Page 36 - ro membanes

2.3 COLLOIDAL FOULANTS 19
water. If the source water contains chlorine, this colloidal iron tends to catalyze the oxidation process caused by chlorine, which in turn enhances the damage of the RO membranes even when residual chlorine in the saline source water is in very small doses (e.g., less than 0.05 mg/L).
2.3.1.2 Silica
Another colloidal fouling compound frequently encountered in brackish groundwater aquifers is silica. Total silica (silicon dioxide) in the source water consists of reactive silica, which is in soluble form, and unreactive silica, which is in colloidal form. While reactive silica is not a challenge for RO membranes, colloidal silica in the saline source water can cause sig- nificant membrane fouling. It should be pointed out, however, that elevated content of silica in colloidal form is mainly found in brackish water sources. Unreactive silica is present in very low levels in seawater and could pose a fouling challenge only when its level in the concentrate exceeds 100 mg/L.
The stability of colloids is reduced with an increase in source water salinity, and therefore, typical saline water with TDS concentration in a range of 30,000e45,000 mg/L would contain silica in dissolved and precipitated forms rather than in colloidal form. Open ocean seawater typically contains silica of less than 20 mg/L, and therefore this compound does not cause colloidal fouling of RO membranes.
However, if the source saline water is collected via a subsurface well intake that is under the influence of a brackish coastal aquifer with a high content of colloidal silica, or it is collected near an area where a silt-laden river enters into the ocean, then colloidal fouling may become a challenge. Colloidal foulants can be removed by coagulation, flocculation, sedimentation, and filtration, similar to particulate foulants.
2.3.1.3 Polymers
Another common source of colloidal fouling is overdosing or poor mixing of polymers used for conditioning (flocculation) of the saline source water prior to clarification or filtra- tion. Such problem usually occurs when the source water has low turbidity (usually less than 0.5 NTU) and low electrical charge (zeta potential lower than 10 mV) and when poly- mer is added to enhance the flocculation of such particles. Practical experience shows that addition of polymers and coagulants to saline waters of low turbidity usually accelerates par- ticulate and colloidal fouling of the RO membranes.
2.3.1.4 Hydrocarbons
The most common organic colloidal foulants are oil-product-based hydrocarbons. Such compounds are not contained naturally in most brackish waters or in open ocean seawater, and their occurrence indicates that the saline water intake area or aquifer is under influence of man-made sources of contaminationdtypically discharge from a wastewater treatment plant, from storm drains collecting surface runoff from urban areas (parking lots, industrial sites, etc.) or from waste discharges or oil leaks released by ships, boats, or near-shore oil storage tanks in port areas.
Even in very small quantities (0.02 mg/L or more), hydrocarbons can cause accelerated fouling of RO membranes. Therefore, it is desirable that the total hydrocarbon (THC) content in the source water fed to the RO be maintained below 0.02 mg/L at all times.