A natural swimming pool (NSP) is a human-made body of water designed to provide a clean, chemical-free swimming environment. Unlike traditional pools that rely on sanitizing agents like chlorine or bromine, NSPs utilize a biological filtration system to purify the water. NSPs mimic natural ecosystems, such as mountain lakes or ponds, to maintain water clarity and quality through natural processes. This approach creates a balanced, living environment where plants and beneficial microorganisms perform the cleaning.
The Core Design: Swimming and Regeneration Zones
The functionality of a natural swimming pool is based on a physical separation into two interconnected areas: the Swimming Zone and the Regeneration Zone. The Swimming Zone is the deep, unobstructed area intended for human use, resembling a conventional pool basin. The Regeneration Zone is the shallow, planted area that acts as the pool’s biological filter.
This design requires a dedicated space where biological activity can thrive without disturbance from swimmers. Water circulation moves continuously between these two zones, ensuring the entire volume is processed regularly. A low-energy pump draws water from the Swimming Zone and directs it through the Regeneration Zone before returning it to the main basin.
The Regeneration Zone is engineered with layers of substrate, typically gravel or crushed rock, which provides the vast surface area needed for the filtration system. This substrate serves as the physical housing for the beneficial microbes that are the true engine of water purification. The continuous flow through this porous material allows the biological components to interact efficiently. The size of this regeneration area often requires up to 50% of the total pool surface to ensure adequate cleaning capacity.
Biological Filtration: The Role of Plants and Microbes
The cleanliness of the water is achieved through a process known as bio-filtration, which leverages the natural nitrogen cycle and nutrient competition. The primary mechanism involves the colonization of the substrate and plant roots by a complex, slimy layer of beneficial bacteria, referred to as biofilm. These microorganisms consume organic contaminants that enter the water, such as skin cells, dust, and pollen.
The bacteria break down these organic materials, converting nitrogenous waste, like ammonia, into nitrate. Nitrate is a compound that can be readily absorbed by the aquatic plants. This process is highly effective because it removes the substances that would otherwise feed undesirable algae growth in the Swimming Zone.
Aquatic plants, known as macrophytes, play a complementary role as nutrient strippers. They actively uptake excess nitrogen and phosphorus compounds from the water before those nutrients can fuel a significant algae bloom. By absorbing these growth-promoting substances, the plants effectively starve the algae, helping to maintain the water’s clarity.
A well-designed and balanced biological filter creates an ecosystem where harmful bacteria, such as Salmonella or Legionella, cannot establish a foothold because they are outcompeted by the beneficial microbes for resources. This constant purification cycle eliminates the need for chemical sanitizers to kill pathogens. The result is a naturally balanced water environment.
Maintaining Water Clarity Without Chemicals
While the biological system is largely self-regulating, consistent mechanical maintenance is required. Regular removal of physical debris is necessary to prevent organic matter from overwhelming the biological filter. This includes skimming the surface to remove leaves and other floating contaminants before they decompose.
Occasional vacuuming of the Swimming Zone floor is also required to remove accumulated fine sediment. This action prevents the buildup of sludge, which can consume oxygen and upset the biological balance. Proper mechanical circulation, driven by the pump, must be maintained to ensure water is passed through the Regeneration Zone for purification.
The plants in the Regeneration Zone require seasonal attention, typically involving pruning. Dead or decaying plant material must be cut back and removed, usually during the colder months. This prevents the nutrients stored in their biomass from leaching back into the water, supporting the ecosystem without chemical intervention.