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Hypochlorite scaling

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tjmurf

tjmurf

Specifier/Regulator
Apr 25, 2003
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Hi,
I was wondering if anyone has had any experience using vacuum injection as a method to supply Hypochlorite? I would like to utilize some of the existing CL2 components while converting from Gas to Hypo. W&T offers a chlorinator for such use, LVN-2000.
In reading the literature, the pit fall seems like the water hardness can scale the injector.
My question is:
If the present injector is using gas without scaling,will there be a problem with Hypo?
The water hardness >400

tjmurf
 
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Don't think that you can use vacuum. Hypochlorite is feed as a liquid.

Consider the use of a peristalic pump. That avoids some of the problems.
 
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Hypo will degas very easily under vacuum and more so the hoter the ambient temperature. May be an issue even if using a peristaltic or diaphragm pump if the suction lift is too high. Helpful to keep suction length to a minium and/or to have the storage tanks at a higher level than the pump.
 
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You will get scaling with using sodium hypochlorite and diluting it with carrier water. Sodium hypo has a pH of around 11?? and water typically has a pH of 7.5-8. If the the pH of the resultant mixture is greater that 7.5-8 you have shifted the equilibrium and the water is now oversaturated with respect to calcium carbonate and you will get calcium carbonate precipitation very very quickly in your lines and your injectors

Chlorine gas on the other hand has a pH of 4. When mixed with water the resultant pH is less than 7.5-8 and the water is now undersaturated with respect to calcium carbonate and that is why you do not get calcium carbonate precipitation.

I designed a large water municipal water treatment plant in the Toronto area 13 years ago and we used sodium hypochorite for zebra mussel control for the 1.5 mile long intake. Scaling was a big concern for me in the sodium hypochorite carrier lines at at the intake diffusers. Part of the main water treatment process involved the use of sulphuric acid. I have designed municpal water softening plants before and I used the reverse logic by preacidifying the carrier water with sulphuric acid so that when the sodium hypochorite was added the resultant pH of the carrier water was around 6. Fast forward...absolutely no scaling in the carrier piping, or at the intake diffuser and best of all there are no zebra mussels inside the intake...but everywhere else outside the intake
 
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One would think that adding acid to a chlorine solution is particularly dangerous and hazardous to personnel since adding acid will drive chlorine out of solution and generate chlorine gas. That practice defeats the whole reason for the use of hypochlorite solution.

You should stop this practice immediately.

If you want to stop the scaling, install a water softener on the dilution line.
 
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There will be no chlorine gas release if you preacidify the water. We are bringing the water from pH 7.5 down to 4.5 by adding sulphuric acid and when it is thoroughly mixed we THEN adding sodium hypochlorite to bring the pH up to 6 and then it is pumped 1.5 miles out to the head of the intake in a 4" line. Coca cola has a pH of 4.5. I hired a University of Toronto chemical engineering professor way back then to do the calculations for me and that question was posed by the client. The municipal water treatment plant is rated for 136 MLD so this is simple solution using simple chemistry. The quantities I am using is probably a lot

With a water softener you will still get scaling but that may be your only alternative because of the scale of the application.
 
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Let me reword my last post....There will be no chlorine gas release if you preacidify the water. We are bringing the water from pH 7.5 down to 4.5 by adding sulphuric acid and when it is thoroughly mixed we THEN adding sodium hypochlorite to bring the pH up to 6 and then it is pumped 1.5 miles out to the head of the intake in a 4" line. Coca cola has a pH of 4.5. I hired a University of Toronto chemical engineering professor way back then to do the calculations for me and the quantities of dilution water I needed. The question of offgassing was brought up and that is why we hired the professor. The municipal water treatment plant is rated for 136 MLD so this is simple solution using simple chemistry with the chemicals already on site. The plant has been in operation for 10 years and there is absolutely no pipe scaling or chlorine gas issues. I didn't have internet back then but if you go to this site you will see a discussion on it.

With a water softener you will still get scaling but that may be your only alternative because of the scale of the application.
 
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The chemistry professor: "The second answer to 'why people mix bleach and vinegar' is that people don't know any better or underestimate the risk."



