Page 35 - ro membanes

18 2. MEMBRANE FOULANTS AND SALINE WATER PRETREATMENT TABLE 2.1 Water Quality Parameters for Characterization of Particulate Foulants
Source Water Quality Parameter
Turbidity (NTU)
Silt density index (SDI15)
Total suspended solids (mg/L)
2.3.1 Description
Pretreatment Issues and Considerations
Levels above 0.1 NTU are indicative of high potential for fouling. Spikes above 50 NTU for more than 1 h would require sedimentation
or DAF treatment prior to filtration.
Source seawater SDI15 levels consistently below 2 (year-around)
indicate no pretreatment is needed. SDI15 > 4dpretreatment is necessary.
This parameter is needed to assess the amount of residuals generated during pretreatment.
TSS does not correlate well with turbidity when it exceeds 5 NTU.
2.3 COLLOIDAL FOULANTS
Colloidal foulants are inorganic and organic compounds that naturally exist in suspension and may precipitate on the RO-membrane surface, thereby causing membrane flux to decline over time. Colloidal solids have a particle size of 0.001e1 mm. For prevention of colloidal fouling, RO-membrane manufacturers usually recommend a feed turbidity of less than 0.1 NTU and an SDI15 lower than 4.
Metal oxide and hydroxide foulants most frequently encountered in brackish and seawater desalination are iron, manganese, copper, zinc, and aluminum. Typically, open ocean seawater contains very low levels of these metal foulants and, therefore, if such fouling is encountered on the membrane elements, the usual sources are overdosing of coagulant (iron salt) or corrosion of pipes, fittings, tanks, and other metal equipment located upstream of the RO system.
2.3.1.1 Iron and Manganese
Iron and manganese fouling may occur if source water is collected via subsurface intake from a brackish aquifer with high content of these metals or from a coastal aquifer, which is under the influence of fresh groundwater that contains high levels of these compounds in reduced form (i.e., iron of more than 2 mg/L as ferrous or manganese of more than 0.5 mg/L).
If iron and manganese are in reduced form and they are below 1.0 and 0.1 mg/L respec- tively, then they can be removed by RO membranes without causing accelerated fouling. However, if iron and manganese are in oxidized form, their levels should be reduced below 0.05 and 0.02 mg/L, respectively, to prevent mineral fouling of the membranes.
In addition to naturally occurring colloidal matter, iron colloidal fouling on the surface of RO membranes may be caused by corrosion of upstream piping and equipment or by over- dosing or poor mixing of iron-based coagulant used for conditioning of the saline source