WHO guidelines

Managing water and air quality
This chapter builds upon the background provided in Chapters 2, 3 and 4 and
provides guidance relating to water and air quality management (risk management
specifi c to certain microbial hazards is covered in greater detail in Chapter 3). The
primary water and air quality health challenges to be dealt with are, in typical order
of public health priority:
• controlling clarity to minimize injury hazard;
• controlling water quality to prevent the transmission of infectious disease; and
• controlling potential hazards from disinfection by-products.
All of these challenges can be met through a combination of the following factors:
• treatment (to remove particulates, pollutants and microorganisms), including
fi ltration and disinfection (to remove/inactivate infectious microorganisms);
• pool hydraulics (to ensure effective distribution of disinfectant throughout the
pool, good mixing and removal of contaminated water);
• addition of fresh water at frequent intervals (to dilute substances that cannot be
removed from the water by treatment);
• cleaning (to remove biofi lms from surfaces, sediments from the pool fl oor and
particulates adsorbed to fi lter materials); and
• ventilation of indoor pools (to remove volatile disinfection by-products and radon).
Controlling clarity, the most important water quality criterion, involves adequate
water treatment, including fi ltration. The control of pathogens is typically achieved
by a combination of circulation of pool water through treatment (normally requiring
some form of fi ltration plus disinfection) and the application of a chemical residual
disinfectant to inactivate microorganisms introduced to the pool itself by, for instance,
bathers. As not all infectious agents are killed by the most frequently used residual
disinfectants, and as circulation through the physical treatment processes is slow, it is
necessary to minimize accidental faecal releases and vomit (and to respond effectively
to them when they occur) and to minimize the introduction of bather-shed organisms
by pre-swim hygiene. Microbial colonization of surfaces can be a problem and is
generally controlled through adequate levels of cleaning and disinfection. The control
of disinfection by-products requires dilution, selection of source waters without DBP
precursors (may include water pretreatment if necessary), pre-swim showering, treatment,
disinfection modifi cation or optimization and bather education.
Figure 5.1 outlines the components and shows a general layout of a ‘typical’ pool
treatment system. Most pools have a pumped system and water is kept in continuous
circulation (see Section 5.6), with fresh water being added for dilution of materials
that are not effectively removed by treatment and to account for losses (often referred
to as make-up water).
5.1 Pre-swim hygiene
In some countries, it is common to shower before a swim. Showering will help to
remove traces of sweat, urine, faecal matter, cosmetics, suntan oil and other potential
water contaminants. Where pool users normally shower before swimming, pool water
is cleaner, easier to disinfect with smaller amounts of chemicals and thus more pleasant
to swim in. Money is saved on chemicals (offset to some extent by the extra cost
of heating shower water, where necessary). The most appropriate setup for showers
(e.g. private to encourage nude showering, a continuously run or automatic ‘tunnel’
arrangement) will vary according to country, but pool owners and managers should
actively encourage showering. Showers must run to waste and should be managed to
control Legionella growth (see Chapter 3).
The role of footbaths and showers in dealing with papillomavirus and foot infections
is under question. However, it is generally accepted that there must be some
barrier between outdoor dirt and the pool in order to minimize the transfer of dirt
into the pool. A foot spray is probably the best of the alternatives to footbaths. Where
outdoor footwear is allowed poolside (e.g. some outdoor pools), separate poolside
drainage systems can minimize the transfer of pollutants to the pool water.
Dilution (5.5) and
make-up water
Swimming pool
Pump Filtration (5.4)
Plant room
Surface water off-take
Bottom off-take
Treated water
alternative disinfection dosing point
Coagulant dosing (5.2)
Water disinfection (5.3)
pH correction dosing (5.10.3)
Balance tank
Figure 5.1. Water treatment processes in a ‘typical pool’ (relevant section numbers are identifi ed
in parentheses)
Toilets should be provided and located where they can be conveniently used before
entering and after leaving the pool. All users should be encouraged to use the toilets
before bathing to minimize urination in the pool and accidental faecal releases. If
babies and toddlers (that are not toilet trained) are allowed in the pool facilities, they
should, wherever possible, wear leak-proof swimwear (that will contain any urine or
faecal release) and, ideally, they should have access only to small pools that can be
completely drained if an accidental faecal release occurs.
5.2 Coagulation
Coagulants (or fl occulants) enhance the removal of dissolved, colloidal or suspended
material by bringing it out of solution or suspension as solids (coagulation), then
clumping the solids together (fl occulation), producing a fl oc, which is more easily
trapped during fi ltration. Coagulants are particularly important in helping to remove
the oocysts and cysts of Cryptosporidium and Giardia (Pool Water Treatment Advisorz
Group, pers. comm.; Gregory, 2002), which otherwise may pass through the fi lter.
Coagulant effi ciency is dependent upon pH, which, therefore, needs to be controlled.
5.3 Disinfection
Disinfection is part of the treatment process whereby pathogenic microorganisms
are inactivated by chemical (e.g. chlorination) or physical (e.g. UV radiation) means
such that they represent no signifi cant risk of infection. Circulating pool water is
disinfected during the treatment process, and the entire water body is disinfected by
the application of a residual disinfectant (chlorine- or bromine-based), which partially
inactivates agents added to the pool by bathers. Facilities that are diffi cult or impossible
to disinfect pose a special set of problems and generally require very high rates of
dilution to maintain water quality. For disinfection to occur with any biocidal chemical,
the oxidant demand of the water being treated must be satisfi ed and suffi cient
chemical must remain to effect disinfection.
5.3.1 Choosing a disinfectant
Issues to be considered in the choice of a disinfectant and application system include:
• safety (while occupational health and safety are not specifi cally covered in this
volume, operator safety is an important factor to consider);
• compatibility with the source water (it is necessary to either match the disinfectant
to the pH of the source water or adjust the source water pH);
• type and size of pool (e.g. disinfectant may be more readily degraded or lost
through evaporation in outdoor pools);
• ability to remain in water as residual after application;
• bathing load; and
• operation of the pool (i.e. capacity and skills for supervision and management).
