Public swimming pools are pools that are used by various subgroups in a non-familial environment. These swimming pools were originally designed to maintain public health. Nowadays, they serve mainly for fitness and recreational activities. Hygiene is subject to Health Authorities’ supervision.
The main goal for swimming pool water treatment of public swimming pools is to constantly provide hygienic swimming pool water, even at peak usage times. Therefore, rational technology and design of the swimming pool water treatment facilities are necessary.
Construction and Design
Principally, the swimming pool consists of a water circuit with a ”basin where the bathing fun takes place”, a filter, where solids are kept back and the disinfection step, where germs, viruses and other unwanted particles are “nipped in the bud“. On the engineering side, a heating system regulates the water temperature while the splash container regulates the water displacement or the balance. Water attractions such as massage nozzles, water slides and integrated whirlpools enrich stays in swimming pools and increase demands upon water treatment technologies.
Design and construction of these swimming pools are controlled by legal regulations and German standards (DIN-guidelines). DIN 19643 defines as priority no.1, fast killing of germs (fore example, E. coli, Pseudomonas aeruginosa) of three decimal powers within 30 seconds.
The most exact determination of being informed on the current status of the swimming pool water would be to take water samples for laboratory inspection and analysis. However, this sampling requires time, and results would be available too late. The bathing guests using the swimming pool in the meantime would be exposed to hygienically poor-quality swimming pool water. Current, direct online-information of the water status can be realized with so-called auxiliary parameters. These parameters show the general conditions having direct influence on the hygienic situation in the swimming pool water.
These important auxiliary parameters are, for instance, pH value and Redox value, the amount of free Chlorine and also the amount of compound Chlorine. The temperature of the swimming pool water is equally of interest, whereas in most cases, the latter is situated between 26°C (pool for swimmers) and 35°C (therapy pool).
The German standard DIN 19643 provides the basis of swimming pool design. Here, all pool types and the current procedures for swimming pool water treatment are defined. This standard is consulted for the assessment of water treatment technologies according to the current status of technology. Depending on the basin type, the standard defines various requirements for swimming pool water treatment. One important criteria is the agitation ratio: with the agitation the filter surface is determined, as the filtering rate should be of 30m/h.
The type of the conditioning stage is an important component for the design of the volume flow. The assignment is determined with the load factor (k). This k factor is a quotient of nominal load and volume flow. For conventional swimming pool water treatment procedures, this value may not exceed 0.5m-3. For swimming pool water treatment with Ozone this value may be 20% higher at 0.6m-3. This should be respected when selecting the technology. If a process stage with Ozone is selected, a smaller agitation pump, filter and possibly also a lesser pipe diameter can be selected. This not only results in initial investment savings but also decreases a high percentage of operation costs. Smaller agitation diminishes the electric acceptance of the agitation pump, the reduced filter size decreases the amount of back-rinsing water (water costs and energy costs of the post feed water to be heated) and the shorter back-rinsing time for Ozone also reduces operation costs accordingly.
pH Value Correction and Regulation of Disinfectant Concentration
pH value and disinfectant concentration of the swimming pool water are the most important parameters for further swimming pool water treatment steps. The pH value not only influences the feeling on the skin but also has a direct influence on the efficiency of the disinfectant and on the flocculant. Moreover, a constant pH value is necessary to protect the value of the facilities and reduce the damage to the system components.
The pH value regulation system strives to keep the pH value between 6.8 and 7.2. This requires constant measuring. Thanks to the combination of measurement/control and dosing made by ProMinent, this pH value is continuously maintained. pH value regulation consists of a pH electrode, a measurement transducer and a respective chemical resistant metering pump. ProMinent supplies all this from one source.
For disinfection of swimming pool water, Chlorine-containing products are used. Measurement is made with amperometric Chlorine probes by ProMinent. These electrodes measure the amount of free and total Chlorine. The deviation of both values shows the amount of compound Chlorine. Limit values are according to the German standard DIN 19643
at 0.2 mg/l (ppm). By means of the Chlorine measurement, the concentration is displayed directly online. The exact measurement helps to keep the concentration as low as possible, however, high enough to ban danger.
