Anyone who has managed commercial swimming pools for a while knows that the water ages over time and becomes very difficult to balance.

While replacing aged water with fresh has long been the best and most viable solution, attitudes toward water are dramatically shifting, probably forever. In California, Arizona and Nevada, especially, this has been brought on largely by historic drought. In response, the pool industry has looked for solutions outside the status quo.

Here’s a quick look at when and why water traditionally is replaced: Water ages for three primary reasons. First, evaporation takes out the pure water, but leaves minerals behind. When tap water replenishes evaporation loss, it introduces more minerals. Second, bathers carry waste with them into the pool. And third, pool chemicals leave behind by-products. All this leads to an increase in total dissolved solids (TDS), which eventually must be addressed.

Unfortunately, most of these problems cannot be treated chemically — TDS must be physically removed. Traditional pool filtration sifts out suspended solids, but nothing that is dissolved into the water, such as calcium and salt. But we can’t just change out our water as freely as we once could.

This is where reverse osmosis (RO) comes into play. A technology long in use for industrial water treatment, RO removes particles down to the ionic level — sodium ions, chloride ions, cyanurate ions and so forth. Relatively recently, the industry has found a way to use RO to clean and recycle pool water.

RO and pools


Studies and trials of RO in the pool industry can be traced back as early as the mid 1990s. Innovators recognized the need to conserve our most precious resource. The process really began to take off commercially in the early 2010s. Now, dozens of companies in California, Arizona, Nevada and even parts of Texas provide this service.

Most of these companies have industrial-grade RO systems built into a trailer, so they can process the water on site. Diesel generators power many, making them self-sufficient. Pumps move the water through a series of hoses, taking it from the pool to the trailer, through the filtration system and back. Generally speaking, the systems incorporate some type of pre-filtration: We want to capture the larger solids first, so they don’t plug the RO membranes.

After passing through the membranes, the water returns to the pool in bottled-water quality. In one example, a 600,000-gallon community pool we treated began with a calcium hardness of 2,300 parts per million and a TDS of more than 6,000 ppm. It was brought down to a calcium hardness of 320 ppm and a TDS of 800 ppm.

Typically, there is a discharge of 10 to 20% of the water that goes to waste. But not all waste rates are the same: The higher the TDS levels of the aged water, the more that will have to be discharged.

Filtration times vary, depending on pool size and TDS levels. A small HOA or apartment pool of 40,000 gallons, for example, could take 6 to 8 hours. On larger pools, we may need multiple trailers. So a half-million-gallon facility might take a couple days to process, using a couple trailers. In some cases, treatment can be performed in stages to prevent pool closures.

It’s always best to do this before the water ages too much. It should be performed when TDS levels are closer to 2,000 to 4,000 parts per million, as opposed to waiting until they reach 8,000 or 15,000 ppm. At these levels, the water becomes very difficult to treat — it’s going to take longer, cost more, and have a higher waste rate.

Additional benefit

This process also offers many benefits over a traditional acid wash — water savings still being first and foremost. It also keeps the plaster submerged, allowing it to remain hydrated, which helps preserve plaster against spalling and check cracking, especially in extremely hot and arid climates.

While our primary goal in the aquatics industry is to give our customers the best swimming experience possible, we must take ownership of all the resources we use and strive to be as efficient as possible. I believe we should be proactive instead of reactive as individual companies and as an industry at large.