CHAPTER ONE

1.0 INTRODUCTION

A swimming pool, a bath, or a wading pool is an artificially created, enclosed

body of water. It may be intended for various kinds of activities ranging from

recreational and competitive to entertainment and health.

Humans have known swimming for long, as archaeological findings tend to

show. Babylonian bas-reliefs and Assyrian wall drawings point to very early

swimming skills. The most ancient and famous of drawings depicting men

swimming are estimated to be about 6,000 years old. Many of the other

worlds ancient civilisations swam, including the Egyptians, the Phoenicians,

Persians, Romans and the Greeks. Plato, the great Greek philosopher once

declared that anyone who could not swim lacked a proper education.

Modern day swimming pools differ greatly from those of the ancient world in

that those of the ancient world were largely baths, which were not meant for

swimming. Their swimming was done in lakes, ponds, rivers etc. Also,

because the baths water was continually drained and refreshed, so it did not

pose health risks unlike modern day pools which use the same body of water

on and on with the attendant risk of continuous contamination from bathers.

1.1 TYPE OF POOLS

Pools can be classified in several ways, based on construction, usage,,

location, source of water etc. Generally, pools are classified as public or

private. All other categories such as material of construction (gunite or

1

poured), inground or aboveground can be classified under the two general

headings.

1.1.1 Public Pools

Public pools are pools, which are meant for every member of the public

usage. They can be fee paying or free. They are of the in ground type and

are usually made from gunite with tile or fiberglass finish.

Fig. 1.1: A public pool hall.

There are different sub-categories under public pools:

Regular Pools: These are used primarily for swimming. They are found in hotels, public parks etc.

Spas: They are public swimming pools designed for recreational and therapeutic uses that are note drained, cleaned, or refilled after each

individual use. Spas may include units designed for hydroject circulation,

hot water, cold water mineral bath, air induction bubbles etc.

Wading Pools: These are public pools designed for use by children, including wading pools for toddlers and childrens activity pools designed

2

for casual water play ranging from splashing activity to the use of

interactive water features placed in the pool.

1.1.2 Private Pools

These are pools, which are not open for every member of the public usage.

They are found in the homes of rich individuals hence the name private or

residential pools. They can be of the above ground or in-ground type (usually

the later) and are constructed from gunite or poured concrete material.

Private pools are costly elaborate and come of different shapes and sizes.

Fig.1.2: A private pool

1.2 SWIMMING POOL POLLUTION

The water in a swimming pool contains microorganisms and unwanted

substances, which derive from the skin and excretion products of swimmers.

Bathers cause many pollutants to enter the water (it is estimated that every

swimmer adds up to a million microorganisms to the water), such as bacteria

from saliva and wounds, excretion products (urine and sweat), pollution from

swimwear, skin tissue, sebum, nose excretion, hairs, cosmetics, dead insects,

leaves, dust and ammonia (NH3. Some of the dissolved pollutants such as

3

sweat and urine are in themselves not harmful to human health but contain

substances such as kreatine, kreatinine and amino acids which when react

with disinfectants in the water, such as chlorine produces unwanted reaction

products consisting mainly of chloramines.

1.2.1 Health Effects of Swimming Pool Pollutants

Swimmers are susceptive to pathogenic microorganisms in swimming pool

water. As a result of cooling and water uptake, the resistance of the mucous

membrane of swimmers to weaken, causing them to become more susceptive

to pathogens in swimming pool water and air, and even to pathogens that are

present in their own bodies. Microorganisms that enter the water through

excretion by swimmers cause a large variety of conditions. Most pathogenic

microorganisms cause diarrhoea or skin rashes. Certain microorganisms (e.g.

poliovirus 1, E. coli bacteria) can cause serious symptoms, such as paralysis,

brain inflammation, heart inflammation, jaundice, fevers, vomiting, diarrhoea

and respirational or eye infections. Pathogenic microorganisms that are

found in swimming pool are bacteria, viruses and parasitic protozoa.

Children, the elderly, and people with damaged immune systems are more

prone to infections caused by these species and will fall ill more easily.

1.3 WATER PURIFICATION

Water purification generally means freeing water from any kind of impurity it

contains, such as contaminants or microorganisms. It is not a very one-sided

process; the purification process contains many steps. The steps that need to

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be progressed depend on the kind of impurities that are found in the water.

This can differ significantly for different kinds of water.

1.3.1 Water Purification Methods

Clean and safe potable water as is distributed in cities is treated extensively.

Specific water purification steps are taken, in order to make the water meet

current water standards.

Purification methods can be divided up into sedimentation, physical/chemical

treatment of colloids and biological treatment.

1. Sedimentation: This is the gravity separation of suspended material from

aqueous solution. Suspensions in which particulate matter is heavier than

water tend to settle to the bottom as a result of gravitation forces. This

process is not used in swimming pool water treatment but reserved for

potable water purification.

2. Physical Water Purification: This is primarily concerned with filtration

techniques. Filtration is a purification instrument to remove solids from

liquids. There are several filtration techniques. A typical filter consists

of a tank, the filter media and a controller to enable backflow.

