If there’s one ingredient that most pool chemical regimens have in common, it’s chlorine. Almost since the beginning of the pool industry, service technicians across the country have been hauling around drums of the sanitizer, using test kits to monitor its concentration, and adjusting water chemistry to maximize its effectiveness.
But over the past decade, a growing number of pools have switched to a somewhat different system: Electrolytic chlorine generation. By using electricity to drive certain chemical reactions in salt water, electrolytic chlorine generators (ECGs) produce chlorine on-site.
Although the chemistry of an ECG-chlorinated pool bears many similarities with that of a traditionally chlorinated pool, it also involves some unique factors.
Here, through the expert advice of scientists and service techs, we examine these differences and provide some field-tested advice for servicing pools with ECGs.
Perhaps the most obvious area in which ECG-chlorinated pools differ from traditionally chlorinated ones is in chemical transportation and storage. Because barrels of chlorine (or chemical compounds including chlorine) don’t need to be trucked to the site and stored there, many safety issues associated with these barrels — such as fumes and spills — are no longer major concerns.
However, the ECG itself adds some new tasks to the traditional maintenance regimen. Among the most important is keeping the salt cell clean. The chemical reactions involved in generating chlorine from salty water also contribute to the accumulation of calcium scale within the ECG — over time, this can lead to less efficient chlorine generation, or even equipment damage. Thus, it’s crucial to perform regular checks on the cell, and address any scale buildup with a light acid wash.
“The first year of a new ECG’s life, you can usually get away with cleaning the cell once every three or four months,” says Cliff Brummett, owner of CTB Pools LLC in Phoenix. But year by year, Brummett goes on to explain, the process of chlorine generation tends to drive the water’s calcium hardness and alkalinity upward, making more frequent cleanings necessary. “By the second year,” he says, “you typically have to start cleaning the cell every month.”
Salt water, and the process of electrolysis, can also contribute to certain kinds of degradation, such as galvanic corrosion. In fact, says Alison Osinski, Ph.D., principal-owner of Aquatic Consulting Services in Avalon, Catalina Island, Calif., “Some manufacturers may say their components were not NSF tested in salt water pools, and therefore [using them in a salt water pool] voids the warranty.”
This is especially a concern for small components in heaters, such as gaskets and O-rings. “You’ll need to pay more attention to those components, and replace them more often than you would in a traditionally chlorinated pool,” Osinski says. Weekly checkups of these components, and replacements of any that are beginning to show signs of damage, will go a long way toward keeping the equipment trouble-free.
Another consideration, which might seem obvious but is often neglected, is the fact that the system’s pump must be running in order for the ECG to produce chlorine. “Since pumps on residential pools usually don’t run 24 hours a day, we can get problems with these residential systems that we don’t see with commercial systems, because they don’t circulate the water enough,” Osinski says. Thus, it’s important to be sure the system is generating enough chlorine to maintain a proper residual in the time it takes the pump to run through one daily cycle.
When it comes to the chemistry of ECG-chlorinated pools, most of the acceptable ranges specified by organizations like the Association of Pool & Spa Professionals and the Independent Pool and Spa Service Association will still apply — in other words, the water’s calcium hardness, total alkalinity, pH and temperature should be maintained in the same ranges as they would for a traditionally chlorinated pool.
However, there’s one important respect in which ECG-chlorinated water differs: its level of total dissolved solids (TDS). Whereas most traditional recommendations place the ideal range for TDS at approximately 300 to 1,800 ppm, salt water often contains 3,400 ppm of TDS due to the salt alone — in addition to as much as 1,000 ppm of other miscellaneous TDS.
ECG manufacturers typically specify an ideal range of salinity for pools using their devices — so it’s important to check the salinity of the water at least once a month. When performing these checks, be sure to use a test method that measures the salinity level in particular; not just the overall TDS — test kit instructions will specify which parameter each test addresses.
“A standard TDS test is going to measure all the salt, plus any other dissolved solids,” explains Ray Denkewicz, worldwide product manager for sanitization and chemical automation at Hayward Industries in North Kingstown, R.I. “So you might get a reading of 5,000 ppm, when in fact the salt contribution to that may be 3,000.”
Thus, distinguishing between these two types of TDS contributions is critical for maintaining balanced water. And an effective way to get a clear sense of the pool’s non-salt TDS is to perform a TDS test when adding salt to the pool for the first time. “That’s your starting TDS,” says Geoffrey Brown, developmental scientist at Pristiva Inc. in Overland Park, Kan. “Once your TDS increases 1,500 ppm above that, then you should start thinking about draining some of the water and replacing it with fresh water.”
