Comparing Operating Costs of Sanitizers: Calcium Hypochlorite and Trichlor
The choice of sanitizer can cost a large aquatic facility a lot of money. This paper will discuss potential costs associated with two common sanitizers, calcium hypochlorite (cal hypo) and trichloroisocyanuric acid (trichlor).
When these sanitizers are added to pool water, along with increasing the available chlorine, they will affect pH, alkalinity and either calcium hardness or cyanuric acid. One way to think about it is that every time these sanitizers are added to the water, additional chemicals “inside” the sanitizer are added as well. Sometimes these “extra” chemicals are beneficial, and sometimes you will have to pay to get rid of them.
An estimate of 2017 Fulton County water and sewage costs combined with 2017 chemical costs shows that costs incurred by facility’s using calcium hypochlorite might be considerably less than the aggregate costs for the same facility using trichlor. The chart below shows estimated costs when adding 10 ppm available chlorine (AvCl2) to a pool using the two sanitizers, and an explanation of the assumptions used for the estimate follows.
Explanation of Assumptions and Estimates When estimating the costs of sanitizing pool water, simply looking at the cost per pound of sanitizer does not take into account less obvious costs and may lead to false assumptions about savings. When added to pool water, cal hypo increases the available chlorine, pH, alkalinity and
calcium hardness (“CH”). Trichlor, when added, increases available chlorine and cyanuric acid (“CYA”), but decreases pH and alkalinity. As a result of the choice of sanitizer, a facility may need to replace some of its pool water to counter the increasing CH or CYA level. Accordingly, the facility’s choice of sanitizer can result in increased water and sewage costs. Further, both sanitizers affect pH and alkalinity, this will typically need to be corrected with additional chemicals, further increasing costs. Every pool environment is different and accounting for every scenario may be impossible. In order to estimate costs incurred by the use of these sanitizers, certain assumptions have been made for this comparison. The following explanations describe the reasoning behind these assumptions. Water Usage Water usage is a big component of running any aquatic facility. As mentioned, cal hypo adds calcium hardness and trichlor adds cyanuric acid. Neither CH or CYA are destroyed by chlorine, nor do they evaporate. Draining and replacing the water is the only practical way to decrease concentrations. Splash out and back washing the filter will reduce these concentrations, however estimating water consumption rates of these activities is difficult and can change from pool to pool, so water usage from these activates were left out of this paper’s estimates. Since evaporation does not remove either chemical, water consumption from evaporation was not factored directly into costs. Cal Hypo:
Calcium hardness (CH) plays a vital role in water chemistry, helping to protect plaster, concrete and metal objects from corrosion caused by aggressive water. A minimum of 150 ppm CH is required for pool water (ANSI/APSP-11 15), with 200 ppm CH recommended for plaster pools (NPC). Between 150 - 1000 ppm CH, the saturation index can be maintained to prevent scale and cloudy water formation (ANSI/APSP-11 15).
Adding 10 ppm AvCl2 using calcium hypochlorite will add 8 ppm CH (APSP Calcium). For a pool starting at 150 ppm CH, adding 10 ppm AvCl2 daily with calcium hypochlorite would cause the CH in the pool water to reach 1006 ppm in 107 days. When the water reaches 1000 ppm CH, removing and replacing 1 % will drop the CH 8 ppm if the source water is 200 ppm CH. This will counter the CH added by the daily dose of calcium hypochlorite at these concentrations and rates. In 100,000 gallon pool this is equal to 1000 gallons.
Cyanuric acid (CYA) helps protect the available chlorine from decomposition caused by UV light. A maximum of 100 ppm CYA is recommended by ANSI/APSP-11 2009, and a maximum 90 ppm CYA by the Model Aquatic Health Code (MAHC 211). CYA is unnecessary and not recommended for indoor pools (ANSI/APSP-11 22).
Adding 10 ppm AvCl2 using trichlor will add 6 ppm CYA (APSP Trichlor). For a pool starting at 30 ppm, adding 10 ppm AvCl2 daily with trichlor would cause the CYA in the pool water to reach 90 ppm in 10 days and 102 ppm in 12 days. At 90 ppm CYA, 6.7% of the water must be removed and replaced to counter this daily addition of trichlor. At 100 ppm CYA, removing and replacing 6.0% of the water will counter the CYA added daily by the use of trichlor. For a 100,000 gallon pool that is 6,700 gallons and 6,000 gallons respectively.
