Determining Cycles of Concentration

“Cycles of Concentration (CoC)” — you either care about it or you don’t! Water treaters care about CoC because it directly relates to water and energy conservation as well as chemical usage rates. The higher the CoCs, the less wastage.

So how do we calculate CoC? This is pretty easy for open cooling water systems. In the old days, it was done by testing the chloride concentration in the cooling water and then dividing that result by the chlorides in the make-up water. However, with more chlorine and bromine products being fed into these systems, the resulting “chloride result” will be artificially high. Silica could work, but I prefer killing two birds with one stone: testing for magnesium.

There is no direct test for magnesium. The magnesium hardness is determined by testing a water for total hardness (TH) and calcium hardness (CaH). The difference between these two results is the magnesium hardness (MgH): TH — CaH = MgH. But how is this killing two birds with one stone? Magnesium is very soluble at the pH levels found in cooling towers, but calcium is not. This is because magnesium generally scales as magnesium hydroxide [Mg(OH)2]. Calcium, on the other hand, scales as calcium carbonate (CaCO3). There should be no hydroxide alkalinity (OH-) in the water, but there could be plenty of carbonate alkalinity (CO32-). So if you have a problem with calcium scaling, your calcium hardness test result will be low, while your magnesium hardness should be correct. This means you should see your true cycles of concentration by dividing the cooling water MgH by the make-up water MgH. In a properly controlled/treated system, that value would be equal to the value associated with the CaH: cooling water CaH divided by the make-up water CaH. In an uncontrolled system, the calculated CaH cycles of concentration will be lower than the MgH value.

You could calculate the CoCs with other soluble minerals, like silica, but why not use magnesium and get the extra bang for the buck? It feels really good to know your program is protecting your system from scale!

Next time we’ll talk about cycles of concentration in boiler systems, which can be a little more complicated.

*Note — The picture below shows the properly formatted formulas: