Treatment of hard water with polyphosphates 8

[PEDARRIN2]

I have a project where there is a desire to add polyphosphates using a metering pump to control hardness and scale in a domestic water system. The system also includes water heaters.

The system is for a school which is located fairly close to to the municipal water treatment plant so it encounters swings in water chemistry. Because of this, the total hardness ranges from 124 ppm to 240 ppm. pH is fairly neutral (~7.2).

They are encountering a lot of scale formation in the hot water generator.

I have done some research on polyphosphates and found the following:

1) They are mainly used for iron control but seem to help with hardness.

2) They lose their effectiveness or may even become counter effective in hot water.

3) There may be problems with the phosphorous in the effluent water.

I personally have some experience with water softening using NaCl regeneration. It seems to work if you can accept the higer Na in the water and the disposal problems associated with NaCl in the waste water. I am not sure though about the corrosiveness of the soft water when heated. There seems to be differing opinions about that. That is why I have not pushed the softener. I do not want to exchange a scaling problem with a corrosion problem if I can help it.

Now all the manufacturer's literature for polyphosphates paint a pretty picture. I am not convinced. I would like some engineering opinions.

 

[bimr]

I don't think you are going to have much success with polyphosphate addition because your water quality is varying so much. As you water quality changes, you would want to vary the amount of phosphate and there is no practical method that can be used to automatically accomplish the addition. You also have a application where the flow is not continuous so that presents problems as well.

According to Water Quality and Treatment, polyphosphates may enhance the solubility of metals such as lead and copper, and interfere with the formation of passivating films, tending to increase corrosion in the piping system.

You should consider using a water softener and only partially soften the water instead of complete softening. You can partially softening the water by bypassing part of the flow around the water softener. A water softener will probably be easier to operate for the limited staff that is expected in a school facility.

The degree of water softening required is determined by calculation of the Langelier Saturation Index. You want to have a slightly positive LSI, no more that 0.5 unit above the saturation pH. You have not provided what the alkalinity level is in the water so it is not possible for me to calculate the LSI.

If you soften the water too much, the water will be corrosive because your pH is low. Calculation of the LSI will show that the water corrosivity increases as you remove the hardness.

The amount of salt in the wastewater from a water softener (as well as the polyphosphate) is generally not an issue in the wastewater. You can confirm that if you contact your local wastewater treatment agency.

 

[PEDARRIN2]

Thank you for replying.

The alkalinity ranges from 132 to 136.

How does softening lower the pH of the water? You are removing Ca+ and Mg+ and replacing them with Na+ so how do you get more H+?

None of the sources I have read mentioned anything about increased solubility. I will look into it more.

They do talk about forming a film on the walls to reduce scaling. I am not sure if that would decrease corrosion.

We have found a new design that does not use a metering pump so the variable concentration should not be a problem.

I have heard of "partial softening". Never used it because it did not seem to be "good engineering" ie removing something and then putting some of them back in.

How do you decide the water chemistry where the "corrosivity of soft water" and the "scaling of hard water" are both minimized?

 

[bimr]

Softening does not lower the pH of the water. Nothing in my post says that. What I said was that the LSI of a softened water will be lower than the LSI of an unsoftened water. That means that a softened water will be more corrosive than an unsoftened water.

The LSI is an indicator of the tendency of a water to scale. You want to have a slight tendency to scale, otherwise the water will be corrosive. A slight tendency to scale means that a water will deposit a light calcium carbonate scale film on the piping system and will thus protect the piping system from corrosion.

If you remove all of the hardness with a water softener, then your water will be corrosive, especially since your water has a low pH. Increasing the pH will tend to decrease the corrosivity of the water.

I did not say anything about solubility. What I said was that a corrosive water will tend to leach more metal off the piping system and not deposit a protective film (the passivating film) of calcium carbonate on the piping system.

What you want to target for with your finished water quality is a slightly scaling water as represented by a slightly positive LSI index.

