Designing For Good Water Treatment

PWTAG Technical notes are updates or new material for the standards and guidance given in the PWTG book, Swimming Pool Water and the PWTAG Code of practice and should be read in association with these publications.

  • Subject: Designing for good water treatment
  • Date: March 2011

Pollution and hygiene

Pollution is introduced almost continuously into the water of swimming pools and spas. Even the mains water that supplies them may contain substances that affect pool water quality. The main source of pollution is the bathers themselves.

Whatever the source of pollution, it should be dealt with by appropriate water treatment, including disinfection and the other processe.

But prevention is better than cure and this section identifies the pollution from bathers, assesses its threat and describes how it should be minimised. It concludes by considering the vital role of dilution in reducing pollution.

Pollution from bathers

Any substance associated with the human body may be introduced to pool water by bathers. There are, broadly, three categories – tissues, excretions and dirt.

The main culprits are sweat, urine, mucus from the nose and chest, saliva, hair and scales from the skin, and faecal matter. These are pollutants in themselves, and most contain microorganisms (bacteria and viruses). To give a crude idea of the scale of human pollution, each day an adult produces a litre of sweat, 1 billion skin flakes and 38g of grease.

Urination, some involuntary, certainly does happen in swimming pools. Research has suggested that perhaps 30ml of urine is released per bather on average. That’s about 200mg of urea. And most bathers will lose significant amounts of sweat in the exercise of swimming – particularly in the higher temperature pool water that has become common). An average of 950ml of sweat per swimmer per hour has been suggested – another 250mg of urea.

This is a good reason for discouraging access from a sauna or gym directly to the main pool: a shower or a plunge pool should be interposed. The reaction between disinfectant and the breakdown products of urea in sweat and urine is the main cause of high combined chlorine residuals (chloramines) in the water, which contribute to eye and skin irritation. Some of these chlorinous compounds pass into the air, producing irritant gas in the atmosphere above the pool. This can affect bathers, poolside staff and other people near the pool (e.g. spectators).

Clearly, dirt of all sorts normally collects on the body before bathing. Openair pools pose a particular problem as dust, leaves, grass, soil, bird droppings, insects, etc can contaminate pool water directly, as well as via swimmers. Cosmetic materials such as powders, creams, lotions, tanning products and oils are a rich source of pollutants, many of them imposing a significant burden on the water treatment system.

Preswim hygiene

In some European countries it is quite routine (even compulsory) to shower before a swim; a swimming cap may even be required as well. In the UK, showers are more likely to be seen as a means of washing off the pool water after a swim. It is ironic, of course, that the disinfectant swimmers want to remove by showering is there in such quantities in order to deal with the body pollution introduced by swimmers who do not use the toilet and shower first.

If preswim showering were the norm, pool water would be cleaner, easier to disinfect with less chemical addition, have fewer irritant byproducts and thus be more pleasant to swim in. There would, too, be money saved on chemicals (offset to some extent by the extra cost of heating shower water). Important recommendations about toilets, showers, footbaths and bather education follow from this.


Toilets should be provided where they can be used conveniently before entering the showers and the pool; explanatory notices and posters should be prominently displayed. Everyone – but children in particular – should be educated to use the toilets before showering and bathing, to minimise involuntary (and voluntary) urination in the pool. People should be encouraged to wash their hands after use; nappy disposal facilities should be available.

Babies should not be allowed to swim in nappies; instead, if old enough, they should be encouraged to empty their bladders before they swim and wear special baby costumes which incorporate nappies and are waterproof both ways. Pool management should include frequent inspection and cleaning of toilets.


Most swimming pools have just one set of showers; some don’t even have separate ones for males and females. Given how important preswim showering is (preferably nude), pool designers and operators should provide dedicated preswim showering facilities (and, of course, encourage bathers to use them – even consider making them compulsory). Newer pools should ideally have been designed like this, although few are in practice, it seems. The design of new pools now should certainly consider incorporating these principles in their design. By hook or by crook, pool managers should ensure that people expect to shower before they swim.

