treatment of swimming pool water

8/8/2019 Treatment of Swimming Pool Water

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23TREATMENT OF SWIMMING POOL

WATER

1. PRINCIPLE

Swimming pool water is regenerated

through a closed-loop system, i.e., the

water polluted by numerous swimmers is

treated and pumped back into the pool.

Renewing the water completely would be

prohibitive owing to the high cost of

purchasing and reheating the feed water,

which needs to be of drinking water

quality.

Fresh water does need to be added daily,

however, to compensate for water losses

and to reduce concentrations of organic,

ammonia and mineral compounds, which

would otherwise increase continuously.

The regeneration treatment for

swimming pool water discussed here is

applicable to public swimming pools and

private collective swimming pools. It is

also suitable for leisure pool complexes

comprising swimming pools, artificial

rivers and various other amusement

attractions.

The installation used to regenerate

swimming pool water must be designed to

offer high standards of health and hygiene

although specific details may vary from

country to country owing to differences in

national legislation. French legislation may

be taken as a reference when no other

legislation exists in a given country.


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Chap. 23: Treatment of swimming pool water

2. FRENCH LEGISLATION

2.1.

GENERAL

The treatment of swimming pool water in

France is governed by decree No. 81.324 of

April 7, 1981, which nullifies the decree of

June 13, 1969 and defines the rules of

hygiene and safety applicable to swimming

pools and similar installations. New facilities

must respect the terms of this decree.

The circular of May 9, 1983 defines the

measures to be taken to bring existing

swimming pools into compliance with the

new legislation. Each pool is studied on a

case-by-case basis in order to determine how

best to bring the installation into line with the

decree of 1981.

In the case of leisure pool complexes, the

overall design is covered by the decree of1981 but specific measures may need to be

taken concerning certain amusements. It is

always wise to submit projects to official

organizations, such as the Ministry of Youth

and Sport (in France) or the Public Health

Department, for approval.

The legislation specifies the

bacteriological and physical-chemical

standards applicable to swimming pool water,

the general layout and construction of

walkways within the establishment and a

certain number of technical questions

concerning water treatment. These technical

aspects are discussed below.

2.2. FLOW RATE, RECYCLINGAND RENEWAL OF WATER

2.2.1. The recycling and treatment

installation

The role of the recycling and treatment

installation is to supply each of the pools

managed with a constant flow of water that

has been filtered and disinfected in

accordance with applicable standards (see

Page 1244).

For swimming pools whose total surface

area exceeds 240 m2, the duration of the

water cycle must be less than or equal to:

- 8 h for a diving pool or scuba diving pool;

- 30 min for a paddling pool;

- 1 h 30 min for other pools or parts of pools

whose depth is equal to or less than 1.50 m;

- 4 h for other pools or parts of pools whose

depth exceeds 1.50 m.

Flowmeters are used to check that the

water in each pool is recycled within the

specified period.

A single installation can treat the water in

several pools provided that each pool has its

own supply, drainage and disinfection

system.

Water running along the edge of the pool

should be drained off to ensure that it does

not flow back into the pool.


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2. French legislation

2.2.2. Fresh water supplies The water in the pool must be renewed

at a rate of at least 0.03 m per swimmer

Fresh water is transferred by overflow daily.

to a separation tank or buffer tank up-

stream of the treatment installation. A Pools must be emptied completely at

meter checks the incoming flow rate. least twice a year.

2.3. WATER CIRCULATION

In new swimming pools, with the exception

of paddling pools and wave pools during wave

production, at least half the flow rate drawn off

the surface for recycling must be reinjected atthe bottom of the pool after treatment.

Skimmers can only be installed in pools

whose surface area is less than or equal to 200

m2. There must be one skimmer per 25 m

2of

water.

Two systems of water circulation are used

for swimming pools:

- In the inverse hydraulics system, the treated

water is reinjected through an axial channel or

a system of inlets at the

bottom of the pool. Channels running down the

length of the pool or around the sides draw off

the polluted water from the surface (Figure

832).

- In the mixed hydraulics system, only part of

the flow rate (at least half) is drawn off from

the surface. Water is drained off partly from

the surface and partly from the bottom of the

pool (Figure 833).

These methods effectively skim off the

pollution concentrated on the surface of the

pool.

Numerous constraints must be taken into

account in the design and manufacture of the

channels installed along the edges of the pool

to draw water off from the surface. However,

inverse hydraulics are simpler and generally

more economical than mixed hydraulics.


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Chap. 23; Treatment of swimming pool water

2.4.

QUALITY OF SWIMMING POOLWATER

Swimming pools obtain their water supplies

from public networks.

