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6.4 ADDITION OF SCALE INHIBITORS 127
hydrochloric acid may be used if the sulfate introduced to the source water with the feed of sulfuric acid significantly affects the system design and cost.
Because source water acidification could be needed for both enhanced coagulation and prevention of mineral scaling, the actual applied dosage should be driven by the larger of the two dosages. Usually, this is mineral scaling for brackish water reverse osmosis desalina- tion applications and enhanced coagulation for SWRO applications. The sulfuric acid solution is dosed proportional to the source water flow.
Sulfuric acid is typically delivered on site in liquid form and in a concentration of 98% by weight. The density of this chemical at this concentration is 1.83 kg/L. The commercial prod- uct is usually diluted to 20% or less before its application to the saline source water to avoid/ minimize corrosion of metallic piping and equipment installed on the conveyance piping and mixing chamber structures.
Applying sulfuric and hydrochloric acids at concentrations over 25% by weight can cause accelerated corrosion of steel equipment and structures and has to be avoided by frequent checking of the application concentration of these acids, which is preferably to be set at 20% or less.
6.4.2 Other Scale Inhibitors
Sodium hexa-metaphosphate (SHMP) has been one of the most commonly used scale in- hibitors in the past but in recent years SHMP is frequently replaced by proprietary chemical formulations because of their improved effectiveness, long storage life without losing strength, microbial growth resistance while in the feed tank, ease of handling, or other rea- sons. SHMP can serve as a bacterial nutrient and because it contains phosphates, its use/ overdose could result in discharge of concentrate with high phosphorus content, which in turn can trigger algal blooms in the discharge area. Therefore, the use of this otherwise popular and effective sale inhibitor is limited.
Phosphonates such as amino-tris-methylenephosphonic acid, 1-hydroxyethyllidene-1, 1,1-diphosphonic acid, and 2-phosphonobutane-1,2,4-tricarboxylic acid have found wide application for high-temperature waters and they are usually very suitable for calcium and barium sulfate scale prevention and calcium carbonate scale inhibition.
Phosphonates decrease the precipitation rate of salts that have exceeded their solubility thresholds. These antiscalants are particularly efficient for SWRO systems with high pH oper- ation for enhanced boron removal. Besides calcium scale, other important scaling compounds that impact system operations are magnesium carbonate and magnesium hydroxide. Phosphonates can also react with and remove low levels of iron in the source water and inhibit silica fouling.
Polymeric dispersants based on polyacrylic acid and maleic acid (polyacrylates) are commonly applied as calcium carbonate scale inhibitors. They act by distorting the crystalline growth of the scales on the surface on the membranes. However, they are incompatible with coagulants used for seawater pretreatment and are, therefore, not recommended for such applications.
It is important to select the correct scale inhibitor for the specific application. For example, the presence of iron in the source water can cause precipitation and membrane fouling with