Separate preswim showers will be located on the route from changing rooms to pool. Like postswim showers, they can be supplied with fresh water (stored at 60oC, piped at 50oC and then mixed to around 40oC), run to waste. All appropriate precautions regarding Legionella should be followed. Where preswim showers are placed on or very close to the pool surround it will encourage use but care will be needed to make sure that shower water goes down the drain to waste and not into the pool.

Postswim showers can be on a different, return route from pool to changingrooms. There should be provision for soap and shampoo and, ideally, cubicles for privacy and nude showering.

A compromise – just one set of showers, for both pre and postswim use – requires thoughtful signposting. They should be on the route from changing rooms to pool; ideally private enough for nude showering (the ideal, before swimming). Their preswim use should be actively encouraged. Swimming is part of the national curriculum. Although pupils’ pool visits may have to be programmed quite tightly, it is important to build in enough time for youngsters to shower before they swim. Apart from the immediate benefits, if they don’t develop good habits when young, they’re less likely to when they’re older.


The role of footbaths in dealing with verrucas and other foot infections is now considered largely irrelevant. It is now well established that footbaths do not help control foot infection. They are often the most infected area because they are the greatest point of concentration of people flow. Avoiding these narrow circulation spaces in bare foot areas, and poster campaigns encouraging better personal hygiene, will minimise the risk of foot infection.

Showers do the cleaning work more effectively, and bring extra benefits. If there really is no alternative, however, especially in an outdoor pool, a properly maintained footbath (or foot spray) is better than nothing. There must in any case be some barrier between outdoor dirt and the pool (while still allowing wheelchair access).

Pollution accidents

There are specific procedures to follow should a pool be contaminated by solid stools, diarrhoea, blood or vomit. These are dealt with on

When not to swim

A swimming pool is not very different from any other public place. Just as people might avoid work, school or public transport when they are not well – for their own sake as much as for others – so there are circumstances when swimming pools should be avoided.

  • Nobody suffering from diarrhoea should swim. That message is crucial; it must be clearly delivered and never compromised. Attention to ill children is particularly important. Gastrointestinal bacteria and viruses are killed by pool disinfectant. With the protozoa Crytosporidium, even after diarrhoea has stopped, chlorineresistant infective particles can be passed for up to 14 days. The risk of contamination will be less, but people should not swim. Solid stools, once removed, are not a problem.
  • People should not go to public pools if they have open wounds, severe eczema or any infectious skin complaint.
  • Colds, flu and other infectious illnesses are a sufficient reason not to swim.
  • People whose illness or treatment makes them susceptible to infection, or affected badly by it, should take medical advice before swimming.
  • Towels should not be shared because of the risk of transfer of skin and other infections.
  • People should not swim if they are affected by drink or drugs.
  • Parents should think twice about taking infants under the age of six months to pools. They may lose heat in cold water, the air may irritate their lungs, and their skin may be too sensitive for the pool chemicals. Children should not be exposed to spas under the age four.
  • When children do start swimming, it should not be in nappies; there are special baby bathing costumes.
  • Parents should make sure that children go to the toilet and then shower before swimming.
  • Everyone would be doing the pool a favour if they showered – preferably nude, and making sure heads, armpits, genitals, backsides and feet are clean – before swimming.
  • Children shouldn’t swim on a full stomach – not because of cramp (no connection) but because they may swallow water and be sick.

Pool managers must decide for themselves how to pass on messages like these. A policy of exclusion may not be enforceable – but there is no harm in gently but firmly letting people know what their responsibilities are.

Dilution with fresh water

Disinfection and filtration will not remove all pollutants. Indeed, there are some complex byproducts that will largely resist both processes. So the design of a swimming pool should recognise the need to dilute the pool water with fresh water. Dilution limits the buildup of pollutants from bathers and elsewhere, the byproducts of disinfection, and various other dissolved chemicals.

To some extent, dilution is effected through the replacement of water used in backwashing. But filter backwashing is often not frequent enough (although it will be at least weekly) to keep the concentration of unwanted pollutants at an acceptably low level. And some pollutants can be reduced only by dilution.

If dilution is inadequate, bather discomfort can result. So pool operators should replace pool water as a regular part of their water treatment regime, at a rate of 30 litres per bather. This is mandatory in some European countries. As well as making bathing (and the pool hall atmosphere generally) more comfortable, proper dilution can help protect the fabric of the building by reducing the level of contaminants in the air above the pool.

