Biological filtration for swimming pools: reed-bed treatment, technical filter, or hybrid system - how to choose

A chlorine-free swimming pool, where the water is purified by plants, gravel, and bacteria: the idea is becoming increasingly appealing. But behind the term “biological filtration” lie very different systems, from a large planted treatment pond that takes up as much space as the swimming pool to a compact technical filter hidden in a utility room. Choosing biological filtration first means choosing a compromise between available space, the amount of technology you are willing to accept, and the level of maintenance: the smaller the planted area, the more powerful the filter must be, and the more central phosphorus control becomes. Here is how to compare the options before you get started.

How biological filtration purifies water

A conventional swimming pool kills microorganisms with chlorine or another disinfectant, and removes particles with a sand or cartridge filter. Biological filtration works in the opposite way: it allows and even cultivates communities of bacteria, attached microalgae, and plants that consume what would otherwise feed suspended algae.

In practice, three mechanisms work together. First, mechanical filtration retains leaves, pollen, and particles. Next, the bacterial biofilm that colonizes gravel, filter media, and roots breaks down organic matter and converts ammonium into nitrates. Finally, plants and the biofilm absorb dissolved nutrients, especially phosphorus, which is the limiting factor for algae growth in most freshwater.

Remember this point, as it determines everything else: in a biological swimming pool, water clarity depends less on the fineness of the filter than on the amount of available phosphorus. A well-designed system keeps phosphorus levels extremely low; an overloaded system, or one supplied with phosphorus by the fill water, leaves, or fertilizer, turns green.

The three main system families

The German FLL recommendations, which serve as a reference for natural swimming pools, distinguish several types of pools, from the most natural, purified solely by plants, to the most technical, where most of the work is done by filters. There is no right or wrong type: there is the one that suits your site, budget, and tolerance for a «pond-like» appearance.

Planted treatment wetland in detail

This is the traditional form of a natural swimming pool. A shallow area, separated from the swimming area by a submerged wall, is filled with washed gravel and planted with helophytes: reeds, marsh irises, rushes, water mint, and purple loosestrife. A pump circulates the water through this bed.

Downflow or upflow

In downflow systems, water is distributed across the surface and flows down through the gravel toward drains placed at the bottom, from where the pump sends it back to the swimming area. In upflow systems, the reverse is true: water is injected from below and rises through the bed. Each configuration has advantages in terms of clogging, oxygenation, and maintenance; our dedicated article at the end of the page compares them in detail.

What needs to be planned

• A generous surface area, broadly comparable to that of the swimming area for a purely planted system.
• Washed gravel, with a suitable grain size, free of fines and friable limestone.
• Native plants, hardy, planted without potting soil or fertilizer. Our selection of aquatic plants for natural swimming pools will help you create the planting beds.
• A low-consumption pump that runs continuously, including throughout the season.
• Access for maintenance: annual pruning of the plants, removal of leaves, inspection of the drains.

The technical biological filter

When space is limited, or when you want water that is as clear as possible with a pool-like rather than pond-like appearance, filtration is entrusted to equipment. The principle remains biological, but it is concentrated in a reduced volume.

The aerated moving bed

In a tank, thousands of small plastic media with a very high specific surface area are continuously stirred by air injected at the bottom. Bacteria colonize these media; the movement constantly renews the water in contact with them and prevents clogging, while the air supplies the oxygen they need. This is a proven technology, derived from wastewater treatment and aquaculture, offering high capacity in little space.

Submerged and media filters

Other systems use specific media or submerged biological cartridges, sometimes combined with materials that bind phosphorus. They require regular washing but remain very compact.

Pre-filtration, always

Whatever the system, an effective skimmer and a sieve or mechanical filter upstream are essential. Every leaf that decomposes in the pool releases phosphorus. Organic matter removed early means fewer nutrients for algae.

The criteria for deciding

One point deserves emphasis for conversions: converting an existing chlorinated swimming pool into a biological pool is often simpler with a technical filter, possibly supplemented by a small planted zone in a container or an adjacent pond. Digging a regeneration zone the size of the pool is not always possible.

