Sodium bisulfite dechlorination

Chlorine addition may be a prerequisite to sanitize the RO RW supply line and oxidize any organics (followed by dechlorination using sodium bisulfite after the MM filtration stage). Where chlorine is required, it is usual to provide a 20- to 25-minute contact period by means of a temporary storage tank. This is followed by a repressurization pump system. 

Any FRC that may be present in the concentrate is known to be toxic, and can have severe impacts on marine hfe. However, following dechlorination with sodium bisulfite, the level of free residual chlorine in the concentrate is often quite low, and quickly decreases after discharge as it dissipates and degrades (Lattemann and Hopner 2008). Chlorine also has potential to form halogenated compounds, and although these can be dangerous to marine life, their concentrations are often well below the FRC concentrations, and hence considered less toxic (Lattemann and Hopner 2008). 

Chlorine can be removed from RO feed water using sodium bisulfite or carbon filtration (see Chapters 8.2.4 and 8.1.4, respectively). As discussed in Chapter 8.1.4, carbon in carbon filters can aide the growth of microbes so carbon filtration is typically not recommended for dechlorination of RO feed water unless the concentrations of organics is high enough to warrant its use, or if the dosage of sodium bisulfite is too low for accurate control. 

Sodium metabisulfite is the most commonly used technique to dechlorinate RO influent. In water, the sodium metabisulfite forms sodium bisulfite ... 

J, negative impact of chlorine containing effluents on receiving water ecosystems thus, effective dechlorination using sodium bisulfite (NaHSOs) solutions is increasing the salinity of effluents ... 

When no dechlorination chemical was added, the chlorine concentration decreased from 1.05 to 0.95 mg/L after 1000 feet (Fig. 2). This indicated that only a small amount (0.1 mg/L) of the chloramines dissipated through chlorine demand of paved surfaces. Sodium bisulfite, sodium sulfite, ascorbic acid, and sodium ascorbate neutralized all detectable chlorine to below 0.1 mg/L within 2 ft downstream of the mixing hose (approx 2 s). Sodium thiosulfate neutralized more than 80% of the chlorine within 2 ft. However, chlorine concentrations decreased below 0.1 mg/L (the discharge limit in most states) after about 500 ft (elapsed time 3 min, 2 s). Calcium thiosulfate neutralized 60% of the chlorine within 2 ft and neutralized 90% of the chlorine after 1000 ft (elapsed time 7 min, 10s). 

To eliminate residual free chlorine from hquid, granular activated carbon adsorption or chemical reduction (with reducing agents, such as sulfur dioxide, sodium bisulfite, and sodium metabisulfite) are the most common processes for dechlorination. Ultraviolet (UV) irradiation process is gaining wider acceptance as a dechlorination process (30,45,46, 60,61). 

Chlorine has been added to the feedwater upstream of reverse osmosis pretreatment. However, since chlorine will depolymerize the polyurea membrane barrier layer in the spiral wound element, with subsequent loss of desalination properties, the chlorine is removed in the pretreatment system dechlorination basin. This removal is chemically accomplished by the addition of sodium bisulfite. The chlorine level in the influent and effluent to the dechlorination basin is continuously monitored. The feedwater is then transferred from the dechlorination basin to the cartridge filter feed pumping station by gravity flow and it is then pumped to the cartridge filters. 

Due to chlorines deleterious effects on polyamide membranes, it [and more specifically, free chlorine (i.e., hypcochlorite, + hypochlorous acid + chlorine gas + trichloride ion)] must be removed to prevent contact with the membranes. Dechlorination is relatively simple, typically using either sodium bisulfite to chemically remove free chlorine or carbon filtration to catalytically remove chlorine (see chapter 8.2.3. and 8.1.4, respectively). 

Chlorine (or other disinfectant) is required to minimize the potential for fouling the membranes with microbes (see Chapters 8.2.1, 8.2.2, and 8.5.2.1). Once membranes are fouled with microbes, it is very difficult to remove them. A free chlorine residual of about 0.5 to 1.0 ppm in the pretreatment system is desirable. Feed water to the RO must be dechlorinated prior to the membranes because the membranes are sensitive to oxidizers, which will degrade the membrane. Sodium bisulfite is the preferred method to dechlorinate unless the RO feed water has a high organic concentration, in which case, carbon filtration at a flow rate of 2 gpm/ft is recommended. (see Chapters 8.1.4 and 8.2.3) Sodium metabisulfite is typically about 33% active, and the stoichiometic dosage of sodium metabisulfite is about 1.8 ppm per ppm free chlorine. So, the stoichiometric dosage of 33% active sodium metabisulfite is 5.4 ppm. For safety, a factor of 1.5 is used to increase the dosage of sodium metabisulfite to ensure complete elimination of free chlorine. 

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