Water Conditioning & Purification Magazine

Making Sense of an Incomplete Water Analysis

By Frank DeSilva, ResinTech, Inc.

The water treatment professional is often required to recommend a treatment scheme to rectify problem water. More often than not, the initial water analysis data that the end user provides is not sufficient to make a valid recommendation.

Here is what you need to get from your customers in order to make a valid recommendation for resin selection and throughput predictions: the influent conditions and also the effluent requirements.
Author’s note: I have gotten into the habit of listing ions in mg/L if they are reported as the ion and in ppm if they are reported as CaCO3. This is a convention used by Bill Bornak in his book, Ion Exchange Deionization.

Information needs by application
Cationic applications (hardness removal, metals removal, radium removal)
pH; TDS or conductivity; hardness (or separate calcium and magnesium numbers); iron; manganese; all metals of concern if metals removal is the application (copper, lead, cadmium, etc.); other cations as needed (radium, for example).

Anionic applications (sulfate removal, nitrate removal, chromate removal, uranium removal, organics removal, perchlorate removal, fluoride removal, dealkalizers, boron removal)
Technically, the same type of resin will remove all of the constituents listed; however, the determination of which anion resin will actually be the best choice is dependent upon the water analysis parameters that are requested. For instance, a type II strong base anion resin will work well for arsenic removal on high pH/low TDS water, while a hybrid strong base anion resin would work well on a low pH/high sulfate water.

pH; TDS or conductivity; sulfate; nitrate; chloride; alkalinity (or HCO3); silica (for arsenic applications).

Of course, for the contaminant of concern (arsenic, chromate, uranium, etc.) you’ll need to know the influent concentration and also the effluent requirement. It is also useful to know if the iron and manganese concentrations are above 0.5 ppm and 0.25 ppm respectively. If so, the client needs to lower the iron and/or manganese level before introducing water to the anion unit.

Deionizer applications
pH; TDS or conductivity; calcium; magnesium; sodium; potassium (if any); sulfate; chloride; alkalinity; silica; CO2.
Deionizer applications will specify effluent quality in terms of conductivity, resistivity, silica or sodium.

Customer provided information
It’s not often that the customer will have all the items you are asking for. TDS or conductivity is easy to test for and you’ll usually be able to obtain those numbers. Here’s an example of roughing up a water analysis from partial information.

The customer provides us with a water analysis that shows the following:

Conductivity 550 microsiemens; hardness 150 ppm; alkalinity 125 ppm; chloride 30 mg/L; silica 15 mg/L; pH 7.

What’s missing? The breakdown of the hardness into calcium and magnesium, the sodium, the sulfate and CO2.

First the cations. If you’re trying to get a cationic water analysis together and the customer only has the inlet conductivity of 550 and the hardness of 150 ppm as CaCO3, here are the assumptions you can make.

Take the inlet conductivity and convert it to TDS ppm as CaCO3 (550/2.53 = 217.4 ppm as CaCO3). By subtracting the hardness of 150 ppm as CaCO3, you find the sodium level as CaCO3 (217.4 – 150 = 67.4 ppm as CaCO3). Now since we don’t have separate numbers for calcium or magnesium, you can use an old rule of thumb that says that calcium is usually two thirds of the total hardness number and magnesium the remaining third. So, the calcium is 100 ppm as CaCO3 and magnesium is 50 ppm as CaCO3.

Cation summary, all as ppm CaCO3: calcium 100 ppm; magnesium 50 ppm; sodium 67.4 ppm.

Now let’s take a look at the anions. Assume that all the customer had for us is the alkalinity (again a simple test for the customer to do), chloride and silica.

Alkalinity 125 ppm as CaCO3. (If the water analysis states alkalinity, it is reported as CaCO3. Sometimes the alkalinity is reported as HCO3 and so you must convert that to ppm as CaCO3); chloride 30 mg/L; silica 15 mg/L.

First thing to do is convert the chloride to ppm as CaCO3 (30 x 1.41 = 42.3 ppm as CaCO3). Now find out what the sulfate level is by subtracting the chloride as CaCO3 plus the alkalinity as CaCO3 from the total cation (217.4 – (125 + 42.3) = 50.1). So the sulfate is 50.1 ppm as CaCO3.

The silica is not incorporated into the ionic balance since it is weakly ionized and does not contribute to the conductivity or TDS.

Here’s a summary of what we have calculated, now shown as ppm as CaCO3:

The total cations and anions should be equal at this point since they all contribute to the electroneutrality of the solution. There may be some potassium present in the cations, but for our calculations, it is lumped in with the sodium since it is also monovalent. On the other hand, there may be low levels of nitrate present; it is lumped in with the chlorides as a monovalent. (This is for DI calculations only. If we are dealing with a nitrate removal job, we need to know precisely how much nitrate is present.)

This is not all of the exchangeable anions, however, since we still have the silica and carbon dioxide to contend with. The silica is reported at 15 mg/L as silica. The conversion to CaCO3 is 0.83 (15 x 0.83 = 12.5 ppm as CaCO3). Adding that to the total anions, 12.5 + 217.4 = 229.9 ppm as CaCO3 as total exchangeable anions (TEA). To be completely thorough, you would want to calculate the CO2 level to see what its contribution would be to the anion loading.

Here is a good link that explains the alkalinity relationships in water. http://www.onlinewater treatment.com/literature/Nalco/docs/Tf-084.pdf. Refer to the chart upper right, page 1. At a pH of 7, the ratio of CO2 to M alkalinity (total alkalinity) is 0.16 (0.16 x 125 = 20 ppm CO2 as CaCO3). That means that our total exchangeable anions are now 229.9 + 20 = 249.9. To calculate the load in grains per gallon, divide the ppm as CaCO3 by 17.1. Total cation load therefore equals 217.4/17.1 = 12.7 grains per gallon (gpg). Total anion load equals 249.9/17.1 = 14.6 gpg.

Here’s our water summary once again. Provided by customer: conductivity 550 microsiemens; hardness 150 ppm; alkalinity 125 ppm; chloride 30 mg/L as Cl; silica 15 mg/L as SiO2; pH 7. Calculated analysis (all shown as ppm as CaCO3):

Of course, any predictive information provided to the customer at this point must clearly show the calculations and assumptions that have been made. The less complete the original water analysis is, the higher the safety factor or engineering factor should be. A typical engineering factor that is used for DI calculations is 0.9 or a 10-percent downgrade for cation or 15 percent for anion, which is applied to the throughput calculations. If we run a projection on the water analysis that we just calculated, you might want to use 0.8 or 0.75 as the safety factor.

Tables 1 and 2 give conversions and information on the common ions.

About the author
Francis J. ‘Frank’ DeSilva is National Sales Manager for ResinTech Inc. of Cherry Hill, N.J. ResinTech is a manufacturer and supplier of ion exchange resin, activated carbon products and the Aries line of laboratory demineralizers and cartridges. DeSilva operates out of Jensen Beach, Fla. He can be reached at (561) 225-0763, (561) 334-1099 (fax) or email: fdesilva@resintech.com

 

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