ECGs’ tendency to drive pH and total alkalinity upward can impact other chemical parameters as well. “Not only can high pH result in bather discomfort, it also makes the chlorine less effective,” Brown says. This means that while a chlorine test might show that the water’s chlorine level is acceptable, if the water’s pH is too high, that chlorine will exist in a much less effective chemical form. Thus, weekly pH checks are essential for effective sanitation.
In these high-pH conditions, some say they’ve found that higher TDS creates a greater potential for calcium carbonate and other soluble calcium compounds to form scale deposits on surfaces throughout the pool and equipment. “The calcium will want to precipitate out of solution,” Osinski explains. “It can start clogging the pipes up, creating milky water, and causing scale.”
However, other scientists point out that a higher TDS would actually lead to more corrosive water, by lowering the water’s Langelier Saturation Index (LSI) value. “Higher TDS makes the water more corrosive,” says Karen Rigsby, leader of technical services at BioLab Inc. in Lawrenceville, Ga. “It’s inside the chlorine generator where you get the likelihood of scale formation, and that’s because of the high pH inside there.”
If calcium scale does become a problem in an ECG-chlorinated pool, experts say it’s generally reasonable to adjust the pH slightly downward with muriatic acid. Still, it’s a smart idea to calculate the water’s LSI value on every visit to the site, and visually inspect surfaces for any signs of corrosion, as well as calcium deposits.
Even if the pool’s water has been balanced into an ideal LSI range, it’s still helpful to be aware of some additional chemical traits of ECG-chlorinated pools. Aside from their higher salt-contributed TDS, the other main chemical distinction of these pools is how their cyanuric acid (CYA) concentration must be managed.
As many service techs know, CYA is a chemical that protects chlorine from breaking down under the sun’s ultraviolet (UV) rays. Many traditionally chlorinated pools are chlorinated with trichlor tablets, which contain both chlorine and CYA. However, the chlorine in ECG-chlorinated pools must also be protected with cyanuric acid (CYA) — industry organizations like the APSP recommend an ideal range of 30 to 50 ppm — which means it’ll be necessary to add this chemical manually from time to time. Techs say approximately once per year is usually sufficient, but it still pays to test the pool’s CYA concentration every month to ensure that the level hasn’t dropped due to splash-out or backwash.
“But CYA doesn’t degrade,” Rigsby says. “It’s not something you have to replace all the time, but you want to keep an eye on it.”
Some service techs even recommend switching to tablets during colder months, when certain ECG models automatically shut down. “We use tabs during the winter, because our water gets colder than 55 degrees, and most cells shut off at 55,” Brummett says. This can help prevent algae blooms and other microbe infestations during the winter.
Trichlor tablets contrast with ECG chlorination in another way, too — while these tablets tend to drive the water’s pH downward, the pH of an ECG-chlorinated pool tends to drift upward (as discussed in the “Balance concerns” section earlier). This means the water balance regimen that keeps traditionally chlorinated pools balanced can send an ECG-chlorinated pool’s LSI value well above the acceptable range.
Sequestrants can lead to a few problems in ECG-chlorinated pools. Some simply aren’t as stable in the presence of high levels of chlorine — in other words, the levels inside the ECG itself — which can make them less effective. Also, some sequestrants are based on phosphates, which break down into orthophosphates — chemicals that combine readily with calcium in the pool to form calcium phosphate on the ECG. In any case, many manufacturers make sequestrants that are designed specifically for use in ECG-chlorinated pools; the packaging will usually specify this.
Some dry acids — such as sodium bisulfate — can leave sulfates in the pool, and these can contribute to scale problems similar to those caused by phosphate-based sequestrants. “And if you’re unlucky enough to live in a part of the country where you’ve got barium in the source water, then you can get barium sulfate in the ECG, and that is next to impossible to get off,” Brown adds. Pool test kits don’t generally include a test for barium; the best way to find out if it’s in the local source water is to consult the municipal water authority.
Bromine may also contribute to ECG trouble. Though this chemical can be a helpful supplemental algaecide in traditionally chlorinated pools.
“But you don’t want to use it in a salt chlorinated pool,” Denkewicz says, “because the bromide ions interact adversely with the electrodes in the cell.”
As the ECG’s electrodes make chlorine from chloride ions, they’ll also make bromine from bromide ions. “Bromide is harsh on the sensitive electrode,” Denkewicz explains; “it can damage it, and decrease the overall lifetime of the cell.”
Though these potential issues can cause problems for ECG-chlorinated pools, keeping them in mind will help ensure that many pitfalls associated with ECGs are avoided. As many ECG experts point out, chlorine is chlorine, no matter how or where it’s generated and introduced into the pool — but even so, a proper understanding of issues unique to ECG-chlorinated pools can extend the life of both the pool and its equipment.