A natural swimming pool, also called eco pool or chemical-free pool, is a type of swimming pool that operates without an array of sanitation chemicals and that can be built with a minimum of materials. Common in Europe, this swimming pool type leans toward an ethos of building with nature and blending into the natural landscape around it and thus may use gravel stone and clay instead of concrete or fiberglass, and aquatic plants in a filtration area instead of chemicals and mechanically-complex filtration systems for pool sanitation. The plants in a natural pool's filtration area planted hydroponically (placed no deeper than 30 cm.) create a low-cost maintenance system for the pool. A conventional pool's complex sanitation and filtration system would often cost its owner more than ten times the maintenance cost spent on a natural swimming pool. Additionally, a natural swimming pool requires only a small amount of management hours and does not require regular draining. Apart from economic reasons, a natural swimming pool seeks to avoid chemicals such as chlorine that can irritate a pool user's eyes and skin, provide a natural aesthetic to a yard, or welcome more wildlife into a given environment, among other reasons.
History In the 1980s, Austria opened the first private natural swimming pool in Europe. In 1998, Germany opened Europe's first public natural swimming pool. In the United States, the first reported natural public swimming pool was completed in 2015 in Minneapolis after ten years of building. The superintendent of the Minneapolis Park and Recreation Board said that the pool is "part of what keeps our Park system the number one park system in the United States," adding that "It's better for the environment and for users." The Minneapolis Board also reported that Europe had, as of that year, already 20,000 public and private natural swimming pools. The Minneapolis pool can hold up to 500,000 gallons of water, recycled every 12 hours. In 2019, Canada opened the first natural public swimming pool in the country, built by gh3*, in Edmonton.
Construction
Water source A natural pool would usually have a natural water source. In one such pool, rainwater is allowed to run down an olive orchard hill through to its filtration pool filled with plants, while other natural pools use bore water.
Filtration area Construction of a natural swimming pool separates the filtration area (where the aquatic plants are placed) from the swimming area. Aside from providing dragonflies and other aquatic life an ecologically diverse small habitat, this filtration area's shallow-water plants—placed at one side of the pool or around it—are not only set there to boost the pool water's oxygen but also to provide support to the beneficial bacteria (decomposer organisms) at the plants' roots as well as to the aquatic life consuming the dross, the potentially unhealthy organisms or contaminants, and the excess nutrients in the water in both the filtration and swimming parts of the pool. This system mimics the same natural system that would present in a stream, lake or wetland. The Canadian natural pool built by gh3* uses both a biological-mechanical system in a sand and stone pond and the wetland and gravel filter it constructed as a hydro botanic pond, filling this latter with zooplankton. Adjacent to these ponds, a granular filter PO4 adsorption unit was installed, running along the gabion walls of the pool, thus allowing the water to laterally circulate from one end to the other.
Filtration area plant distribution In the design of the plant zone for the pool's filtration, which may take up 30-70% of the total pool area, taller aquatic plants are positioned near the edge of the zone's water, while submerged and floating aquatic vegetation would occupy the gradually deeper water area near the border with the pool's swimming zone. The shallow area with the tall plants provides a habitat for invertebrates that eat mosquito larvae. In one natural pool, the following plants are used: water lily, water hyacinth, phragmites australis, caltha palustris, and mentha aquatica for purification, and hippuris vulgaris and waterweed for oxygenation.
Natural and synthetic materials used To augment its ecological intent of using only natural materials as much as possible, a natural swimming pool's construction would avoid a conventional swimming pool's need for an iron framework beneath cement walls, meant to prevent the soil around the pool from caving in, simply by digging the pool with sloping sides. A layer of bentonite clay, ethylene propylene diene monomer, RPE-reinforced polyethylene or enhanced PVC (fish- and plant-safe polyvinyl chloride) liner is then applied to seal the soil before cleaned gravel stones are placed on top of the walls. The gravel layer also becomes a habitat for beneficial bacteria that help biodegrade leaves and other natural materials that have sunk to the bottom of the pool. Sand may also be added. To help protect the pool's sides, cobblestone steps or a cantilevered board may be placed for access into and out of the pool.
Option to build with concrete Other builders of a natural pool may still opt to use cement or Rastra blocks. Although a cement pool would be less eco-friendly than a pool built with gravel and stone, the plant-based filtration system that would be used to operate the pool would still reduce chemical and energy usage.
Maintenance
Water circulation and aeration To enable the roots of the plants in a natural pool's filtration area to clean the pool, the pool's water must continuously circulate. The water must also be aerated every now and then to address the oxygen needs of the water organisms. Lack of circulation and aeration would produce water stagnation, which would in turn nurture foul-smelling anaerobic bacteria.
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