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Pools for all

To accompany our Swimming pools water treatment systems section (S18), Technical author
Sarah Delany provides some background to selecting the treatment.

Gone are the days of the standard rectangular pool. Today, pools come in all sizes – from 50 m Olympic pools to small private pools – and shapes, and cater for everything from water theme parks with many slides and rides, to diving pools for clubs and competitions, and learner pools for teaching babies and children. Fortunately, the principles of water treatment are the same for them all.

Selecting water treatment method

The quality of swimming pool water is not at present governed by any regulatory requirements in the UK, although the quality of drinking water and coastal waters is regulated. The BSI code of practice, developed through the Pool Water Treatment Advisory Group (PWTAG) (www.pwtag.org) (PAS 39:2003 Management of public swimming pools. Water treatment systems, water treatment plant and heating and ventilation systems. Code of practice), is not to be regarded as a British Standard and defers to the PWTAG publication Swimming pool water treatment and quality standards as being more comprehensive. The PWTAG publication also forms the basis of the guidance in CIBSE Guide G and is to be revised this summer.

So, looking to PWTAG, where do we start?

• What type of facility is it in? Leisure complex, health club, school, hospital, hotel or sports centre?
• Who is it likely to use it? The general public, school children, medical patients, families or competitive swimmers?

Having established these factors, further information on times of use, water temperatures, and expected usage can be determined.

In order to make the water safe and comfortable for swimmers the pool water will need to be filtered, disinfected and – probably – heated. A detailed knowledge of the quality of the source water is essential for selecting the correct treatment method. The pH value and alkalinity of the source water will affect the choice of chemicals for disinfecting the water and the coagulants added to the water to improve efficiency of the filtration process.

Other factors affecting the choice of water treatment include the desired water quality, the pool hall environment and the skills available for operating the system.

Choosing a disinfectant

The better the disinfectant suits the source water, the more efficient it will be and the fewer other chemicals will be needed. The softer the source water, the easier it is to adjust the chemical treatment to suit the disinfectant. Also the acidity or alkalinity of the disinfectant will affect any pH adjustment.

The rough guide given by PWTAG is as follows:

• For soft waters (typically: hardness under 50 mg/L CaCO₃; alkalinity under 30 mg/L CaCO₃; pH value easily influenced): An alkaline disinfectant is the likeliest, e.g. sodium hypochlorite plus calcium chloride to increase hardness a little, or calcium hypochlorite. To lower the pH value, use a gentle acid such as carbonic acid (from carbon dioxide).
• For hard waters (typically: hardness over 250 mg/L CaCO₃; alkalinity over 150 mg/L CaCO₃; pH value stable): The disinfectant should be one that operates well at the pH of the source water. Chlorine gas (with the proper safeguards) is worth considering for large pools, trichloroisocyanuric acid for smaller pools and lidos; otherwise, use calcium or sodium hypochlorite. Avoid carbon dioxide for pH value control.

Ozone and ultraviolet treatments operate by purifying the water as it passes through the plant room. They are used in conjunction with conventional disinfectants, but the initial cleansing means that less disinfectant is used than in disinfectant-only schemes.

Chemical control

For the disinfectant to work properly, and to maintain good quality water, it is often necessary to dose with other chemicals. The use of coagulants can improve the removal of suspended matter by filtration. Where water is cloudy or turbid it is important to check the interaction of chemicals and to dilute the pool water.

If the problem persists new methods of filtration, such as reverse osmosis, can be used. The pH value and alkalinity of the water can be adjusted using dosing plant. The aim is to maintain the water balance so that it is neither scale forming nor corrosive.

All these functions need to be monitored and controlled by automatic systems in public pools. For small private pools, test kits are available for the monitoring of water conditions.

Ultimately the main cause of pollution in swimming pools is the bathers themselves, particularly their creams and cosmetics, and sweat and other bodily secretions. Much of this could be reduced by encouraging people to shower before swimming. In other European countries there is a far better culture of washing before swimming, which pool managers are trying to introduce here.

Factors affecting building fabric

If stainless steel is used for building components the choice of grade is important, to avoid stress corrosion cracking which may cause failure. The Nickel Institute publishes Stainless steel in swimming pool buildings: A guide to selection and use (publication 12010), which is available via their website (www.nickelinstitute.org).

There have been problems with grout being attacked, although there is no clear cause for this. Low hardness levels are cited and water hardness can be boosted to compensate. However, in other countries the problem does not appear so often. This is attributed to the use of epoxy grout instead of cementitious grout. Sulfates can also affect grout – it may be necessary to use sulfate-resistant Portland cement or epoxy grout where sulfate levels are high.

Size and position of the plant room

There are a number of key issues which affect the success of the operation of the water treatment plant:

• Location
  The hydraulic design is affected by the location of the plant room in relation to the pool. Pumps should operate under flooded suction conditions and be near the balance tank and the extract points from the pool. Therefore it is better to have the plant room below the pool surround.
• Size and access
  The treatment method used will largely dictate the size of plant room required, but it is also important to allow access for equipment operation, maintenance and replacement. Filtration plant is large and it is difficult to allow sufficient access for the removal of the complete plant. The filter media may need replacing every six years or so, but the plant itself may last 25 years.
• Segregation
  Chemicals require separate, secure spaces for storage. Some equipment, such as electrical control panels and ozone generators, needs to be in clean, dry areas, away from chemicals.
• Environment
  HSG 179 Managing health and safety in swimming pools is a publication produced by the Health and Safety Executive and Sport England, and gives advice on how to comply with the COSHH Regulations for handling and storing chemicals. These affect the environment of the plant room.

Sustainability

In recent years there have been some moves towards investigating sustainable aspects of swimming pool design. Research has been carried out by BRE to assess the thermal performance of swimming pools. Their report (number 15959, August 2004) considers the effect of heat loss to the ground through the base and sides of the pool. Another development has seen glass from recycled green wine bottles used as a filter media – AFM® (active filter media) – with results indicating that an improvement in filter efficiency can be achieved. Further information is available on the Dryden Aqua website (www.drydenaqua.com).

Swimming pools are ideal installations for using heat reclamation plant and other energy efficient services for environmentally friendly design. The new 50 m pool at Loughborough University uses techniques for reducing energy consumption, including natural ventilation in conjunction with heat recovery plant. Rainwater is also collected from the roof and used in the pool's balance tank. The pool was designed by architects FaulknerBrowns (www.faulknerbrowns.co.uk) with services by Max Fordham (www.maxfordham.com).

In conclusion

In these days of awareness of health issues and a drive to involve people in various forms of exercise, it is important that pools for all provide a healthy and enjoyable experience. Designing systems that minimize the use of chemicals can achieve this, and help to reduce the impact on the pool's water, air and fabric.

Related NBS information:

Articles:

• Down to the wire: automated timing and scoring systems

Selected links:

• Recreational buildings
• Water supply

Written January 2007

 

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