Water balance is a crucial part of pool maintenance. Without it, your pool water could corrode metal, be so cloudy you can’t see through it, or, most importantly, be a breeding ground for bacteria.
You may be familiar with the Langelier Saturation Index (LSI). This index is a “go-to” calculation for pool managers to determine if their water is balanced, safe and healthy. The LSI of water is determined by the water’s temperature, calcium hardness, total alkalinity (TA), total dissolved solids and most importantly, the pH. A pH range between 7.2 and 7.6, and a free chlorine between 1 to 4 ppm will help ensure the healthiest, safest water.
How does pH affect safety? First, having high pH pool water, around pH 8 or higher, can increase the chance that the water may become cloudy. While the water still may be somewhat chemically safe, it now presents a bigger risk for drownings.
Next, high pH pool water presents a health risk. When any form of chlorine is put into water, it becomes two different things: Hypochlorous acid (HOCl), and Hypochlorite ion (OCl). Hypochlorous acid is the power behind chlorine disinfection. While hypochlorite ion can still disinfect, it is not nearly as effective and takes much longer to oxidize contaminants. Hypochlorous acid is the bacteria-killing power that keeps your pool clean, free of pathogens and algae, and safe to swim in. At a pH of 7.5 (within that industry-standard range), there is a 50:50 mix of HOCl and OCl- in the water (with no CYA stabilizer present). However, as the pH increases, the amount of hypochlorous acid in the water drops rapidly. This means that as the pH gets higher, the less killing power your pool water has, making it susceptible to all kinds of diseases.
Acids are one important way to keep your pool water within the recommended pH range, with three main options: muriatic acid, carbon dioxide (CO2) and sodium bisulfate.
Muriatic acid, aka hydrochloric acid, is the most common. It will effectively reduce your pool’s pH, but also its total alkalinity. In a 100,000-gallon pool, it takes about 2.8 gallons of muriatic acid to decrease the pH from 8 to 7.4, and also decrease the total alkalinity by about 14 ppm. It is considered a strong acid with a pH below 1 and can be dangerous if not properly handled. Personal protective equipment (PPE) is always needed when applying muriatic acid. If spilled on pool equipment, it can damage or corrode these materials. If spilled on skin or clothes, or splashed in eyes, a severe acid burn could occur. Muriatic will also create harmful fumes when exposed to air, especially humid air.
Carbon dioxide, or CO2 is another pH adjustment option. No PPE is required when using CO2 as it is a gas that comes in pressurized tanks, so there is no contact between the user and the chemical. Using CO2 does, however, require more equipment, which increases the cost. However, CO2 does not require separation from oxidizers (chlorine) in storage like the other two acids do.
A major consideration when using CO2 is the safety aspects when using compressed gas cylinders. First, an enclosed space can quickly fill with unbreathable gas that can poison or asphyxiate anyone inside. Next, while the chances are small, there is a possibility of a cylinder exploding.
When CO2 is introduced to water it partially forms carbonic acid, which is a much weaker acid than either muriatic or sodium bisulfate. Since it is weaker, the addition of CO2 will decrease the pH in a gradual fashion. CO2 does not decrease or increase total alkalinity. Do keep in mind that unlike muriatic or sodium bisulfate, which help keep TA in check, with CO2 your total alkalinity will tend to drift upward with the addition of chemicals like calcium hypochlorite and liquid bleach. In a 100,000-gallon pool, 120 ounces of carbon dioxide is needed to decrease the pH from 8 to 7.4. While this form of pH control does not create any unwanted byproducts like sulfates or chlorides, it does gas off readily from aeration of the water from swimmers splashing or pump usage. Once the CO2 gasses off, it must be replaced.
Sodium bisulfate (pH ~1) is a dry acid powder (granule) that is applied by dissolving it in a bucket and pouring around the perimeter of the pool. A tableted bisulfate form, as part of an erosion dispensing system, is increasing in popularity based on safety advantages. Both have a concentration of more than 90%. While granular sodium bisulfate is more common, the tableted version has been on the market for a few years and provides a safer experience since it reduces spills and handling concerns. Around 30 pounds of sodium bisulfate will lower the pH of a 100,000-gallon pool from a pH of 8 to 7.4, and will lower the total alkalinity by about 14 ppm. When using bisulfate, operators should monitor sulfate levels in their pool. Too much sulfate (above 300 ppm) can cause metal corrosion or plaster/concrete damage. Fumes are not a consideration, and spill concerns are fewer. However, PPE is still recommended when handling this type of acid, as dealing with a dusty material can be dangerous.
Each form of acid has its benefits and drawbacks. If you have a well-trained team that is water-balance-focused, muriatic or CO2 may be your best option. If safety is your foremost concern, then granular or tableted sodium bisulfate may check your boxes.
ABOUT THE AUTHOR:
Samantha Greig has been a research chemist with Westlake Water Treatment for the last 12 years.