The disinfectant used as part of swimming pool water treatment should ideally
meet the following criteria:
• effective and rapid inactivation of pathogenic microorganisms;
• capacity for ongoing oxidation to assist in the control of all contaminants during
pool use;
• a wide margin between effective biocidal concentration and concentrations resulting
in adverse effects on human health (adverse health effects of disinfectants
and disinfection by-products are reviewed in Chapter 4);
• availability of a quick and easy measurement of the disinfectant concentration
in pool water (simple analytical test methods and equipment); and
• potential to measure the disinfectant concentration online to permit automatic
control of disinfectant dosing and continuous recording of the values measured.
5.3.2 Characteristics of various disinfectants
1. Chlorine-based disinfectants
Chlorination is the most widely used pool water disinfection method, usually in the
form of chlorine gas, a hypochlorite salt (sodium, calcium, lithium) or chlorinated
isocyanurates. While chlorine gas can be safely and effectively used, it does have the
potential to cause serious health impacts, and care must be taken to ensure that health
concerns do not arise.
When chlorine gas or hypochlorite is added to water, hypochlorous acid (HOCl)
is formed. Hypochlorous acid dissociates in water into its constituents H+ and OCl–
(hypochlorite ion), as follows:
The degree of dissociation depends on pH and (much less) on temperature. Dissociation
is minimal at pH levels below 6. At pH levels of 6.5–8.5, a change occurs from
undissociated hypochlorous acid to nearly complete dissociation. Hypochlorous acid
is a much stronger disinfectant than hypochlorite ion. At a pH of 8.0, 21% of the free
chlorine exists in the hypochlorous acid form (acting as a strong, fast, oxidizing disinfectant),
while at a pH of 8.5, only 12% of that chlorine exists as hypochlorous acid.
For this reason, the pH value should be kept relatively low and within defi ned limits
(7.2–7.8 – see Section 5.10.3). Together, hypochlorous acid and OCl– are referred
to as free chlorine. The usual test for chlorine detects both free and total chlorine; to
determine the effectiveness of disinfection, the pH value must also be known.
The chlorinated isocyanurate compounds are white crystalline compounds with a
slight chlorine-type odour that provide free chlorine (as hypochlorous acid) when dissolved
in water but which serve to provide a source of chlorine that is more resistant to
the effects of UV light. They are widely used in outdoor or lightly loaded pools. They
are an indirect source of chlorine, and the reaction is represented by the equation:
Free chlorine, cyanuric acid and chlorinated isocyanurate exist in equilibrium.
The relative amounts of each compound are determined by the pH and free chlorine
↔ + OCl–
x = 1 (mono-); 2 (di-); 3 (tri-)
+ H + 2O

concentration. As the disinfectant (HOCl) is used up, more chlorine atoms are released
from the chloroisocyanurates to form hypochlorous acid. This results in an enrichment
of cyanuric acid in the pool that cannot be removed by the water treatment process.
Dilution with fresh water is necessary to keep the cyanuric acid concentration at a
satisfactory level.
The balance between free chlorine and the level of cyanuric acid is critical and can
be diffi cult to maintain. If the balance is lost because cyanuric acid levels become too
high, unsatisfactory microbial conditions can result. Cyanuric acid in chlorinated
water (whether introduced separately or present through the use of chlorinated isocyanurates)
will reduce the amount of free chlorine. At low levels of cyanuric acid, there
is very little effect; as the cyanuric acid level increases, however, the disinfecting and
oxidizing properties of the free chlorine become progressively reduced. High levels of
cyanuric acid cause a situation known as ‘chlorine lock’, when even very high levels
of chlorine become totally locked with the cyanuric acid (stabilizer) and unavailable
as disinfectant; however, this does not occur below cyanuric acid levels of 200 mg/l.
It means, however, that the cyanuric acid level must be monitored and controlled
relative to chlorine residual, and it is recommended that cyanurate levels should not
exceed 100 mg/l. A simple turbidity test, where the degree of turbidity, following
addition of the test chemical, is proportional to the cyanuric acid concentration, can
be used to monitor levels. For effective disinfection, the pH value must also be monitored,
because the infl uence of pH on disinfection effi ciency is the same as described
for chlorine as a disinfectant.
2. Bromine-based disinfectants
Elemental bromine is a heavy, dark red-brown, volatile liquid with fumes that are
toxic and irritating to eyes and respiratory tract, and it is not considered suitable for
swimming pool disinfection.
Bromine combines with some water impurities to form combined bromine, including
bromamines. However, combined bromine acts as a disinfectant and produces
less sharp and offensive odours than corresponding chloramines. Bromine does not
oxidize ammonia and nitrogen compounds. Because of this, bromine cannot be used
for shock dosing. When bromine disinfectants are used, shock dosing with chlorine
is often necessary to oxidize ammonia and nitrogen compounds that eventually build
up in the water (MDHSS, undated). Hypobromous acid reacts with sunlight and
cannot be protected from the effects of UV light by cyanuric acid or other chemicals,
and thus it is more practical to use bromine disinfectants for indoor pools.
For pool disinfection, bromine compounds are usually available in two forms, both
of which are solids:
• a one-part system that is a compound (bromochlorodimethylhydantoin –
BCDMH) of both bromine and chlorine, each attached to a nitrogen atom of
dimethylhydantoin (DMH) as organic support for the halogens; and
• a two-part system that uses a bromide salt dissolved in water, activated by addition
of a separate oxidizer.
BCDMH is an organic compound that dissolves in water to release hypobromous
acid (HOBr) and hypochlorous acid. The latter reacts with bromide (Br–) (formed by a
reduction of hypobromous acid) to form more hypobromous acid:
HOBr Br–
HOCl + Br– HOBr + Cl–
It can, therefore, be used both for treatment (oxidation) and to provide a residual
disinfectant. Like the chlorinated isocyanurates, failure to maintain the correct relationship
between the disinfectant residual and the organic component can result
in unsatisfactory microbial conditions. The level of dimethylhydantoin in the water
must be limited and should not exceed 200 mg/l. There is no poolside test kit available,
and the need to regularly monitor dimethylhydantoin by a qualifi ed laboratory
is a disadvantage of the use of BCDMH. On the other hand, BCDMH is relatively
innocuous in storage, is easy to dose and often does not need pH correction (as it is
nearly neutral and has little effect on the pH values of most water). It is mostly available
as tablets, cartridges or packets. BCDMH has a long shelf life and dissolves very
slowly, so it may be used in fl oating and erosion-type feeders.