Most municipalities do not completely soften the water because it is not necessary for consumer acceptance and it is also more corrosive to the municipal piping distribution systems. A municipal target would be around 110 to 120 hardness as a low point.

Partial softening is not removing something and putting it back in. If you install a bypass pipe with a preset rate valve in parallel with the water softening, you will be partial softening. If you design the bypass to pass 50% of the water, then your blended product water will have about 50% of the raw water hardness. This bypass arrangement has been used by thousands of users since the beginning of the water treatment industry. Partial water softening is very, very commonly used.

The variable water hardness that you have is a concern because there is no simple online method available (for small systems) that will automatically test the water hardness and then adjust your chemical feed system with the appropriate response.

A reexamination of your application in light of the new information (alkalinity of 132 to 136) shows that your water is slightly corrosive from 55 to 140 Deg F. with a hardness of 124 ppm and will tend to be scaling as your approach 240 ppm hardness and 140 Deg F. I have assumed that your water heater is heating to 140 Deg F. and that your calcium hardness is about 80% of the total hardness.

Since your water quality hardness varies, you are probably depositing some hardness at times and then resolubilizing at others. The net depositing of calcium carbonate on your heating system and piping depends on how much time you are operating at the higher hardness or at the lower hardness.

If the scaling situation is not something that you can live with, then the recommendation is to install a water softening system and reduce the calcium hardness to say 40 ppm as well as increasing the pH to a range of 8-8.5. This recommendation will minimize the scaling and corrosion. You should use a chemical such as sodium carbonate to raise the pH since it is relatively nonhazardous.

If you just water soften, the water is going to be corrosive because of the lower pH. The water will be even more corrosive, when it is heated. If you use just the polyphosphate, then the water is probably going to continue to scale, at least where the water temperature is the most hot (the heat transfer areas in the water heater).

To summarize, you have 2 problems with your water quality, not one:
1. High hardness
2. Low pH

If you want to research this further, 2 excellent textbooks address these issues:
1. Water Treatment by Eskel Nordell
2. Water Quality and Treatment by the AWWA.
Both of these books are probably available on bookfinder.com.

 

[PEDARRIN2]

Thank you for your post.

I have a question about the pH you are suggesting. Since this is a domestic water system, is it allowable to have a pH of 8.0-8.5 and still drink the water?

 

[bimr]

The National Secondary Drinking Water Regulations calls for a pH of 6.5-8.5.

Note that we have only been discussing treating (water softening) just the hot water. Presumably, you would be drinking only cold water or using a blend of the hot and cold for cooking purposes. The blended pH of the hot and cold waters would be lower.

Regardless, a pH range of 8-8.5 in the water is acceptable for human consumption.

 

[PEDARRIN2]

Thank you for your reply.

Most of the discussion has been the effects of polyphosphates in water heaters/boilers and whether they are better than softening the water.

Whether we would use polyphosphates or water softening, we would be treating the whole domestic water system, not just the hot water.

It makes no sense to me as a plumbing engineer to protect the water heater, although that is very important, but not protect the fixtures that use a blend of hot water, i.e. sinks, showers, etc. from the scale of hard water.

 

[bimr]

The original post indicates some concern for the hot water heater. In fact, water heater was mentioned twice.

Information that is missing includes items such as the size of the system (gpm), layout (length of pipe runs), complete water analysis, cost to repair system due to use of existing water, and project budget ($).

If you are concerned about the entire water system and desire to treat the entire system, the previous comments still apply.

The water that you have is slightly corrosive (because of the low pH) from 55 to 140 Deg F. with a hardness of 124 ppm and will tend to be scaling as your approach 240 ppm hardness and 140 Deg F. The water heater is going to scale at the heat transfer areas because of the relatively high hardness. The rest of the water system is going to corrode because of the low pH.

Steps to take:
1. Reduce the hardness to less than 100 with a water softener. (You can evaluate whether it is feasible to remove all of the hardness or partially soften.)

Benefits of partially softening the water include:
Less waste (brine in sewer)
No need for water to be totally softened.
Less expensive capital cost (water softener purchase).
Treated water is not as corrosive.


