In aquatics facilities, pH is the most critical element of water balance. If you’re like most commercial pool operators, you’re probably using a high pH form of chlorine as your primary disinfectant. The two most popular disinfectants used in aquatics facilities are calcium hypochlorite and sodium hypochlorite, and they both can have a pH up to 11 and 13. Since the ideal pool water pH is somewhere in the 7.2 to 7.6 range, this means we must add something to bring that pH down. But does it really matter which chemical you choose?
As responsible pool operators know, this is another important decision that has safety, cost and water quality implications. Here’s some helpful information on today’s top three pH-lowering chemicals: muriatic acid, carbon dioxide, and sodium bisulfate.
Muriatic acid, the most commonly used pH reducer, is a relatively expensive liquid acid that has a pH of less than 2 at the 31.45 percent concentration that is typically used in pools. Muriatic acid is extremely corrosive and presents a significant health and safety hazard if exposed to skin or eyes, or if vapors are inhaled. Muriatic acid contributes greatly to TDS (total dissolved solids) levels by the addition of chlorides. For instance, one gallon of muriatic acid added to a 20,000-gallon pool will increase the TDS by 18 ppm. In addition, muriatic acid aggressively strips alkalinity from the water which requires the addition of other chemicals, usually sodium bicarbonate, to maintain water balance.
Sodium bisulfate is the most expensive of the three chemicals but is safer to handle as it is a dry acid, which can come in a granular form or in tablets that can be erosion fed. Sodium bisulfate contributes about 6 ppm of TDS for every pound added to 20,000 gallons of water but it takes about 10.5 pounds to do the same job as one gallon of muriatic acid, which would result in a TDS increase of 63 ppm. The sulfates that are introduced with this acid alone can be corrosive, but in the presence of high calcium levels can precipitate out of solution as calcium sulfate crystals which can be particularly difficult to remove. In addition, sodium bisulfate can lower alkalinity in the water.
Carbon dioxide is the least expensive of the three and is a gas that can be delivered in small portable tanks or pumped from a delivery truck into a bulk tank on site. There are limited safety concerns as low concentrations of CO2 are not harmful to humans, but high concentrations can displace oxygen so it is important to store in a well-ventilated area in case of any leaks. CO2 does not contribute to an increase in TDS but it will gradually increase alkalinity.
So far, it would seem like CO2 is a clear winner; however, it can require manual additions of another acid, usually muriatic, to lower the alkalinity. These manual additions bring along the safety concerns of handling acid as well as the risk of damaging coping or plaster if spilled or added incorrectly. If sodium bisulfate is used for the manual additions, then it should be properly mixed into a slurry prior to adding it to the pool. Manual acid additions must be done frequently and in small amounts in order to maintain alkalinity without lowering pH too much – this can be a challenging and inconvenient endeavor.
Employing a system that utilizes CO2 along with one of the other acid systems may be the solution that maximizes the benefits and minimizes the downsides of both systems. This type of system can be set up using a crossover switch to switch from one system to another.
For example, the switch can be set to feed acid until the alkalinity reaches 80 and then flipped over to CO2 until it climbs back up to 120, and back and forth. This system can also be set so that both CO2 and acid feed at the same time so that alkalinity remains relatively constant.
Alkalinity tends to rise more slowly on CO2 than it lowers on acid so you end up spending more time on the cheaper chemical. This also prevents the need for additional alkalinity adjusting chemicals such as sodium bicarbonate. Not only does this method result in the most consistent pH and alkalinity levels but also results in the smallest increase in TDS over time.
Getting the correct pH balance can be as much of an art as it is a science, but choosing the right pool treatment system for your facility can help. The best solution requires first lowering chlorine levels and then taking a dual-process approach using CO2 along with another acid system. But, no matter which solution you use, safely lowering your chlorine consumption will help keep your water more balanced overall and reduce the need for additional pH-lowering chemicals.
In order to lower your chlorine consumption while keeping your pool safe and healthy, you can use a secondary or supplemental sanitation system such as an Advanced Oxidation (AOP) pool treatment system in conjunction with chlorine. The AOP pool system will eliminate disinfection by-products while destroying organic and inorganic pollutants in the water, including chlorine-resistant parasites such as Cryptosporidium. This will allow you to significantly lower your chemical consumption while improving air and water quality and making your pool easier to balance.
Maintaining the proper pH balance can be a difficult job for aquatics facility operators, but it is an important one. By minimizing overall chemical consumption and taking a dynamic, multi-process approach to pH remediation, you can achieve the best water quality possible and avoid many of the problems that arise from imbalanced water.
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