Sodium Hypochlorite Dosing & Calcium Deposits 2

[tipp79]

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

 

[bimr]

You can have scaling if you have high water hardness. If you are using PVC piping, the scale probably will not stick to the PVC.

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.

 

[djs]

Most bleaches contain an excess of caustic soda. For that reason they have a high pH. Adding bleach to the water stream will then raise the pH and the CSI or calcium saturation index. When the CSI goes too high scaling will occur.

 

[semo]

The injection point of the sodium hypochlorite will scale and plug. Just a matter of how soon/often. Doesn't matter what material it is.

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.

 

[QualityTime]

The water does not have to be "hard" for scaling to occur. It will stick to any pipe. Scaling will occur in the pipe. I designed a 40 usmgd direct filtration plant (ultimate size 80 usmgd) in the Toronto area. It has been in operation for 13 years. We used sodium hypochlorite for dosing for zebra mussel in the 1.5 mile 84" diameter intake using carrier water in a carrier line. We also use the sodium hypochlorite for disinfection. The plant uses sulphuric acid for pH control to ensure low aluminum residuals in the treated water. I decided to use the sulphuric acid to preacidify the carrier water to lower the pH so that after the sodium hypochlorite was added the reusltant carrier water had a pH less than when it started. Therefore there is no calcium carbonate precipitation anywhere!!!!

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".

 

[QualityTime]

To me, using a water softener in the scale of a municpal water treatment plant is not the best approach for the following reasons:

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 ;-)

 

[bimr]

NaOCl dissolves and chemically breaks apart in water.

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.

 

[QualityTime]

I am well aware of mixing acids and sodium hypochorite. You have to think out of the box to get around this problem. It is all about safe pH ranges to work with. There is no chlorine gas generation

 

[tipp79]

Thanks for everyone's advice. Much appreciated!

 

[ch81pc]

We operate on softened carrier water on plants up to 160 Mg/d. Scale will form at the point where hypo meets unsoftened water forming calcium or magnesium carbonates due to the pH of the hypo - If your total hardness is <40ppm you shouldbe able to use un softerned carrier water without too many problems. The other thing key thing you can do is ensure a high volume of carrier water to ensure the hypo/carrier mix is always <9 pH units. Softening may be your safest bet as realsing you have an issue later on and trying to retro fit would be costly. Maintaining a softener and buying the salt accounts for around 4% of our total OPEX at the 160Mgd plant. For us, savings in terms of labour, predictable performance and reduced down time is well worth it.

 

[QualityTime]

The cost of using sulphuric acid is peanuts compared to trying to soften water . There is no ".....you should be able to use unsoftened carrier water without too many problems...." qualification. All piping in the entire system is clean all of the time

 

[QualityTime]

One other thing, there is no handling of salts, no calcium carbonate sludge from the softening operation, no mechanical mixing. If you do it right you have a metering pump for the 93% sulphuric acid and for the sodium hypo and a passive non mechanical mixing system. All are liquids. Sulphuric acid is commonly used in washing beer bottles in large breweries. I would not be suprised if the cost of using the sulphuric acid approach is fraction of a percent as opposed to the 4% OPEX quoted in the above example. Sulphuric acid is very cheap.

 

[ch81pc]

We use ion exchange No sludge.

 

[QualityTime]

The Lake Ontario water I conditioned at my water treatment plant had an alkalinity of 110 mg/L as CaCO3, a hardness of 135 mg/L as CaCO3 and the pH could vary from 7.9 to 8.3. Water temperature could vary from 20 oC to 4 oC.

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"?

 

[QualityTime]

Sulphuric acid is also commonly used for pH adjustment of the effluent before it is sent to the city sewer system

 

[bimr]

Sulphuric acid is not commonly used to wash beer bottles. See the attached reference for a list of chemicals that are used.