The water in swimming pools must comply

with the requirements below:

- The water must be transparent, i.e., the lanes

marked on the bottom of the pool must be

clearly visible and a dark object (measuring

0.30 m along each side) placed in the deepest

part of the pool must be easily discernible.- The water must not irritate the eyes, skin or

mucous membranes.

- The quantity of substances that oxidize in hot

potassium permanganate in an alkaline

medium, expressed as oxygen, must not exceed

the levels present in the water used to fill the

swimming pool by more than 4 mg.l-1.

- The water must contain no substances liable

to damage the health of swimmers.

- The pH must be comprised between 6.9 and

8.2.

- In one ml of water, the number of aerobic

bacteria revivable at 37C must be less than

100.

- In 100 ml of water, the number of total

coliforms must be less than 10 and no fecal

coliforms must be present.

- The water must contain no pathogenic germs.

A full 90% of the 100 ml samples taken must

be free of pathogenic staphylococcus germs.

- The water must be disinfected and also act as

a disinfectant.- Indoor swimming pools must be heated to

between 25C and 27C. The recommended

temperature for open-air pools is 23C.

- Monthly reports must be made to the regional

health authorities concerning the quality of

swimming pool water and the relevant

analyses, which must be performed by an

officially approved laboratory.


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3. Closed-loop treatment of swimming pool water


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3. CLOSED-LOOP TREATMENT OF SWIMMING

POOL WATER

The water in the pool is treated in the installation used comprises preliminary

regeneration installation and reinjected. filtration, pumping, filtration, heating

The treatment comprises two separate (where necessary) and disinfection (Figure

stages: filtration and disinfection. The 835).

3.1.

PRELIMINARY FILTRATIONAND PUMPING

The water drawn off from the surface by the

channels running along the edge of the pool

is conveyed to a special tank and

transferred to the feed pumps in the

regeneration circuit.

One or more preliminary filters must of be

placed immediately upstream of the


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Chap. 23; Treatment of swimming pool water


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pumps to ensure that no waste particles of these flow rates, which may be mod-

enter the circuit. Each preliminary filter ified in certain cases according to the

comprises a simple removable basket that numbers of swimmers using the pool. It is

is easy to inspect and dean. important to select the type of pump bestThe number of pumps and preliminary suited to the specific characteristics of

filters depends on the extent of the recycle each pool.

flow rates and the possible combinations

3.2.

FILTRATIONPressure sand filters are generally used.

As the turbidity of the water is low, fine-

grained sand is used for the filteringprocess, which can be performed at very

high rates. Filters backwashed with water

alone are therefore suitable for this

application (see Page 766).

Specially designed for rapid filtering of

swimming pool water, the Hydrazur sand

filter operates at up to 40 m.h-1

. The

Hydrazur dual media filter, another type of

filter, uses one layer of sand and one layer

of pumice stone or hydroanthracite, which

makes it possible to reach filtration rates of

up to 55 m.h-

'.Filters are backwashed at a flow rate of

between 30 and 40 m3/m

2.h irrespective of

filtration rate. The amount of water

consumed to wash the filters (over several

cycles) is roughly the same whatever the

filtration rate.


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In certain cases, the type of pressure

filter featuring backwash and a combined

air scour may be more suitable. This type of

filter is faster to wash and only requires a

small amount of water.

It is important to note that water, filtered

through sand, will not be perfectly clear

unless a water coagulant is used. In

consequence, a very small dose of

aluminium sulphate should be injected prior

to filtering to coagulate the colloidal

suspended solids, which thereby remain on

top of the filter rather than circulating

through the whole filtering bed. If no

coagulant is used, the consumption of

chlorine (and disinfectant in general) will

increase owing to the binding of the latter

with non-retained matter.

The higher consumption of disinfectant

is harmful in that it increases theconcentration of chlorides in the water and

favours the constitution of undesirable

organic chlorine compounds.

The coagulant is injected through a

metering pump as is the sodium carbonate

or any acid that may be required to bring

the pH of the water into compliance with

applicable standards.

3.3. DISINFECTION

Disinfection is an extremely important

part of the treatment. The objective of the

process is to make the water both clean and

attractive, i.e., to avoid the transmission of

contagious illnesses and to prevent the

development of microscopic algae, which

turn the water green.

Numerous illnesses can be transmitted if

the water is badly or inadequately treated.The most frequent are:

- conjunctivitis caused by a virus;

- ear, nose and throat infections owing to

the presence of streptococcus or

staphylococcus propagated through nasal

mucus;

- certain types of enteritis caused by the

same germs or certain viruses, when water

is swallowed;

- certain skin infections (eczemas)

sometimes caused by Koch bacillus.