- Screens: Filtration through screens is usually done at the beginning of the

water purification process. The shape of the screens depends on the

particles that have to be removed. Screens do not find application in pool

water treatment.

- Sand Filtration: Sand filtration is a frequently used, very robust method

to remove suspended solids from water. The filter medium consists of a

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multiple layer of sand with a variety in size and specific gravity. When

water flows through the filter, the suspended solids precipitate in the sand

layers as residue and the water, which is reduced in suspended solids,

flow out of the filter. When the filters are loaded with particles the flow

direction is reversed (backwashing), in order to regenerate it. Sand

filtration finds very useful application in swimming pool water treatment.

- Cross Flow Filtration: Cross flow membrane filtration removes both

salts and dissolved organic matter, using a permeable membrane that only

permeates the contaminants. The remaining concentrate flows along

across the membrane and out of the system.

- Cartridge Filtration: Cartridge filtration units consist of fibres. They

generally operate most effectively and economically on applications

having contamination levels of less than 100 ppm. For heavier

contamination applications, cartridges are normally used as final

polishing filters.

3. Chemical Water Purification: Chemical water purification is concerned

with a lot of different methods. Which methods are applied depends on

the kind of contamination in the (waste) water. Below, many of these

chemical purification techniques are briefly described.

- Clarification: Clarification is a multi-step process to remove suspended

solids. First, coagulants are added. Coagulants reduce the charges of ions,

so that they will accumulate into larger particles called flocs. The flocs

then settle by gravity in settling tanks or are removed as the water flows

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through a gravity filter. Particles larger than 25 microns are effectively

removed by clarification. Water that is treated through clarification may

still contain some suspended solids and therefore needs further treatment.

- Disinfection: Disinfection is one of the most important steps in the

purification of water from cities and communities. It serves the purpose of

killing the present undesired microorganisms in the water; therefore

disinfectants are often referred to as biocides. There are a variety of

techniques available to disinfect fluids and surfaces, such as: ozone

disinfection, chlorine disinfection and UV disinfection.

Chlorine-based disinfectants are among the most frequently applied

disinfectants and oxidizers for swimming pool treatment. Chlorine is

added as hypochlorous acid (HOCl) or hypochlorite (OCl-). Chlorine kills

pathogenic microorganisms that are present in the water. Chlorine

dioxide is an effective biocide at concentrations as low as 0.1 ppm and

over a wide pH range. ClO2 penetrates the bacteria cell wall and reacts

with vital amino acids in the cytoplasm of the cell to kill the organism.

The by-product of this reaction is chlorite. Toxicological studies have

shown that the chlorine dioxide disinfection by-product, chlorite, poses no

significant adverse risk to human health.

Ozone has been used for disinfection of drinking water in the municipal

water industry in Europe for over a hundred years and is used by a large

number of water companies, where ozone generator capacities up to the

range of a hundred kilograms per hour are common. When ozone faces

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odours, bacteria or viruses, the extra atom of oxygen destroys them

completely by oxidation. During this process the extra atom of oxygen is

destroyed and there are no odours, bacteria or extra atoms left. Ozone is

not only an effective disinfectant, it is also particularly safe to use.

UV-radiation is also used for disinfection nowadays. When exposed to

sunlight, germs are killed and bacteria and fungi are prevented from

spreading. This natural disinfection process can be utilised most

effectively by applying UV radiation in a controlled way.

- Distillation: Distillation is the collection of water vapour, after boiling

the wastewater. With a properly designed system removal of organic and

inorganic contaminants and biological impurities can be obtained,

because most contaminants do not vaporize. Water will than pass to the

condensate and the contaminants will remain in the evaporation unit.

- pH-adjustment: Treated water is often pH-adjusted, in order to prevent

corrosion from pipes and to prevent dissolution of lead into water

supplies. The pH is brought up or down through addition of hydrogen

chloride, in case of a basic liquid, or natrium hydroxide, in case of an

acidic liquid. The pH will be converted to approximately 7 to 7.5, after

addition of certain concentrations of these substances.

4. Biological Water Purification: Biological water purification is

performed to lower the organic load of dissolved organic compounds.

Microorganisms, mainly bacteria, do the decomposition of these

compounds. There are two main categories of biological treatment:

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aerobic water treatment and anaerobic water treatment. The Biological

Oxygen Demand (BOD) defines the organic load. In aerobic systems the

water is aerated with compressed air (in some cases merely oxygen),

whereas anaerobic systems run under oxygen free conditions. This

method of purification is not used in swimming pool water treatment.

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 INTRODUCTION

Historically, water was considered clean if it was clear. Without the analytical

chemistry of todays world, visual clarity and appearance were the only real

indicators of how pure a water source was. People who lived in prehistoric

times built their homes on lakeshores or along rivers so they would have

water to drink and wash in. the water in lakes and rivers was much cleaner

back then because many of the impurities of today did not exist then. There

are no records of how water was cleaned in prehistoric times.

2.2 ADVANCES IN WATER TREATMENT

Before 500 B.C.

The Egyptians were the first people to record methods for treating water.

