Water chemistry is the most important and least understood phase of pool or spa care. It can be complex and confusing, but it can be simplified.  There are many variables in water we must contend with.  By using common sense and basic reasoning after you learn the basics, you can become an expert. No Ph. D. in water chemistry is necessary; but many hours of on-the-job training are needed. Knowing the water in your area and the mineral concentration is a must. Pool owners can find out what mineral concentrations are in their water supply by calling their local water department. Professionals should be able to test for these and have the proper test kits needed. Once you determine the quality of the supply water, you can determine the basic sanitizers needed to make balancing easier. Before I start up new services, I test water for water hardness, cyanuric acid, total dissolved solids (TDS), pH, and total alkalinity (TA). I can then determine if I must change the water, or at least some of it.

A professional should have a basic 4-in-1 test kit, plus the DPD for combined chlorine, CYA, water hardness, and TDS meter.  A test kit for copper and iron should also be considered. If you intend to service commercial, municipal, or even semi- commercial pools, you should consider one of the Oxygen Reduction Potential (ORP) testers. The inexpensive ones I have tried did not give a true reading, and are evidently not calibrated for the lower chlorine concentrations used in residential pools. Even with a 4.0 chlorine reading, the ORP tester did not show in the range it required for enough sanitizer. Do not rely on the OTO chlorine tests, as these are not for combined chlorine, but for total chlorine only. Use the DPD tests to determine combined chlorine. You can buy combination test kits with most of the needed tests, or they can be purchased separately. I use a kit that has free available chlorine, combined chlorine, total chlorine, pH, acid demand, base demand, cyanuric acid, total alkalinity, and water hardness. I also have a TDS meter and a kit for copper and iron.

Water Hardness
Contrary to what many experts write on balancing water, I start with the water hardness. This is the calcium and magnesium concentrations in water. Higher concentrations of these can provide scale formation; lower concentrations can have corrosive tendencies. The range of water hardness can determine where total alkalinity should be maintained. If the water has higher water hardness, maintain a lower total alkalinity range; for lower water harness, maintain a higher TA range. This allows a buffer zone between the corrosive and scaling tendencies of water. Plus, TA in proper range can help maintain pH range much more easily. Most experts refer to this buffering zone, or area, as the saturation point or index. Refer to Langelier index on this. Regardless of the formula or method you use, water hardness will play an important part. I prefer water hardness in the 200-400 ppm range. This is considered moderate, if not ideal. In this area, it is easier to maintain TA and, down the line pH. If below 200 ppm, chemicals can be added to raise the water hardness. If above 600 ppm, change the water (if supply water has water hardness lower than this, it can lower total water hardness).

Total Alkalinity (TA)
This is the amount of alkaline materials in the water. Lower or higher TA can give us the same scaling or corrosion as low or high water hardness. The TA acts as the stabilizer or a buffer for the pH.  Here again, sanitizers will play an important role in maintaining these ranges according to the pH factors of the sanitizers. I use high pH liquid chlorine along with low pH trichloro tablets, and prefer my TA in the 90-130 ppm range. Our local supply water has an average 200-250 ppm water hardness, 7.7 pH, and TA around 150 ppm. These concentrations can and will often vary from one water source to another, and often only a few miles apart. Some experts recommend lower TA, other according to the sanitizers and their pH factors, lower pH factors, higher TA, and vice versa. But if you combine several sanitizers, as most of us do, then we have another ball game. If TA is lower or higher than the recommended range, you can have difficulty keeping the pH in the recommend range.

pH
pH is in reference to the acidic or alkaline properties of materials in water. A pH of 7.0 is neutral; above 7.0 is alkaline; below 7.0 is acidic. With my water supply’s concentrations in the range mentioned earlier, I maintain a pH of 7.4-7.6- the middle of the pH scale- and the range recommended by most experts (7.2-7.8). Here again, water hardness can determine if you will use the lower end of the scale (7.4-7.2 for higher water hardness), the higher end (7.4-7.8 for lower water hardness), or somewhere within this range. Test water hardness at least every six months; if it is higher than average route, compensate by slightly lowering TA and slightly lowering the pH range. If above 600 ppm, consider changing some or all of the water. If this is not possible, consider using a calcium inhibitor to hold the calcium in solution to reduce or prevent scale formation. Too many service pros do not test water hardness often enough and don’t compensate for higher water hardness when they maintain TA or pH ranges. Low pH, as well as low TA and low water hardness, can contribute to corrosive tendencies directly, and can cause staining damage to equipment (plaster, vinyl, and fiberglass). Higher ranges of these can cause scale formation in heater, plumbing, and filter equipment. Chlorine is more effective as a sanitizer if the ranges above are in the recommended areas. Test for water hardness at least twice each year, and compensate for higher concentrations if water is not changed.

Total dissolved solids (TDS)
Total dissolved solids are the combined organic and inorganic minerals in water. These consist of the residue from the chemicals we add, as well as from other sources. Dirt, soap, leaves, iron, other metals, and anything introduced into the water anywhere along the way. Higher concentrations of these can be bad news. High TDS can slow down the killing power of the sanitizers, can cause more problems controlling algae, promote scale formation, and even give a false pH reading. According to experts, safe levels of total dissolved solids; (TDS) can vary, However most agree  that is the TDS count is above 2000 ppm present in your fresh water prior to the use of this water in your swimming pool, this thought accepts that we know that TDS will naturally increase the longer the water exists in the swimming pool.  The only way to reduce the TDS levels in the swimming pool is to remove and replace the water; this represents the scientific principal of saturation transference. While water in the pool is evaporating, only the fresh water is evaporating, the components of the total dissolved solids will never evaporate; these solids will remain in the water and continue to increase. So, if you do change water to reduce the TDS, change at least 25%, as make-up water from splashing or evaporation added is not going to reduce these concentrations, only add to them. Spa and hot tub water should be changed often to reduce the TDS.

Sanitizers’ Contributions to TDS
If we can change water as often as needed to significantly reduce the TDS, then the percentages each of the sanitizers can add to these build ups is not that critical. But in areas with water shortages or the potential for them, then we should know this information and use sanitizers that contribute the least. Following is a quote from the June 22, 1990, issue of the Service Industry News. The additional chemicals needed along with these to balance or compensate for the higher of lower pH factors of each sanitizer is included in the percentages.

“Basic chemicals used for chemically treating and balancing pool water and spa water (national average): Bromide salts (sodium bromide), 8 ppm; bromine tablets, 10 ppm; calcium hypochlorite, 9 ppm; liquid chlorine (sodium hypochlorite), 20 ppm; gas chlorine, 20 ppm; dry acid (sodium bisulfate), 8ppm; lithium hypochlorite, 9 ppm; trichloro, 10 ppm; dichloro, 8ppm; sodium bicarbonate, 8 ppm; soda ash, 8 ppm; sodium sesqui, 8 ppm, cyanuric acid, 8 ppm, non-chlorine shock (potassium monopersulfate), 8 ppm”
All this good information we cannot learn on this route. I appreciate the efforts of the people who publish these facts that those of us in the service industry need. I find more publications are now more service minded, and we are finally receiving the respect we deserve. All water is not fresh; it will contain many variations of minerals and materials. Test your pools and spas often. As dissolved solids and water hardness build up, change some water if possible. If not, then consider using a sanitizer that can contribute the least to these build ups.