Estimating Water and Sewage Costs Water and sewage costs are a big opportunity for pool operators to save money. The examples above show that to provide the same amount of available chlorine, it can take 6 times or more water to keep the CYA in a recommended range when using trichlor than the amount of water that is needed to keep the CH in a recommended range when using calcium hypochlorite. Since this water must be drained and refilled, it stands to reason that in locations where sewer costs are based on consumption, not only would the water costs be 6 times higher, but sewer costs would be as well. Water and sewage costs are quite different around the country; calculating them can be complex. Water and sewage prices change from location to location. In this analysis, we used Fulton County Georgia’s 2017 water and sewage costs. Prices scale up depending on monthly usage (Water & Sewer Rates). To compare apples to apples, we assumed that the source water contains 200 ppm CH, the maximum recommendations of CH or CYA had been reached (so draining and refilling is necessary), and the top tier water price had been reached by water consumption not associated to the sanitizer (water cost $0.0100 per gallon and sewage cost $0.0055 per gallon). Water loss due to evaporation, splash out and filter backwashing was excluded. The below table estimates the daily and monthly (30 day) water and sewer costs that would be charged by Fulton County, Georgia for the amount of water replacement needed to maintain recommended CH or CYA levels for a pool if adding 10 ppm AvCl2 daily:
Chemical costs In addition to changing the CH or CYA levels, these sanitizers affect the pH and alkalinity of the pool water. Calcium hypochlorite contains small amounts of calcium hydroxide and calcium carbonate, which results in slightly increased pH and total alkalinity. In theory, 10.5 ounces of calcium hypochlorite added to a 10,000 gal pool (i.e. 5.1 ppm AvCl2) will increase the pH by 0.009 and carbonate alkalinity by 0.29 ppm (pH 7.5, 100 ppm carbonate alkalinity, 100 ppm CA, and 1000 ppm TDS) (Wojtowicz). Conversely, in the same pool, 7 oz. of trichlor (i.e. 4.7 ppm AvCl2) will theoretically decrease the pH by 0.14 and decrease the total alkalinity by 3.3 ppm (Wojtowicz). Muriatic acid and sodium carbonate are common choices for large commercial and municipal pools to neutralize pH changes in pool water. Although there are other methods to neutralize the pH changes, this paper assumes the use of muriatic acid to neutralize the increased pH caused by calcium hypochlorite, and sodium carbonate to neutralize the pH decrease cause by trichlor. A study performed by Olin Corporation found, on average, 1.56 oz. of 32% muriatic acid neutralized the pH of one pound of calcium hypochlorite (Oberson). In the same Olin study, 0.93 pounds of sodium carbonate was found to neutralize the pH of one pound of trichlor (Oberson). Combined chemical costs were determined using the above chemical usage rates for pH neutralization and the following chemical costs: Calcium hypochlorite - $2.30 per pound 32% muriatic acid - $0.055 per ounce ($7.00 per gal) Trichlor - $2.16 per pound Sodium carbonate - $1.80 per pound
The chemical cost to counter the pH effect of 1 lb. of cal hypo adds $0.09, where the chemical costs to counter the pH effect of 1 lb. of trichlor adds $1.67 per pound. The combined chemical costs per pound of cal hypo would be $2.39, and per pound of trichlor would be $3.83. With cal hypo at 68% AvCl2, and trichlor at 90% AvCl2, the combined chemical costs per pound of AvCl2 for cal hypo would be $3.51 and trichlor would be $4.26. With 10 ppm AvCl2 is provided daily, the chart below estimates the daily and monthly costs for several pool sizes:
Estimated Water, Sewer and Chemical Costs The chart below combines the daily chemical the water and sewage costs that would be charged by Fulton County, Georgia:
References ANSI/APSP-11. American National Standard for Water Quality in Public Pools and Spas. Alexandria: The Association of Pool and Spa Professionals, 2009. APSP. "Calcium Hypochlorite." June 2010. APSP.org. The Association of Pool and Spa Professionals. Fact Sheet. 18 April 2017. <http://www.apsp.org/Portals/0/RWQ%20Fact%20Sheets/Calcium%20Hypochlorite%20- %20July%2029,%202014.pdf>. "Trichloro-S-Triazinetrione (Trichlor)." June 2011. apsp.org. Fact Sheet. 18 April 2017. "MAHC." The Model Aquatic Health Code. Atlanta: The Centers For Diseas Control and Prevention, 15 July 2016. 211. Web. 18 April 2017. <https://www.cdc.gov/mahc/pdf/2016-mahc-code-final.pdf>. NPC. "Start-up Cards." Vers. 3. 18 April 2017. npconline.org. National Plasterer's Council. pdf. 18 April 2017. <https://c.ymcdn.com/sites/npconline.site-ym.com/resource/resmgr/Docs/startup-card-current-rev3_wa.pdf>. Oberson, S.R. "Amount of Chemicals to Maintain a Balanced Pool Water with Chlorine Sanitizers." Inter Office Memo. 1981. Water & Sewer Rates. n.d. City of Atlanta Department of Watershed Management. Web. 18 April 2017. <http://www.atlantawatershed.org/customer-service/rates/>. Wojtowicz, J.A. "Swimming Pool Water Balance - Part 2: Factors Affecting the Calcium Carbonate Satrutation Index." Journal of the Swimming Pool and Spa Industry 1.2 (1995): 9-16. Print.