Chemical imbalance: why one water park switched its water-chemistry system

Parks & Recreation, Dec, 2003 by Mike Moss, Clay Finney

For more than a century, Columbian Park in Lafayette, Ind., has been a place for visitors to enjoy high-quality leisure time with family and friends. In June 1999, the Lafayette Parks & Recreation Department enhanced the visitor experience at the 43-acre park by opening the Tropicanoe Cove family aquatic center.

The $4.6-million city parks project, part of a master plan developed in 1998 through open public meetings and surveys, encompasses 16 acres inside the park. The aquatic center features a 640-foot lazy river, a 168,000-gallon zero-depth leisure pool, a 33,000-gallon plunge pool, plus many other aquatic features, including a 300-foot tube slide, a water playground and two high-energy drop slides. The center is open Memorial Day through Labor Day and, besides family fun, offers scheduled activities such as aquatic fitness programs. Attendance at the center in 2002 was 76,000.

Maintaining proper water chemistry in the center's three distinct water bodies (leisure pool, plunge pool and lazy river) is of paramount importance. State regulations require 1.0 to 3.0 parts per million free chlorine residual and pH values of between 7.2 and 7.8 at all times. With highly fluctuating and, often disparate bather loads between the plunge pool and leisure pool, maintaining consistent chlorine residual presented a major challenge for facility staff. Because of this challenge, last year the center changed water-chemistry systems. Included in that change was switching from sodium hypochlorite to calcium hypochlorite as the sanitizing agent.

Problems in the Pool

The center wanted to find an alternative to using sodium hypochlorite For several reasons. Although effective in performing its sanitizing task, commercial sodium hypochlorite has certain problematic inherent characteristics. Chief among these was safety. The center's staff was concerned about the potential for personnel exposure to injury using the high-solution strength liquid chemical under a pressurized system.

There were also maintenance concerns associated with sodium hypochlorite usage. Commercial sodium hypochlorite is highly corrosive to metals, and presented a high potential for creating a corrosive environment inside the equipment building, adversely affecting pumps, piping valves and surroundings. There were also concerns regarding the common problem of chemical metering pumps becoming air-bound owing to sodium hypochlorite off-gassing.

Another concern was the inconsistent concentration of sodium hypochlorite. Commercial sodium hypochlorite solutions are subject to degradation and byproduct formation because of time, temperature and other factors. The concentration of chemical delivered to the aquatic facility and stored on site in two 500-gallon tanks often varied. Although the facility's water-chemistry control system compensated for losses in solution strength, it required pumping more solution to meet the chlorine-residual requirements in the pool. In the long run, using the degrading product cost more because the facility had to use higher volumes of the chemical to achieve the desired results.

In addition, commercial sodium hypochlorite contains sodium hydroxide (caustic soda), which is added by manufacturers to help stabilize the bleach solution. High sodium hydroxide levels in the pool water can produce higher levels of total dissolved solids, which can adversely affect water clarity, feel and taste.

Because chlorine residual values in the water would fluctuate, operators often had to set the facility's oxidation-reduction potential controller's set-point (or operating range) high to ensure that state regulations were met; this practice further increased chemical usage. These fluctuations were caused by a number of factors, including limitations of the original pool chemistry-control system in adjusting feed rates to address the disparate bather loadings between the leisure pool and plunge pool. Because of head pressure coming off the liquid metering pump, unwanted siphoning in the chemical injectors also occurred, and often resulted in feeding sodium hypochlorite into the system when it was not needed.

Switching Sanitizers

After comparing other sanitizing agents commonly used in recreational aquatic facilities, including chlorine gas, bromine and ozone, the aquatic center decided to switch to calcium hypochlorite.

Calcium hypochlorite briquettes are a solid form of chlorine, with an available chlorine concentration of 65 percent. Because it is a solid product, it must either be pre-dissolved or added by a chemical feeder. The product is basic and, because it is slightly alkaline, requires the use of an acidic chemical for pH adjustment. The primary advantages of calcium hypochlorite briquettes are excellent water appearance (clarity), safety and ease in handling, and low total dissolved solids build-up. In addition, its calcium base helps balance water chemistry and protects pool surfaces from damage.

Total dissolved solids build-up with calcium hypochlorite is less than half that of liquid bleach. In addition, calcium hypochlorite requires less than half the amount of pH adjustment chemicals than sodium hypochlorite. Calcium hypochlorite briquettes also have up to a two-year shelf life, as opposed to commercial sodium hypochlorite, which can lose one-third of its strength in only 30 days.