The two-part bromine system consists of a bromide salt (sodium bromide) and an
oxidizer (hypochlorite, ozone). The sodium bromide is dosed to the water, passing
through the treatment processes, upstream of the oxidizer, which is added to activate
the bromide into hypobromous acid:
Br– + oxidizer HOBr
Disinfectant action returns hypobromous acid to bromide ions, which can again
be reactivated. The pH value should be between 7.8 and 8.0 using this disinfection
system (see also Section 5.10.3).
3. Ozone
Ozone can be viewed as the most powerful oxidizing and disinfecting agent that is
available for pool water treatment (Rice, 1995; Saunus, 1998); it is generated on site
and is potentially hazardous, particularly to the plant room operators. It is unsuitable
for use as a residual disinfectant, as it readily vaporizes, is toxic and is heavier than air,
leading to discomfort and adverse health effects (Locher, 1996). Ozonation is, therefore,
followed by deozonation and addition of a residual disinfectant (i.e. chlorine- or
bromine-based disinfectants).
All of the circulating water is treated with suffi cient amounts of ozone (between
0.8 and 1.5 g/m3, depending on the water temperature) to satisfy the oxidant demand
of the water and attain a residual of dissolved ozone for several minutes. Under these
conditions, ozone oxidizes many impurities (e.g. trihalomethane [THM] precursors)
and microorganisms (disinfection), thereby reducing subsequent residual disinfectant
requirements within the pool water. Lower disinfectant demand allows the pool operator
to achieve the desired residual with a lower applied chlorine (or bromine) dose. As
ozone can be inhaled by pool users and staff, excess ozone must be destroyed (forming
oxygen and carbon dioxide) by deozonation (using granular activated carbon, activated
+ 2 H + 2O
↔ H-(DMH)-H
heat-treated anthracite or thermal destruction), and an ozone leakage detector should
be installed in the plant room. As residual disinfectants would also be removed by the
deozonation process, they are, therefore, added after this. Microbial colonization of the
deozonation media (especially granular activated carbon) can occur; this can be avoided
by ensuring that there is residual disinfectant in the incoming water stream from the
pool, maintaining the correct fi lter bed depth and an appropriate fi lter velocity.
Chloramines are oxidized by ozone into chloride and nitrate (Eichelsdörfer &
Jandik, 1979, 1984), and precursors of disinfection by-products are also destroyed,
resulting in very low levels of THMs (<0.02 mg/l) (Eichelsdörfer et al., 1981; Eichelsdörfer,
1987) and other chlorinated organics. The use of ozone in conjunction with
chlorine (to ensure a residual disinfectant throughout the pool or similar environment)
is, however, considerably more expensive than that of chlorine alone.
An ozone system in combination with BCDMH is also in use. However, the practice
is to add only small amounts of ozone to this system to oxidize only the bromide (resulting
from the spent hypobromous acid) back to hypobromous acid. Therefore, this
BCDMH/ozone combination allows less BCDMH to be added. Ozone can also be used
in combination with sodium bromide, as described above, as an oxidizer.
4. Ultraviolet (UV) radiation
Like ozone, the UV radiation process purifi es the circulating water, without leaving a residual
disinfectant. It inactivates microorganisms and breaks down some pollutants (e.g.
chloramines) by photo-oxidation, decreasing the oxidant demand of the purifi ed water.
UV disinfection can be achieved by UV irradiation at wavelengths between 200
and 300 nm. The following criteria are important in the selection of an appropriate
UV system:
• type of microorganisms to be destroyed;
• water fl ow rate to be treated;
• type of lamps (low or medium pressure);
• UV dose;
• water temperature; and
• rate of disinfection.
For UV to be most effective, the water must be pretreated to remove turbidity-causing
particulate matter that prevents the penetration of the UV radiation or
absorbs the UV energy (Saunus, 1998). The UV lamps need to be cleaned periodically,
as substances that build up on the lamps will reduce their pathogen inactivation
effi ciency over time. As with ozone, it is also necessary to use a chlorine- or brominebased
disinfectant to provide a residual disinfectant in the pool.
5. Algicides
Algicides are used to control algal growths, especially in outdoor pools. Algal growth
is possible only if the nutrients phosphate, nitrogen and potassium are present in the
pool water. Phosphate can be removed from the pool water by good coagulation and
fi ltration during water treatment. Algal growth is best controlled by ensuring effective
coagulation/fi ltration and good hydraulic design. In such properly managed swimming
pools, the use of algicidal chemicals for the control of algae is not necessary
(Gansloser et al., 1999). If problems persist, however, then proprietary algicides can
be used. Quaternary ammonium and polyoximino compounds and copper salts can
be used, but any based on mercury (a cumulative toxic heavy metal) should not be
added to swimming pools. All should be used in strict accordance with the suppliers’
instructions and should be intended for swimming pool use.
5.3.3 Disinfection by-products (DBP)
The production of disinfection by-products (see Chapter 4) can be controlled to a
signifi cant extent by minimizing the introduction of precursors though source water
selection, good bather hygienic practices (e.g. pre-swim showering – see Section 5.1),
maximizing their removal by well managed pool water treatment and replacement
of water by the addition of fresh supplies (i.e. dilution of chemicals that cannot be
removed). It is inevitable, however, that some volatile disinfection by-products, such
as chloroform and nitrogen trichloride (a chloramine), may be produced in the pool
water (depending upon the disinfection system used) and escape into the air. While
levels of production should be minimized, this hazard can also be managed to some
extent through good ventilation (see also Section 5.9).
5.3.4 Disinfectant dosing
The method of introducing disinfectants to the pool water infl uences their effectiveness,
and, as illustrated in Figure 5.1, disinfectant dosing may occur pre- or post-fi ltration.
Individual disinfectants have their own specifi c dosing requirements, but the following
principles apply to all:
• Automatic dosing is preferable: electronic sensors monitor pH and residual disinfectant
levels continuously and adjust the dosing correspondingly to maintain
correct levels. Regular verifi cation of the system (including manual tests on pool
water samples) and good management are important. Section 5.10 describes the
monitoring procedures.
