| Water quality can be improved eminently and costs can be saved on the long term when ozone is applied for swimming pool water treatment. |
Swimming pool water pollutants
Swimming pool water polluntion is mainly caused by swimmers. This makes it a very dynamic pollution, which is dependent on the number and types of swimmers. Swimming pool pollutants can be divided up into three groups: microorganisms, undissolved pollutants and dissolved pollutants.
Each swimmer carries a large number of microorganisms, such as bacteria, fungi and viruses. Many of these microorganisms may be pathogenic and can cause disease.
Undissolved pollutants mainly consist of visible floating particles, such as hairs and skin flakes, but also of colloidal particles, such as skin tissues and soap remains.
Dissolved pollutants can consist of urine, sweat, eye fluids and saliva. Sweat and urine contain water, but also ammonia, ureum, kreatine, kreatinine and amino acids. When these substances are dissolved in water, they cannot harm swimmers. However, when these compounds react with chlorine in swimming pool water, incomplete oxidation can cause chloramine formation. This causes the so-called chlorine-scent, which irritates the eyes and respiratory system. In a number of cases, stable compounds can be formed, which can only be removed from swimming pool water by water refresment [13,14].
Essential parts of water treatment systems
Generally, water treatment systems for swimming pool water consist of filtration, oxidation, disinfection and refreshment . The conventional purification process is shown in figure 1. Raw pollutants, such as hairs and plasters are removed from water by filters. Smaller, undissolved particles are removed by sand filtration. To improve this removal system, coagulation may be applied. This makes the smallest undissolved particles easier to remove. Dissolved particles are removed by oxidation by the disinfectant. The disinfectant also deactivates the microorganisms. Substances that cannot be decomposed must be gradually removed by water refreshment.
Figure 1: conventional swimming pool water treatment system
Additional water treatment technique: ozone
Because of its strong oxidation and disinfection mechanism, ozone is very suitable for swimming pool water treatment. Safety measures cause ozone to only be permitted for use as a supplementary measure.
Application of ozone for filtration; increase of filter and coagulant efficiency
In order to be able to use the ozone mechanism optimally, ozone is injected on the filter, before the addition of a coagulant (figure 1). After filtration, a chlorine product is added to remain a residual concentration in the system. The amount of chlorine that is added will be less when it is combined with ozone.
Increased efficiency of the filter
By treating water with ozone, the biodegradation rate of natural organic matter (NOM) is increased. This means, that large molecules are diminished to smaller molecules, which can be decomposed by a filter more easily. The organic matter will cause an increase in bacterial growth in the system, because it is a bacterial nutrient. Regrowth of bacteria in a distribution system is an unwanted effect. This can be turned into a positive effect by injecting ozone on a sand filter. Research has shown that preozonization can increase filter efficiency. Ozone increases biodegradability of materials and microbial growth, but it also increases the oxygen concentration in the filter. Total Organic Carbon (TOC) removal can be increased to 35% in a sand filter when ozone is added .
The amount of organic matter in the filter is decreased, causing regrowth of bacteria in the system to be reduced. Figure 2 shows the distiction between algae removal in a sand filter with ozone application and without ozone application.
Figure 2: global rates of removal of various species of algae by slow sand filtration and slow sand filtration combined with ozonization . (parameters: hight sand bed: 0,9 m; particle size: 0,5-1,2 mm; filtration speed: 10 m/h)
Increasing coagulant efficiency
In a filter, a coagulant is usually added to prevent clogging and increase filter capacity. An extra benefit of ozone addition to filter systems is that coagulant activity is increased. Ozone itself can also act as a coagulant. Research has shown that preozonization can increase the adsorption of organic matter to coagulants, such as: activated aluminum , aluminum hydroxide , aluminum polychloride  and calcium carbonate . A possible explanation for this is that the organic matter becomes more polar by ozonization. Figure 3 illustrates the effect of various ozone concentrations on turbidity removal by coagulants, such as aluminum polychloride.
Figure 3: turbidity removal by aluminum polychloride, with and without (various concentrations of) ozone in the Seine (France)
Benefits of ozone application
Swimming water quality can be sufficiently increased by ozonization. This is not only a benefit when it comes to swimming, but it also guarantees healthy swimming water. Recent research has shown that children’s immune systems may be affected by swimming in chlorinated swimming pools. The health risks are also increased for swimmers that train twice a day .
Allthough purchasing an ozone system is relatively expensive compared to other techniques, one can save costs on a long term.
These are the main benefits of ozonisation:
- Decrease in chlorine use
- Improval of the filter and coagulant capacities. This leads to a reduction of coagulant use and less backwashing of the filter is required
- Water use can be decreased, because of an increase in water quality
- Ozone oxidizes organic and inorganic matter in the water, without the formation of unwanted byproducts, such as chloramines (which cause a chlorine-scent)
- Chlorine scents can be fully diminished by ozone application
- Ozone is a more powerful oxidant and disinfectant than chlorine. Certain chlorine-resistant pathogens (see ozone disinfection: resistant microorganisms) cannot multiply in water that is treated with ozone