Mar 20, 2026
Algae in the swimming pool: tackling growth with ozone water
Green water discolouration in a swimming pool is a recognisable sign of algae growth. Algae are single-celled organisms that multiply in pool water when conditions allow: sufficient nutrients, light and an organic load that the water treatment system does not fully break down. The green colour is the visible sign of a built-up algae concentration in the water. Ozone water works on algae via the oxidation mechanism. The dissolved ozone reacts with the organic structures of the algae cell and chemically breaks them down. The reaction is non-selective: ozone reacts with all organic compounds in the water including algae cells. The rate and extent of the effect depend on the ozone concentration in the water, the contact time of the ozone with the water and the ratio of the pool volume to the production capacity of the ozone system. An undersized system gives a slower and less complete reaction at high algae concentrations than a system correctly sized for the water volume to be treated. Prevention of algae growth is more effective than curative treatment of existing growth. A water treatment routine in which ozone treatment is structurally deployed keeps the organic load of the pool water at a lower level and thereby provides fewer nutrients for algae growth. This article describes the mechanism of ozone water with algae growth, the practical approach and the factors that determine effectiveness.
Tackling algae in the swimming pool with ozone water: how ozone oxidation works on algae, the practical approach to green water discolouration and the factors that determine effectiveness.
Tackling algae in the swimming pool with ozone water: mechanism and practice
Algae as organic compounds in pool water
Algae are single-celled organisms belonging to the category of organic compounds. They contain carbon, nitrogen and phosphate compounds that serve as nutrients for their growth and multiplication. In pool water algae find conditions for growth when sufficient nutrients are present and the water treatment routine does not fully break down the organic load. Green water discolouration is the visible consequence of a built-up algae concentration. More on the basic mechanism of ozone in pool water: ozone treatment swimming pool water.
How ozone water works on algae cells
Ozone water works on algae via the oxidation mechanism. The dissolved ozone reacts with the organic structures of the algae cell and chemically breaks them down through oxidation. The cell wall of an alga consists of organic compounds that react to the oxidising action of dissolved ozone. When the cell wall is damaged by the oxidation reaction the algae cell loses its structural integrity. The broken-down algae fragments remain as solid particles in the water.
After the oxidation reaction the broken-down algae fragments are removed from the water by the mechanical filter. The combination of ozone oxidation and mechanical filtration therefore works together in tackling algae growth. A well-maintained filter with sufficient filtration capacity is an essential component of effective algae management in pool water. More on filtration systems: swimming pool water filtration ozone.
Three factors that determine effectiveness
The effectiveness of ozone water with algae growth is determined by the ozone concentration in the water, the contact time of the ozone with the water and the ratio of the pool volume to the production capacity of the ozone system. Higher ozone concentration at sufficient contact time gives a faster and more complete oxidation reaction on the algae cells present. At high algae concentrations the organic load is temporarily elevated, requiring a higher ozone dosage for an effective reaction.
An ozone system correctly sized for the pool volume can handle the temporarily higher load at algae growth by running multiple treatment cycles. An undersized system gives a slower and less complete reaction at high algae concentrations. More on the machine and capacity selection: ozone water machine.
The course of treatment with green water discolouration
With visible green water discolouration treatment with ozone water proceeds step by step over multiple circulation cycles. After each treatment cycle in which the water passes through the ozone system the algae concentration in the water decreases. The green colour visibly diminishes as more treatment cycles are completed and more algae cells are damaged and broken down by the oxidation reaction.
The rate of that colour change depends on system capacity, pool volume and the initial algae concentration. A pool with a low initial algae concentration and lightly green water reacts faster than a pool with a high concentration where the water is strongly discoloured. After the water has visibly cleared a check of water values is recommended to confirm that the organic load has sufficiently decreased.
Ozone and chlorine together with algae problems
The combination of ozone treatment with a minimum chlorine concentration gives a more effective result with algae problems than ozone treatment alone. Chlorine has a residual effect that remains active in the water even after the active ozone treatment cycle. That residual effect provides an additional barrier against renewed algae growth. A low but measurable residual chlorine concentration combined with effective ozone treatment gives the most robust approach to existing algae problems in pool water. More on the ozone-chlorine relationship: ozone swimming pool vs chlorine.
Preventive approach: preventing algae growth
Prevention of algae growth is more effective than curative treatment of existing growth. A water treatment routine in which ozone treatment is structurally deployed as part of daily or weekly maintenance keeps the organic load of the pool water at a lower level. A lower organic load provides fewer nutrients for algae growth and thereby reduces the chance of visible green water discolouration.
