Biological water treatment

How natural swimming pools and swimming ponds work

Natural pools use mechanical and biological treatment instead of conventional continuous chlorination. Clear water depends on correctly sized swimming and treatment zones, adequate flow, suitable filtration and regular maintenance.

Natural swimming pool with separate swimming and planted regeneration zones — How the water circulates

The basic distinction

How is a natural pool different from a conventional pool?

A natural pool is defined by its treatment system, not its shell. It contains a deliberately designed aquatic ecosystem that uses biological and mechanical processes to maintain water quality. Suppliers use different names, so the quote must explain the actual technology.

Swimming pond

The swimming area and a visible planted regeneration zone form part of the same landscape. Seasonal changes, occasional insects and a thin biofilm are normal in a living system. Persistent cloudy or unpleasant-smelling water is not.

Natural pool with a separate treatment zone

A geometric swimming area is connected by pipework to a separate planted or filtration zone. This creates a more formal appearance, but the biological system still needs appropriate care.

Bio-pool or naturpool

These can look much like conventional pools. Biological treatment takes place in a concealed mineral filter or another purpose-designed filter, often with few or no visible plants.

Public natural swimming pool

Higher bather numbers require dedicated design calculations, monitoring and compliance with local public-health rules. A system designed for a private garden cannot simply be used for a public swimming facility.

Before drawing up plans

How the location affects water quality

Runoff, soil, leaves and the composition of the fill water can continually add to the treatment load. Assess the surroundings before selecting the system and sizing the filtration.

Sunlight and temperature

Excessive warmth can accelerate biological activity and algae growth, while too little sunlight limits the choice of plants. The designer needs to account for water depth, shade, circulation and the local climate.

Leaves and surface runoff

Leaves, fertiliser, soil and runoff from lawns introduce organic matter and nutrients. Pool edges, skimmers and ground slopes should keep this material out of the water.

Fill water

Well water, mains water and rainwater have different mineral and microbiological characteristics. Testing before design helps establish phosphorus levels, hardness and a suitable top-up strategy.

Space and access

A planted zone may increase the total footprint. A concealed filter still needs room for servicing. Plan access to equipment, a place to collect removed plant material and safe access for routine maintenance.

Structure, circulation and treatment

What to plan before excavation

The watertight structure holds the water, pipes and pumps circulate it through the filter, and biological treatment removes excess nutrients. A serious design fault in any one of these cannot be permanently compensated for by the other two.

Nutrient control is particularly important. Designers often assess phosphorus before construction because it can enter with fill water, soil, dust, plant debris and swimmers. No single plant solves this: nutrient input, removal and filter capacity all need to be considered.

Natural does not mean pump-free, maintenance-free or test-free. A simple landscaped pond and a compact high-tech bio-pool can require very different equipment, electricity use and routines.

Project stages

Planning a natural swimming pool

This is a planning checklist, not a DIY construction guide. A qualified designer must specify dimensions, circulation and water treatment for the actual site and use.

1Decide what you want the pool to look like, how many people will use it, when you will swim, how clear you expect the water to be and which seasonal changes you are comfortable with.
2Assess the site, soil and fill water. Then consider sun, wind, falling leaves, surface runoff and local construction and safety requirements.
3The designer can then select a system and calculate the zone sizes, water volume, pipework, mechanical treatment and biological filter capacity.
4Design the structure and waterproofing separately from the biological treatment. The shell must remain stable, penetrations must be watertight and soil must be kept out of the water.
5Install pipework, skimmers or overflow channels, suction and return fittings, filters and inspection points. Measure flow rates before covering the installation.
6Use only the specified plant species and growing media. Do not add ordinary garden soil or fertiliser to the water system.
7Commission the system gradually, with initial measurements, records and a first-season maintenance plan. The biological treatment needs time to establish.
8At handover, ask for a circulation diagram, pump settings, acceptable water parameters, a maintenance schedule and clear instructions for dealing with problems.

Pool care

How to maintain a natural pool

Natural pools need different care from chlorinated pools. You still need to remove debris regularly, maintain the specified flow, care for plants and monitor water quality.

Every week during the swimming season

Remove floating debris, check the skimmer and pre-filter, inspect flow and water level, brush accessible surfaces and record any sudden change in clarity or smell.

At scheduled intervals

Clean the filter as specified, remove sludge from designated collection points, check pumps and aeration, and test the water parameters specified by the designer.

Plants and dead vegetation

Dead plant material releases nutrients back into the water. Prune, thin and remove vegetation according to the plant species and season. Do not add fertiliser without the designer’s approval.