It is common knowledge that NaOCl solutions degrade when the solution pH falls below 6 and the chlorine-producing reaction becomes unstable. You are wasting chemical.

"Avoid situations that would lower the pH of the solution. A pH value between 9.5 and 10.5 produces the most stable solution. Excess acidity will enable the hypochlorite to form chlorine gas and vapourise from the solution."



Use of a water softener is common in this application:

10. NOTE: Scaling has the potential to occur when hypochlorite is injected into a dilution stream, which is subsequently piped to the point(s) of application. The sodium hypochlorite solution is at a high pH, due to the use of caustic soda (sodium hydroxide, NaOH) in the manufacture of the sodium hypochlorite. The addition of this high pH hypochlorite solution to the dilution water, may increase the pH of the dilution water (if the source water pH and it’s corresponding alkalinity is not of sufficient strength to buffer the addition of the hypochlorite) to a level which promotes scale formation of calcium and magnesium based minerals. When the dilution water source has pH values above 9, with hard water characteristics, and moderate to low alkalinity values, scale will form on the inner aspects of the downstream pumps, piping, and valves. Passing the dilution water through water softeners prior to the addition of the hypochlorite solution, and/or pH adjustment to keep the pH below 8.9 to 9, and feeding the concentrated hypochlorite solution directly into an "open point (channels, etc)" of application have proven to be solutions to scale formation. Concentrated solutions of sodium hypochlorite pumped directly into "closed points of application (such as pipes) may experience plugging/scaling at the junction of the two pipes.



"HAZARDOUS MIXTURES WITH OTHER LIQUIDS, SOLIDS, OR GASES: Reacts violently with acids liberating chlorine gas."
 
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Actually the most stable pH for sodium hypochlorite is 11 and greater. That is why it is manufactured and comes delivered from the plant that way.

I don't smell any chlorine gas....nor has anyone else for the last 10 years. No clogging of the diffuser and no zebra mussels either. No one has complained about wasted chemical. We even have chlorine leak detectors in the room. The process is a continuous process. I am well aware of what W&T and the disinfection properties of sodium hypochlorite at different pH's and I am well aware of why they elevate the pH of sodium hypochorite in the manufacturing process which is why we hired a chemical engineering professor in the first place. It is all about the quantities and concentrations. I think what you may not be considering is the amount of sodium hypo being used, the dilution water quantities, what is being added to what, the size of the mixing tankage that is employed,the time element involved in the process and the fact that water even at low pH's has a certain solubility. It does not all go up into the air immediately. Careful thought has to be given to all of these factors when engineering something. That is why professionals are hired. Otherwise we would all hire a recently graduated engineer with his school texts in hand to give the industry solutions. I am not saying this is the solution for this problem because we don't know enough about it. It is a comment and it will be up to the questioner to decide if there is any merit in this approach and he will then have to decide if he wants to hire someone to do the calculations and decide what equipment is required.

If I read correctly, the water being discussed here has a hardness of 400 mg/l as CaCO3. This is extremely hard water. Water softeners will work but it does not remove all of the calcium carbonate hardness. How much precipates is dependant on pH and if the water is over saturated or undersaturated with resepct to calcium carbonate or magnesium hardness at the initial and final pH.



 
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Bimr, I would add one more comment when looking at the graph you referenced in the W&T paper. I think the graph says it all. At pH=4, all of the chlorine exists as 100% hypochlorus acid (HOCl).....NOT 99% hypochlorus acid (HOCl)sitting in solution with 1% chlorine gas (Cl2)being discharged into the atmosphere (or in extreme case 0% hypochlorous acid in solution and and having a chlorine leak with 100% chlorine gas (Cl2)being discharged to the atmosphere. The text goes on further to say "At extreme dosing it is possible to lower the pH to the point where the chlorine gases off." To me that means the receiving water is less than pH=4 and that is why they do not publish the graph with the pH being less than 4.
 
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Since this post is in the waste forum, one would assume that the use of an ejector may be avoided by changing the design of the system. The use of an ejectors is more common with water mains.

