Advanced sodium bisulfite dechlorination guide
Sodium bisulfite is used to reduce free or combined chlorine before discharge, reuse, or membrane treatment. The goal is simple. Add enough reducing agent to remove the measured chlorine residual. Avoid large overfeed, because excess sulfite can affect dissolved oxygen and downstream chemistry.
Why the dose changes
The chemical dose depends on five items. These are flow, chlorine residual, target residual, solution strength, and field safety margin. A higher flow increases the daily chemical mass. A higher residual increases the required dose per liter. A weaker solution needs more pump volume. Lower purity also raises the feed requirement.
Practical design method
This calculator starts with the chlorine removal target. It subtracts the desired residual from the measured residual. It then multiplies that value by the sodium bisulfite stoichiometric ratio. The default value is 1.46 mg of NaHSO3 for each 1 mg of chlorine as Cl2. A safety factor is applied after the theoretical dose.
Using solution data
Many plants feed liquid sodium bisulfite. The strength is entered as percent by weight. Density converts the required solution mass into volume. This gives liters per day, gallons per day, liters per hour, and milliliters per minute. These values help set a metering pump.
Contact time and checks
Good mixing matters. Fast reaction does not fix poor injection. Place the injection point where turbulence is strong. Use contact time to estimate a required mixing or reaction volume. Always confirm performance with field residual tests. Adjust the pump after real samples, not only from calculation.
Best operating practice
Use fresh residual readings. Check the chemical certificate for strength and purity. Inspect pump calibration. Verify that the sample point is downstream of full mixing. Keep records of flow, residual, dose, and measured final chlorine. These records make future trimming safer and faster.
Field verification
Operators should also consider the form of chlorine present. Free chlorine often reacts quickly. Chloramines may need more attention and careful residual testing. Temperature, pH, and mixing can change field response. The calculator gives a strong starting dose, but jar testing is still useful. When permits require very low residuals, use conservative monitoring and calibrated instruments. Recheck settings after any major flow or chemical delivery change occurs onsite.