• Hand-dosing (i.e. putting chemicals directly into the pool) is rarely justifi ed.
Manual systems of dosing must be backed up by good management of operation
and monitoring. If manual dosing is employed, it is important that the
pool is empty of bathers until the chemical has dispersed.
• Dosing pumps should be designed to shut themselves off if the circulation system
fails (although automatic dosing monitors should remain in operation) to
ensure that chemical dispersion is interrupted. If chemical dosing continues
without water circulating, then high local concentrations of the dosed chemical
will occur. On resumption of the circulation system, the high concentration
will progress to the pool. If, for example, both hypochlorite and acid have been
so dosed, the resultant mix containing chlorine gas may be dangerous to pool
• Residual disinfectants are generally dosed at the end of the treatment process.
The treatment methods of coagulation, fi ltration and ozonation or ultraviolet
serve to clarify the water, reduce the organic load (including precursors for
the formation of disinfection by-products) and greatly reduce the microbial
content, so that the post-treatment disinfection can be more effective and the
amount of disinfectant required is minimized.
• It is important that disinfectants and pH-adjusting chemicals are well mixed
with the water at the point of dosing.
• Dosing systems, like circulation, should operate 24 h a day.
Shock dosing
• Using a shock dose of chlorine as a preventive measure or to correct specifi c
problems may be part of a strategy of proper pool management. Shock dosing
is used to control a variety of pathogens and nuisance microorganisms
and to destroy organic contaminants and chloramine compounds. Destroying
chloramines requires free chlorine levels at least 10 times the level of combined
chlorine. As a preventive measure, routine shock dosing (which is practised
in some countries) typically involves raising free chlorine levels to at least
10 mg/l for between 1 and 4 h. Intervention shock dosing for a water quality
problem (such as an accidental faecal release) may involve raising the free
chlorine residual to 20 mg/l for an 8-h period while the pool is empty (see
Section 5.8).
• Trying to compensate for inadequacies in treatment by shock dosing is bad
practice, because it can mask defi ciencies in design or operation that may produce
other problems.
• If not enough chlorine is added, the combined chlorine (chloramines) problem
may be exacerbated, and conjunctival irritation and obnoxious odours in the
pool area may be raised to high levels. If too much chlorine is added, it may
take a long time to drop to safe levels before bathing can be resumed. Chlorine
levels should return to acceptable levels (i.e. <5 mg/l – see Section 4.4.1) before
bathers are permitted in the pool.
5.4 Filtration
The primary function of fi ltration is to remove turbidity to achieve appropriate water
clarity. Water clarity is a key factor in ensuring the safety of swimmers. Poor underwater
visibility is a contributing factor to injuries (Chapter 2) and can seriously hamper
recognition of swimmers in distress or a body lying on the bottom of the pool.
Disinfection will also be compromised by particulates. Particles can shield microorganisms
from the action of disinfectants. Alternatively, the disinfectants may react
with certain components of organic particles to form complexes that are less effective
than the parent compounds, or the disinfectants may oxidize the organic material,
thereby eliminating disinfection potential. Filtration is often the critical step for the
removal of Cryptosporidium oocysts and Giardia cysts (see Section 3.3). Filtration is
also effective against microbes, notably free-living amoebae, that harbour opportunistic
bacteria such as Legionella and Mycobacterium species.
5.4.1 Filter types
There are a number of types of fi lter available, and the choice of fi lter will be based
on several factors, including:
• the quality of the source water;
• the amount of fi lter area available and number of fi lters> Pools benefi t greatly
from the increased fl exibility and safeguards of having more than one fi lter;
• fi ltration rate: Typically, the higher the fi ltration rate, the lower the fi ltration
effi ciency;
• ease of operation;
• method of backwashing: The cleaning of a fi lter bed clogged with solids is referred
to as backwashing. It is accomplished by reversing the fl ow, fl uidizing
the fi lter material and passing pool water back through the fi lters to waste.
Backwashing should be done as recommended by the fi lter manufacturer, when
the allowable turbidity value has been exceeded, when a certain length of time
without backwashing has passed or when a pressure differential is observed; and
• degree of operator training required.
1. Cartridge fi lters
Cartridge fi lters can nominally fi lter down to 7 μm and last up to two years. The fi lter
medium is spun-bound polyester or treated paper. Cleaning is achieved by removing
the cartridge and washing it. Their main advantage is the relatively small space requirement
compared with other fi lter types, and they are often used with small pools
and hot tubs.
2. Sand fi lters
Medium-rate sand fi lters can nominally fi lter down to about 7 μm in size with the
addition of a suitable coagulant (such as polyaluminium chloride or aluminium hydroxychloride).
Cleaning is achieved by manual reverse fl ow backwashing, with air
scouring to remove body oils and fats to improve the backwash effi ciency. For indoor
heated pools, the sand medium typically has a life of between fi ve and seven years.
Medium-rate sand fi lters are comparatively large-diameter pressure vessels (in a horizontal
or vertical format) and require large plant rooms. Drinking-water treatment
has shown that when operated with a coagulant, sand fi lters can remove over 99% of
Cryptosporidium oocysts. Studies in a pilot sand fi ltration plant under swimming pool
fi ltration conditions have shown that without the addition of coagulant, removal of
the Cryptosporidium oocyst surrogate (fl uorescent polystyrene particles sized between
1 and 7 μm) was less than 50%. Using coagulants, polyaluminium chloride and polyaluminium
silicate sulfate improved the removal up to 99% (Pool Water Treatment
Advisory Group, pers. comm.).
3. Ultrafi ne fi lters
Ultrafi ne precoat fi lters (UFF) use a replaceable fi lter medium that is added after each
backwash. Filter media include diatomaceous earth, diatomite products and perlite.
The benefi t of precoat fi ltration is that it can provide a particle removal of 1–2 μm
and, as such, provide good removal of Cryptosporidium oocysts. Table 5.1 compares
the alternative fi lter types.