2. Raise the pH of the water so that the water is not as corrosive.

Note that you need to water soften and raise the pH at the same time. If you do one or the other, you will make the problem worse.

Polyphosphates are not the answer. Polyphosphates will enhance corrosion at low dosages. Polyphosphates will prevent calcium carbonate formation and deposition (scaling) in the system, but will not stop the scaling in the hot water heater.

You might also consider calling the local utility and finding out what their treatment consists of and confirm that the pH is accurate.

 

[kenvlach]

PEDARRIN2,
You don’t seem to fully understand bimr’s advice, but it is the best.
Softening doesn’t lower pH – your initial pH is low. Municipalities typically supply water ~7.8-8.0 pH and LSI (slightly) greater than one to minimize corrosion. Scaling and corrosion occur under opposite conditions; thus, treatment for hot water heat heater differs from cold water system.

1) Forget polyphosphates – not appropriate for your system.
2) Raise the pH of entire system to minimize plumbing corrosion.
3) Soften or partially soften the water to the hot water heater. Millions of installations.

Re the AWWA book Water Quality and Treatment. Best prices are at Amazon.com, 5th edition (1999) $65.32 (used):

http://www.amazon.com/exec/obidos/t...=lpr_g_1/102-3533511-2621741?v=glance&s=books

or the 4th edn. (1990) $19.99 :

http://www.amazon.com/exec/obidos/t...f=sr_1_2/102-3533511-2621741?v=glance&s=books

 

[PEDARRIN2]

Thank you for your response.

One of the problems we know of is that the school is located near to the water treatment facility. So they see all the swings before the water in the water distribustion system has attenuated to more stable levels. Pursuing the municipality to try to get them to do something is not an option.

It is hard to design a good system with the swings they are encountering.

I know that municipalities adjust the pH but I have never seen a user having to do it. Is that common?

I cannot treat just the water going to the water heaters for scaling. If I do not treat the cold water also, I will get scaling on the faucets, shower heads, etc. From an appearance standpoint, that would be unacceptable if a softener was already installed.

I want to use a softener since I have more experience with them. But there is a push for the polyphosphates for this system in my office.

I have found one vendor that states that his product(which never states exactly what it is, i.e. propriatary) can and has been used successfully in hot water applications. It could be snake oil, but I am not sure.

The literature compares itself to "normal" polyphosphates in that the "normal" polyphosphates degrade in hot water and his product doesn't. He does have some "testimonials" from similar situations. That is about the only reason I am still hanging onto the polyphosphate option.

Also, we had the water sampled and analyzed, the company that performed the analysis recommended polyphosphates. This company is not a vendor which sells polyphosphates so has less of a bias toward them.

Our office has a copy of the AWWA publication. The only problem is it is the 1971 edition. You can guess how useful that is.

 

[semo]

I believe Bimr's advice to be your best option. Understand that you may not be able to treat the hot water at one central location. With that in mind, you can treat the whole water supply. It only means you have a larger softener.

I've not had a lot of experience with polyphosphates and what I have seen has not had the best results so I would trust those on here that have the experience.

If you or your company are insistent on the polyphosphates, ask the chemical supplier to put his money where his mouth is. He can set up a pilot test to see how his chemical affects the problem. If it doesn't work, you can then go to the softener.

 

[PEDARRIN2]

I do not understand what you mean by "not treating the hot water at one central location".

Where else would I treat it?

I don't want to treat it downstream of the water heaters. Most softeners do not work with hot water influents.

Please elaborate what you meant.

 

[semo]

I may have misunderstood your statement "I cannot treat just the water going to the water heaters for scaling."

I saw the next sentence; but, I thought you may also have multiple water heaters at various locations throughout the building. Unless they have their own separate water feed line (which is unlikely), the hot water would require a separate softener prior to each heater.

My misunderstanding.

