SWP 08 Chapter 1:swp 26/02/2009 09:33 Page 1
Chapter 1
What is a swimming pool?
In pre-historic times people swam, if they did, in
natural waters. The Indus Valley (in modern
Pakistan) has the remains of a 60-metre mud
brick pool lined with tar, built about 3,000BC.
Swimming pools were built by the ancient Greeks
and Romans and used mainly for athletic and
military training. Roman emperors had private
pools in which they also kept fish.
A typical swimming pool needs four major
components:
• a tank (basin, shell)
• a circulation system – pumps, inlets and
outlets, pipework
• filtration
• a dosing system for treatment chemicals.
The first indoor pools in England were built early
in the 19th century. Largely untreated, early
swimming pools got dirtier as the days went by
and had to be emptied and refilled. When
charging came in, the price of a swim might fall
as the water got dirtier. Another way round the
problem of pollution was to build pools that water
flowed through on a continuous basis, usually
from a nearby stream.
The basic idea is to pump water in a continual
cycle, from the pool via filtration and chemical
treatment and back to the pool again. By this
circulation, the water in the pool is kept relatively
free of dirt, debris and microorganisms (bacteria
and viruses). Other processes are included –
heaters, for example. And it must be possible to
make up water lost by evaporation, backwashing the filters and bathers carrying it out of
the pool on costumes etc.
These days all but the humblest pool is quite a bit
more complicated than that.
How pool tanks are built
How swimming pools work
Swimming pools come in all shapes and sizes,
but nearly all of them, from the domestic to the
Olympic, from the rectangular to the variously
shaped leisure, work in the same basic way.
Anything with more than about 5 cubic metres of
water normally uses a combination of filtration
and chemical treatment to clean continually by recirculation a large volume of water.
The main difference between pools is how the
tank is constructed. There are several different
pool styles, each with its own advantages and
disadvantages. This is not the place to give
construction details, but a summary may be
useful background.
While these pool construction methods are quite
different, they all rely on similar basic circulation
and treatment systems.
Concrete pools
Concrete pools are usually finished in tiles or
mosaic. As these are not flexible, the pool tank
FIG 30. TYPICAL POOL WATER CIRCULATION SYSTEM
pH CORRECTION
BASE OUTLETS
CHLORINE &
pH MEASUREMENT
CIRCULATION PUMP
PRESSURE
SAND FILTER
STRAINER
SWIMMING POOL
BALANCE TANK
A typical
pool water
circulation
and
treatment
system
DISINFECTANT
INJECTION
COAGULANT
DOSING
CALORIFIER/HEAT
EXCHANGER
Chapter One
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A domestic
pool under
construction
and
complete
must be reinforced concrete, designed as a
monolithic structure by a structural engineer and
fit to be built into the geological conditions of the
site chosen
The standard for reinforced concrete structures is
British Standard 8110-1: 1997 Structural use of
concrete. British Standard 8007: 1987 Design of
concrete structures for retaining aqueous liquids
is an adjunct to BS8110.
Generally speaking the vast majority of
commercially built swimming pools and a small
percentage of domestic pool tanks are built to BS
8007. This requires that the pool tank is inherently
watertight (strictly, up to 10mm loss per week). It
does not rely on waterproof renders or applied
finishes to stop leaking. It must be water tested
after construction and before the internal finishes
are applied to confirm this.
Pool tanks built to BS8110 are steel reinforced
structures but are not in themselves watertight;
this is achieved by the application of a waterproof
render and screed.
The method of construction can vary depending
on the structural engineer, the capabilities of the
pool builder and cost considerations. Generally
speaking pools built to BS8007 are more
expensive due to the higher degree of civil
engineering. The general methods of construction
are permanent shuttering, temporary shuttering
and sprayed concrete.
There are a whole range of surface finishes which
can be applied either direct onto the concrete, or
onto renders applied to the concrete when a
smooth true surface or fine dimensional tolerance
is important. The most common finish used is a
ceramic tile or mosaic which will give a long
service life and should be designed and applied
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in accordance with British Standard 5385 Wall
and floor tiling, with particular reference to part 4
which covers the application of ceramics in wet
or submerged conditions.