These records date back more than 1,500 B.C. The records, some of which

are paintings indicate that the most common ways of cleaning water were

boiling it over fire, heating it in the sun, or dipping a heated piece of iron into

it. Filtering boiling water through sand and gravel and then allowing it to cool

was another common treatment method. This early treatment was performed

only to improve taste and appearance of water. The use of alum to remove

suspended particles is also attributed to the Egyptians.

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Fig.2.1: Egyptian drawings depicting siphoning

Among other early advances, Mayan civilizations developed remarkably

complex hydraulic systems for water distribution. An ancient Hindu source

gives what may have been the first drinking water standard, written at least

4,000 years ago; it directed people to heat foul water by boiling and exposing

to sunlight and by dipping seven times into a piece of hot copper, then to

filter and cool in an earthen vessel.

500B.C.-1000A.D

The Greek physician Hippocrates (considered as the Father of Medicine),

invented the Hippocratic Sleeve, a cloth bag to strain rainwater in the 5th

century B.C. He stated in one of his writings that water contributes much to

health. Hippocrates focused more on selecting the healthiest water source,

rather than expending energy and resources on purifying less desirable

sources.

The Romans, borrowing Hippocrates idea of selecting the healthiest water,

built extensive aqueduct system to bring in pristine water from far away to

their cities. But other than the incidental mild disinfection effect of sunlight

on water in open aqueducts, no major treatment was provided.

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In the 8th century A.D., Arabian alchemist Geber distilled water to purify it

for the imbibitions of alcohol and clean medicines according to The Quest for

Pure Water. In the 11th century, a Persian physician named Avicenna, after

performing several tests and experiments on water found out that straining

water through a cloth is effective in removing impurities. He therefore

recommended that travellers strain water through a cloth or boil it.

1000-1500A.D.

As in other scientific arenas, little progress was made in the Middle Ages

toward an understanding of water treatment and its importance to public

health. Sir Francis Bacon, the great Elizabethan philosopher, chronicled only

10 scientific experiments in the preceding 1,000 years (prior to 14th century

A.D.), which related to water treatment. There was little progress in water

treatment and its connection to public health.

1600A.D.

In the 17th century, British philosopher and scientist Sir Francis Bacon

applied his scientific method of making empirical observations and drawing

conclusions from them to a vast array of subjects, including water. In 1627 he

published thousands of experiments detailing water purification methods,

including percolation, filtration, boiling, distillation, and coagulation. In

1684, Dutch naturalists, Anton van Leeuwenhoek published sketches of his

wee animalcules, a common form of bacteria viewed with a simple

microscope that he invented himself.

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Fig.2.2: van Leeuwenhoek microscope

Also in the same century, 1685 to be precise, an Italian physician named Lu

Antonio Porzio designed the first multiple filter. These two unrelated events

were to play important parts in the future of water treatment.

Van Leeuwenhoek was accused of inaccuracy. The scientific community

regarded his sketches of microscopic organisms as unimportant

curiosities.

Then 200 years later, the scientists of the 19th century made the connection

between these "animacules," water, and health. Porzio's filter used plain

sedimentation and straining followed by sand filtration. It contained two

compartments (one downward flow, one upward).

1700A.D.

In the 18th century, called the Age of Enlightenment, natural philosophy

(now termed science) began to be viewed as something that could have

practical value to humans. In 1703, Parisian scientist Phillippe La Hire

presented a plan to provide a sand filter and rainwater cistern in every

individual household. He also documented that groundwater was rarely

contaminated. In 1746, fellow Frenchman Joseph Amy was granted the first

patent for a filter design. Amys filters consisted primarily of sponges and

sand in a variety of configurations, the smallest of which provided for the

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passage of water through sponges in a perforated plate. By 1750 his filters for

home use could be purchased. Later in the century, filtered water was sold on

a small scale, but no large commercial plants were built. James Peacock, a

British architect, was granted a patent in 1791 on a three-tank, upward-flow

backwash filter.

1800A.D.

In 1804, Paisley, Scotland, became the site of the first filter facility to deliver

water to an entire town. It was built by John Gibb to supply his bleachery and

the town, and within three years, filtered water was even piped directly to

customers in Glasgow, Scotland.

In 1806, a large water treatment plant opened in Paris, using the River Seine

as a source. The water was settled for 12 hours prior to filtration then run

through sponge prefilters that were renewed every hour. The main filters

consisted of coarse river sand, clean sand, pounded charcoal, and clean

Fontainebleau sand. The filters were renewed every six hours. A simple form

of aeration was also part of the process, and pumps were driven by horses

working in three shifts (steam power was too expensive). This plant operated

for 50 years. The year 1832 saw the first slow sand filtration plant in the

United States built in Richmond, Virginia. By 1833, the plant had 295 water

subscribers, showing a growing awareness of the relationship between clean

water and health. The next US plant to open was in Elizabeth, N.J., in 1855.