 

[PEDARRIN2]

The reason I made the statement is that some engineers do soften only the water going to the water heaters. Or they may only soften the water going to the water heaters for the kitchen and/or laundry but not the water heaters serving the rest of the building.

They do this as a cost savings measure in that it requires a smaller softener, but I could not accept this because at nearly all fixtures using domestic hot water, cold water also is used. This cold water would still be hard and scaling would occur at the faucets, shower heads, etc.

 

[GarySCWSVI]

PEDARRIN2: "Older" engineers soften only the water going to the water heater, as you said, as a "cost savings measure". Today's younger engineers understand that capital cost savings will typically result in higher operating costs when it comes to scale prevention by applying an ion exchange Water Softener. Therefore in most instances they will recommend ion exchange water softening to all hot water feed,softening cold water that will be blended with soft hot water and softening any of the cold water where scale formation is the concern. This results in the most positive result.

Regarding LSI. LSI does not apply to "softened water" since one of the factors required is Ca in the equation. Therefore it is not applicable to softened water. If the LSI of the raw water indicates the water is corrosive that wlll not change by softening the water. Softening does not increase corrosivity. Therefore if corrosivity is a concern then you need to treat for that in addition to softening the water.

Operating "cost savings" are the highest when both hot and cold water are softened by ion exchange. When you mix hot soft water with cold hard water the result is hard warm water. The cost of using that lesser hard water is greater than the costs related to soft water use. My suggestion is to soften the feed water to any water heater and to soften any cold water than, if not softened, will result in scale formation or will be mixed with hot soft water. That will likely mean that some of the cold water will be softened that may not need softening. If that is a concern then design the plumbing scheme to not provide soft water to those locations.

IMHO softening all water is best. Where the raw water is corrosive, and that is the concern, then treat the water accordingly.

Gary Schreiber, CWS VI
The Purolite Co.

 

[bimr]

GarySCWSVI, whether the engineer is "older" or not has no bearing on the feasibility of using a water softener. Maybe I am having a senior moment, but I do not recall ever having seen a engineering factor for the age of the engineer doing a process evaluation.

Information that is needed by PEDARRIN2 to determine the feasibility of partial or total softening includes: items such as the size of the system (gpm), layout (length of pipe runs), complete water analysis, cost to repair system due to use of existing water, and the project budget ($). Since this information has not been supplied, how can you be so sure of your recommendation?

Are you saying that "today's younger engineers" blindly apply technology without an understanding of what they are doing?

The "LSI" is a general corrosion index and does apply to softened water. Since 1980, the amendments to the NIPDWR have required the LSI or AI to be used and the reports of the values must be submitted to the state regulatory agencies. Refer to page 1074 of Water Quality and Treatment published by the AWWA.

If all of the hardness is unnecessarily removed, then the costs of chemical treatment to make the water less corrosive would be expected to increase.

"When you mix hot soft water with cold hard water the result is hard warm water" is also an incorrect statement. The predicted hardness as a result of mixing hot and cold water depends on the enthalpies of the water. If you assume that the water heater is 140 Deg. F and the raw water is 55 Deg. F, mixing 50% hot water and 50% cold water will give you a tap water of about 100 Deg. F. Therefore, mixing hot soft water with cold hard water is going to give you a water with approximately 50% of the initial hardness, not a "hard warm water".

I disagree that maximizing capital costs and maximizing operating costs (the cost of corrosion treatment) is the way to go.

The recommended approach should be a partial softening of the incoming water as well as raising the pH up so that the water is not corrosive. The extent of partial water softening that is necessary will depend on the type of water heating equipment that is installed. That information is probably available from the water heater manufacturer. The LSI is used to predict how high the pH must be raised to prevent corrosion. After startup of the system, the predicted pH should be doublechecked to ensure that the water pH is appropriate to prevent corrosion.

 

[semo]

I agree with Bimr, you do not want to remove all of the hardness. I see it increasing your construction, operating, and chemical costs.

Most systems use bypassing to retain some hardness for this and other asthetic reasons.