Pre-formed sectional (PFS) pools
Over the last 25 years or so the design and
installation of PFS pools have developed
substantially. They can be made from a range of
materials including welded stainless steel plate,
stainless steel, galvanised panels and glass
reinforced plastic panels that are bolted together.
Water retention is achieved with a heavy-duty
uPVC membrane.
PFS pools can be prefabricated offsite and
assembled quickly onsite. They have been used
particularly for 25 and 50m pools, notably as
temporary (demountable) pools for international
swimming competitions.
Vinyl-lined in-ground pools
The vinyl-lined pool has no structural strength
to speak of, as it is watertight only because of
its uPVC flexible liner. The structure’s wall
assembly can be blockwork, preformed plastic,
composite or galvanised tin panels or even
timber. The floor is porous to allow the ground
water pressure to move the liner and push into
the pool, thus preventing the pool assembly
being distorted.
The factory-made vinyl lining may be secured just
at the top, under the paving slabs, or the onsite
vinyl lining secured to the structural wall.
These pools are almost only used in the domestic
market.
Swimming Pool Water
SWP 08 Chapter 1:swp 26/02/2009 09:33 Page 3
Pre-formed (one-piece) pools
water pressure. This last is a SPATA requirement.
These are made from glassfibre-reinforced plastic
or ceramic, moulded in one piece into a tank
shape, with the necessary plumbing fitted
according to the manufacturer’s instructions. To
install the pool, an appropriately sized hole is
prepared and the preformed pool lowered into
the hole and a surround constructed. Such pools
can be installed within a week or two, and are
popular in the domestic market.
Care must be taken emptying or filling pools (see
page 38). A tiled pool must be filled or emptied at
a constant rate no greater than 750mm in 24
hours and the water heated at a constant rate no
greater than 0.25ºC per hour to avoid thermal
shock.
Above-ground pools
These can be the cheapest construction option,
as well as the easiest to build. Most above
ground pools are made from prefabricated kits,
designed for an amateur to put together (though
some people use professional installers). The
main disadvantage of this sort of pool is that it's
less durable than other designs, and generally
less attractive. Their impermanence can be a
good thing: it's relatively easy to disassemble the
pool and move it to a new location. Large ones
have been used in public swimming competitions.
Full and empty
A concrete in-ground swimming pool may seem
like a solid, immovable structure, but it is actually
rather like a boat: it can float where the
surrounding ground water table is high. Unless
the tank has been designed to withstand uplift
when the pool is empty, the ground water
pressure can actually push the pool up out of the
ground.
Partly in order to resist this pressure, in-ground
pools are generally kept full all year round. Many
in-ground pools have a special hydrostatic relief
valve near the main drain to guard against
damage from ground water pressure. Basically, if
the ground water pressure is great enough, it will
push up on a small float, which opens the valve.
When the valve opens, ground water flows into
the main drain and equalises the pressure. The
seal on this type of valve needs regular careful
maintenance to keep it free of grit and silt and
stop pool water leaking into the surrounding
ground. This approach to equalising the
pressures will restrict the opportunity for
maintenance to times when the water table is at
its lowest, which unfortunately is likely to be mid
summer, when the pool is most in demand.
Most publicly owned pools, and an increasing
number of private ones, are constructed to
BS8007 (see page 10) and designed by a
structural engineer to resist the uplift from ground
Chapter one
Leaks The commonest leakage problem in small
pools is when pipework and fittings (skimmers,
underwater lights, main drain etc) run through the
walls and floor of the pool tank. So great care
needs to be taken to ensure a good watertight
joint at these interfaces.
The circulation system
This subject is dealt with in detail in Chapter 6,
but for the purpose of understanding simply how
pools work, here is a summary.
Outlets and inlets
During normal operation, pool water is removed
at the bottom of the pool through two or more
main drain outlets and from the surface perimeter
of the pool into deck-level transfer channels,
skimmers or overflow channels. The bottom
outlets are usually on the lowest point in the pool,
so that the entire pool floor surface slopes toward
them. Most of the dirt and debris, heavier than
water, will sink and leave the pool through these
outlets. Some debris will usually have to be
vacuumed up.