Up until the late 1860s, only 136 waterworks operated in the US. Many of

these delivered what was considered to be pure water that did not require

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filtration. Following the American Civil War of 1865, waterworks

construction increased significantly. Slow sand filters were introduced in

Massachusetts in the mid-1870s. Sand filters and other treatments were

primarily designed to improve the aesthetic quality of water. It took major

developments in bacteriology during the 1870s and 1880s to demonstrate that

microorganisms that exist in water supplies can cause human and animal

diseases. This led to the realization that water treatment could help prevent

disease. Robert Koch, the German physician and microbiologist who

postulated the germ theory of disease, and the Scottish surgeon Joseph Lister

were major players in this work. In 1881, William Stripe, superintendent of

waterworks at Keokuk, Iowa, issued an invitation to all persons concerned

with waterworks design, construction, operation, maintenance, and

management to gather at Washington University in St. Louis, Mo. The 22

respondents to this call to exchange information pertaining to the

management of water works, mutual advancement of consumers and water

companies, and to secure economy and uniformity in the operation of water

companies, together founded the American Water Works Association. By the

1890s filtration was gaining recognition for not only straining out undesirable

particles, but also removing deadly germs. For instance, towns and cities

along the Hudson River in New York State that used filtration for water

purification had fewer outbreaks and incidences of typhoid than communities

that did not filter the Hudson River water. In the mid 1890s, the Louisville

(Kentucky) Water Co. combined coagulation with rapid sand filtration,

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reducing both turbidity and bacteria in the water. Significant improvements to

water treatment in the 1880s and 1890s included development of rapid sand

filters, which were mechanically driven and could handle larger volumes,

improved operation of slow sand filters, and the first applications of chlorine

and ozone for disinfection. At an 1894 meeting of the American Public

Health Association, waterworks engineer George Warren Fuller suggested

that a cooperative effort toward standardization of bacteriological testing was

needed so that results from different laboratories could be compared. The

result was an 1897 report that evolved into the Standard Methods text used

today.

1900A.D.

The year 1906 saw the installation of slow sand filters in Philadelphia, United

States and the use of ozone as a disinfectant in Nice, France. In the early

1900s, ozonation for disinfection became common in Europe, but was less

prevalent in the US. Ozonation equipment was more complex and costly than

that used for chlorination, but ozone caused fewer taste and odor problems.

Many Europeans also were reluctant to use chlorine after World War I

because it had been used as a chemical warfare agent. In 1908, Jersey City

(N.J.) Water Works became the first utility in the US to use sodium

hypochlorite for primary disinfection, and the Bubbly Creek plant in Chicago

instituted regular chlorine disinfection (electrolytic generation of chlorine and

hypochlorites was by then a readily available technology). In that same year

information became available on bacterial kill rates, which led to the Chick

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and Watson model of chemical inactivation of microorganisms. It was

observed that numbers of typhoid cases often plummeted following

introduction of chlorine. In 1914, the US Department of Treasury

promulgated the countrys first drinking water bacteriological standard, a

maximum level of 2 coliforms per 100 mL. This only applied to interstate

systems, as the authority to establish such a regulation was created under the

1893 Interstate Quarantine Act, intended to prevent the spread of disease

from one state or possession to another. Chlorination was first used in 1917

in Ottawa, Canada and Denver, Colorado. Initially, chlorine was applied for

disinfection on a dosage basis. In 1919, Americans Abel Wolman and L.H.

Enslow demonstrated that chlorine consumption varied dramatically

depending on the characteristics of the water and developed the concept of

chlorine demand as the amount added minus the residual present after a

specified time period. By the 1920s and 1930s, use of filtration and

chlorination had virtually eliminated epidemics of major waterborne diseases

such as typhoid and cholera from the American and European landscape. In

1925, the US bacteriological standard was revised to 1 coliform per 100 mL,

and standards for lead, copper, zinc, and excessive soluble mineral substances

were added. These two decades also saw the development of dissolved air

flotation (patented 1924), early membrane filters (primarily for analytical

use), floc-blanket sedimentation, and the solids-contact clarifier. A major

step in the development of desalination technologies came in the 1940s

during World War II when various military establishments in arid areas

17

needed water to supply their troops. In 1942, the US Public Health Service

adopted a set of drinking water standards that included bacteriological

sampling in the distribution system and maximum permissible concentrations

for lead, fluoride, arsenic, and selenium. Hexavalent chromium was added to

this list in 1946, and the membrane filter process for bacteriological analysis

was approved in 1957. By the early 1960s, more than 19,000 municipal water

systems were in operation throughout the US. Most of these facilities used

chlorine for disinfection. Although ozone was in common use in continental

Europe throughout the 20th century, by 1987 only five US water treatment

facilities were using it, primarily for taste-and-odor control or trihalomethane

precursor removal. With the exception of the coliform standard in interstate

commerce, US drinking water standards were basically non-enforceable

guidelines until the Safe Drinking Water Act of 1974. The SDWA came

about in large part because of concerns about organic contaminants, and the

law laid out the process that the US Environmental Protection Agency would

use to set health-based maximum contaminant levels (MCLs) and the

aesthetic-related secondary MCLs. Although the focus of USEPA

regulations in the 1980s was on minimization of disinfection by-products,

concern for both chemical and microbial contaminants dominated the water

industry in the 1990s. The 1993 Cryptosporidium outbreak in Milwaukee,

Wisconsin, served as a reminder that another pathogen always exists that may

cause acute health effects if a breakdown in treatment occurs. The 1996

amendments to the SDWA were a step in the direction of stronger

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cooperation between utilities and USEPA in establishing new regulations.