5.4.2 Turbidity measurement
Turbidity is a measure of the amount of suspended matter in water, and the more
turbid the water, the less clarity. Turbidity needs to be controlled both for safety and
for effective disinfection. For identifying bodies at the bottom of the pool, a universal
turbidity value is not considered to be appropriate, as much depends on the characteristics
of the individual pool, such as surface refl ection and pool material/construction.
Individual standards should be developed, based on risk assessment at each pool,
but it is recommended that, as a minimum, it should be possible to see a small child
at the bottom of the pool from the lifeguard position while the water surface is in
movement, as in normal use. An alternative is to maintain water clarity so that lane
markings or other features on the pool bottom at its greatest depth are clearly visible
when viewed from the side of the pool. Operators could determine these indicators as
a turbidity equivalent through experience and then monitor routinely for turbidity.
In terms of effective disinfection, a useful, but not absolute, upper-limit guideline for
turbidity is 0.5 nephelometric turbidity unit (NTU), determined by the nephelometric
method (ISO, 1999).
5.5 Dilution
Coagulation, fi ltration and disinfection will not remove all pollutants. Swimming
pool design should enable the dilution of pool water with fresh water. Dilution limits
the build-up of pollutants from bathers (e.g. constituents of sweat and urine), of byproducts
of disinfection and of various other dissolved chemicals. Dilution rates need
to account for the replacement of water used in fi lter backwashing, evaporation and
splash-out. As a general rule, the addition of fresh water to disinfected pools should
not be less than 30 litres per bather.
5.6 Circulation and hydraulics
The purpose of paying close attention to circulation and hydraulics is to ensure that
the whole pool is adequately served by fi ltered, disinfected water. Treated water must
Table 5.1. Comparison of fi lter types
Filter type
Criteria UFF Medium-rate sand Cartridge
Common fi lter sizes Up to 46 m2 Up to 10 m2 Up to 20 m2
Design fi lter fl ow rate 3–5 m3/m2/h 25–30 m3/m2/h 1.5 m3/m2/h
Cleaning fl ow rate 5 m3/m2/h 37–42 m3/m2/h Not applicable
Cleaning Backwash and media
Backwash Manual, hose down
Average wash water 0.25 m3/m2 pool water 2.5 m3/m2 pool water 0.02 m3/m2 mains water
Filter aid None Optional coagulants None
Cleaning implications A backwash tank may
be required. Separation
tank required to collect
used fi lter media with
periodic sludge removal
A backwash tank
may be required
Hose-down and waste
drain facility
Particulate collection Surface Depth Degree of depth
Nominal particle removal 1–2 μm 10 μm, 7 μm with
7 μm
Pressure rise for backwash 70 kPa 40 kPa 40 kPa
Comparative running costs High Low Medium
Comparative installation
High High Low
UFF = ultrafi ne fi lter
get to all parts of the pool, and polluted water must be removed – especially from
areas most used and most polluted by bathers. It is recommended that 75–80% be
taken from the surface (where the pollution is greatest – Gansloser et al., 1999), with
the remainder taken from the bottom of the pool. The bottom returns allow the
removal of grit and improved circulation within the pool. Without good circulation
and hydraulics, even water treatment may not give adequate pool water quality.
The circulation rate is defi ned as the fl ow of water to and from the pool through all
the pipework and the treatment system. The appropriate circulation rate depends, in
most cases, on bathing load. There are, however, some types of pool where circulation
rate cannot realistically be derived from bathing load – diving pools and other waters
more than 2 m deep, for example, where the bathing load relative to water volume
may be very low. Circulation rate is related to turnover period, which is the time taken
for a volume of water equivalent to the entire pool water volume to pass through the
fi lters and treatment plant and back to the pool. Turnover periods must, however, also
suit the particular type of pool (see Box 5.1 for an example of guidance); this is related
to the likely pollution load based on the type of activity undertaken and the volume
of water within the pool. Where pools have moveable fl oors, the turnover should be
calculated based upon the shallowest depth achievable. Formulae are available for
calculating turnover rates, and these should be employed at the design stage. Box 5.1
gives some examples of turnover periods that have been employed in the UK.
In the United Kingdom (BSI, 2003), the following turnover periods for different types of pools have
been recommended:
Pool type Turnover period
Competition pools 50 m long
Conventional pools up to 25 m long with 1-m shallow end
Diving pools
Hydrotherapy pools
Leisure water bubble pools
Leisure waters up to 0.5 m deep
Leisure waters 0.5–1 m deep
Leisure waters 1–1.5 m deep
Leisure waters over 1.5 m deep
Teaching/learner/training pools
Water slide splash pools
3–4 h
2.5–3 h
4–8 h
0.5–1 h
5–20 min
10–45 min
0.5–1.25 h
1–2 h
2–2.5 h
0.5–1.5 h
0.5–1 h
5.7 Bathing load
Bathing load is a measure of the number of people in the pool. For a new pool, the
bathing load should be estimated at the design stage.
There are many factors that determine the maximum bathing load for a pool; these
• area of water – in terms of space for bathers to move around in and physical safety;
• depth of water – the deeper the water, the more actual swimming there is and
the more area a bather requires;
• comfort; and
• pool type and bathing activity.
Pool operators need to be aware of the maximum bathing load and should ensure
that it is not exceeded during the operation of the pool. Where the maximum bathing
load has not been established, it has been suggested in the UK that the fi gures in Table
5.2 (BSI, 2003) can provide an approximation. These fi gures may not be appropriate
for all pool types or all countries.
5.8 Accidental release of faeces or vomit into pools
Accidental faecal releases may occur relatively frequently, although it is likely that most
go undetected. Accidental faecal releases into swimming pools and similar environments
can lead to outbreaks of infections associated with faecally-derived viruses, bacteria and
pathogenic protozoa (Chapter 3); vomit may have a similar effect. A pool operator
faced with an accidental faecal release or vomit in the pool water must, therefore, act
If the faecal release is a solid stool, it should simply be retrieved quickly and discarded
appropriately. The scoop used to retrieve it should be disinfected so that any bacteria
and viruses adhering to it are inactivated and will not be returned to the pool the next
time the scoop is used. As long as the pool is, in other respects, operating properly (i.e.
disinfectant levels are maintained), no further action is necessary. The same applies to
solid animal faeces.