Factors that increase the organic load are intensive pool use, higher water temperatures, leaf fall and other organic input from the garden. A preventive approach anticipates these load variations and adjusts treatment intensity accordingly. During peak season with intensive use a higher treatment frequency is worthwhile. More on the ozone water system: ozonewater.
Algae on the rim and terrace around the pool
Algae growth is not limited to the pool water. On the rim of the pool, the surrounding terrace and walls above the waterline green deposits from algae and moss can also develop. For treating algae on surfaces around the pool the two-cloth method with ozone water is the appropriate procedure. Ozone water breaks down the organic compounds in the algae cell on the surface through the same oxidation reaction as in the water. Regular treatment of the rim and terrace with ozone water interrupts algae growth on the surface before deposits spread further. More on the two-cloth method: two-cloth method.
Water values and monitoring during algae treatment
During and after curative treatment of green water discolouration regular measurement of water values is an essential part of the approach. The relevant parameters are ozone concentration in the water, free chlorine concentration, pH value and total organic load. A falling organic load combined with a diminishing green water colour is a reliable indicator of effective treatment. After treatment maintaining a low organic load via structural ozone treatment is the best preventive measure against return of algae growth.
Water temperature and algae growth: seasonal pattern
The growth rate of algae in pool water increases at higher water temperatures. During summer months higher temperature combines with more intensive use and greater biological input of organic material from the surroundings. That combination increases the organic load of the pool water and thereby the pressure on the water treatment system. An ozone system deployed at higher frequency during peak season compensates for the increased load. Regular monitoring of ozone concentration and free chlorine concentration in the water gives the insight needed for adequate adjustment of treatment intensity during the peak season.
After treatment: preventing return
After successful curative treatment of green water discolouration preventing return is the next step. A structural ozone treatment routine after curative treatment keeps the organic load of the water at a lower level and thereby provides fewer nutrients for renewed algae growth. A combination of ozone treatment, a low residual chlorine concentration and regular measurement of water values gives the most robust protection against return. Timely detection of a rising organic load and corresponding adjustment of treatment intensity is more effective than waiting until green water discolouration is again visible.
The role of filter type in algae treatment
After oxidation of algae cells by dissolved ozone the broken-down algae fragments remain as solid particles in the water. A filter with a finer filtration grade removes those particles more efficiently than a filter with a coarser grade. The combination of a filter with fine filtration grade and a correctly sized ozone system gives the best overall result in tackling algae growth. A cartridge filter or diatomaceous earth filter retains smaller algae fragments better than a sand filter with a coarser filtration grade.
pH value and algae treatment
The pH value of the pool water indirectly influences algae growth. Algae thrive better at higher pH values. A pH value in the optimal range for pool water between 7.2 and 7.4 makes the environment less favourable for algae growth and simultaneously increases the effectiveness of chlorine as a complementary water treatment agent. pH correction is therefore a fixed part of the water treatment routine when dealing with algae in pool water. Regular measurement of the pH value and timely correction at deviations support the effectiveness of ozone treatment with algae problems.
Algae species and response to ozone treatment
Pool managers encounter multiple algae species in practice, of which green algae are the most common. The response of different algae species to ozone oxidation differs in rate but not in principle: all algae species contain organic compounds in their cell structure that react to the oxidising action of dissolved ozone. Green algae respond quickly to ozone treatment at sufficient ozone concentration and contact time. Blue-green algae and filamentous algae can have a thicker cell wall structure that requires a higher ozone concentration or longer contact time for effective treatment.
Combination with cleaning pool walls
Algae growth on the underwater walls of the pool often goes hand in hand with algae growth in the water. Manual cleaning of the walls and bottom of the pool loosens algae deposits that then float as particles in the water. Those loosened particles are led by the circulation pump through the filter and the ozone system and are thereby removed from the water and chemically treated. A thorough mechanical cleaning of the walls followed by a treatment cycle via the ozone system therefore gives a more effective overall result than purely chemical treatment without mechanical cleaning of the walls.
Costs and affordability
An ozone system structurally deployed for pool water maintenance reduces the required chlorine concentration and the frequency of curative treatments for algae growth. Savings on chemical consumption and time savings on curative maintenance with consistent preventive use outweigh the investment in the system over the lifetime of the installation. Questions about the approach? get in touch.
Testimonials
💬 "Last year we had green water discolouration every spring. Since we consistently use the ozone system as part of our water treatment routine we no longer see that discolouration returning to the same extent." — Private pool owner
Further reading
Full overview: ozone water knowledge guide. Filtration and ozone cooperation: swimming pool water filtration ozone. Ozone versus chlorine: ozone swimming pool vs chlorine.