How does Sodium bisulfite reduce Chlorine in Commercial RO plants?

Reverse osmosis (RO) is now one of the most extensively used water purification processes in both the public and private sectors.

RO plants are manufactured for both large and small communities, and they are also widely used in the commercial sector, such as the food and beverage industry. Netsol Water ensures that they work smoothly because of their role in the commercial sector and in protecting public health. Many plant owners and operators still assume that normal maintenance is sufficient to keep their plants in a good working order. On the other hand, Commercial RO plant manufacturers urge for a proper pretreatment system. We routinely deal with extremely unclean water that requires extra care before usage. The stress on the RO membrane, which is the most valuable and susceptible, is reduced by pretreatment.

Pretreatment concerns

The major difficulty with today's polyamide thin-film RO membranes is the removal or annihilation of any chlorine or other potentially oxidative chemicals. Free chlorine (found in many municipal water sources) has a low tolerance for this membrane, while chloramines have a low tolerance (in other municipal water sources).Reducing-agent injection and activated carbon filtration are the most prevalent ways for breaking down chlorine.

Sodium bisulfite (NaHSO₃) is the most common reducing agent, as it preferentially interacts with free chlorine to convert it to the harmless chloride ion.

Sodium bisulfate as reducing agent of chlorine in RO Plants

For bigger RO systems, sodium bisulfite [SBS] is the most common chlorine reducing agent. Free chlorine is reduced to sodium bisulfite and hydrochloric acid by sodium bisulfite.

SBS is a reduction-capable chemical molecule with the molecular formula NaHSO₃. As a result, it's employed in water/wastewater and industrial applications to eliminate residual chlorine. It's also utilized in boiler water treatment as an oxygen scavenger. It is a preservative used in the food sector. The solution has a pH of 4.6 at 1.0 percent (by weight) solution strength and is made by dissolving solid sodium metabisulfite into water.

In order to make a 10% (by weight) SBS solution, 0.51 pounds of solid sodium metabisulfite must be added to one gallon of water. The sodium metabisulfite is commercially available in purity ranges from 97.5 to 99 percent and can be safely stored for up to six months in a dry environment.Because the SBS solution is unstable in the presence of air and reacts with both oxygen and chlorine, batches of less than 2% by weight should be used within 3 to 7 days, and batch solutions of less than 10% should be used within 7 to 14 days.

SBS Concentration in RO Plants

To add an industrial safety factor for brackish water RO systems, designers have been known to employ a dosage rate of 1.8 to 3.0 ppm SBS per 1.0 ppm chlorine. SBS must be injected far enough upstream of the RO elements to ensure a response time of at least 20 seconds.

Na₂S₂O₅ (sodium metabisulfite) + H₂O 2NaHSO₃ (sodium bisulfite)

NaHSO₃ + HOClNaHSO₄ (sodium bisulfate) + HCl (hydrochloric acid)

NaHSO₃ + Cl₂ + H₂O NaHSO₄ + 2 HCl

SBS de-chlorinating has several advantages over carbon filters, including cheaper capital costs for large systems, elimination of reaction by-products by the RO, and removal of residual SBS by the RO.

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