If the stool is runny (diarrhoea) or if there is vomit, the situation is more likely to be
hazardous, as the faeces or vomit is more likely to contain pathogens. Even though most
disinfectants deal relatively well with many bacterial and viral agents in accidental faecal
releases and vomit, the possibility exists that the diarrhoea or vomit is from someone infected
with one of the protozoan parasites, Cryptosporidium and Giardia. The infectious
stages (oocysts/cysts) are resistant to chlorine disinfectants in the concentrations that are
practical to use. The pool should therefore be cleared of bathers immediately.
The safest action, if the incident has occurred in a small pool or hot tub, is to empty
and clean it before refi lling and reopening. However, this is practically impossible in
many larger pools, for reasons of cost and extended periods of closure. If draining down
is not possible, then a procedure based on the one given below should be followed (it
should be noted, however, that this is an imperfect solution that will only reduce but
not eliminate risk):
• The pool should be cleared of people immediately.
• As much of the material as possible should be collected, removed and disposed
of to waste; this may be done through netting, sweeping and/or vacuuming
(provided the equipment can be adequately disinfected after use).
Table 5.2. An example of maximum bathing loadsa
Water depth Maximum bathing load
<1.0 m 1 bather per 2.2 m2
1.0–1.5 m 1 bather per 2.7 m2
>1.5 m 1 bather per 4.0 m2
a Adapted from BSI, 2003
• Disinfectant levels should be maintained at the top of the recommended range
or chlorination to 20 mg/l at pH 7.2–7.5 for 8 h (shock dosing) should be
• Using a coagulant (if appropriate), the water should be fi ltered for six turnover
cycles; this may mean closing the pool until the next day.
• The fi lter should be backwashed (and the water run to waste).
• The fi nal residual disinfectant level and pH value should be checked, and if
satisfactory, then the pool can be reopened.
The willingness of operators and lifeguards to act is critical. Pool operators are unlikely
to know with certainty what has caused a diarrhoea incident, and a signifi cant
proportion of such diarrhoea incidents may happen without lifeguards being aware
of them. The most important contribution a pool operator can make to the problem
is to guard against it. There are a few practical actions pool operators can take to help
prevent faecal release into pools:
• No child (or adult) with a recent history of diarrhoea should swim.
• Parents should be encouraged to make sure their children use the toilet before
they swim, and babies and toddlers that have not been toilet trained should ideally
wear waterproof nappies or specially designed bathing wear.
• Young children should whenever possible be confi ned to pools small enough to
drain in the event of an accidental release of faeces or vomit.
• Lifeguards should be made responsible for looking out for and acting on accidental
faecal release/vomit incidents.
5.9 Air quality
Air quality in indoor swimming pool facilities is important for a number of reasons,
• Staff and user health. The quantity of water treatment by-products, concentration
of airborne particulate matter and fresh air need to be controlled. The two
areas of principal concern for health are Legionella and disinfection by-products,
particularly chloramines. Although Legionella should primarily be controlled
in the water systems, areas housing natural spas (thermal water) and hot tubs
should also be well ventilated. Reducing exposure to disinfection by-products in
air should be pursued in order to minimize overall exposure to these chemicals,
as inhalation appears to be the dominant route of exposure during recreational
water use (see Chapter 4). As concentrations of disinfection by-products decrease
rapidly with distance from the water, this has implications for ventilation
design, which involves both mixing and dilution (i.e. with fresh air), and building
codes should stipulate appropriate ventilation rates (at least 10 litres of fresh
air/s/m2 of water surface area).
• Staff and user comfort. The temperature, humidity and velocity of the pool hall
air should be appropriate to provide a comfortable environment.
• Impact on the building fabric. The air temperature, concentration of airborne
particulate matter and quantity of water treatment by-products need to be controlled
in order to avoid an ‘aggressive environment’ that may damage the building
5.10 Monitoring
Parameters that are easy and inexpensive to measure reliably and of immediate operational
health relevance (such as turbidity, residual disinfectant and pH) should
be monitored most frequently and in all pool types. Whether any other parameters
(physical, chemical and microbial) need to be monitored is, in practice, determined
by management capacity, intensity of use and local practice. However, microbial
monitoring is generally needed in public and semi-public pools.
There should be pre-established (clear, written) procedures set up by managers for
acting on the results of monitoring, including how to act on any unexpected results.
Operators must know what to do themselves or how to ensure that appropriate action
is taken by someone else. Management should review data and test systems regularly
and ensure that pool operators have taken appropriate remedial action.
5.10.1 Turbidity
Turbidity testing is simple; approaches to establishing appropriate, facility-specifi c
turbidity standards are described in Section 5.4.2. Exceedance of a turbidity standard
suggests both a signifi cant deterioration in water quality and a signifi cant health
hazard. Such exceedance merits immediate investigation and should lead to facility
closure unless the turbidity can rapidly be brought within standards.
5.10.2 Residual disinfectant level
National or other standards for minimum and maximum levels of residual disinfectant
vary widely. The main factor is that the residual disinfectant level should always
be consistent with satisfactory microbial quality.
Failure to maintain target residual disinfectant should result in immediate investigation
and follow-up testing. If residuals cannot be rapidly re-established and
maintained, then full investigation of cause and prevention of repetition are merited,
and public health authorities should be consulted in determining whether the facility
should remain open.
1. Chlorine-based disinfectants
For a conventional public or semi-public swimming pool with good hydraulics and
fi ltration, operating within its design bathing load and turnover and providing frequent
(or online) monitoring of chlorine and pH, experience has shown that adequate
routine disinfection should be achieved with a free chlorine level of 1 mg/l
throughout the pool. Free chlorine levels well above 1.2 mg/l should not be necessary
anywhere in the pool unless the pool is not well designed or well operated – if, for
example, circulation is too slow, distribution is poor or bathing loads are too heavy;
where this is the case, it is more appropriate in the long term to deal with the underlying
problem, rather than increasing disinfection levels.
Experience suggests that the combined chlorine level within pool water (chloramines)
should be no more than half the free chlorine level (but combined chlorine should be as
low as possible and ideally less than 0.2 mg/l). If the levels are high, then it is likely that
there is too much ammonia in the water, indicating that bathing loads or pollution from
bathers may be too high, that dilution is too low or that treatment is suboptimal.
