PWTAG Code of Practice for Pool Water Treatment
In this pool guide the water management experts at WTS review the popular Code of Practice for pool water treatment published by the influential Pool Water Treatment Advisory Group (PWTAG).
The guide highlights the role of PWTAG within the pool sector, their aims and objectives, and considers some of the key points and guidance from the Code of Practice itself. This includes the importance of identification and treatment of waterborne pathogens such as cryptosporidium, E. coli, coliforms and pseudomonas. It also looks at water sampling and chemical testing for chlorine, pH, alkalinity, hardness, dissolved solids and sulphates.
What is PWTAG?
PWTAG is an abbreviation for the Pool Water Treatment Advisory Group. It is a UK organisation dedicated to setting, maintaining and raising standards for the management of safe pool water. It does this through the publication of detailed standards and guidance covering the management of swimming, spa, hot tub and hydrotherapy pools. This includes the PWTAG Code of Practice for the Management and Treatment of Swimming Pool Water which we’ll be reviewing here.
What is the PWTAG Code of Practice for pool water treatment?
The PWTAG Code of Practice for swimming pool water treatment is a detailed document that gives people who either own a swimming pool, spa, hot tub or hydrotherapy pool, or who are responsible for managing a pool on behalf of others a set of very detailed guidelines for the management of safe pool water.
The guidelines list all the minimum quality standards which will make sure that anyone using the pool has a safe and hygienic experience, and embraces the best current knowledge in terms of design, maintenance and engineering. Following the recommendations in the Code gives the industry and the people using the pool peace of mind – they know that you are doing what you need to do in order to maintain healthy water standards.
Management of cryptosporidium risk
One of the main aspects of the PWTAG Code focuses on the management of one of the most important health challenges faced by pool operators… contamination by cryptosporidium.
What is cryptosporidium and how does it affect pool water safety?
Cryptosporidium is a microscopic parasite which causes cryptosporidiosis, an infection of your bowels (gastroenteritis) which can lead to diarrhoea and sometimes cause vomiting. It is resistant to chlorine and can be harmful to health if found in large quantities.
The PWTAG guidance advises having samples of water drawn from the swimming pool tested monthly to look for any microorganisms which may cause issues, including cryptosporidium. Some pool operators carry out this testing themselves, but there is also the option of sending samples to an external laboratory for testing.
Total Viable Count (TVC) testing
Also known as Aerobic Colony Count, this is one of the basic tests which pool operators will use to look at the quality of the water as it gives a measure of the total quantity of bacteria found in the sample water. The results of this TVC test will tell pool operators whether their current disinfection and filtering protocols are effective. The results of the TVC test should be less than 100 cfu per millilitre. Any result above 100 shows that the disinfection and filtration isn’t working. Repeated high results require further investigation.
E. coli in pool water
Unlike other bacteria, E. coli doesn’t grow in water, and can only get into samples of pool water from faeces from humans or animals. If E. coli is detected in a sample, then this indicates that there has been recent faecal contamination of the pool. There should be no E. coli bacteria found in a standard 100 ml sample of pool water.
Testing for coliforms
Coliform bacteria are related to E. coli, but also occur naturally in soil and plants. If you detect coliforms in your pool water sample, this means that you have a potential issue with external contamination in the pool. Ideally, total coliforms should be undetected in a 100ml sample. A result of less than 10 cfu per 100ml isn’t necessarily a safety issue, as long as this is a one-off and not detected in repeated samples and there is no E. coli detected.
Pseudomonas aeruginosa testing
Pseudomonas aeruginosa is another organism which can grow in water, at lower temperatures than other sorts of bacteria. Pseudomonas might start to develop in your pool filters, tanks or channels, and can also live on flexible pool covers, tubes or inflatable pool equipment. For the majority of pool users, Pseudomonas aeruginosa won’t cause any health issues, but it can cause ear infections (swimmers ear) or rashes (hot-tub rash). Levels of pseudomonas should also be zero in a 100ml sample. If levels are over 10 cfu per 100ml but under 50, check both the pH value and free chlorine levels in the pool, and repeat the pseudomonas test.
If you have repeated test results indicating the presence of Pseudomonas aeruginosa, then look more closely at your pool maintenance plan, filtration, disinfection and cleaning regime to try to identify the parts of the pool where the organism might be taking hold. Remember to also check your pool cover, and any swimming aids or inflatable toys used in the pool.
