Disinfection Continued

Natural Water Hardness, High Evaporation Rates and Total Dissolved Solids:

Let's explore some of the ways that water chemistry can negatively impact and modify the efficacy of disinfectants. One of the greatest issues we have here in the Sonoran Desert is a two fold problem. The first part of the problem is that the water we have is very hard. In other words, the water here has a lot of dissolved minerals in it, the the largest being calcium carbonate. It is not uncommon for calcium carbonate to come out of Phoenix's, Scottsdale's or Glendale's water system at a concentration of 300 to 400 parts per million. In addition to this, the fact is that the evaporation rate in the Phoenix area is twelve feet a year. So the water you have had in your pool last year has evaporated away, but has left behind all the minerals that it had contained.

As the water continues to evaporate and minerals continue to concentrate, the water level is maintained by the pool owner or by means of a automatic pool leveler. So highly mineralized water continues to enter the pool do to refilling activity and distilled water continues to evaporate. How does this impact the disinfection process? Well basically, it just gradually gets really difficult for the oxidizing agent to find the organic molecules. It is as though you are trying to find a friend of yours in a crowd, the larger the crowd and the more densely packed the crowd is, the more difficult it will be for you to make contact with you friend. Hypoclorous acid, in water that is highly mineralized, just keeps bumping into other stuff instead of the organic matter.

When you have for instance, a water concentration of calcium carbonate in the range of 1000 parts per million, you can assume that the actual total amount of everything dissolved in the water is approaching 2000 parts per million or more.There is a name for this "everything dissolved", it is called Total Dissolved Solids (TDS), when your pool's TDS gets too high, the chlorines ability to do its job or function can get reduced tremendously. So high TDS levels are caused by not only the hard water being added to a pool, combined with high evaporation rates (high temperatures and low humidity), but also everything else that gets put in the pool.

This includes stuff that is supposed to be in the water, like cyanuric acid (stablizer), sodium from any liquid bleach that has been used and regular sodium chloride, if you have a chlorine generator. One the other hand, left over calcium residues from shock treatments, or glue from chlorine tablets, atomized suntan oils, dissolved plaster, copper based algaecide residues, are all examples of stuff that we don't want in the water.

I knew of a man who bought or built his swimming pool in 1963, and refused to ever drain his pool. This pool had a sand filter which he obviously did backwash, so that the full effect of his situation was allevated somewhat. I first swam in his pool in 1976, and noticed an salty taste in his pool water: because he liked to use liquid chlorine as a disinfectant. The pool as the years went by became increasingly difficult to keep clear, and every summer would turn green until he whacked it back into shape. Finally an entire summer went by without his being able to turn the pool clear.

Finally, the pool turned a solid green all year and turned into a pond. Still he was not interested in draining his pool. The last straw was when the walls of the pool turned black. I was called in and drained the pool in 2008 and performed a chlorine wash. It took me about eight hours of work, but I got the pool back in shape.

The walls of the pool were layered with calcium ion scale, that was smooth to the touch, with a light grey coloring. This was caused by extremely mineralized pool water being run at a very high pH, and by using alkaline liquid chlorine without using acid on a regular basis to adjust the pH down. The only upside to this, was that his plaster, after all these years was still in surprisingly good shape, the high pH levels and scaling evidently protected the plaster from erosion.

This was the worst example, I have ever known, of allowing the TDS to increase to the point of collapsing the sanitation effort. So, as an absolutely critical part of your sanitation effort, be sure that you drain your pool often enough. I drain my pool every winter, around Christmas, during the coldest part of the year, and am happiest if I can drain the pool out on a cold rainy day. This will protect the pool's plaster from drying out and delaminating. Pool draining techniques will be covered in a later post.

Potential Hydrogen (pH) and Chlorine Effectiveness:

Another factor that impacts disinfectant efficacy and chlorine in particular is the water's pH, potential hydrogen. We can all remember the acid/base concept. Water that has a high pH, is base. Water that has a low pH, is acidic. The pH of your pools water is directly related to chlorine effectiveness.

When you pour liquid chlorine into the pool or introduce a granular chlorine or put chlorine tablets in the pool, chlorine breaks into two separate chemical forms. The first is called hypoclorous acid. Hypochlorous acid is the actual oxidizing agent that destroys organic matter in your pool. The other chemical that forms is hypoclorite ion. Hypochlorite ion does not oxidize organic material in any appreciable amount. These two forms of chlorine exist simultaneously in the water, one does the disinfecting, the other just acts as a type of reservoir for the hypoclorous acid.

The next idea is where you are going to learn to get interested in your pools pH. There is a direct relationship between the amount of hypchlorus acid and hypochlorite ion and the pH of your pools water. The higher your pool water's pH is, the less there is of hypochlorous acid in the water and the more there is of hypochlorite ion.

Remember we said that hypochlorous acid was the actual oxidixing agent? So if you run your pool with a high pH, no matter how much chlorine you put in the water, the killing ability of that chlorine is going to be diminished. Conversely, if you keep your pool's pH low, you will increase the amount of hypochlorite ion in the pools water, and it would appear that is what we want.

But things are a little more complicated than that. Like I stated previously, the quanity of the hypochlorite ion in your pool acts as a kind of reserve for hypoclorous acid, sort of a type of storage repository. As the hypoclorous acid gets used up, more is automatically made from the hypochlorite ion. If you run the pools pH down too low the amount of measured chlorine in your pool will disappear rather quickly, because hypochlorous acid is unstable and tends to gas off into the atmosphere, Hypochlorite ion in contrast is more stable, but unfortunately is not a very good disinfectant.

So the answer here is a type of compromise, where you keep the pH down but not too low. Most experts recommend keeping the pH between 7.2 and 7.6. This range will give you a sufficient amount of hypoclorous acid while also keeping a healthy reserve of hypoclorous acid in the form of hypochlorite ion. An additional benefit that accrues to you is this: by keeping the pools water to within this pH range, the minerals remain somewhat sequestered or held in the water instead of precipitating out in the form of scale onto your pools surfaces. If you let the pH move up for any extended period of time you may begin to notice that your plaster, whether it is white or black, will start to stain up. If you don't have a plaster pool, you will still accumulate staining on whatever surface you have and will have staining occurring around the pool's water line. So by keeping the pH lower you suppress two problems at once; you enhance the chlorines ability to disinfect your pool and you keep problem staining at bay.

How to adjust the pH:

When we want to lower the pH of our swimming pool's water, the proper method is to walk the acid, this is done by pouring the acid into the pool as you are walking along the perimeter of the pool. You want to have the pool pump running. As you are pouring the acid in, lean down towards the water in order to reduce the splashing of acid onto the cool deck, your skin or clothing. Sometimes the splashing of the acid can reach up to your face. So it would be a good idea to wear goggles.

The best method is to get down on your knees, and while holding onto the bottle of acid, let it float in the pool water near an water inlet and slowly pour the acid into the stream of water from that inlet. Never pour more than a pint of acid into the pool at one time. Wait a day then test the pH again then add more if needed. Oftentimes, it is only necessary to add a much smaller amount than a pint. Never pour acid onto your steps, or love seat areas, it will ruin the surface over time. Don't pour acid into just one area, will reduce the total alkalinity of the water. In the next post we will cover a situation where that is exactly what you want to do, and there will be a particular reason for pouring it into the water in that way.

Next, I am going to discuss total alkalinity and how it relates to the water's pH.