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126 6. CONDITIONING OF SALINE WATER
20% as weight and then injected into the pretreated water. Antiscalant mixing with the source water is achieved by in-line static mixer. Sometimes antiscalant is fed upstream of the cartridge filters to use them as a mixing system.
Conditions accelerating membrane scaling are high content of scaling minerals (mainly cal- cium and magnesium salts) combined with low overall mineral content of the saline water (i.e., low TDS), operation at very high RO system recovery (over 50% for seawater desalina- tion and over 65%e70% for brackish water desalination), increase in source water pH (usually pH increase above 8.8), and increased source water temperature (usually scaling rate accelerates dramatically for temperatures above 35C (95F)). The scaling thresholds listed earlier may change from one source water to another depending on the presence and concentration of compounds prone to precipitation at normal operational conditions.
In most cases, scaling could be avoided by lowering the RO system recovery. However, such operational strategy would result in reduced desalination plant productivity and may not be easy to use as a sustainable long-term solution. In the case of carbonate scale, lowering pH to 7.6 or less could inhibit its formation. However, pH decrease is not a remedy for most of the other types of scales.
It should be noted that antiscalants are intentionally designed to be biodegradable because they are removed from the RO system with the concentrate and are disposed to the receiving water body along with it. Therefore, antiscalant overdosing typically causes accelerated RO membrane biofouling. To avoid overdosing, it is recommended to apply the minimum viable dosage of this chemical. Such a dosage could be determined by pilot testing by starting at a minimum dosage of 0.25 mg/L and applying incrementally higher dosages of antiscalant un- til they reach levels of 3e4 mg/L (or higher if recommended by potential supplier of antisca- lant based on their specialized software). For most antiscalants, the optimum feed dosage varies between 0.25 and 1.0 mg/L.
It should be pointed out that most antiscalants can be oxidized by chlorine and other strong oxidants such as bromamines. Therefore, these source water conditioning chemicals should always be added after source water dechlorination. Exposure to chlorine will not only inactivate the applied antiscalant but will also break down its complex organic mole- cules into easily biodegradable compounds that would accelerate RO membrane biofouling.
6.4.1 Acids
Sulfuric acid feed upstream of RO membrane systems is commonly used for calcium car- bonate control. Calcium carbonate is the most common scaling compound in brackish water (Wilf et al., 2007) and, therefore, brackish water desalination plants are usually equipped with acid addition systems.
The addition of acid lowers the carbonate concentration by converting bicarbonate to carbon dioxide. The carbon dioxide passes through the RO membranes and is removed or used in the posttreatment system. Sulfuric acid is only effective against carbonate scale and because this chemical adds sulfates to the source water, it actually increases its calcium sulfate scaling potential.
Sometimes hydrochloric acid is used as a scale inhibitor instead of sulfuric acid. Sulfuric acid is usually preferred over hydrochloric acid because of cost and safety reasons. However,