Although 30 litres per bather might seem an intimidating amount of water, operators are likely to be diluting already by as much as half of that just by backwashing. Fresh water should, ideally, be metered to allow accurate measurement and monitoring of dilution rates; and added gradually throughout opening hours, derived from bather throughput.

Water rationing and shortages may affect these judgements. In any case, it may be worthwhile to recycle some of the water drained from the pool into a grey water system for toilets etc.

Design in detail

Swimming and leisure pools are complicated buildings, making serious demands in terms of design, construction, operation and maintenance. So design is critical to successful operation. It comes first chronologically (though a brief and even a feasibility study may precede it), and needs to be considered by all the relevant professions. This applies equally to the design of new buildings and alterations to existing ones.

Water treatment systems are an integral part of the architectural, structural and mechanical design, and must be addressed from the very start of the project. Water treatment plant design must take into account:


  • bathing load, circulation rate, turnover period and hydraulics
  • filtration
  • choice of treatment system
  • disposal of effluent and backwash water

This technical note deals with those aspects of building design not directly connected with the water treatment plant but important, nevertheless for the safe, comfortable and efficient running of a swimming pool. That includes ventilation and heating.

Overall, the design must conform to the appropriate guidelines for maintaining safety and the chemical and microbiological quality of the water. The HSE book, Managing Health & Safety in Swimming Pools, is relevant here. Obviously the type of pool being planned eg freeboard or decklevel – needs to be taken into account. Although this note does not prescribe details of design, it does indicate how the necessary issues should be covered. And there is advice about getting specialist design and contracting help.

Changing rooms

When people visit a swimming pool they bring with them dirt and bacteria on their bodies and from outside, which they deposit on the floor by way of footwear, pram wheels etc. As they remove clothes and change into swimsuits they become more susceptible to inadvertently picking up or depositing a whole range of contaminants which may adversely effect an individual’s wellbeing, or in turn the wellbeing of others via contaminated surfaces or the pool water. The risk of cross contamination can be minimised by careful design, good housekeeping and bather education. Barefoot and outdoor wear areas should be as separate as possible.

Floors in wet areas

When designing floors for wet barefoot traffic the designer needs to give careful consideration to three factors.

Bacterial infections can be transmitted via the floor: shod feet to wet feet, wet feet to wet feet and from wheelchair and pram wheels. There needs to be a protocol for dealing with blind swimmers’ guide dogs.


  • Good floor drainage and under floor heating will promote cleanliness and minimise crosscontamination.
  • Slip resistant finishes are important. Corundum tiles have very good slip resistance, drain well but are difficult to clean. Profiled tiles have good slip resistance, are not so difficult to clean but don’t drain as effectively. Unglazed and matteglazed mosaics have a good slip resistance, drain well and are relatively easy to clean.

Design to facilitate cleaning

Proper cleaning by the operator is essential but the design can also make the task easier by considering:

  • cantilever fixings for benches, WC pans and wash basins to allow easy cleaning underneath
  • the areas that are difficult to clean are often the areas which are not walked on. Do they need to be slip resistant and therefore harder to clean?
  • smooth borders and cove skirting on floors • good floor falls and drainage
  • underfloor heating
  • plinth instead of legs on lockers.

Access for all

Legislation and regulations concerning the needsof people with disabilities include:

  • Building Regulations Part M 2010
  • Disability Discrimination Act 1995
  • Special Education Needs & Disability Act 2001 • Code of Practice BS8300

But to be inclusive and fully accessible, a swimming pool must do much more than simply comply with these. A Sport England guidance note on the subject has details of what should be provided. The aim is full integration of disabled swimmers in all activities, including providing them with the same route through the pool complex as other users. This clearly requires close attention to many detailed design issues – both avoiding the use of any inherently challenging surfaces, fittings etc and providing much specialist equipment and expertise. There are fixed and moveable poolside chairlifts for disabled access.

The plant room

The importance of good plant room design is frequently underestimated. It is important that it be considered as early as anything else. There are clear design considerations around the five key issues of location, size, access, segregation and environment.

