Dechlorination 4

[redgreen]

I am working on a treatment plant that will be using sodium bisulphite to dechlorinate waste water that will be discharged to a lagoon. Does anyone have any information on the dosing rates of sodium bisulphite vs chlorine in order to effectively "dechlorinate" the water?

 

[kmm]

Dear Friend

I have no values for sodium bisulphite, but I can give you values for the five compounds used for dechlorination. Once you know the chlorine required to be removed (in mg/l), you multiply it with the following factors to get the dosages:

1. Hydrogen Peroxide 0.479
2. Sulphur Dioxide 0.901
3. Sodium Thiosulphate 2.225
4. Sodium Sulphite 1.775
5. Sodium Pyrosulphite 1.338 (Sodium Metabisulphite)

You can then convert it using the % commercial grade and specific gravity of the above chemicals

Khalid
kmehmood@amwater.com

 

[Jlevie]

Sodium Bisulfite, very common in dechlorination operations. Typically (38wt% sol'n NaHSO3, 1.3 s.g.) stoichiometry is around 1.46 mg/mg. Rule of thumb would be to oversize the dosing requirements to account for poor mixing kinetics and non-ideal reaction parameters, 5-10% oversize (the unreacted materials will react with dissolved oxygen BUT at a rate that will not significantly deplete DO in the effluent). Note that for each part of chlorine removed, you will also lose 1.38 parts alkalinity as CaCO3.
Suitable contact materials for s.bisulf. are stainless (304, 316), PVC, FRP and similar. Diaphragm pumps are prefered for metering applications.

In comparison, sodium metabisulfite is 1.34 mg/mg, and again 1.38 parts alkalinity are required.

Both materials react very rapidly with residual chlorine, both free and combined forms and contact times are very short.

Hope this helps,
John