2000 and Beyond Today, the processes of filtration and disinfection are still

in use, but are continually being refined based on better understanding of the

complex web of physical and chemical interactions that make these processes

work. Particles can now be measured in microns, and compounds can

detected to part-per-billion and part-per-trillion levels. Regulations now

require not only proper disinfection but also careful control of disinfection

by-products. Membranes are starting to provide the same functions as

conventional treatment and alternative disinfection methods such as

ultraviolet light are coming into focus. In addition to water treatment

practices, water systems must work toward solutions to the formidable

problems of source water protection and water scarcity, as well as how to

replace an aging infrastructure. The challenges of supplying an increasingly

higher quality of water to an increasing human population on a planet with a

limited freshwater supply will shape the future of water utilities and advanced

treatment processes in the 21st century.

2.3 HISTORY OF POOLS

Swimming as organised activity dates back as far as 2500BC, ancient Egypt

and later in ancient Greece, Rome, and Assyria. In Rome and Greece,

swimming was part of the education of elementary age boys. Until the

Romans built the first pools, what was obtainable in most of the ancient

world could be considered as baths. The transition to the present day

swimming pool happened over several centuries.

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2.3.1 Baths and Spas

The first in the category of artificially enclosed body of water was the bath,

which came about as a result of socialization. Social bathing was an

important cultural process practiced by Mesopotamians, Egyptians, Minoans,

Greeks, and Romans whenever they sought health and relief from their pain

and diseases. As a result, baths and adjacent gymnasiums became popular and

were places of socializing. With the completion of a new Roman aqueduct in

19 BC to supply water, the Thermae Agrippae was the first public bath in

Rome. The largest of all Roman baths was the Diocletian, completed in A.D.

305 and covered an area of 130,000 sq. yards. Engineers of the ancient times

still cause modern man to marvel and ask: "How did they do that"? Gaius

Maecenas of Rome, a rich Roman lord, built the first heated swimming pool,

in the first century BC.

As the Roman Empire fell, the Roman thermae fell into disrepair and disuse.

The bath gained and lost popularity in different parts of the world Asia,

Europe, Africa, and North America through the present day. Baths were

often built near natural hot or mineral springs. According to Professor de

Vierville, Charlemagne's Aachen and Bonaventura's Poretta developed as

important social bathing and healing places around thermal springs during the

Middle Ages. In the Renaissance era, Paracelsus' mountain mineral springs at

Paeffers, Switzerland, and towns like Spa, Belgium, Baden-Baden, Germany,

and Bath, England, grew up around natural thermal waters considered to have

healing properties. The use of saunas and steam baths also emerged. As these

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springs and spas were discovered, forgotten, and rediscovered, the healing

power of the water was often enhanced and formalized. In 1522, the first

scientific book on the Czech Karlovy Vary treatment for disease was

published in which a regimen of baths and drinking the waters of the springs

was recommended. In the 1890s, Father Sebastian Kneipp developed holistic

herbal and water therapy in the German spa village of Bad Worishofen.

The King's Bath

The Kings Bath was built, using the lower walls of the Roman Spring

building as foundations, in the 12th century. The bath is so called because a

statue of King Bladud overlooks it. The bath provided niches for bathers to sit

in, immersed up to their necks in water. On the south side of the bath is a seat

beneath the waterline, known as the Master of the Baths chair that was

donated in the 17th century. Although modified and encroached upon by the

building of the Grand Pump Room in the 18th century and subsequent 19th

century developments the Kings Bath continued in use for curative bathing

until the middle of the 20th century.

American Sweat Houses

They had also sweathouses and menstrual lodges. The permanent sweathouse

was a shallow subterranean excavation, roofed with poles and earth and

bedded with grass, in which the young and unmarried men slept during the

winter season, and occasionally sweated themselves by means of steam

produced by pouring water upon hot stones placed in the centre. The

temporary sweathouse used by both sexes was a framework of willow rods,

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covered with blankets, and with the heated stones placed inside. The

menstrual lodge, for the seclusion of women during the menstrual period and

for a short period before and after childbirth, was a subterranean structure,

considerably larger than the sweathouse, and entered by means of a ladder

from above. The occupants thus secluded cooked their meals alone and were

not allowed even to touch any articles used by outsiders.

2.3.2 Modern Swimming Pool, Hot Tubs and Spas

The modern hot tubs and swimming pools of today have come a long way.

The transition from the baths and spas of the ancient world to the present day

artificial pools and spas has been greatly assisted as Kings, Emperors, rulers

and the fabulously rich have constantly tried to out do one another thereby

encouraging pool designers to come up with new and improved design.