The location of the filtration and water treatment system in relation to the pool critically affects hydraulic design. Circulation pumps should, ideally, operate under flooded suction conditions and be sited near the balance tank, and near the extraction points from the pool. (If the pumps have to be some distance from the balance tank, increasing the suction pipe size may improve pump performance.) If the plant room has to be at pool surround level, a pump pit will give flooded suction conditions. If there is no balance tank, the connection between pool water and pumps must be designed to keep the system free of air entrainment.

The size of the plant room (water treatment plant only) will typically be between 15 and 30% of the pool water area (depending on the treatment method used).

It should be sized to ensure good access, both to the plant room itself and for plant room equipment operation, delivery of chemicals, maintenance and replacement. Filters must be accessible. It is difficult, in most plant rooms, to allow for the removal of complete filters; instead, they will usually be removed in sections, and replacements put together in situ. Filter lining and media may need to be replaced after, say, seven years, filters themselves after perhaps 25 years. Such work may demand knowledge of Confined Spaces Regulations.

Certain equipment needs segregation. For example, chemicals should be stored in separate, secure rooms and different liquid chemicals stored in separately bunded areas. Electrical control panels, chemical control units, ozone generators etc should be in clean, dry areas away from chemical stores, and on raised concrete work plinths.

Certain plant items demand a specific environment. For example, there are important requirements (under COSHH) for ventilating ozone plant, electrolytic chlorine and chlorine dioxide generating equipment and chemical mixing areas. Most electrical items will require limits on plant room air temperature and humidity. There is also a requirement for effective and suitable plant room ventilation in the Workplace (Health, Safety and Welfare) Regulations 1992.

The design process

Water treatment is just one factor within the design of what may be a multiuse leisure complex. The starting point for design is a full assessment, by the client responsible for providing the pool, of the needs and demand involved, taking into account existing provision etc. Depending on the particular project, there may have been a strategy prepared, a multidisciplinary project team appointed, a feasibility study commissioned, and decisions taken on location and finance. By the end of this stage, a project manager and an architect experienced in pool design should, ideally, have been appointed. They will then work together to produce a design brief. Consultation on the design brief should include the local authority if is to be the enforcing authority for a new installation.

A design brief is a broad description of what is to be provided within the design; it is not a specification. The purpose of the brief is to establish the performance requirements of the design and hence a framework within which the detailed design work can progress. The starting point is a decision on the type of facility required – e.g. the type and amount of use envisaged; swimming or leisure; moveable floors/booms to convert competition/diving pools into learner and leisure; other jointuse arrangements; how to arrange the treatment plant for different pools.

The design brief, preferably accompanied by a basic schematic diagram and system layout, should cover the parameters dealt with in the next section. It can also usefully cover essential operational and staffing factors.

Construction (Design and Management) Regulations 2007
These CDM regulations are a revision of the 1994 regulations made under the Health and Safety at Work, Etc Act 1974. They place specific requirements on the designer – whether an architect, engineer, contractor or from some other professional discipline. They have a responsibility, when designing, to assess site hazards and deal with them where possible; and pass on information to keep workers safe and reduce accidents on construction sites.
There is now a distinction between nonnotifiable projects (e.g. domestic – with minimum obligations) and notifiable projects. The latter require the client to appoint a CDM coordinator, as well as the principal contractor, to cover the whole construction phase.

Designers’ duties can be summarised:


  • design for safety
  • advise clients on their responsibilities under the regulations
  • check that a CDM coordinator has been appointed
  • provide any information for the health and safety file

The CDM coordinator on a project, who is responsible for coordination of design, should ensure that those responsible for building and engineering design consider health and safety (including that of swimmers). The CDM coordinator is also responsible for:

  • advising the client about their duties
  • liaising with HSE and the principal contractor
  • coordinating the health and safety aspects
  • preparing/updating the health and safety file

SI Code of Practice

In January 2004 the British Standards Institution published a PWTAG sponsored code of practice, based on the 1999 version of Swimming Pool Water. Its full title is Publicly Available Specification PAS 39:2003

Management of public swimming pools – Water treatment systems, water treatment plant and heating and ventilation systems – Code of practice (out of print but stll available, and with an update This is a useful checklist, in CoP form, of PWTAG principles and practices – and a standard to which pools can aspire and be accredited.