The surface water draw off system works the
same way as the main outlets, but they should
take water only from the very top of the pool. As
a result, any pollution that more or less floats –
skin cells, hair etc – leaves the pool from the
surface. Today most public pools work on the
deck-level principle: up to 80% of water removal
is actively drawn from the surface of the pool,
overflowing via transfer channels round the pool.
Inlets and outlets also need to be safely
designed, with a safe water flow rate. The
designer specifies the number and arrangement
needed to achieve the recommended flow rates.
To keep people from getting their hair or limbs
caught in the pipework, the outlets are covered
with grilles or special covers.
Water is drawn out of the pool by circulation
pumps through the pool outlets, and is returned
to the pool via the filtration and treatment
systems, through the pool inlets.
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Pumps
The heart of the pool system is the water
circulation pump. In a typical pool one or more
electric pumps will draw water from the pool, pull
it through a strainer and push it through the
filtration system and back into the pool. At
appropriate points chemicals are dosed to treat
the pool water.
Filtration
This is the subject of Chapter 7. The filters shown
here are sand filters. Sand filters consist of a large
circular tank, usually made of glass fibre,
reinforced plastic or steel, containing a bed of
special, even-graded sand, which has sharp
edges.
During filtration, dirty water from the pool is
pumped down through the filter bed and the tiny
sand particles capture the many insoluble
pollutants that can be filtered. At the bottom of
the tank, the filtered water flows through the
collection system and out of the outlet pipe.
Over time, the debris collected in the sand slows
down the water flow. If gauges show an increase
in pressure on the inlet pipe compared to the
outlet pipe, the operator knows there's a lot of
collected debris in the sand. This means it's time
to backwash (cleanse) the filter. To backwash, the
operator (usually having to switch off the
circulation pump first) adjusts a number of valves
to reverse the water flow, so water from the
pump pushes up through the sand, dislodging
the debris. Automatic backwash systems are also
available. At the top of the filter tank, the dirty
water flows out through the inlet pipe and into the
sewer.
Domestic and small commercial pools normally
have a single-handle ‘multiport’ valve to operate
the various filter functions. Larger pools have a
four or five-valve frontal system.
Diatomaceous earth filters use filter membranes
(or septa) coated with this fine powder. When it
gets dirty, it has to be removed, everything
cleaned and the membranes recoated with fresh
diatomaceous earth.
In a cartridge filter, the polyester cloth (usually) or
(in some domestic pools and spas) corrugated
paper filter cartridges are removed, soaked in a
bucket of cleaning chemical, hosed off and dried.
So at least two sets of filter elements are needed.
After up to a year, the filter element is replaced.
For a filter to do its job, all the water in the pool
(or more accurately, the equivalent volume) must
12
pass through the filter in a certain amount of time
– typically between 30 minutes and six hours –
depending on the type of pool (spas, 6-15
minutes). That time is called the turnover period,
and has to be related to the bathing load (number
of bathers) the pool can take. The pool and filter
system is usually connected (via a break tank) to
the mains so fresh water can be added to the
pool. This is necessary to replace water lost to
evaporation, backwashing and the water that is
carried out on people's bodies and swim suits.
Fresh water is added also to dilute the pool water
in order to remove some of the chemical
contaminants that accumulate. In the UK
replacing up to 30 litres of pool water with fresh
water per bather is recommended (see page 24
for more on this).
Chemical disinfection
The pool's filter system does the hard work in
keeping the water clean, but it takes chemistry to
make sure pool water is safe and suitable to
swim in. Again, this subject is dealt with in detail
elsewhere (Chapters 8, 9, 10 and 11) but in
summary it's important to control the pool’s
chemistry for four reasons.
• Some disease-causing microorganisms – eg
bacteria and viruses – thrive in untreated water.
Without disinfection, there can be cross-infection
among bathers.
• pH values can affect the activity and efficiency
of disinfectants.