Modern swimming pool only became popular in the middle of 19th century in

Britain and this was largely due to competitive swimming. Indeed by 1837,

six indoors pools had been built in London, England. As the sport grew in

popularity many more pools were built, and when a new governing body, the

Amateur Swimming Association of Great Britain, was formed in 1880, it

numbered more than 300 member clubs. The Olympic games further

popularized swimming when swimming became a medal-winning event at

games.

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CHAPTER THREE

3.0 SWIMMING POOL WATER TREATMENT

The prime purposes of applying water treatment equipment to pool water

(other than for sterilization) are three in number:

- To maintain continuously a satisfactory standard of cleanliness

- To ensure that the bottom of the pool is at all times clearly visible to the

attendant staff, as a safeguard against drowning accidents

- To achieve clarity and sparkle which will make the water attractive to

the bather.

3.1 WATER TREATMENT TECHNIQUES

A typical swimming pool comes with seven major components and these are:

- a basin

- a motorized pump

- a water filter

- a chemical feeder

- drains

- returns

- uPVC plastic plumbing connecting all these elements

The contamination of swimming pool water to some extent is inevitable,

water treatment techniques therefore must be established to make the water

safe for bathers. Such treatment is accomplished by the operation of three

interrelated and interacting systems as regards the seven components listed

above:

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- A system for the re-circulation and distribution of pool water

- A system for feeding chemicals for disinfection and control of pH

- A system for the removal of particles by filtration

The basic idea is to pump water in a continual cycle from the pool through

the filtering and chemical system and back to the pool again. In this way, the

pumping system keeps the water in the pool relatively free of dirt, debris and

bacteria. Some pools also include heaters in the mix, in order to keep the

water at a certain temperature.

Fig.3.1: A typical pool system

3.2 POOL RECIRCULATION SYSTEMS

The function of the swimming pool re-circulation system is probably best

described as a type of transportation system. Water is transferred from the

pool, delivered to a station where it is filtered and chemically treated, and

then returned to the pool. The round trip the water takes is described by the

term turnover. Turn over is expressed as the number of hours necessary to

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circulate a volume of water equal to the volume contained in the pool.

Another method of expression is the number of times in 24hours that the

volume of water in the pool is circulated i.e. turn over in 8hours is a turnover

of three. Both in theory and practice, it has been determined that the typical

public pool should be re-circulated continuously at a rate equal to one

turnover in each 6 to 8hour period. The law of dilution as developed by Gage

and Bidwell suggests that such a turnover rate will provide 95 to 98% dilution

of soiled pool water with water that has been filtered and chemically treated.

Gage and Bidwells law has been largely upheld in practice and the 6 to

8hour turnover rates have generally become a standard for the operation of

the public pools.

The basic problems inherent in the circulation of the filtered and treated pool

water are not unfamiliar to the heating and ventilating engineer. As with so

many systems, the production of a conditioned agent is but part of the task, its

conveyance to and distribution within the occupied area often represent the

key to success or failure. In the case in question, the output of the plant must

be introduced into the pool in such a manner as to avoid stagnation and to

provide optimum conditions at all times to suit varying occupancy. Four

principal methods have been developed to meet these requirements; these

basic principles may be summarized as follows:

The Orthodox System: This has the fundamental merit that at all times the

whole of the water delivered to the pool passes through the shallow end and

is thus available to deal with the contamination which, as has already been

25

said, occurs predominantly in this area, further, this principle is one which

can not be altered by maladjustment or misuse. The system is simple; it calls

for a minimum of pipe work and valves and of connections through the pool

wall, these are such that the pool is inherently drainable through the outlet

connections.

The Cross-flow System: This a more complex arrangement, has longer

pipelines, more valves and connections and is therefore inherently more

expensive. Since flow is across the pool the throughput of water in the

crowded shallow end is less than with the orthodox system to the extent that

water is delivered direct to the deep end of the pool. The deep end has a very

low rate of turnover since a relatively small water quantity is delivered and

the volume of this area of the pool is large.

With the many distributed inlet and outlet points, a potential facility exists by

manipulation of the valve on each such point, for adjustment of the flow rate

over any portion of the pool at will and thus of regulating the pattern of water

movement to suit the conditions for the time being, or of recovering a

condition of lost breakpoint by local treatment. Such regulation, however

could only be made if rate of flow indicators were provided on each inlet and

outlet branch and even with this facility, alterations to flow patterns would be

difficult to set up, doubtful in effect and disastrous if misused or improperly

understood. A practical disadvantage is that the system does not naturally

provide pool drainage facilities and in consequence, a separate connection

must be made for this purpose.

26

Fig 3.2 Circulation in conventional pools

The Surflo System: In effect, this system provides a preset varying rates of

turnover highest in the shallow ends and decreasing towards the deep ends.

The shallow end turnover as in the cross-flow system is necessarily less than

in the orthodox system. Circulation generally within the bath is good and the

surface flow (hence the name) of water towards the edge weirs is conducive

to the removal of surface contamination. The need for a standing head of

water over the weirs when running necessitates a balance tank to

accommodate the surplus water when the circulation ceases and this could in

some circumstances become a depository for pollution.