Design issues These issues are presented here simply as headings; the subjects are dealt with in more detail in Swimming Pool Water. It is unlikely that one person – consultant, architect, manager, pool operator or whatever – would be fully conversant with all these technical issues. But anybody with some responsibility for a new building or alteration of an existing one does need to be aware that these technical issues must be taken into account. (Maintenance, too – see Swimming Pool Water – needs to be included.)

Bathing load, circulation rate, turnover

Bathing load affects circulation rate; this, with pool volume, governs turnover period; the two together determine the size of filtration plant etc, and affect the selection of the water treatment system. So these issues are central.


The design of water movement demands attention to:

  • pool size and shape (including profile)
  • size, number and location (including safety considerations) of pool water inlets and outlets
  • size and routing of circulation pipework • size and location of balance tank
  • water circulation within the balance tank • transfer channels
  • pumping and location of pump sump (if needed)
  • design and correct sizing of the filtration plant, including filters, filter media, filtration rates and backwashing system
  • integration of water features
  • moving floors and booms
  • effect of evaporation (normal and induced by water features) on relative humidity in the pool hall
  • effect of water movement on noise levels in the pool hall

Water treatment

In design terms this means considering:

  • source water characteristics – bearing in mind that these may change
  • pool type
  • pool temperature
  • removal of suspended and colloidal matter • disinfection and oxidising agents
  • coagulants
  • pH adjustment
  • water balance
  • fresh water dilution
  • effects on air quality
  • plant size and operation
  • plant personnel; training
  • water testing and recording
  • plant monitoring and control
  • energy and operation costs
  • guidelines on water quality

Plant room

The principles are dealt with earlier in this technical note. Many detailed issues need to be considered at the design stage (some of them to do with chemical safety):

  • size and location of plant room, taking into account filter specifications, the scale of other water treatment plant, and the need for short, flooded suction pipework
  • location of other plant items and ductwork
  • plant layout for ease of operation and maintenance
  • interfaces and coordination with other building elements – including ventilation intakes (well away from plant room and chemical stores or where people have access)
  • access for plant replacement/refurbishment
  • access for chemical deliveries, including safety considerations
  • secure bunded storage areas for chemicals
  • backwash water and drainage requirements
  • health and safety requirements
  • plant room environment relating to temperature, humidity, ventilation and noise
  • builders’ work requirements
  • electrical requirements (the current IEE regulations).

General environment

There are a number of such design issues that affect water quality:

  • movement of people, reception, changing room lockers etc
  • how much of the operation is inside and how much outside the building
  • preswim hygiene, toilets, showers
  • pool finishes, etc (which can affect algal growth)
  • how floors are arranged on poolside, in changing rooms, etc. – including drainage
  • cleaning around the pools – especially if there can be a separate route for draining cleaning products
  • separation of function in changing rooms, so that shoes don’t get wet and spread dirt
  • how far polluting features (like flumes) spill into main pools (say, via waterfalls)
  • heating and ventilation.

Specialist help

Building or substantially refurbishing a swimming pool, to be a success, demands a proper understanding of the distinction between design and installation. It is also important that the responsibility for issues of design and installation is clearly identified at the beginning of the project.

Who designs?

Water treatment design requires specialist engineering knowledge, which needs to be recruited at the same time as the architect and structural and environmental services engineers. That knowledge, which is critical to producing a satisfactory design, is available from two sources:

  • consultants – who can provide independent specialist advice and who are appointed as full members of the design team
  • contractors – who can work to a consultant’s brief, or to their own or a client’s.

Normally, an architect would be the lead consultant – coordinating design, construction, building services, etc. A specialist building services engineer would advise on water treatment design and liaise with specialist companies before tender.

Choosing a swimming pool consultant

A water treatment consultant with pool experience will develop the brief, produce a competent design, detailed drawings and specification, monitor the installation work on site and oversee final commissioning. When competitive tenders are needed, the consultant should be particularly valuable in ensuring that they are based on an equivalent level of specification and scope of work.

It is important that the water treatment consultant is a member of a recognised engineering institute and is not linked to any particular manufacturers or suppliers. Lists of water treatment consultants are available from professional and other institutes.