Spring start-up and winter care

Start-up, pruning, frost protection and circulation settings depend on the climate and system. Do not routinely drain the pool or add treatment products without the designer’s approval.

Problems and solutions

What to check if the water changes colour or smells

Do not add conventional pool treatments at random. Start by checking circulation, incoming debris and the biological treatment zone.

Green or cloudy water

Check circulation, mechanical filtration, nutrient input, bather numbers and filter condition. Adding an algaecide at random can damage the biological system and hide the underlying problem.

String algae

Small amounts are not unusual in a living system. If growth suddenly increases, remove the algae by hand and check sunlight, phosphorus, dead vegetation, circulation and the filtration zone.

Low water flow

A full skimmer, blocked pre-filter, air in the pipework, closed valve or pump fault can reduce flow. Running below the design flow rate reduces treatment performance.

Sludge and unpleasant smells

If organic material accumulates or areas become short of oxygen, check circulation, remove sediment and find where the contamination is coming from. An unpleasant smell is not normal in a well-maintained natural pool.

Dying plants or excessive growth

The species may be unsuitable for the depth, light or water chemistry. Excessive growth can indicate too many nutrients. Choose plants as part of the treatment design, not as a final decorative addition.

Water quality after a storm

If muddy runoff enters the pool, first correct the site drainage, levels and pool edges. Then remove the deposited sediment and check the treatment system.

Initial cost

The quote should include the shell, waterproofing, treatment zones, filters, earthworks, plants and landscaping. A price that excludes finishing work cannot be compared directly with a complete project price.

Operation and energy

Allow for pumps, skimmers, robotic cleaning or vacuuming, pruning and professional inspections. Ask for equipment power ratings and operating schedules, not promises that the system costs nothing to run.

Long-term costs

Budget for pump servicing, consumable parts, filter media replacement where specified, replacement plants, sludge removal and possible repairs to the edges or waterproofing.

Reviewing the quote

Eight questions to ask the designer and contractor

Ask for written, measurable answers specific to your site. Manufacturer claims are useful only when they identify the system and conditions they apply to.

1Exactly which system are you proposing, and how are the swimming, regeneration and filtration zones arranged and connected?
2What measurements and assumptions were used to size the filter and set flow rates and operating times? How many bathers is the system designed for?
3What soil and fill-water tests have been carried out? How will soil, fertiliser, leaves and surface runoff be kept out?
4Which parameters will be measured at commissioning and afterwards, how often, and who will interpret the results?
5Which plants, substrates and filter media are specified? Who supplies them, and how are they maintained or replaced?
6Does the quote cover excavation, the shell, equipment, planting, edge landscaping, commissioning and owner training?
7Who guarantees the structure, waterproofing, equipment and biological treatment, and what conditions apply to each warranty?
8What local servicing is available, and what happens if water quality deteriorates during the swimming season?

FAQ

Natural pool questions answered

Does a natural pool mean no chemicals at all?

The principle is to treat water mechanically and biologically rather than maintain a conventional residual disinfectant. Water chemistry still needs testing, materials must be compatible, and any corrective treatment must suit the particular system.

Does a natural pool need plants?

Swimming ponds usually have a visible planted regeneration zone. Some modern bio-pools use a concealed biological filter with very little visible vegetation. Names vary between suppliers, so ask how the water is treated rather than relying on a product name.

Will it attract mosquitoes?

Mosquitoes mainly breed in still, stagnant water. Proper circulation, surface maintenance and a functioning ecosystem reduce suitable breeding spots. Check other standing water around the garden too.

Can the water be completely clear?

The water can be very clear, but a natural pool is not a sterile environment. Seasonal changes are possible. Persistent cloudiness, unpleasant smells or heavy algae growth need investigation.

Is it cheaper to maintain?

Not necessarily. Disinfectant use may differ, but pumps, filters, cleaning, plant care, testing and occasional specialist servicing still cost money. Compare lifetime costs, not just chemical consumption.

Can an existing pool be converted?

Sometimes, but the shell, pipework, available space and options for a separate filtration zone all need assessment. Adding a few plants to a conventional pool does not turn it into a properly designed natural system.

Is it safe for children?

Any open water requires supervision and multiple layers of protection. Natural edges, varying depths and planted zones can make it harder to see swimmers. Plan safe entry points, slip-resistant surfaces, barriers and rules from the outset.

How much space does a natural pool need?

It depends on the treatment system. A swimming pond with a large regeneration zone needs more space; a compact biofilter can reduce the visible treatment area. The designer should calculate the proportions for the specific system.

Related guides

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Pool Explained editorial team

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