Lower free chlorine concentrations (0.5 mg/l or less) will be adequate where
chlorine is used in conjunction with ozone or UV disinfection. Higher concentra-
tions (up to 2–3 mg/l) may be required to ensure disinfection in hot tubs, because of
higher bathing loads and higher temperatures.
If the chlorine source is chlorinated isocyanurate compounds, then the level of
cyanuric acid must also be monitored and controlled; if it becomes too high (above
100 mg/l), microbial conditions may become unsatisfactory, and increased freshwater
dilution is required.
2. Bromine-based disinfectants
Total bromine levels in swimming pools, should ideally be maintained at 2.0–2.5 mg/l.
When bromine-based disinfectants are used in combination with ozone, the concentration
of bromide ion should be monitored and maintained at 15–20 mg/l. If BCDMH
is the bromine source, the level of dimethylhydantoin must also be monitored; it should
not exceed 200 mg/l.
3. Sampling and analysis
In public and many semi-public pools, there will be continuous monitoring of residual
disinfectant levels as the disinfectant is dosed (see Section 5.3.4). In addition to this, samples
should also be taken from the pool itself. In public and semi-public pools, residual
disinfectant concentrations should be checked by sampling the pool before it opens and
during the opening period (ideally during a period of high bathing load). The frequency
of testing while the swimming pool is in use depends upon the nature and use of the
pool. It is suggested that the residual disinfectant concentration in domestic pools be
checked before use. All tests must be carried out immediately after the sample is taken.
Samples should be taken at a depth of 5–30 cm. It is good practice to include as a
routine sampling point the area of the pool where, because of the hydraulics, the disinfectant
residual is generally lowest. Occasional samples should be taken from other
parts of the pool and circulation system.
The tests employed should be capable of determining free chlorine and total bromine
levels (depending upon the disinfectant used). Analysis is generally performed
with simple test kits based on the N,N-diethyl-p-phenylenediamine (DPD) method,
using either liquid or tablet reagents. This method can measure both free and total
disinfectant and is available as both colorimetric and titration test kits.
5.10.3 pH
The pH of swimming pool water should be controlled to ensure effi cient disinfection
and coagulation, to avoid damage to the pool fabric and ensure user comfort. The
pH should be maintained between 7.2 and 7.8 for chlorine disinfectants and between
7.2 and 8.0 for bromine-based and other non-chlorine processes. The frequency of
measurement will depend upon the type of pool. It is suggested that for public pools,
the pH value should be measured continuously and adjusted automatically; for other
semi-public pools and public and semi-public hot tubs, it is suggested that monitoring
be conducted several times a day, during operating hours; for domestic pools, it is
advisable to measure prior to pool use. Actions to be taken on failure to maintain pH
within the target range are similar to those for disinfectant residual.
5.10.4 Oxidation–reduction potential (ORP)
The oxidation–reduction potential (also known as ORP or redox) can also be used
in the operational monitoring of disinfection effi cacy. In general terms for swimming
pools and similar environments, levels in excess of 720 mV (measured using a silver/
silver chloride electrode) or 680 mV (using a calomel electrode) suggest that the water
is in good microbial condition, although it is suggested that appropriate values should
be determined on a case-by-case basis.
5.10.5 Microbial quality
There is limited risk of signifi cant microbial contamination and illness in a well
managed pool with an adequate residual disinfectant concentration, a pH value
maintained at an appropriate level, well operated fi lters and frequent monitoring of
non-microbial parameters. Nevertheless, samples of pool water from public and semipublic
pools should be monitored at appropriate intervals for microbial parameters.
Such tests do not guarantee microbial safety but serve to provide information with
which to judge the effectiveness of measures taken.
1. ‘Indicator’ organisms
As outlined in Chapter 3, monitoring for potential microbial hazards is generally
done using ‘indicator’ microorganisms, rather than specifi c microbial hazards (see
Box 3.1). Microorganisms used to assess the microbial quality of pools and similar
environments include heterotrophic plate count (HPC), thermotolerant coliforms,
E. coli, Pseudomonas aeruginosa, Legionella spp. and Staphylococcus aureus. Where
operational guidelines are exceeded, pool operators should check turbidity, residual
disinfectant levels and pH and then resample. When critical guidelines are exceeded,
the pool should be closed while investigation and remediation are conducted.
The HPC (37 °C for 24 h) gives an indication of the overall bacterial population within
the pool. This should be monitored in public and semi-public disinfected swimming
pools. It is recommended that operational levels should be less than 200 cfu/ml.
Thermotolerant coliforms and E. coli
Thermotolerant coliforms and E. coli are indicators of faecal contamination. Either
thermotolerant coliforms or E. coli should be measured in all public and semi-public
pools, hot tubs and natural spas. Operational levels should be less than 1/100 ml.
Pseudomonas aeruginosa
Routine monitoring of Pseudomonas aeruginosa is recommended for public and semipublic
hot tubs and natural spas. It is suggested for public and semi-public swimming
pools when there is evidence of operational problems (such as failure of disinfection
or problems relating to fi lters or water pipes), a deterioration in the quality of the pool
water or known health problems. It is recommended that for continuously disinfected
pools, operational levels should be <1/100 ml; where natural spas operate with no
residual disinfectant, operational levels should be <10/100 ml.
If high counts are found (>100/100 ml), pool operators should check turbidity,
disinfectant residuals and pH, resample, backwash thoroughly, wait one turnover
and resample. If high levels of P. aeruginosa remain, the pool should be closed and a
thorough cleaning and disinfection programme initiated. Hot tubs should be shut
down, drained, cleaned and refi lled.
Legionella spp.
Periodic testing for Legionella is useful, especially from hot tubs, in order to determine
that fi lters are not being colonized, and it is recommended that operational levels
should be <1/100 ml. Where this is exceeded, hot tubs should be shut down, drained,
cleaned and refi lled. Shock chlorination may be appropriate if it is suspected that
fi lters have become colonized.