References should be pursued, and the consultants’ skill and experience in watertreatment verified by interview. Having enough professional indemnity insurance is critical: it must cover the specific work being undertaken.

Choosing a water treatment contractor

If the client has a good design brief or specification, then a water treatment contractor can be appointed to design as well as install the plant. The choice of contractor then becomes particularly important.

In any case, the contractor must be responsible, for the supply, installation and commissioning of the system, and for incorporating equipment from reputable manufacturers.

So it is a challenging brief and there is no single method for finding the right contractor. But there are at least four useful pointers for selecting contractors (and manufacturers).

  • Contractors may offer accredited quality assurance – perhaps to EN ISO 9000. Where contractors provide a design warranty, they should be qualified to Part 1 of this standard. This should imply a quality system, though not necessarily a quality product.
  • A long and successful record of quality work is a positive indication. Previous work can be checked by inspecting installations and by taking up references from clients, architects and engineers. Referees should be asked to comment on design ability, performance during contract, reliability of equipment recommended and used, commissioning and staff
  • training record, standard of operating and maintenance anuals, and aftersales service.
  • Members of professional (and associated) affiliations should be considered first. CIBSE, ISPE and ISRM belong in the first category; BISHTA, British Water and SPATA are in the second.
  • Good contractors, like good consultants and good suppliers, will be familiar with the PWTAG book, Swimming Pool Water: Treatment and Quality Standards for Pools and Spas.

Heating and air circulation

Maintaining satisfactory environmental conditions in the pool hall and all other areas of the building is essential for the health and comfort of bathers, lifeguards, staff, spectators, teachers, etc – and for the pool to operate successfully over a reasonably extended working life.

The heating of the pool water, and the heating and ventilation of the pool hall, need to take into account a wide range of factors such as bathing load, water temperature and quality; plant room location; integration with the building structure; materials and insulation of the pool hall envelope; capital, operating and life cycle costs.

Then the temperature of the air and the water need to be linked and balanced so as to maintain the right humidity, optimise user comfort and minimise evaporation from the pool water. Air temperature should be no more than 1 degree celsius above that of the water – even if this increases evaporation. It is also necessary to ensure that the air circulation system removes used air from the hall and particularly from just above the pool water surface (where disinfection byproducts, including chloramines, are at greatest concentration). Fresh air must be distributed effectively over the whole of the pool hall area.

Pool water heating

The actual heating of the pool water is a relatively simple operation. It is generally carried out by a heat exchanger (normally a lowpressure hot water system at 82oC flow, 71oC return) to transfer heat from the primary heating system, sometimes via heat recovery systems, to the pool water. The heater is generally sized on the basis of raising the pool water temperature by 0.5 degrees per hour (to adjust temperature, make good heat lost by evaporation, backwashing, etc). If a pool is being heated from cold, the rate must be no more than 0.25 degree per hour, otherwise rates of expansion of materials may cause problems to the pool structure or lining (see BS53854:1992).

Particularly on a new pool, the precise rate of temperature rise should be determined by its designers.

The heating control system must be capable of coping accurately with a wide range of temperatures. It may be possible, through the use of mixing valves and associated equipment, to serve different pools at different temperatures from a single heat exchanger. But it is recommended that a separate heat exchanger with controls is provided for each separate pool water area (so they can have different temperatures) – unless the pools share one filtration and circulation system.

Spa water is normally heated electrically (almost 66% less efficient than a lowpressure hot water system), to a temperature slightly above body heat, maximum 40oC.


There has been a consistent trend towards higher water temperatures in recent years, encouraged by the substantial growth in leisure pools and special swimming sessions for young children. Operators tempted to join the move towards higher temperatures should bear in mind that they do create a number of problems.

  • Microorganisms multiply faster – up to twice as fast for a rise of 10 degrees C; filters are increasingly likely to become colonised.
  • Bathers get hotter – limiting serious swimming and increasing sweat and grease in the water.
  • Energy costs, direct and indirect, are higher – whatever efficiency or conservation methods are used.
  • Air temperatures, which are linked to those of the water, rise too – making the atmosphere less comfortable for staff and others (as can the higher moisture levels).
  • There is more moisture in the pool atmosphere, even when relative humidity is controlled at the same level – with a risk of condensation and possibly corrosion and deterioration of the building fabric, structure and equipment.
  • Dissolved gases become less soluble – more bad smells (chloramines) and potentially harmful trihalomethanes; and pH value rises as carbon dioxide escapes.

