Page 37 - ro membanes

20 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT
It should be pointed out that except for DAF clarification, which can reduce THC to less than 95% of their source water level, all other pretreatment systems provide very limited (5%e15%) hydrocarbon removal. Hydrocarbons cause practically irreversible fouling of the RO membranes, and if saline water with hydrocarbon levels over 0.1 mg/L is fed to the RO membranes, they will be permanently destroyed within 1 to 2 h after the hydrocarbons reach the membranes.
2.3.2 Measurement Parameters and Methods
Silica, colloidal iron, and manganese are measured by standard laboratory tests (APHA, 2012). Usually colloidal (also referred to as unreactive) silica is determined by measuring total and reactive silica in the source water.
Both oil & grease and THC are used as measurement parameters for hydrocarbon content in the saline source water. However, measurement of THC concentration is preferable because it could be completed continuously using online monitoring analyzer, which can detect THC content at levels significantly lower than the threshold level that can cause per- manent fouling of the RO membranes. For comparison, oil and grease can be measured by laboratory analytical methods only and therefore is not suitable for continuous monitoring of hydrocarbon content in the source water.
Most desalination plants using surface water sources (e.g., saline river estuary or ocean water) are equipped with online THC analyzers because they allow for practically instanta- neous detection of elevated THC content in the source water, which in turn permits the auto- matic shutdown of the desalination plant intake facilities as soon as THC concentration approaches the critical threshold level of 0.02 mg/L.
2.3.3 Threshold Levels of Colloidal Foulants
Table 2.2 presents the threshold levels of key colloidal foulants. If the saline source water contains colloidal foulants exceeding the threshold levels in this table, the RO membranes processing this water will be exposed to accelerated and potentially irreversible fouling.
TABLE 2.2 Water Quality Parameters for Characterization of Colloidal Foulants Source Water Quality
Parameter (mg/L)
Iron Manganese
Silica
Total hydrocarbons
Pretreatment Issues and Considerations
If iron is in reduced form, RO membranes can tolerate up to 2.0 mg/L.
If iron is in oxidized form, concentration >0.05 mg/L would cause accelerated fouling.
If manganese is in reduced form, RO membranes can tolerate up to 0.1 mg/L.
If manganese is in oxidized form, concentration >0.02 mg/L would cause accelerated fouling.
Concentrations higher than 100 mg/L in the concentrate may cause accelerated fouling. Concentrations higher than 0.02 mg/L would cause accelerated fouling.