We’ve been conditioned in the United States to think that the only safe water is water that has been sanitized by powerful chemicals and/or devices using ozone or ultraviolet light. In fact, it seems that our industry has promulgated a doctrine that we need to kill everything in the pool (except for swimmers).

While it is certainly true that disinfected water is safe for swimming, we need to keep in mind that we mammals evolved on our planet over millions of years both drinking and swimming in water that was clarified and purified naturally in ponds, streams, rivers and freshwater wetlands.

This makes it interesting to speak with prospective clients about the benefits of natural swimming pools — that is, pools in which biological processes treat the water in place of familiar chemical treatments — and to describe to wary homeowners how plants and microbial activity actually work to make the water perfectly safe for swimming.

Originally a skeptic myself, I have spent a tremendous amount of time understanding the science behind natural swimming pools and observing their operation in the field. Fortunately, the science at the heart of the natural swimming pool concept is well-established. Indeed, it stands at the core of how all healthy freshwater systems work in nature (at least on this planet).

Nature’s Waste-Management System

These days, few of us consider that humankind developed and thrived without knowing how to apply chlorine, ozone or any of the other chemicals or devices used to treat water.

Our ancestors spread across the planet drinking natural water from a variety of sources without much trouble at all. And the waterborne diseases that eventually required chemical water treatment didn’t become a problem until cities became densely populated and the detritus of human occupation contaminated local water supplies. Similarly, the copious quantity of fertilizer used in farming has made its way into water systems, polluting them and knocking them out of their natural ecological balance.

The science behind the biological water treatment in natural swimming pools is called limnology, which some refer to as the “oceanography of fresh water.” It is the study of biological systems present in the fresh water we’ve relied on since the dawn of man. The idea behind natural swimming pools is an extension of this science, bringing common natural processes to manmade systems.

By mimicking the natural processes in a specially constructed and controlled environment, we are able to clarify and purify pool water without the use of chemicals and/or devices. The key to all of this is the nitrogen cycle, which might be best described as nature’s waste-management system.

Here’s how it works: Organic matter made up of debris and detritus enters the water in the form of all the compounds associated with human bathing and other sources (including leaves and fertilizer). These compounds give rise to microbial growth and algae that, since early in the 20th century, we have sanitized and oxidized using chlorine and other substances in our chemically treated pools.

In natural swimming pools, by contrast, beneficial agents called heterotrophic bacteria break down these compounds and convert them to carbon dioxide (CO₂) and ammonia. Next, another beneficial bacteria known as nitrosomonas bacteria converts the ammonia into nitrites (NO₂). Finally, in the third step of this elegant process, nitrobacter bacteria convert the nitrites into nitrate (NO₃).

Nitrate is the compound that makes up the bulk of the fertilizer we buy to feed plants. In traditional swimming pools, this material can be a major problem, as nitrates are the primary food for algae. In a natural swimming pool, however, nitrates are removed by aquatic plants that we place in what we call regeneration zones.

In effect, the detritus that would otherwise require oxidation is instead used to grow plants, and the plants removing the nutrients from the pool water results in pool water that is perfectly clear.

Regeneration Zones

Again, in achieving this effect we are relying on ecological relationships that exist in natural bodies of water. In this context, bacteria are essentially “reducers” that take detritus and transform it into oxygen and nutrients for plants and phytoplankton such as algae. The phytoplanktons and oxygen are then consumed by a host of tiny animal species categorized as zooplanktons.

Natural swimming pools include the abovementioned regeneration zones (or what some call constructed wetlands) to harbor these biological processes and make the water safe for swimming. And that is just one application of this approach: In fact, the “wetlands effect” is now being used with great success to treat mass quantities of wastewater and to restore natural bodies of water to healthy, balanced conditions.

(In fact, one of my friends has successfully used constructed wetland technology in lieu of a septic system at a number of homes in the Colorado Rockies, and many municipalities are now planning systems that use constructed-wetland technology for municipal sewage and wastewater treatment.)

The conceptual foundation for all of this is the nitrogen cycle as it’s applied in closed-loop systems. Here, the compounds that might otherwise engender growth of harmful pathogens instead wind up inside the plants, which we can harvest and remove from the system. Through this harvesting, we actually reduce the volume of nutrients carried by the overall system — although once the nutrients are bound up in the plants, they no longer are a source of potential problems.

Harvesting is just one of the tactics we use to control the nutrient burden. Another key detail of these systems is the fact that the plants in our constructed wetlands are not rooted in soil; instead, they grow in gravel in a hydroponic system. The only place these plants can gather nutrients is from the water itself, so they’re very hungry.

Not only do these concepts make sense, they can be applied with just a few variations on the techniques that are already used to build conventional swimming pools. These natural systems, for example, require proper hydraulic design with respect to flow rates, turnover rates and pump/plumbing sizing — just as is the case with their chemically treated cousins.

As for differences, there are two main ones, both of which are related to the design and construction of the regeneration zones. First, the gravel must be right for the job; second, the system must be set up so the aquatic plants stay hungry — meaning we are sure to eliminate any surface runoff into the pool that can carry fertilizers, pesticides or other organic compounds into the water from the surrounding landscape.

Additionally, the water plants must be able to out-compete the algae for the nutrients in the water, thereby limiting the amount of algae that can grow in the system.

It also bears mentioning that proper regeneration zones will attract animals in the form of amphibians, insects, birds and other species that thrive in natural wetlands. A family of happy frogs living in a regeneration zone is a good indication that the system is in equilibrium. In fact, we use skimmers that are frog-friendly; instead of finding bleached-white dead frogs in our skimmer baskets, the frogs are able to easily escape.

Setting Accurate Expectations

In speaking with clients about these systems, we’ve come to recognize that it is extremely important to set accurate expectations.

Most of us have had the experience of swimming in natural bodies of fresh water in the form of rivers, streams, lakes and ponds — and we survived with little concern about the presence of plants, microbial life forms, biofilm and other animals. Nonetheless, once you start talking about a “swimming pool,” all sorts of suppositions about how that body of water will look and how it will perform crop up almost instantly. The natural swimming pools that I have seen — designed and built correctly, of course — all had water that was transparently clear, and I was perfectly able to see right to the bottom of the deep end.

We never make any bones about it: The inescapable fact is that natural swimming pools teem with life, and the distinction we work to convey is that the nitrogen cycle our clients’ children are learning about in their science classes is what makes that water safe for swimming.

We also point out in some cases that our own bodies teem with microbial life. In fact, every healthy human body plays host to trillions of microbes of perhaps a thousand different species. The list of those that are harmful is infinitesimal compared to the vast majority that are beneficial. We couldn’t survive without them. Indeed, our bodies host about 10 times more bacteria cells than human cells — so all in all, we’re about 90 percent microbes!

Ultimately, we keep coming back to a simple fact: The processes we use to keep the water safe in natural swimming pools are akin to the reliable set of enduring biological relationships that enabled our ancestors to flourish around the globe without ever giving a passing thought to the plants and microbes that keep us safe.