27

The Deck Level System: In some respect the circulation arrangements are

similar to those of the surflo system but important differences arise in that the

peripheral outlet takes the form of a channel, covered by a grating, actually

on the pool surround. When there are no bathers in the pool, the water level

there lies an inch or so below the surround level and return circulation is from

a deep end floor grating to the balance tank. With increasing occupation, the

tank water level rises to due to displacement and a float valve restrict the

outflow from the floor grating to bring the peripheral channel into use. At a

maximum load the entire outflow is via the channel. It is claimed that bathers

can enter and leave this kind of pool with such ease due to the literal identity

of water and surround levels, that steps and ladders are unnecessary.

3.3 CIRCULATION EQUIPMENT

Simplicity of installation, resistance to corrosion and economy of labour and

materials are the dominating factors for all good circulation installations. The

Greek used timber and terra-cotta, the Incas gold, the Romans silver and lead

and the Victorians copper and cast iron for their pool water circulation lines

and fittings. Todays pool plumber uses plastic pipes and sometimes cast iron

or asbestos-cement when large bore plastic fittings is difficult to get. Large-

bore systems in plastic also provide strength and easy fixing plus excellent

durability.

Pool pipe work is a low pressure, low temperature re-circulation system but

where extremes are involved, below freezing and above 400C- special plastic

grades will be required. Most pool system try to standardize between 25 and

28

100mm lines with their relevant fittings, keeping larger diameter bores and

their more costly fittings for main lines only. For facings, panels, grilles,

grids and drains, detailed specification are usually necessary, they must be

tough and durable, and they must not trap fingers or toes nor catch skin. They

should not be adjustable by the swimmers, nor in anyway corrodible; main

drain grilles especially must be designed never to allow excess suction or to

be removable by bathers. Maximum flow through a main drain grille can be

0.3m/s but 0.2 or lower is better.

Inlets and outlets, skimmers and overflows, offer diverse design arrangements

to suit all circumstances and need to be professionally installed since most

leakages occur around them.

3.3.1 The Drains

It is inevitable that the water in a swimming pool needs to circulate through a

filtering system to remove dirt, debris and soil particles. During normal

operation, water flows to the filtering system through two or more main

drains at the bottom of the pool and multiple skimmer drain around the top of

the pool. The main drains are usually located on the lowest point in the pool,

so the entire pool surface slants towards them. Most of the dirt and debris that

sinks exit the pool through these drains. To keep bathers from getting their

hair or limbs caught in the plumbing, the drains are almost always covered

with grates or antivortex covers (a cover that diverts the flow of water to

prevent a dangerous vortex from forming). The skimmers as suggested earlier

on, draw water the same way as the main drains but they suck only from the

29

very top of the pool (the top eight of an inch typically). Any debris that

floats- leaves, suntan oil, hair- leaves the pool through these drains.

Fig. 3.3: The Skimmer

In the drain system, the floating weir i.e. the door at the inlet passage way

swings in and out to let a very small volume of water in at a time. To catch

debris effectively, the goal is to skin just the surface level, the water flows

through the strainer basket, which catches any larger debris such as twigs and

leaves. In addition to main inlet the skimmer system has a secondary

equalizer line leading to a drain below the surface level, this line keeps the

skimmer from drawing air into the pump system if the water level drops

below the level of the main inlet. The water is pumped through the filtering

system and back out to returns inlet valves around the side of the pool. The

system involves a lot of suction but if the pool is built and operated correctly

there is no risk of suction holding somebody against one of the drains. The

only way the plumbing system could apply this sort of suction is if there were

only one open drain. In a safe pool, there are always multiple main drains so

if somebody or something blocks one drain, the plumbing system will pull the

30

water from one of the other drains, this eliminates the suction from the

blocked drain.

3.3.2 Balance Tanks

A balance tank is required to take up displacement caused by bathers and

wave surge and to provide a source of backwash water so that water in the

pool remain at a constant level. This facility can be accommodated within a

level deck design, though it is usual to provide a separate tank, either

freestanding or as part of the main pool construction.

The balance tank is usually provided with high and low level switches to

control the make up supply; a make up solenoid valve opening to provide a

water supply to the tank on the low level switch and closing on the high level

switch. Sufficient available volume remains at the low level mark to provide

for a filter backwash likewise, there should be sufficient volume at any point

between low and high level marks to provide for maximum bather

displacement and wave surge. A facility in the suction line from the balance

tank is required to regulate the flow quantity.

3.3.3 Water Make-up Supply

The water treatment process produces pollutants that can only be controlled

by dilution of the pool water with fresh make-up water, this make-up water

may be derived directly from sources other than the water company mains

supply as in the case with borehole, spring or sea water fed pools. To some

extent, the dilution is achieved by water replacement to offset water lost to

evaporation, to bathers and during backwashing but further dilution is usually

31

necessary to control pollutants especially where bather loading is high.

Intensively used leisure pools which incorporate significant areas of shallow

water may require a weekly water replacement in excess of 50% of the total

pool volume to control levels of dissolved solids and combined chlorine

within the normal range.

