Page 149 - ro membanes

132 6. CONDITIONING OF SALINE WATER
To protect membrane integrity, residual chlorine or other oxidants, which are not consumed by the source water impurities, are typically removed by the addition of reducing compounds (oxidant scavengers), which react with the oxidants in the seawater and create nonoxidizing side products.
The most commonly applied reduction chemical is sodium metabisulfite (Na2S2O5). When introduced to the source water, it creates sodium bisulfite (NaHSO3), which reduces hypo- chlorous acid to sulfuric acid (H2SO4), hydrochloric acid (HCl), and sodium bisulfate (Na2SO4), all of which are not oxidizing compounds:
Na2S2O5 þH2O/2NaHSO3 (6.iii) 2NaHSO3 þ2HOCl/H2SO4 þ2HClþNa2SO4 (6.iv)
Approximately, 3.0 mg/L of sodium metabisulfite is needed to remove 1.0 mg/L of free chlorine. Typically, the sodium metabisulfite dosage is optimized based on the reading of the ORP of the saline source water at the entrance to the RO system trains, which should be maintained to less than 200 mV to protect the RO membrane integrity.
Sodium metabisulfite is usually introduced to the feed water immediately after cartridge filtration on the suction side of the booster pumps, which feed pretreated source water into the RO system high-pressure feed pumps. Because the reaction between residual chlorine and sodium bisulfite is practically instantaneous, no separate mixing device is needed. Mix- ing provided by the booster pumps and high-pressure RO pumps is typically adequate to achieve practically compete removal of chlorine or other strong oxidants before membrane separation.
Overdosing of sodium metabisulfite is not recommended for two reasons: (1) after consuming chlorine and other strong oxidants in the source water, this reducing compound will react with oxygen naturally occurring in the source water and will reduce the content of oxygen in the desalination plant concentrate, which in turn may have a negative impact on the marine environment receiving this concentrate; (2) sodium bisulfite could serve as food to some of the biofouling bacteria growing on the RO membranes and, therefore, could exacer- bate membrane biofouling.
Another compound, besides sodium metabisulfite, which can be used as reducing agent, is activated carbon. Activated carbon, however, is more costly and the reaction is slower. There- fore, this compound has not found a wide use for dechlorination.
6.8 PLANNING AND DESIGN CONSIDERATIONS FOR SOURCE WATER CONDITIONING
All chemical feed systems for source water conditioning chemicals have two key componentsdstorage and feed solution preparation tanks and chemical feed pumps. These chemicals are stored on site in areas, which allow their safe loading, containment, and handlingdstorage buildings or contained storage areas.