Pool Water-Treatment Systems and Processing Myths

May 1, 2014, Department, by Rich Young and Kent Williams

Filter efficiency is dependent upon turnovers and recirculation flow. This chart, based on Gage & Bidwell’s Law of Dilution, shows the percentage of turbidity remaining after each turnover.There are many and various water treatment systems and processes that provide the excellent water quality we have come to expect in our public pools. Some are simple, others complex. Some are located or do their work in the mechanical room, and some are out in the pool. It is important we have a firm grasp on what the system or process is designed to do, is capable of doing and — as important — what it cannot do.

Having said that, there are some systems and processes that fall short of published expectations, all of which are worthy of careful review. We call these unrealistic expectations “myths.” With budget dollars becoming increasingly tough to come by, aquatic facility managers and directors need to make sure they spend their money wisely. There are many myths out there. In this article, we look at a few regarding “outboard treatment systems and processes.”

Outboard Treatment Myths

Expectations for some processes exceed reality. It is important we are aware of some of the myths having to do with outboard treatment systems and processes. 

The term “outboard” describes those processes and systems that treat pool or spa water someplace other than in the pool. Most often they are found in the pump room, mechanical room or whatever name is given to that space where the pumps, filters, heaters and other equipment are found.

So why do we include outboard treatment systems in our discussion of myths? We must first review what and where various water treatment processes are designed to take place.

Filtration is a good example of an outboard process that most often occurs in the pump room, not in the pool. Any facility might install the latest mega-dollar, microprocessor-controlled filter system with every bell and whistle imaginable, be it diatomaceous earth, high-rate sand, cartridge or other new snazzy filter media, often with the addition of a filter “aid,” usually a clarifier or polymer that will make a good filter better by removing even the smallest of particles. It is important to remember that bacteria range in size from .2 to 5 microns, viruses from .005 to .2 microns and protozoan cysts from .5 to 10 microns, squeezing through even smaller spaces. So, the filter is probably not the place we would expect to handle the removal of those items.

Heating is another outboard process completed in the pump room. There are low- to high-efficiency systems and of course solar options. 

The point is that these outboard systems are totally dependent upon the circulation flow and the frequency of turnover. The definition of a “turnover” is “to circulate the amount of water equal to the total number of gallons of the vessel.” Note we did not say “all the water!” More on that soon.

Other outboard processes that are just as dependent upon recirculation and frequency of turnover include ozone use and all types of ultraviolet treatment.

Ozone systems are fantastic at destroying bacteria, viruses and protozoan cysts many times faster than even high levels of chlorine. Ultraviolet systems, including amalgam high-intensity, low-pressure and medium-pressure systems, not only inactivate the same pathogens, but do so in a single pass if set up correctly.

So what is the big deal? Let’s look further into the two important water treatment processes that are absolutely necessary and must take place in the pool: sanitation and oxidation.

Sanitation and Oxidation

Sanitation is the inactivation of bacteria, viruses and protozoan cysts. We sometime refer to this process as “killing the bugs,” although that is not entirely true.

Oxidation is the dismantling (breaking down) of organic compounds such as saliva, sweat and urine (and other “stuff” we won’t bother to describe).

Both processes must be ongoing in the pool. Bacteria such as E. coli, staphylococcus, pseudomonas and viruses such as the rhinovirus must be inactivated in seconds; protozoan cysts such as giardia and cryptosporidium might take hours. Organic compounds such as saliva, sweat and urine should be broken down in minutes.

So, again you ask, what is the big deal? Where is the myth?

With an understanding that outboard treatment systems only treat the water that reaches the pump room, we must look closer at the recirculation process. 

This demands us to carefully examine and fully understand Gage and Bidwell’s Law of Dilution. Look at the graph above.

Examine the “real” amount of water that reaches the pump room after the first turnover of all those gallons: less than half! Even more important is how much water has not been through the pump room: almost 58 percent. Keep in mind, the basis for this Law of Dilution is the proven removal of particles placed in suspension that were introduced to the water in one large dose.

After a single code-required turnover of a typical swimming pool, which is usually six hours, there is 58 percent of the original load of particles remaining in suspension. So, be it particles of dirt, bacteria, viruses or protozoan cysts, we must realize no one is really protected for the first quarter of the day. But wait! Look further at the second, third and even forth turnovers. Even after four full turnovers, 2 percent of the original dose still remains in suspension — out there with the swimmers! For all the water to reach the pump room at least once we must experience more than four turnovers.

These concepts lead us to the teeth of outboard treatment system myths. Filters, filter additives, ozone and UV systems do little to protect our patrons for the first 24 to 30 hours in a typical pool, or just four to six hours in a wading pool and less than two hours for a spa.

With this understanding, we realize in order to continually protect patrons from most pathogens, an effective residual of a sanitizer/oxidizer (usually chlorine or bromine) is required at all times. Keep in mind “effective” implies not just how much, but the present “work value” of our chlorine/bromines. Of course we are talking ORP (oxidation reduction potential), a subject for another day.

In addition, claims of significant chlorine and bromine savings with the use of filter aids, ozone and UV also fall short of reality. Answer this simple question: Besides the sun, what is the cause for the greatest loss of your sanitizer/oxidizer? Any good AFO or CPO can tell you it is oxidation, the breakdown of organics. That activity is followed by disinfection. Where do these occur? We better hope it is out in the pool! By the time the water reaches the pump room to see the results of the outboard treatment processes, most of the pathogens and organics had better be handled. That is not to say that in heavy organic load situations, ozone, and especially new UV systems, offer tremendous help in removing the combined organic chlorine compounds we call “chloramines.” They also offer a “secondary line” of disinfection and oxidation — but clearly a distant secondary line.

So, with all that said, when the sales person either boasts of, or provides you with, literature that includes claims of immediate protection for your patrons with these outboard treatment processes, along with great chlorine/bromine savings, you can put those claims in a proper, minimized perspective. 

Rich Young is the Owner/Manager of Aquatic Commercial Consulting, an independent pool and waterpark industry firm, as well as a contractor, lecturer and NRPA AFO instructor. Young was the managing editor for the 6th edition of the AFO Manual. Kent Williams is a longtime independent pool and waterpark industry consultant, speaker, researcher, AFO instructor trainer and author of the NRPA AFO Manual.