“Chlorine regulation“ consists of an electrode, a measurement transducer and a metering pump. In-house development and production by ProMinent integrate this control circuit not only from the technical aspect but also as far as support is concerned. No interface problems/incompatibilities arise between probe, controller and metering pump.
As comparison measurement, Redox measurement is used. This high-impedance signal supplies an mV signal. The latter indicates the ratio of the reductants to the oxidants. The higher the value, the faster the germs are removed in the basin water. For completion, the temperature is measured. The latter is consulted exclusively for basin water temperature measurement as the necessary temperature compensation is integrated in the Chlorine probe during Chlorine measurement.
With DULCOMARIN II, ProMinent offers a new and worldwide unique measuring and control concept for the public swimming pool sector.
Disinfection of Swimming Pool Water
Operators of public swimming pools use various solutions for disinfection of swimming pool water. The German standard DIN 19643 lists the chemicals admissible for disinfection of swimming pool water. These are:
- Chlorine gas
- Calcium hypochlorite according to E DIN EN 900 as granulate or tablets
- Sodium hypochlorite according to E DIN EN 901 as ready-made 150 g/l solution
- Sodium hypochlorite solution produced on the place of use by means of electrolysis
Worldwide, Chlorine gas is used the most frequently in swimming pool water treatment. It is an economical solution especially when processing large amounts of water. During the last years, however, it has lost ground in Germany, and more so in Switzerland, due to its danger potential.
For Calcium hypochlorite, powder/granulate is released in water. In most cases a 1% disinfection solution is mixed. The product becomes slightly cloudy through insolvable 7% of the weight of the solid particle solution.
Sodium hypochlorite is used often for smaller swimming pools, where handling with containers and the respective chemical costs do not carry much weight. Chlorine electrolysis is an interesting version of disinfection of swimming pool water with sodium hypochlorite.
For Chlorine electrolysis, caustic soda is generated on the spot from salt water, current, Chlorine and hydrogen. We differentiate between tubular cell electrolysis and diaphragm electrolysis. Tubular cell electrolysis combines the reaction products to a sodium hypochlorite solution with a high content of residual brine (non-transferred salt of the reaction). In salt-water pools, this is of use, however, in normal swimming pools it is not desired. The diaphragm electrolysis works with separated electrode chambers, so that entry of residual brine into the swimming pool water is avoided. The Chlorine generated can either be fed into the swimming pool water as hypochloric acid or be intermediately stored together with the caustic soda generated as sodium hypochlorite. This sodium hypochlorite can be dosed by means of metering pumps in various circulation systems. This solution is especially suitable for swimming pools with several basins. The dosing amount of 2g/m3 in indoor swimming pools or 10 g/l in open-air bathes is to be ensured with the metering pumps.
Disinfection or oxidation steps with UV light and Ozone can be used as complement to conventional disinfection of swimming pool water with Chlorine. With UV and Ozone plants, complex general conditions can be controlled or the use of Sodium Hypochlorite for disinfection of swimming pool water can be reduced to a minimum. With the use of these additional conditioning chemicals, reaction by-products can be minimized. Unpleasant odor or the generation of trihalomethanes is avoided. The bathing pleasure increases.
Don’t be satisfied with a simple water treatment technology, where there is a risk of expensive retrofitting in the years to come.
For further and effective support of the swimming pool water treatment, flocculant dosing is an important treatment step. Flocculants are needed to render non-filterable particles filterable! With potentials, not only filterable particles are stored, but also so-called macro-flocs are generated. These can draw the unwanted particles from the water circuit. “Flocculant dosing plants” can be switched off during the night at ideal water values, in order to minimize chemical consumption.
For decades, ProMinent has been one of the leading partners in swimming pool water treatment and we offer comprehensive solutions. Be it flocculation, filtration, pH value correction or disinfection of swimming pool water, ProMinent systems are integrated and individually combinable. Profit from these advantages!