Pump and circulation: the system's engine

Biological filtration never stops during the season, and often not even in winter, albeit at a reduced rate. Bacteria need a continuous flow of water and oxygen; an interruption lasting several days in summer may be enough to kill part of the biofilm. The pump therefore runs for thousands of hours per year, making its power consumption decisive.

Choose a high-efficiency pump sized for the flow rate actually required, rather than the most powerful one possible. Excessive flow passes through the gravel too quickly and reduces contact time; insufficient flow leaves dead zones. Our article on pumps for natural pools explains the typical figures and the technologies to prioritize.

For a moving bed, a membrane air pump provides mixing and oxygen. It also runs continuously: its reliability and power consumption matter just as much as its airflow.

Start-up and seeding

A new biological filter is devoid of life. It takes several weeks for a functional biofilm to establish, longer if the water is cold. During this period, the water may turn green or become cloudy: this is normal, and it is precisely when you should not panic or add chemicals.

1. Fill with water containing as little phosphorus as possible. Have it tested if you use well water or stored rainwater.
2. Start continuous circulation as soon as filling begins.
3. Seed it with bacteria specific to natural swimming pools, following the manufacturer's instructions. This shortens the establishment phase.
4. Plant the regeneration zone in spring, when the plants start growing, so they can quickly take over.
5. Wait one season. A biological pool generally finds its balance during its first year and stabilizes in the following years.

To choose the strains and understand their role, see our guide to bacteria for natural pools and treatment ponds.

The mistakes that ruin biological filtration

• Undersizing the planted area without compensating with a technical filter: the system is overwhelmed as soon as the first hot weather arrives.
• Using potting soil or fertilizer for planting: it's a phosphorus bomb.
• Turning off the pump at night to save money: the biofilm lacks oxygen and water quality deteriorates.
• Adding chlorine or an algaecide at the first sign of greening: you kill exactly what was supposed to purify the water.
• Letting leaves accumulate on the bottom in autumn: they release their nutrients throughout the winter.
• Neglecting external inputs: runoff from a fertilized lawn, soil washing into the pool, excessive sunscreen.

Most disappointments come from a system that is undersized or from impatience during the first year. With realistic design and a little time, a biological pool offers clear, skin-friendly water and a swimming experience very different from that of a treated pool.

Also read

To learn more about how a planted filter works: Downflow or upflow treatment for my natural pool.

READ THE ARTICLE

Frequently asked questions

— Can a chlorinated pool be converted into a natural swimming pool?

Often yes, by adding a regeneration zone or a technical biological filter and stopping all chemical treatment. The lining must be checked, suitable pre-filtration must be provided, and you must accept living water, whose appearance varies somewhat with the seasons.

— How much treatment-wetland area is needed for a natural swimming pool?

With purification exclusively by plants and gravel, the regeneration zone is, as a general rule, an area comparable in size to the swimming zone. With a high-performance technical biological filter, it can be significantly reduced, sometimes limited to a small decorative planted filter.

— Should bacteria be added to a biological filtration system?

Adding starter bacteria speeds up the commissioning of a new system and helps restart it after winter. After that, a well-established biomass maintains itself if filtration runs continuously and no biocidal products are used.

— Can a natural swimming pool have perfectly clear water?

Yes, very clear water is possible, especially with a technical system and well-controlled phosphorus levels. But the water remains alive: a slight biofilm on the walls and some variations in spring are normal.

In summary

• Biological filtration does not mean a single system: planted treatment wetland, technical filter or hybrid system.
• The less planted surface you have, the more technical and efficient the filter must be.
• Pre-filtration (skimmer, sieve) prevents organic matter from feeding algae.
• Phosphorus is the key factor: no fertilizer, controlled filling water, phosphate trap if needed.
• Filtration must run continuously; a low-consumption pump is essential.
• No chlorinated products or biocides: bacterial biomass is your treatment.

Last updated: October 3, 2026.