A number of cases of

meningoencephalitis, several of which have

proved fatal, have been attributed to

Naegleria gruberi, an amoeba that is

destroyed by free chlorine or ozone.

Last, certain cutaneous infections

(epidermophytosis caused by a fungus that

appears on the skin between the toes and

verrucas caused by a papilloma virus) can

be caught by swimmers walking along the

side of the swimming pool. The area around

the pool must therefore be cleaned and

disinfected regularly.

In order to prevent the transmission of

illnesses, the water in the pool must have

powerful and lasting disinfectant properties.

Three types of disinfectant are commonly

used.

3.3.1. Chlorine and derivatives

Chlorine can be distributed in gas form

by a chlorinator from a bottle of liquefied


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chlorine or else as a solution of sodium

hypochlorite (bleach) fed by a metering pump.

The water should contain between 0.4 mg.l-1

and 1.4 mg.l-1 of free chlorine.

The amount of total chlorine should not exceed

the free level by more than 0.6 mg.l-1.

Chlorine is the most commonly used

disinfectant. It does not

inconvenienceswimmers, even in large

quantities, when the pH of the water is correct,

i.e., between 6.9 and 7.7. At this level, the

bactericidal properties of chlorine are

maximized and possible irritations

minimizedprovided that the dose corresponds to

the breakpoint. Relatively large quantities may

be required in certain cases (see Chapter 3,Paragraph 12.3.2).

Compounds from the cyanogen chloride

family may also be used. Prepared by

chlorinating cyanuric acid, these compounds

include trichloroisocyanuric acid and sodium

dichloroisocyanurate. The products obtained

hydrolyze in water and are in equilibrium with

the hypochlorous acid.

The balance depends on the concentration

and the pH. The hypochlorous acid is also in

equilibrium:

The cyanogen chloride family therefore

constitutes a reserve stock of disinfectants,

which may be used to disinfect swimming pool

water. The approval of the health authorities

should nevertheless be obtained before using

these substances. The legislation of the country

concerned should also be studied.

3.3.2. Bromine

Bromine-is used in certain cases,

particularly for swimming pools filled with sea

water.

The average total bromine content in swimming

pool water should be between 1and 2 mg.l -1.

The water must have a higher pH than for

chlorine, ideally between 7.5 and 8.2.

The use of bromine is, however, a controversial

issue.

3.3.3. Ozone

Ozone is the most powerful disinfectant

known. A particularly efficient amoeba

destroyer, ozone also breaks down certain

amino acids and turns the water an attractive

shade of blue. Ozone does not favour the

formation of products liable to irritate the

mucous membranes nor does it affect the taste

or smell of the water. No traces of ozone should

be present in water reinjected into the pool.

Residual ozone must be destroyed (filtering on

granular activated carbon, etc.). Swimming

pool water cannot be treated with ozone alone

as the remaining traces would make the water

sensitive to subsequent contamination. A low

dose of chlorine or bleach is required to

complete the disinfection process.

Ozonized air is produced electrically. The

ozone is dissolved in a contact column. A

residual ozone content of 0.4 mg.l-1

must be

maintained for 4 minutes if the ozone is to act

effectively.

Without preliminary ozonation, the second

disinfectant would need to be fed in much

larger quantities and the irritant effects of

chlorine would be greater.

An ozone-based disinfection system is

relatively expensive but operating costs are

modest and the effects greatly appreciated by

swimmers.


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3.3.4. Other disinfectants

Other disinfectants such as chlorine

dioxide, iodine and silver are sometimes

used. Although a number of these products

are suitable for certain applications in

water, they cannot be considered as totally

reliable for use in public swimming pools

and are rarely recommended. Before using

any of these products, it is advisable to

check applicable legislation. Certain

countries authorize the use of a calcium

hypochlorite solution instead of bleach

(sodium hypochlorite).

It is possible to buy disinfectant products,

often in pellet form, to reinforce the

disinfectant power of swimming pool water

where necessary. Before purchasing aproduct, it is advisable to check that it is

authorized for use.

3.3.5. Destruction of algae

A swimming pool cleaned by a well

designed and well operated water treatment

installation should be free of algae. Any

algae that do appear can be destroyed using

copper salts alone or a solution of copper

salts mixed with a small dose of chlorine.

Algae are removed by covering the walls

of the pool with a solution containing 10%

of regular powdered copper sulphate. Thisshould be done when the pool is closed. A

dose of 250 g of powder (or even 1 kg if the

pool is very green) is required for every 50

m3 of pool capacity. The pool should be

cleaned with the sweeper after a few hours.