• Low free chlorine and/or high pH values can
result in cloudy water; this is unpleasant and can
be dangerous if a swimmer gets into trouble and
can’t be seen.
• Failure to maintain appropriate chemical
concentrations in the pool water can affect users’
health and damage parts of the pool and its
associated equipment.
Disinfectants
The most popular pool disinfectant is the
element chlorine, usually in the form of a
chemical compound such as calcium
hypochlorite (a solid) or sodium hypochlorite
(a liquid). When the disinfectant is dosed into
the water, it reacts to form various chemicals,
most notably hypochlorous acid, the prime
disinfectant. It must be monitored, in milligrams
per litre (mg/l) or parts per million (ppm) – exact
equivalent measures of what is called free
chlorine. Depending on the pool and its
clientele, the free chlorine residual might be
anything from 0.5–2.5mg/l, in order to keep
the microorganisms under control.
Swimming Pool Water
SWP 08 Chapter 1:swp 26/02/2009 09:33 Page 5
Because hypochlorous acid is an oxidant as well
as a disinfectant, it tends to combine with
pollution from bathers to form byproducts that
don’t disinfect and can irritate eyes, noses and
chests. In particular, ammonia from sweat and
urine combines with chlorine to form chloramines.
This is the irritant smell associated with swimming
pools – not chlorine (which is virtually odourless).
Pool operators have to ensure through an
effective system of engineering and control that
the level of chloramines is as low as possible.
They must in any case be less than half those of
the free chlorine value measured in the pool. It is
also important to minimise the ammonia and
other pollution going into the pool by encouraging
bathers to use toilets and showers before
swimming.
Chlorine is normally introduced into the pool
water in the plant room, usually just before or
after the filter (but see page 82). The whole
process can be automated so that the pool water
is monitored and dosed appropriately –
continuously.
One problem with hypochlorous acid is that it can
degrade rapidly when exposed to ultraviolet light
from the sun. Some organic disinfectants include
a stabilising agent, usually cyanuric acid, that
reacts with the chlorine to form a more stable
compound that does not degrade as easily when
exposed to sunlight. Or cyanuric acid can be
added separately alongside an inorganic
disinfectant (hypochlorite) to achieve the same
effect.
Water balance
Pool water chemical values must be right for
disinfection, safe for swimming and good for pool
materials. That means attention to pH, alkalinity,
hardness and dissolved solids. See Chapters 10
and 11. They need to be measured regularly (and
of course the pool water has to be tested for
microbes – Chapters 12, 13 and 14).
Overlapping circles of chemical treatment
and filtration – result, disinfection
FIG 31. DISINFECTION
CHEMICAL
TREATMENT
DISINFECTION
FILTRATION
TREATMENT
than 200), to act as a buffer that helps keep the
pH value relatively stable.
Calcium hardness is controlled so that the pool
water is neither corrosive to pool materials nor
scale producing. The ideal range is 80-200 parts
per million.
Total dissolved solids (TDS) should be no more
than 1,000 parts per million above the TDS of the
mains water. Dilution with mains water is likely to
be the only way to reduce it.
Temperature also has an effect – see page 36.
Working out the cost
The budget for building the pool is only one
consideration. The operating cost will be ongoing
in terms of labour, chemicals, water, energy,
maintenance and periodic replacement of
equipment. The whole of life costs should be
considered before reaching a decision on the
specification.
The pH value is a measure of how acid or alkaline
(strictly, basic) the water is. Neutral water is pH
value 7; lower than that the water is increasingly
acid, above it basic. Depending somewhat on the
disinfectant used, the pool’s pH value should be
maintained (by adding alkalis or acids as
necessary) at 7.2-7.4, ideally. Below that and the
water may be irritant to bathers and corrosive to
pool materials; higher and disinfection won’t be
so effective (so more chlorine is needed).
Total alkalinity is a measure mainly of alkaline
bicarbonates and carbonates in the water. It
should be more than 80 parts per million (but less
Chapter One
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A public pool
during
construction
and in use
14
Swimming Pool Water