I don't think the LMI was very high at all, in fact, I have many municipal clients with higher values that do not complain about a scale build up. There is buildup; but, since it is not process water, it is not a problem.

If the water is too soft, you will also have complaints that users cannot wash the soap from their hands. That will become a problem. I've seen systems where a bypass has been added later to another firms design because of this.

 

[GarySCWSVI]

I will agree that we all have good points. I know that we are all smart enough to also agree to disagree.

My comment about "older" engineers comes from experiences I have had over the years. I definitely fall into the "older" category. New young engineers are more amenable to considering the value of reduced operating costs, as well as, to not cling to the old fashioned idea of just softening hot water.

Regarding LSI, if one is going to blend hard and soft water with a resulting less hard water then one can use LSI since there would be an appreciable amount of Ca in the blended water. Without an aprreciable amount of Ca, LSI will always show a negative result indicating the water to be corrosive. That is, in my mind, one of the fallacies of using it to make the determination. Other factors come into play that will work to keep corrosion down or to indicate a corrosive condition. Just modifying pH is not the total corrosion control answer.

Regarding a "50% less hard water" by blending will still put the majority of water sources in the category, as most understand it, of "hard water". Maybe I should have said instead of "hard warm water" better to have said "hard water of a lower temperature". All of this, of course, applies to residential and industrial use of water. Municipal water treatment plants are another story and they are best served by following AWWA guidelines.

The reduced operating costs I referred to are lower operating costs of water heating systems and the savings that result in the use of soft water such as lesser corrosion control chemicals, lesser cleaning chemicals in any washing or cleaning processes and lessor water heating costs.

Now, regarding, a complaint about "cannot wash the soap from their hands". That is easily resolved. Use less soap. Another money saving benefit of softened water.

Soften water where it needs softening. Don't where it doesn't. Consider all of the factors involved and in more cases than not softening water is a money saving and problem preventing service.



Gary Schreiber, CWS VI
The Purolite Co.

 

[PEDARRIN2]

Thank you all for your very helpful tips and guidance.

I would like to ask two additional questions for response if possible.

1) When getting into the initial stages of a project what water chemistry parameters should be examined to determine the need if any for water treatment. Now, of course, I understand that I, as the engineer, need to ask questions up front which will help determine what, if any, specific analysis must be required. But, what do I require when nobody really knows what is in the water that will be used?

2) I would like some outside opinions on the product that we are considering using. To me, it seems to "solve" all the problems associated with polyphosphates. It is called Hydroblend(TM)and the company that produces it is Southeastern Filtration & Equipment Systems. The website is

http://www.sfes.com.

Please let me know what you think.

 

[GarySCWSVI]

Question 1: It is a mistake to treat a water supply without knowing what is in the water to be treated. As complete a water analysis as one is willing to have done is necessary. Any treatment of the water should be based on whatever analysis is available and any additional notice given by the treater that any other contaminants present may effect the outcome of the treatment or he use of the water.

Question 2: Be aware that the information on that web site is from the supplier of the product. There are some indications there that leaves some question of doubt in my mind. First, is that at one point they say the product will perform without knowing the water analysis while in another section (FAQ) it says that it is best for you to allow the Hyrdoblend rep to review the water analysis. Second, they only say that it is a "proprietary blend" and do not provide information as to what the product makeup is.
Third, they make two contradictory statements regarding scale accumulation effect on fuel consumption. In one place they say "1/4 inch of scale results in 55% more fuel consumption" and in another place "1/4 inch of scal results in 39% more fuel consumption".

I have used products of this type (not specifically Hydroblend) in the past. Some work. Some don't. I have usually found that it is very important to have a water analysis and treat accordingly. Further, I do not believe what Hydroblend says about the "water softening effect" of the product. It may or may not be a good corrosion inhibitor but with 32 years of water treatment experience I see that kind of comment (softening effect) made by many vendors of different "alternative" products (alternate to ion exchange water softening) with no proof other than "anecdotal evidence" and not scientific evidence.

Gary Schreiber, CWS VI
The Purolite Co.