Consider increasing volume of dilution water so that the hypochlorite solution will have less effect on the combined diluted solution. A more dilute solution will have a lower pH.

Also consider minimizing the suction and discharge piping lengths.



 
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I think the reason why they are considering the use of the LVN-2000 is because with conventional metering pump piping systems the have a tendancy to collect off gas and it air locks the metering pump at the back pressure valve. They then have to back off the back pressure valve to let the bubble pass to unlock the pump. This is a common complaint with metering of sodium hypochlorite

It has been my experience that you will still get scaling at the point where you have neat sodium hydroxide meeting with your water. It will occur with an eductor and it will occur using a submerged diffuser. Can you live with dosing the process using neat sodium hypo dropped onto the surface of the water and have something like a water champ sitting in the channel to provide the mixing. That way you will never have calcium carbonate buildup
 
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Gentlemen,
Your discussions have been most informing. Let me start off with the reason why the question was posted. At present, we are using gas to chlorinate two waste water effluents. We are entertaining the idea of changing over to hypo. The water delivery system is already in place and the changeover would be VERY simple . I want to utilize as much of the existing equipment as possible.

In the spirit of keeping this discussion going how about this! With the stated hardness, how often would one need to clean the ejector, once or twice a month? If this is possible it is simply a maintenance issue. If not, excavation will be necessary to install the necessary piping to the point of application(s), The existing delivery piping is 2” PVC.

Thank you again for your comments!
 
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Hi, I don’t think anyone will be able to give you an answer other than the eductor and dilution line will most likely will plug up. It depends a lot on if the dilution water has a lot of alkalinity in it because the more alkalinity in the water the more it will resist pH changes (i.e. buffering capacity of the water), how much you are dosing and how much dilution water you are using. The answer is not that easy because believe it or not it is not a well discussed topic and you have to have a chemical background to do proper calculations. If you don’t understand what buffering capacity means I will give you the following example:

In northern Ontario we have a lot of small lakes. The lakes have very little alkalinity (i.e. 5-7 mg/l as CaCO3) aka “no buffering capacity” and the worry was that “acid rain” was causing the pH in the lake water to drop. But why don’t we hear about acid rain causing the pH in Great Lakes water, Lake Ontario for example, to drop? It is because it Lake Ontario water has a lot of buffering capacity. Lake Ontario water has a hardness of pH ranging from 7.5-8 depending on the time of the year, Hardness= 125 mg /l as CaCO3 and alkalinity = 110 mg/l as CaCO3.

We heard of a water plant in eastern Ontario where they were using carrier water dosed with sodium hypochlorite and conveyed in had a 400 ft long 2” diameter line to the head of the water plant intake for zebra mussel control. It clogged SOLID with calcium carbonate in a three month period. The characteristics of the St Lawrence River water at that plant (it really is Lake Ontario water) is about 7-7.5 pH, Hardness= 125 mg /l as CaCO3 and alkalinity = 110 mg/l as CaCO3 (the numbers are close enough for our purposes). At my plant we were mandated to use sodium hypochlorite and not gaseous chlorine and in order to not encounter the same problem I dreamt up this solution and this was well before the age of the internet. But rather than try to solve the problem myself by boxing myself to my own way of thinking…which may be incorrect. I went to others (i.e. the chemical engineering professor) to work out the numbers for me. I am a civil engineer and not a chemical engineer but I have been around the block a few times. I however provided him an envelope of design conditions that will be encountered(i.e. range of alkalinities, pH, water temperatures, dosing range, water flows etc). He came up with the how much dilution water I needed, how much acid I needed to add to the dilution water etc. I at the end of the day still questioned the professor about how he went about to get the answer before I was comfortable with the answers.

Many years ago I scanned a book titled “Alkalinity-pH Changes with Temperature for Industrial Waters”. For chemistry novices like me I use it to figure out how much the pH will change if you dose a quantity of acid or alkaline solution etc. The scanned copy is in my laptop and it is pretty accurate….I did jar tests to prove the results…I have a copy that can be sent by email if you want
 
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Just to be clear....it is not just the injector point you have to worry about....it is also the line that carries the sodium hypochorite/carrier water line
 
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