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With recent outbreaks of cryptosporidium across the nation this past year, many operators may be wondering what they can do to protect themselves against this insidious and hard-to-kill bug. And manufacturers have no shortage of solutions, such as ultraviolet and ozone systems that destroy crypto, as well as special filters and filtration enhancements designed to trap it.

Before you invest in any of those options, however, it’s important to understand how pool circulation and filtration works, and why pump room remedies cannot guarantee crypto-free water at any given time.

Devices or processes that work in the pump room are called “outboard treatment systems.” This simply means that these systems handle only pathogens that enter the pump room via the pool’s circulation system. They do not destroy or catch crypto in the body of water — out there in the pool where the swimmers are. This distinction becomes important when you talk about turnover rates and how pool filtration works.

To do so, consider Gage and Bidwell’s Law of Dilution, which may best be described in the National Recreation & Park Association’s Aquatic Facility Operator Manual by Kent Williams. Simply put, much less than half of all the pool’s liquid contents actually sees the pump room in any one turnover (circulation of water equal to the total capacity of the pool). Assuming the presence of foreign material such as suspended soil or crypto in the water, only 42 percent of that foreign material has any potential to be either removed by enhanced filtration or destroyed by other processes during any single turnover of the pool — six hours in most full-sized aquatic venues. Thus, patrons remain at risk for many hours after infection, regardless of which killing or catching device is working in the pump room.

Furthermore, most conventional pool filters allow the organism to pass right through, ending back again in the pool. Even the top-of-the-line filtration processes (2- to 3-micron catch, oversized and very expensive) that do catch crypto do little to help protect people in the pool for most of a full day following an infected fecal event. In addition, this process would only catch the invader, not kill it.

Specific and broad-based “polymers” and “filter aids” that enhance the performance of a conventional filter (that is, make the filter catch smaller particles) eventually will produce a significant loss of flow. This results in a longer turnover and again little or no protection for more than a day or maybe two. This process only catches the cysts; it does not kill them.

Ozone and medium-pressure UV systems are other options; both have proven effective in killing crypto. However, because they are installed in the pump room, they must be classified as an outboard treatment system. Therefore, the same dilution law applies, resulting in prolonging the risk, long before the crypto is destroyed. Patrons remain unprotected for the first day or so even on full-flow treatment.

Sprayparks also must take precautions. National attention recently was triggered by two major incidents of crypto infection occurring in sprayparks — those above-grade, interactive play sites with fountains, sprayers and water cannons, where lots of toddlers play. Nationally, the design and application of reservoir hydraulics vary greatly, with some systems having two- to four-hour turnovers; most patrons remain at risk for many hours after a crypto-infested fecal accident.

It’s important to understand that ozone and medium-pressure UV should be applied to all of the water returning to waterfeatures, not simply in the circulation loop of the filtered reservoir. In this way, the water that children come in contact with is less likely to carry the dreaded crypto.