The Standard
From the Chemist
Last month I promised this issue would cover what happens to your pool between the last swim and the first day back next spring. That promise comes in two parts: why the chemistry matters in the first place, and why it doesn’t stop mattering once the season does.
Closing season is underway in the Kansas City metro right now. Covers are going on, water that sat in the 80s°F (upper 20s°C) all summer is cooling by the week, and for most pool owners, this is the moment the pool drops off the list until April. On our field visits it’s the opposite. The water doesn’t stop changing when the swimming stops, and some of the most expensive damage we see all year starts in the months nobody’s looking.
That’s the reason I wrote The Pools Scientific Guide to Off Season Maintenance. Too many pools get opened in spring already carrying damage that began in December, and every bit of it was preventable. The book puts everything we do for our own clients through the winter into one place, so any pool owner can do the same. You’ll find it further down in this issue.
John Cooper, Pools Scientific
The Science
Pool chemistry does two completely different jobs at the same time, and mixing up the two is where most of the industry’s bad habits come from.
Sanitization kills whatever swimmers bring into the water. Every person who gets in carries bacteria, viruses, and parasites on their skin and in their bodies, and the pool becomes a shared bath for all of it. Chlorine’s working form, hypochlorous acid, crosses a microorganism’s cell wall and oxidizes it from the inside. In CDC testing, E. coli dies in under a minute at ordinary chlorine levels. Hepatitis A takes about 16 minutes. Giardia takes about 45.
Those times come from water with no stabilizer in it. Most outdoor pools carry cyanuric acid, which protects chlorine from sunlight but also slows it down, so real kill times in a real backyard pool run longer than the lab numbers.
Cryptosporidium, a parasite wrapped in a hard outer shell, survives more than ten days even in that stabilizer-free lab water. It’s the leading cause of disease outbreaks in treated swimming water in CDC surveillance data, and it does all of this in water that looks perfectly clear. Clear water tells you nothing about whether the water is safe. Sanitization isn’t a big dose once a week. It’s chlorine present and working every hour of every day, so everything short of Crypto dies long before it reaches the next swimmer.
Water balance protects the investment itself: the plaster, the gelcoat, the vinyl, the equipment, all of it. The number that ties it together is the Langelier Saturation Index, or LSI. LSI tells you whether your water wants to deposit scale (LSI too high) or pull calcium and material out of your surfaces (LSI too low, and yes, that includes plaster, over months and years). Every pool we service stays between negative 0.3 and positive 0.3, because that’s the range where water is genuinely balanced instead of quietly doing damage in one direction.
Sanitizing and balancing aren’t the same test. A pool can pass one and fail the other completely.
The Technology
Ultraviolet light kills Crypto in the seconds water spends passing through a UV unit. It doesn’t need to break through that hard outer shell by chemical force. It damages the parasite’s DNA directly, so it can’t reproduce and can’t make anyone sick. Public health codes call for UV or ozone as a secondary disinfection system at higher risk public pools, like splash pads and pools built for small children, for exactly this reason.
That’s half of why AOP, UV light plus ozone, is the strongest complement to a salt chlorine generator. Combined chlorine is the other half. It forms when free chlorine reacts with organic contamination, sweat, sunscreen, skin oils, and it’s a far weaker disinfectant, somewhere between 25 and 100 times weaker than free chlorine depending on which compound forms. That sharp “chlorine smell” everybody blames on too much chlorine comes from this, and it’s the opposite problem. An AOP system’s hydroxyl radicals break that organic contamination down before it reacts with your chlorine, which keeps more of your chlorine doing its real job.
UV and ozone only treat the water that passes through them. Anything sitting in the pool waits until circulation brings it around, which is one more reason the pump at every pool we service runs 24 hours a day. The AOP system doesn’t replace your chlorine. Chlorine covers the whole pool all the time, and the AOP system covers what chlorine can’t.
Poolside
Most pool companies treat a closed pool as a problem that goes away until spring. It doesn’t. Water temperature in the Kansas City metro can fall from the 80s°F (upper 20s°C) in July down into the 30s°F (near freezing) by January, and that swing pushes LSI in one direction the whole time, negative, even if nothing else about the water changes. A pool that closed up balanced in October can be actively etching by January with nobody watching.
Vinyl and fiberglass pools have it worse than plaster here. Plaster carries a deep, built in reserve of calcium in the material itself, so it has some cushion when the water turns aggressive. Vinyl and fiberglass don’t have that buffer, so the same cold weather LSI drop hits them harder and faster.
This is exactly why Pools Scientific clients stay on an active off-season water balance program all winter, instead of closing up and walking away until spring. It’s also why I wrote The Pools Scientific Guide to Off Season Maintenance, a guide that walks any pool owner through exactly what’s happening to their water all winter and how to manage it instead of hoping for the best.
On the Show
Episode 23, “Metal Staining, Sequestrants, and the Ecosystem of Metals in Pool Water,” aired September 22. It’s built around a genuinely rare field case, a Pools Scientific client’s pool turned purple for the better part of a week, one of the strangest calls I’ve ever taken. The episode walks through where metals in your pool come from, why they stain the way they do, and what works to prevent and remediate it.
Next up: what your plaster’s been quietly trying to tell you the whole time.
John