With an increasingly wide variety of pool uses, and operators attempting to introduce more flexibility into programming of pool operation, it is obviously difficult to select a single appropriate or optimum operating temperature for any particular pool. The large volumes of water involved make it impossible to vary water temperatures rapidly in any one water area. This means that the selection and accurate control of the optimum water temperature for each pool is essential.

The temperature of the pool hall air should normally be maintained at the water temperature – or no more than 1 degree C above or below. But it is recommended that air temperatures over 30oC should generally be avoided. Clearly there may have to be compromises where, for example, mothers and toddlers have to be accommodated in the same area as fitness swimming.

These are PWTAG’s recommendations on water temperatures for different types of pool. They are maximum temperatures; operators may be able to run pools a degree or two less than the figures quoted.

Competitive swimming and diving, fitness swimming, training 26-28oC, Recreational, adult teaching, conventional main pools 27-29oC, Leisure pools 28-30oC, Children’s teaching 29-31oC, Babies, young children, disabled 30-32oC, Hydrotherapy 30-35oC, Spa pools 30-40oC.

Ventilation and air circulation

This is a complex and critical area – and the most important way of controlling smells, air temperature, and humidity, condensation and environmental quality generally. It is generally recommended that fresh or treated air is well distributed over the whole area, and that air movement within the occupied zone is maintained within acceptable conditions for bather health and comfort.

The ideal ventilation rate for a pool hall, taking into account varying external conditions, bathing loads, evaporation rate, water quality, etc is very difficult to estimate and will, by necessity, change with varying circumstances. A  recommended guideline figure of 10 litres of ventilation air per second per m2 of total pool hall area (water area plus all wet surrounds) has proven to be acceptable in a wide range of pools. This should also control humidity at a satisfactory level, provided adequate fresh air is introduced.

This ventilation rate normally results in an overall total of approximately six air changes per hour depending on the height of the pool hall, but this may need to be increased to eight or ten air changes per hour for leisure pools with extensive water features.

It is generally recommended that the relative humidity is maintained between 50% and 70% throughout the pool hall area (recommended control level of 60% ± 10%). Levels above 70% produce a risk of discomfort and condensation, and levels lower than 50% can increase evaporation and energy use. Humidification control (using dehumidifiers, etc) can help control pool hall conditions, but this does not replace the need for adequate ventilation to control air quality.

Separate areas

Areas for eating, drinking, etc within the pool building are a potential problem. Their individual requirements should be assessed carefully. Those areas do not necessarily need to be physically separated from the pool hall, but environmental conditions different from those around the pool must be considered. Furniture, for example, can be affected by the pool hall atmosphere.

Sources of ventilation

The best source for ventilation is fresh air; this should be the first consideration for all pools. There should be a minimum of 12 litres per second of fresh air provided for each occupant of the pool hall (bathers, staff, spectators, etc).

Safeguarding the fabric of the building

There are various ways in which pool water can have a damaging effect on the fabric of pool buildings.


Perhaps the most dramatic effect is stress corrosion cracking of stainless steel, seemingly provoked by disinfection byproducts. At its worst, this has caused the failure of the supporting structure of suspended ceilings, which have collapsed as a result. The most susceptible grades of stainless steels are the grades in common use in swimming pools. Unfortunately the grades which offer greater resistance to stress corrosion cracking are more expensive and not commonly available as components. Using stainless steel in pool buildings demands careful attention to the selection and availability of the correct grades, as well as regular and easy inspection and maintenance regimes. Air temperatures above 30°C must also be avoided. Designers need to evaluate critically even the smallest stainless steel components against the risk of stress corrosion.

In general, well chosen stainless steel elements in and around the pool water which are easily accessible and not safety critical can be effectively maintained and are protected against staining and pitting.

There have been discoveries of corrosion of mild steel in the reinforced concrete of the pool tank itself. This is the result of chloride attack following penetration of the grout and the concrete covering the mild steel reinforcement. The steel reinforcement can corrode as a result, but the damage remain undetected.


