cartridge filters mainly serve as a protection device to capture small sand or silt particles, which may be released to the source water stream when individual wells are turned on as well as to capture rust released from the intake wells or other equipment upstream of the RO membranes.
Combination of cartridge filtration and single-stage single or dual media pressure filtration is typically used only when the source water quality is expected to change seasonally or if the coastal aquifer is unconfined and hydraulically connected to potential sources of contamina- tion, which could be immobilized over the useful life of the desalination plant. Under such con- ditions, the pretreatment filters are often run without the addition of coagulant of flocculant.
In addition, pressure filters are sometimes installed to reduce the frequency of RO system clean-in-place (CIP) frequency and enhance the useful life of the RO membranes. Practical experience shows that the installation of granular media pressure filtration system versus car- tridge filters only allows to increase the time between two CIP cleanings of the RO system from once every 6 months to once every 12 months and to extend the useful life of the RO membranes from 7 to 10 years. Such trade-offs between additional capital costs for installa- tion of pretreatment filters and reduced operation and maintenance (O&M) costs for RO membrane cleaning and replacement are warranted when the cost of cleaning chemicals is relatively high or the desalination plant is at a remote location and chemical delivery would be difficult and costly.
Use of pressure-driven UF or MF pretreatment filters instead of pressure-driven granular media filters for this application is usually more costly and is only practiced if periodically the source water contains contaminants such as iron and manganese in reduced form, which are easier to remove with membrane instead of granular media filters.
It should be pointed out that the saline source water of some desalination plants with subsur- face intakes contains iron and manganese in reduced form above their threshold levels of 0.05 and 0.02 mg/L, respectively. Such conditions typically occur when the subsurface intake of the desalination plant is located near the estuary of a large river entering the saline water body (e.g., ocean, sea). In this case, to protect the RO membranes against colloidal fouling, the iron and manganese will have to be oxidized (typically by chlorination or permanganate addition), precipitated, and removed by filtration. In this case, the preferred pretreatment option is UF or MF filtration. Alternatively, pressure granular media filtration with special greensand media has to be used to remove iron and manganese below the threshold levels indicated earlier.