Reverse osmosis cartridge filters

A simplified flow scheme for a brackish water reverse osmosis plant is shown in Figure 5.24. In this example, it is assumed that the brackish water is heavily contaminated with suspended solids, so flocculation followed by a sand filter and a cartridge filter is used to remove particulates. The pH of the feed solution might be adjusted, followed by chlorination to sterilize the water to prevent bacterial growth on the membranes and addition of an anti-sealant to inhibit precipitation of multivalent salts on the membrane. Finally, if chlorine-sensitive interfacial composite membranes are used, sodium sulfite is added to remove excess chlorine before the water contacts the membrane. Generally, more pretreatment is required in plants using hollow fiber modules than in plants using spiral-wound modules. This is one reason why hollow fiber modules have been displaced by spiral-wound systems for most brackish water installations. 

FPG-type deep cartridge filtering elements of 20 pm and 5 pm filtering capacity are used in microfilters. Element ERU-100-1016 (filtering capacity 10 nm) at a pressure < 0.3 MPa is used in ultrafilters in reverse-osmosis filters element ERO-KM-100-1016 (selectivity for NaCl 95-99%) at a pressure <7.0 MPa is used. Ion-exchange filters are... 

Chlorine has been added to the feedwater upstream of reverse osmosis pretreatment. However, since chlorine will depolymerize the polyurea membrane barrier layer in the spiral wound element, with subsequent loss of desalination properties, the chlorine is removed in the pretreatment system dechlorination basin. This removal is chemically accomplished by the addition of sodium bisulfite. The chlorine level in the influent and effluent to the dechlorination basin is continuously monitored. The feedwater is then transferred from the dechlorination basin to the cartridge filter feed pumping station by gravity flow and it is then pumped to the cartridge filters. 

After chemical addition, the feedwater is routed to cartridge filters which serve to mix the chemicals which have been added upstream and to insure that any particles that may have escaped the gravity filters, such as sand or other particulate matter is removed. In general, the cartridge filters do not improve the quality of the reverse osmosis feedwater to a large degree and they are not intended as continuous duty filters. The effluent from the cartridge filters is routed to the primary reverse osmosis feed pump wet well. 

About 100 gallons/hour (GPH) are pumped from the first rinse tank through a cartridge filter and into a reverse osmosis unit. The reject stream contains 99% (59,400 mg/E) of the nickel in the feed stream with 1% (32 mg/E) remaining in the product stream. The reject stream is routed through an activated carbon column to the plating bath. The reverse osmosis product stream is combined with 5 GPH of tap water makeup, which is added to compensate for surface evaporation in the plating tank, and the combined stream is returned to second rinse tank. The waste stream (10 GPH) is sent to waste treatment which is a precipitation process. 

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