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]

Low dosages of phosphates such as hexametaphosphate have long been used to solve red water problems. The phosphates mask the color and the water appears clear because the iron is tied up as a complex iron. The corrosive symptoms are removed, but the corrosion rates are not reduced. However, controlling metal loss requires up to 10 times that low dosage of hexametaphosphates. Orthophosphates has also been used to reduce the "measured" corrosion rates of metals and limit aesthetic problems resulting from corrosion.

The choice of a particular type of phosphate in a corrosion program depends on the water quality and piping material. Phosphate inhibitors require particular zones of pH, alkalinity, and phosphate level to be effective for corrosion control. This is often not understood. The complexation or solubility side reactions of calcium, magnesium, and iron with orthophospates and polyphosphates can alter the dosages, alkalinity and pH required for best performance. Generic phosphate formulations will not work in all places in all types of water applications.

My opinion is that the products that you have proposed are those of a deluded inventor and that these people, strange as it may seem, actually may believe in them. The most frequent delusion is that they will remove old scale and prevent the formation of new scale, and the germ of these ideas is probably due to a phenomenon that is well known to the water treatment professional but is apparently is not know to any extent outside of it, namely that a change in the composition of boiler salines will frequently shell off the old scale right down to the bare metal.

The hydroblend product appears to be a formulation of generic phosphate products. However, the seller states that the generic formulations have special properties. The literature states that hydroblend forms some kind of microfilm and "This microfilm does not increase in thickness as it is continually washed off and replaced." No particular reason for this phenomenon has been claimed by the manufacturer. I do not recall any reputable chemical company making similar claims about similar phosphate products.

In summary, the phosphate product is not likely to prevent corrosion in the dosages that will be used. The phosphate product will not prevent the scaling in the hot water tank either that you have noted.

This company's target market is most likely the uninformed. This market can be characterized as the indifferent equivalence of everything with everything else, for an audience that has no concern for that difference, and no discernment of quality. If you have any doubts, read the testimonials.

 

[bimr]

The water quality parameters that you should obtain for a project are the basic drinking water quality parameters.

Most labs sell a Basic Water Quality Chemistry Package that contains:

pH
Conductivity and
Corrosion Index
Total Dissolved Solids
Total Suspended Solids
Total Coliform
Alkalinity
Iron, Manganese,
Copper,
Sodium
Calcium,
Magnesium ,
Potassium,
Zinc
Nitrate and Nitrite
Chloride,
Sulfate,
Chlorine
Total Hardness

The labs do this complete package for less than $100.

 

[PEDARRIN2]

I found a resource that gives guidance on determining LSI and RI (Ryznar) index. I have been told that the RI is better than the LSI at determining the direction a particular water will lean.

But I am not sure I am doing it right because it is giving me results that are not indicated in the field.

I have two separate water analysis results:

#1: ph = 7.2, Total Hardness = 124, Calcium Hardness = 84, Magnesium Hardness = 40, Alkalinity = 132, Conductance = 601 micromhos. (I am not sure I have the right prefix. It is a mu character)

From my calculations at 50 F, LSI = -1.0, RI = 9.1
at 140 F, LSI = -0.1, RI = 7.3

This tells me that at the lower temperatures, I am either slightly corrosive or intolerably corrossive depending upon whether LSI or RI is indicative. At the higher temperatures, I am very close to balanced or significantly corrosive.

#2 ph = 7.3, Total Hardness = 214 (I mistakenly listed it as 240) Calcium Hardness = 136, Magnesium Hardness = 78, Alkalinity = 136, Conductance = 577 micromhos.

From my calculations at 50 F, LSI = -0.7, RI = 8.7
at 140 F, LSI = 0.2, RI = 6.9

This tells me that at the lower temperatures, I am either somewhat corrosive or heavily corrossive depending upon whether LSI or RI is indicative. At the higher temperatures, I am very close to balanced or little scale or corrosion.

