Sodium Hypochlorite Dosing & Calcium Deposits
Sodium Hypochlorite Dosing & Calcium Deposits
(OP)
Hello all,
As part of a WTW upgrade, we're changing from chlorine gas to sodium hypochlorite. The sodium hypochlorite dosing requires carrier water to maintain velocity in the dosing lines. This carrier water is final water from the WTW and as such has been dosed with sodium hydroxide. At a recent HAZOP, the water authority informed us that there had been several projects where the presence of caustic in carrier water for sodium hypo resulted in deposits of calcium in the dosing line and hence blockages.
My question is how would the having carrier water dosed with caustic for hypo dosing result in calcium deposits? The water is not hard.
Cheers,
James
As part of a WTW upgrade, we're changing from chlorine gas to sodium hypochlorite. The sodium hypochlorite dosing requires carrier water to maintain velocity in the dosing lines. This carrier water is final water from the WTW and as such has been dosed with sodium hydroxide. At a recent HAZOP, the water authority informed us that there had been several projects where the presence of caustic in carrier water for sodium hypo resulted in deposits of calcium in the dosing line and hence blockages.
My question is how would the having carrier water dosed with caustic for hypo dosing result in calcium deposits? The water is not hard.
Cheers,
James





RE: Sodium Hypochlorite Dosing & Calcium Deposits
I have experienced scaling of the PVDF injection nozzle at the nozzle tip with a very high hardness nozzle.
You can add a water softener to the carrier water supply if you expect problems.
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
We always call out for a retractable injection quill to be installed. It too will scale; but, a spare quill can be purchased and every so often swapped out. The old quill can be cleaned in a weak acid solution and then reused.
You can look up injection quills on the internet and find several sources and they can be made of PVC, stainless, or other steels and plastics.
The scaling doesn't typically occur on the inside of the main pipe, just on the end of the injector. The retractable one can be removed and replaced without taking the line out of service.
RE: Sodium Hypochlorite Dosing & Calcium Deposits
I have designed water softening plants before using hydrated lime. The logic that I used is the reverse of water softening. You just have to know the safe design boundary limits when mixing acids and bases and carrier water. I am pretty sure I am the only person to have ever done this. This is a very practical low cost solution using a "known" water treatment chemical".
RE: Sodium Hypochlorite Dosing & Calcium Deposits
1) It will still result in calcium carbonate precipitation
2) You will be adding more sodium to the water
3) Operational costs
Hire me as a subconsultant
RE: Sodium Hypochlorite Dosing & Calcium Deposits
NaOCl + H2O OCl- + Na+ + OH- + H+
The hypochlorite ion (OCl-) reacts with the hydrogen ions in the water to form hypochlorous acid.
OCl- + H+ +OH - HOCl + OH -
With a drop in pH, another change occurs, resulting in an equilibrium reaction between hypochlorous acid and free chlorine. The chlorine molecules form chlorine gas (Cl2) according
to the following reaction:
HOCl + H+ + Cl - Cl2 + H2O
This is a reaction to be aware of since chlorine gas is highly toxic. For this reason, never add acid to a chlorine solution without monitoring its pH.
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits
What were the characteristics of the water that you softened? Are you injecting your sodium hypo into a pressureized line or gravity feeding it into an "open tank"?
RE: Sodium Hypochlorite Dosing & Calcium Deposits
RE: Sodium Hypochlorite Dosing & Calcium Deposits