Staphylococcus aureus
The routine monitoring of Staphylococcus aureus is not recommended, although monitoring
may be undertaken as part of a wider investigation into the quality of the
water when health problems associated with the pool are suspected. Where samples
are taken, levels should be less than 100/100 ml.
2. Sampling
Guidelines on routine sampling frequencies, along with a summary of operational
guideline values, are outlined in Table 5.3. In addition to routine sampling, samples
should also be taken from public and semi-public facilities:
• before a pool is used for the fi rst time;
• before it is put back into use, after it has been shut down for repairs or cleaning;
• if there are diffi culties with the treatment system; and
• as part of any investigation into possible adverse effects on bathers’ health.
Table 5.3. Recommended routine sampling frequenciesa and operational guidelinesb for microbial
testing during normal operation
Pool type
plate count
coliform/E. coli
Disinfected pools,
public and heavily
(<1/100 ml)
When situation
(<1/100 ml)
(<1/100 ml)
Disinfected pools,
(<1/100 ml)
When situation
(<1/100 ml)
(<1/100 ml)
Natural spas n/a Weekly
(<1/100 ml)
(<10/100 ml)
(<1/100 ml)
Hot tubs n/a Weekly
(<1/100 ml)
(<1/100 ml)
(<1/100 ml)
a Samples should be taken when the pool is heavily loaded
Sampling frequency should be increased if operational parameters (e.g. turbidity, pH, residual disinfectant concentration) are not maintained
within target ranges
Sample numbers should be determined on the basis of pool size and complexity and should include point(s) representative of general
water quality and likely problem areas
b Operational guidelines are shown in parentheses
c e.g. when health problems associated with the pool are suspected
The most appropriate site for taking a single sample is where the water velocity
is low, away from any inlets. Depending on the size of the pool, it may be advisable
to take samples from multiple sites. Many leisure pools will have additional features,
such as fl umes, islands and backwaters with a complex system of water fl ow; representative
samples should be taken.
Misleading information on pool water quality will result from incorrect sampling
procedures. Sample containers must be of a material that will not affect the quality of
the sample either microbially or chemically. Although a good-quality glass container
will meet these requirements, the risk of broken glass in the pool environment as a
result of breakage has favoured the use of shatterproof plastic-coated glass containers.
All-plastic containers can be used provided they do not react with microorganisms or
chemicals in the water; not all are suitable.
For microbial examination, the bottle must be sterile and contain an agent that
neutralizes the disinfectant used in the pool water. Sodium thiosulfate (18–20 mg/l)
is the agent used for chlorine- and bromine-based disinfectants. Clearly, the testing
laboratory must be advised before sampling if any other disinfectant is being used.
Bacteria in pool water samples and especially those from disinfected pools may be
‘injured’, and normal analytical ‘resuscitation’ procedures should be fully adhered to.
5.10.6 Other operational parameters
Several parameters are important for operational purposes. These include:
• alkalinity: Alkalinity is a measure of the alkaline salts dissolved in the water. The
higher the alkalinity, the more resistant the water is to large changes in pH in
response to changes in the dosage of disinfectant and pH correction chemicals.
If the alkalinity is too high, it can make pH adjustment diffi cult.
• calcium hardness: Calcium hardness is an operational measure that needs to be
monitored to avoid damage to the pool fabric (e.g. etching of surfaces and metal
corrosion) and scaling water.
• total dissolved solids: Total dissolved solids (TDS) is the weight of soluble material
in water. Disinfectants and other pool chemicals as well as bather pollution
will increase TDS levels. The real value of detecting an increase in TDS levels is
as a warning of overloading or lack of dilution, and TDS levels should be monitored
by comparison between pool and source water. If TDS is high, dilution is
likely to be the correct management action.
5.11 Cleaning
Good water and air quality cannot be maintained without an adequate cleaning programme.
This should include the toilets, showers, changing facilities and pool surroundings
on at least a daily basis in public and semi-public pools. Public and semi-public
hot tubs should be drained and the surfaces and pipework cleaned on a weekly basis.
Heating, ventilation and air-conditioning systems should be cleaned periodically (e.g.
weekly to monthly for those serving hot tubs). Features such as water sprays should be
periodically cleaned and fl ushed with disinfectant (e.g. 5 mg/l hypochlorite solution).
5.12 References
BSI (2003) Management of public swimming pools – water treatment systems, water treatment plant and heating
and ventilation systems – code of practice. British Standards Institute, Publicly Available Specifi cation
(PAS) 39: 2003.
Eichelsdörfer D (1987) [Investigations of anthropogenic load of swimming pool and bathing water.] A.B.
Archiv des Badewesens, 40: 259–263 (in German).
Eichelsdörfer D, Jandik J (1979) [Ozone as oxidizer.] A.B. Archiv des Badewesens, 37: 257–261 (in German).
Eichelsdörfer D, Jandik J (1984) [Investigation and development of swimming pool water treatment. III.
Note: Pool water treatment with ozone in long time contact.] Zeitschrift für Wasser- und Abwasser Forschung,
17: 148–153 (in German).
Eichelsdörfer D, Jandik J, Weil
(1981) [Formation and occurrence of organic halocarbons in swimming pool water.] A.B. Archiv des
Badewesens, 34: 167–172 (in German).
Gansloser G, Hässelbarth U, Roeske W (1999) [Treatment of swimming pool and bathing water.] Berlin,
Beuth Verlag (in German).
Gregory R (2002) Bench-marking pool water treatment for coping with Cryptosporidium. Journal of Environmental
Health Research, 1(1): 11–18.
ISO (1999) Water quality – Determination of turbidity. Geneva, International Organization for Standardization
(ISO 7027:1999).
Locher A (1996) [Non-chlorine treatment of pool water.] Gesundheits- und Umwelttechnik, 3: 18–19 (in
MDHSS (undated) Swimming pool and spa water chemistry. Missouri Department of Health and Senior
Services, Section for Environmental Health (http://www.health.state.mo.us/RecreationalWater/Pool
Rice RG (1995) Chemistries of ozone for municipal pool and spa water treatment. Journal of the Swimming
Pool and Spa Industry, 1(1): 25–44.
Saunus C (1998) [Planning of swimming pools.] Düsseldorf, Krammer Verlag (in German).


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