WHAT IS A NATURAL SWIMMING POOL?

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A natural swimming pool (NSP) is a swimming pool that keeps its water clean through biology instead of chemical disinfection. It doesn't hold a chlorine or bromine residual. Instead, water circulates through a biological filter where plants, beneficial bacteria and other microorganisms remove the nutrients and organic matter that would otherwise feed algae and cloud the water.

You may also see NSPs called swimming ponds, natural swimming ponds, swim ponds, natural pools, organic pools or biological pools. Some people also call them chemical-free pools. That label is useful shorthand, but it shouldn't suggest the water is sterile. Like a lake or river, an NSP holds living organisms, and keeping it healthy depends on good design and ongoing care.

ASPNSP uses a definition based on limnology, the study of inland waters. Under this definition, a natural swimming pool has four attributes:

1. A contained vessel. No water enters from outside the system except makeup water, which replaces evaporation and other losses.
2. A separate swimming zone. The swimming area is always separate from the biological filter.
3. No disinfecting chemicals or devices. The pool doesn't use chlorine or other chemicals, or devices such as UV lights, ozone generators, ionizers or salt systems, to disinfect the water. Purification is entirely biological.
4. Built like a pool. The NSP is designed, engineered, permitted and built to the swimming pool guidelines that apply where it's located.

Definitions differ across the industry. For example, some trade sources describe "natural" pool systems that include UV clarifiers. Those systems would not meet ASPNSP's definition. When you compare builders or products, ask exactly how the water is treated.

Most designs combine three processes:

• Mechanical capture. Skimmers and pre-filters remove leaves, pollen and other debris before they break down and release nutrients.
• Biological filtration. Water flows through gravel or another filter medium. Bacteria living on that surface break down organic matter, turning wastes into nitrate.
• Nutrient uptake. Aquatic plants and/or biofilm take up nitrate and phosphate. With fewer nutrients available, algae have less to grow on.

The gravel-filled area is often called the regeneration zone. In some designs it sits beside the swimming area in the same basin, behind a submerged wall. In others, the biological filter is a separate, engineered chamber that can be out of sight, which lets the swimming area look like a conventional pool. Designs that rely on a large planted zone generally need more land. Designs with a compact engineered filter use less space but depend more on pumps and filter backwashing.

An NSP isn't just a pond you can swim in. Ordinary ponds and lakes take in runoff, groundwater and wildlife, and their water quality can change from one day to the next. An NSP is a closed, engineered system with a defined filter, controlled circulation and a swimming zone built to pool standards.

Compared with a chlorinated pool, an NSP works differently rather than simply "without chemicals." A chlorine pool relies on a disinfectant residual that is intended to kill all of the microorganisms in the water. An NSP relies on filtration, competition between microorganisms and nutrient control.

Natural swimming pools are long established in Europe. Germany's FLL, a landscape research and standards society, publishes detailed recommendations for planning, building and operating outdoor pools with biological water purification, and many North American designers draw on that guidance.

In North America, the Webber Natural Swimming Pool in Minneapolis opened in 2015. The Minneapolis Park and Recreation Board describes it as the first public natural swimming pool in North America. It has lifeguards and a planted regeneration basin, and its water is tested regularly for bacteria.

An NSP may suit you if you want to swim without chlorine and you like the idea of a pool that works as a small, managed ecosystem. Before you decide, think about:

• Space: whether you have room for a regeneration zone or a separate biological filter
• Rules: your local permit and barrier rules
• Upkeep: seasonal maintenance, such as leaf control in fall and plant care
• Budget: whether it fits

To find professionals with natural pool experience, see the ASPNSP Member Directory. You can send other questions to info@aspnsp.org.