Now my field results are telling me that there is signigicant scaling at the domestic water heating boilers. This is not indicated by my calculations.

I see one of two things here.

1) My calculations are incorrect.
2) The water analysis is not indicative of what is really going on in the system.

What do you think?

 

[JoeDenny]

The wide variation in hardness of the water produced by your local municipality is exceptional, to say the least.

Out of curiosity, exactly where are you located, please?

 

[PEDARRIN2]

It is a school district in Southwest Ohio.

They were experiencing problems with their recently installed boilers.

During a down time in July/Aug of this year, they noticed moderate scale and debris build up in the bottom of the water heater storage tank. They have also noticed a hard, green colored film which is thought to be copper oxides. Use of descaling chemicals does not affect the film other than turning it pink.

Water samples from the cold water supply and the hot water return loop were sent to a lab. The initial thought of the lab was high chlorine content. I would expect this since the school is very close to the municipal treatment plant and would expect to see spikes.

 

[bimr]

You should note that the LSI or RI are indicating measures of scaling and are not absolute measurements. The LSI or RI measurements should be used as guidelines.

You should also not that the temperature at the heat transfer points in the water heater are much higher than 140 Deg. F. In order to get 140 Deg F water out of the heater, you have to have a higher temperature on the heat transfer areas to heat the water. That is why you will experience scaling on the heat transfer surfaces.

I see that you are now saying that you have a boiler. Before, you talked about a water heater. Note that the equipment manufacturers typically provide guidelines for the water supply to the equipment.

Reread my post of Dec. 9. "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.....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

 

[PEDARRIN2]

Raising the pH using chemical feeding is not an option for this project.

This is a school, not a water treatment facility. They are not going to hire a trained and certified water treatment technician just to operate one piece of equipment.

I looked at other options but they would not work since I do not have acidic water just corrosive. My problem is not the H+ ion, but the other ions. So I am stuck with my pH. I will have to try to control the hardness/corrosivity without adjusting the pH.

 

[bimr]

There are third party firms that handle all of the treatment and maintenance issues. One contracts with these firms and they handle everything. These firms usually have multiple sites (schools, boilers, etc.) that they service so it is usually more economical than trying to do the water treatment yourself.

You might ask around the location that you are working in to find one of these firms.

 

[botan]

I would be extremely carefull of adding anything to water that is or could possibly be used as drinking water. Softeners are acceptable but any chemical addition might cause legal problems if the water ended up in the drinking supply.
Also DI beds are a hot bio growth area.

 

[countryham]

I have been building a file on phosphates for over thirty five years and I have not found one true "authority" on this subject.The generally accepted view has been in use since Calgon patented the use of hexametaphosphate in municipal water supplies many,many years ago. Hexametaphosphate has since been recognized as both a sequesterant and a corrosion inhibitor.Also, it is recognized as a threshhold treatment,i.e. it's efficacy is not stiochiometric and it's performance is relatively poor at values less than three parts per million.It has both a physical as well as chemical effect on corrosion prevention . Some calcium is needed to lay down a calcium phosphate physical barrier. I have seen this chemical,at the low level of 1ppm, actually promote corrosion in absence of moderate hardness. Betz Laboratories incorporated phosphate into a zinc-chromate corrosion inhibitor formulation to significantly reduce the total amount of chromate used in industrial water supplies over forty years ago while improving inhibition. They named the product "dianodic".
Myths have several characteristics:::1)They are not completely untrue;there is usually a measure of truth in them,but they are not completely or literally true either.(2)They were often more true in the past than they are now.(3)They are deeply ingrained.When our reason for believing in them are shown to be invalid,we look for other reasons,or continue to act as if the myths were still true. Once in the mind, they are there to stay.......Source:-Improving Chemical Engineering Practices,authored by T.A.Kletz

 

[smile1]

BIMR makes the best case, and if you followed his advice, I would see few problems. Check that deposit for Aluminum though.

Fine thread!!