Grout attack can be a problem. Some pools have had to have grout expensively replaced within a few years of construction. The cause is not clear. Low calcium hardness has been blamed, but the circumstantial evidence was always equivocal. Given the variability in water supplies and experience with pools around the country, it seems difficult to summon up good empirical evidence that hardness is the issue. Acids are known to attack grout. Water movement (wave machines, for example) will erode grout as might automatic cleaners. Pool water with high sulphate levels may attack grout; resistant cement and epoxy grout may be necessary.

Grout loss seems to be less of a problem abroad, without boosting hardness into the hundreds. On the other hand, epoxy grouts which do resist attack – seem to be used more commonly abroad. The cost of tiling and grouting a pool rises by only around 10% if epoxy is used instead of cementitious grout. This extra could be less than the chemical cost of increasing hardness. Research by PWTAG indicated that properly prepared, applied and cured grout should stand up to waters of any hardness. So the problems some pools have had seem more likely to be the result of unsuitable grout, acid attack, water movement, cleaners, changing tile sizes and, perhaps most likely, poor application methods.

For example, it can be difficult to achieve the recommended 90% fill of the void behind tiles; there is also the question of whether grout is given enough time to dry and cure before the pool is filled. It is important to follow the times recommended – which can be some weeks. The different gaps allowed between different sizes of tiles may even be part of the problem.

Filling and emptying

A pool should not be emptied unless absolutely necessary, and never without taking expert advice, as enormous damage can be done to the structure. Great care must also be taken refilling and reheating the water. There is a British Standard BS5385 on this and an ISRM code of practice. Operators should notify the relevant water authorities about the discharge and refilling.

Recirculation of pool air

Although air recirculation conserves some energy, ot does produce a risk of increased buildup of contaminants in the pool environment. This can aggravate respiratory complaints, especially in staff. It also increases the potential for deterioration in equipment and components made of metal, wood or nylon – structural timber, steelwork, roof and ceiling fittings, air handling plant and equipment, etc. So any recirculation introduced should be carefully controlled (and the results monitored where possible) – for example, restricted to periods when the pool is very lightly loaded or unoccupied, and when pool covers are in use to reduce evaporation.

If recirculation is used with the pool in use, a 30% minimum of fresh air should be provided; with 100% fresh air available when necessary (e.g. very high bather loads and/or high levels of contaminants in the pool atmosphere). There are in any case other ways of conserving energy, through energy management.

Energy management

Swimming pools are one of the few building types operating at such high temperature and humidity throughout the year. This results in potentially high heat losses and means that all pool buildings should be well insulated – considerably better than basic building regulation standards if possible. And they should be well sealed from the outside and surrounding areas.

Heating the ventilation air will generally be one of the major energy loads for a pool. So a simple heat exchange device such as a plate heat exchanger or runaround coil should be provided to reclaim as much energy as possible from the exhaust air, in order to optimise energy efficiency. Other energy efficiency devices can be considered, such as thermal wheels, heat pumps, desiccant wheels and combined heat and power units – but these should be carefully evaluated over the projected life cycle of the building services installation.

The ventilation system should operate all the time the pool is in use, and it may be needed even when the pool is not in use, to maintain environmental conditions within the pool hall and prevent condensation.

An effective pool cover can normally reduce the need for the ventilation system to operate at full loading out of hours (it may even allow it to be shut off ) and therefore substantially reduce energy use.

Liquid pool cover manufacturers claim the product spreads a monomolecular layer of wax, delivered via an alcohol carrier, over the surface of the pool water. In theory, this would reduce evaporation in calm waters. This effect is likely to be compromised if bathers are present, and by surface water removal in decklevel pools. So its performance is likely to depend on the type and use of pool. Some pool operators have found it satisfactory, some not. Operators should watch out for the possibility of automatic dosing sensors needing extra cleaning if they get coated in wax.


There are more details of heating and air circulation in pools in the Sport England’s Design Guidance Note on Swimming Pools. For further guidance on energy management, the Building Research Establishment Energy Conservation Support Unit (BRECSU) has a series of publications.