A metering pump or similar device,

operating on an intermittent basis to prevent

the tolerance of algae, can be used to

introduce copper sulphate directly into the

regeneration circuit in doses of 1 to 2 g per

m3.

Lastly, when the swimming pool is

closed, a large dose of chlorine

(approximately 20 g.m-3

with an acid pH)

can be added to the water periodically.

The new micro- and ultrafiltration

membranes filter and disinfect the water

effectively. Adding an oxidizing agent

suffices to maintain the disinfectant power

of the water.

3.4.

CLEANING SWIMMING POOLS

During the night, suspended solids settle

on the bottom of the pool. They must be

removed before the pool opens the next day

to ensure that they do not rise to the surface

again. This operation is carried out using a

pool sweeper, a device that works on the

same principle as a vacuum cleaner.

A floating hose equipped with a suction

device is fitted to a number of quickfit

connectors around the walls of the pool.

The connectors are linked to a system ofpipes around the pool, which run

back to the intake of a fixed pump equipped

with a preliminary filter. The pump musthave its own power unit as the units

powering the recirculation system are

unable to reach a sufficiently low pressure

to suck out impurities effectively.

Certain sweepers have an immersed

pump running on a low or very-low voltage.

The water is sucked out through a floating

hos e or sent through a bag or cartridge filter

attached to the sweeper before being

returned to the pool.

The walls of the pool should also be

cleaned. This task can be performed

manually or with the sweepers describedabove.


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3.5. SPECIAL CASES

3.5.1. Correcting the pH

The pH of the water in the regenerationcircuit frequently needs to be adjusted to

optimize the effects of the disinfectant

used. A metering pump is used to inject

alkaline salt (sodium carbonate) or

hydrochloric acid diluted 20 times.

3.5.2. Removing iron and manganese

The water used to fill swimming pools

sometimes contains iron or manganese,

which will form reddish or blackish

deposits on the walls of the pool unless

removed.

3.5.3. Sea water pools

Certain swimming pools use sea waterinstead of fresh water. The principle

remains the same but certain technical

precautions must be taken to protect the

machinery from corrosion. A certain

quantity of fresh water must also be added

to prevent a progressive increase in the

concentration of salt.


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3.6.

MONITORING AND

CONTROL

To maintain its disinfectant properties,

swimming pool water must possess a

certain pH and, more importantly, a

specific residual content of chlorine, which

must be respected for as long as the water

remains in the pool.

Figure 838 shows a control system

developed by Degrmont for this purpose.

As the water is discharged from the pumps

in the recirculation circuit, a sample is

removed for analysis in a special unit (2),which determines the pH, the residual

chlorine content and, if required, the rH.

Disinfectant can be metered on the basis of

the rH in certain cases if desired but this is

not indispensable. The measurements taken

are sent to the control and regulation unit

(3), which activates the chlorine metering

pump (4) or the chlorinator as well as the

metering pump containing the pH

adjustment reagents) (5). Reagents are

added to the water before it flows back into

the pool. Doses are determined on the basis

of the deviation from the set point.

Differences in the deviation are also taken

into account, which makes it possible to

anticipate how the level is likely to vary in

accordance with the number of swimmers

using the pool. This system can easily be

adapted to cover several pools.


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3. Closed loop treatment of swimming pool water

3.7.

LEISURE POOLS

With the development of amusementparks, leisure pools are becoming

increasingly widespread. The water is

recycled on the same principle as for sports

pools but the following specific factors

must be taken into account:

- The pools are used by large numbers of

visitors.

- The shape and depth of certain pools may

be irregular. It is vital to ensure that the

entire area is effectively covered by the

renewal circuit.

- The temperature of the water is oftenconsiderably higher than in traditional

- The pools are often set in parks or wooded

areas where visitors are able to consume

food. The water may therefore contain a

variety of suspended solids liable to block

the filters. This will create a need for more

chlorine.

- The pH of the water is modified by the

large quantities of dissolved CO2

discharged by amusements such as wave

pools, waterfalls, flumes and water

cannons. Chlorine levels also fall.

- The type of materials used, e.g., artificial

rocks, make it more difficult to dean the

surfaces in contact with the water.

In certain cases, greater doses of reagents

may need to be added to the water.

Figure 839. The Lauzieres sports and leisure pool at Chateau-Arnoux in the French Alps.

Recycled flow rate: 300 m3.h

-1. Filters: 2 Hydrazur dual media filters, dia. 2 m.


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treatment of swimming pool water

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