Pseudomonas and biofilm
Pseudomonas grows readily in water systems and can form biofilm. The recommended method of cleaning and disinfecting is by using a solution of 50mg of free chlorine in 1 litre of water. If you detect Pseudomonas at more than 50 cfu per 100ml, then the pool should be closed as these levels can pose a significant health risk to people using the water.
Pool water sampling and chemical testing
Some larger swimming pools manage disinfection and pH levels automatically, with the water treatment plant automatically releasing chemicals in to the pool as required. If this is not happening as it should, then samples will have to be manually taken and tested using special test kits. Your pool risk assessment should determine how often water sampling and testing needs to be done.
In most cases, the recommended regime for chemical testing and sampling is to take a water sample before the pool opens to the public, every 2 hours during the operation hours, and then again after it closes.
Pool water samples should be drawn from the water at depths of between 100mm and 300mm, preferably from the deepest part of the pool, and away from the water inlets.
Monitoring free chlorine levels in your pool water
If you are using hypochlorite to disinfect your pool, and assuming the pH level is the advised 7.2, then the aim is to keep the free chlorine levels at 1mg per litre or below, with 0.5mg per litre as an absolute minimum.
The PWTAG Code suggests other disinfection methods such as UV or ozone can be used to reduce free chlorine levels. It will be easier to maintain the correct levels of free chlorine in a pool which has been properly designed, well-maintained and which is not allowed to become overloaded with swimmers.
The upper safe limit for free chlorine is 3mg per litre and if this level is passed, then chlorine dosing should be reduced. If the levels still rise, then the pool should the closed for use until the situation has been resolved.
Recommended combined chlorine levels
The levels of combined chlorine levels should be under 50% free chlorine, and never more than 1mg per litre. If you are finding unsuitable ratios of combined to free chlorine, then you may need to take action.
Pool pH levels
The exact pH level required in the pool will depend on which sanitisation method you are using, but for pools which use a chlorine based disinfectant, the ideal pH range is typically 7.2 to 7.4. If you find levels higher than this, then levels of free chlorine have to be increased to ensure it is working effectively.
Controlling pool alkalinity
Alkalinity levels ensure that the pH level is stable when acid-based disinfectants are used to clean the pool water. The PWTAG guidance recommends that levels of alkalinity should be stable at between 80mg and 200mg per litre.
Alkalinity should be tested weekly, and there are plenty of commercially available kits to help operators with this task. Dilute acid can be used to lower the levels of alkalinity in the pool water where required.
Managing calcium hardness levels in your pool
Calcium hardness in pool water should be between 75mg and 150mg per litre, but in areas with hard water this can be very difficult to achieve. It is therefore important to make sure that any chemicals you are adding in to disinfect the pool do not make the water even harder and increase the levels of calcium. If you are regularly finding levels of calcium hardness at over 300mg per litre, this can cause scale to build up on some pool surfaces, which can also affect the pH and temperature.
Measurements of calcium hardness should be taken each week, using commercial kits and testing tablets.
Testing for Total Dissolved Solids (TDS)
High levels of total dissolved solids (TDS) can be destructive and should be avoided. Levels should be kept under 1000mg per litre. If necessary, TDS levels can be reduced by dilution. TDS measurements should be taken weekly, using an electronic meter which has been correctly calibrated.
Testing for sulphates
Sulphates also should be tested weekly, with levels ideally being less than 360mg per litre.
The importance of swimming pool water balance
The key to successful pool water maintenance is keeping all of those different factors in balance. One way of looking at the overall balance of the water in your pool is with the Langelier Saturation Index (LSI), a special formula which brings all of the different elements together. Best practice is to calculate the result weekly, at the same time as measuring alkalinity, TDS and calcium hardness.
What is the Langelier Saturation Index?
The Langelier Saturation Index is an approximate measure of the degree of saturation of calcium carbonate (CaCO3) in water. It is calculated using pH (Hydrogen Ion), Alkalinity (CaCO3), Calcium concentration, Total Dissolved Solids (TDS), and temperature of a water sample.
The Langelier Index (LSI) is calculated as follows:
LSI = pH – pHs
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- pH – pH Value
- pHs – Saturation pH
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