The make-up supply line is usually fitted with a water meter to enable the

quantities to be monitored. The water is introduced via a break tank and the

tank supply, tank and feed must be of sufficient size to refill the pool after

backwashing or dilution in a practicable period of time. Associated heating

and dosing equipment also needs to be able to maintain satisfactory operating

conditions during and after refilling.

3.3.4 The Filter

After making its way into the various drains, the water flows on to the

filtering stage. The filters are of different types but these would be discussed

later under filtration.

3.3.5 The Pump

The pump is the heart of the swimming pool system. It must operate reliably

and economically, reasonably quietly and be compact. The pumping power

must be greater than the total resistance for the complete circulation system

including the restriction from the filtration. This total head resistance

comprises static head i.e. vertical distance to be overcome from pool water

level to the point of delivery, plus dynamic head i.e. friction resisting flow in

suction from within the filter. The best rule is to keep the static and the

32

frictional losses to the minimum, rather than having to upgrade the pump to

overcome them.

3.3.6 Pump Types

As regards the swimming pool, pumps could be classified into two; the

centrifugal pump and the positive displacement pump, there being many

different types within each category. However, the main characteristics

referred to below can be regarded as generally applicable.

Centrifugal Pump

A centrifugal pump in its simplest form consists of an impeller and a volute

casing. It usually includes an integral strainer basket before the impeller and

volute. The volute casing has to be filled completely with liquid when the

pump is in operation, the impeller throwing the liquid to the outside of the

volute thus imparting kinetic energy. In this way a centrifugal pump is

capable of generating a certain head, which varies according to the pump

speed and the accepted method of expressing the relationship between

capacity and head by means of a characteristic curve often referred to as the

Q/H curve, where Q is the quantity (flow rate) and H is the head. The main

characteristics of centrifugal pumps can be summarized as follows:

- Capacity varies with head

- Capacity proportional to pump speed

- Head proportional to the square of the pump speed non self-priming.

- Suitable for low viscosity liquids.

33

Fig. 3.4: A centrifugal water pump

Positive Displacement Pumps

Positive displacement pumps usually consist of a casing containing gears,

vanes, pistons, lobes, screws, and sliding shoes etc. Operating within

minimum clearance, the liquid being positively transferred from suction to

discharge port. Due to the fine clearance involved, most positive pumps are

self-priming and some can handle entrained gas or air.

Neglecting leakage, they deliver almost constant capacity irrespective of

variations in head. It is not usual to provide Q/H curves for positive pumps.

The main characteristics of the positive displacement pumps can be

summarized as follows:

- Capacity substantially independent of head

- Capacity proportional to speed

- Self-priming

- Suitable for viscous liquids (reduced speed usually necessary for high

viscosities)

Generally, about 90 to 95% of the worlds pumping is carried out using

centrifugal pumps and wherever the conditions are suitable, a centrifugal

34

pump is normally the simplest and most economical type available, also

where large volumes of water have to be moved at relatively low heads, the

centrifugal pump is the natural choice, and this is the case with a swimming

pool.

3.4 FILTRATION

Filtration is of some value for its capacity to remove bacteria and disease

producing organisms. However, its primary function is to remove dirt, debris

and soil particles which if not removed would increase the need for chemical

treatment and reduce the germ killing and oxidizing power of disinfection

chemicals. The filter deals with particulate matter, it strains out suspended

solids down to sub-micron size in order to retain water clarity. It does not

remove dissolved salts, nor does it filter microorganisms. Filtration combined

with disinfection produces effective water purification that keeps water clear

and non-toxic, odourless and tasteless, free of bacteria and algae, and

balanced to prevent corrosion or scale formation. The working capacity, in

general, can be determined by the amount of dirt it is capable of holding

without blocking or missing more than say, 10 micron sized particles in a

given time.

3.4.1 Filters

In order to maintain the pool water in the required condition, it is necessary to

provide a system of filtration to remove contaminant matter (and heating,

which is optional) to maintain the required temperature. Filters deal with the

removal of suspended colloidal materials and/or particulate matter, which

35

would otherwise cause excessive turbidity. The most consequential source of

these pollutants being the bathers themselves, although outdoor pools also

experience the addition of atmospheric debris such as dust, leaves etc.

Large suspended matters are removed by passage through a bucket type-

strainer fitted on the suction line from the pool at the pump inlet. The strainer,

usually have a free area of at least six times that of the suction pipeline.

3.4.2 Types Of Filters

There are three general or principal types of main filter commonly used

although a number of refinements and differences are available within each

type:

- Pressure sand filter

- Pre-coat filters and

- Cartridge filters

Of these, the pressure sand filter is by far the most commonly used, having

been applied in substantially the same form for very many years, while the

pre-coat filter, well established in the field of industrial water treatment has

more recently been applied to certain swimming pools. The cartridge filters

are for lightly loaded pools. Regardless of the type of filters selected, it must

be constructed of materials that are compatible with the chemical water

treatment employed. For instance, mild steel filter shells are suitably treated

internally to withstand the corrosive nature of the water.

36

Pressure sand filters:

Pressure sand filters are the most universally available and are suitable for all

types and sizes of pools. They are capable of filtering water down to 5 to 10

microns. A pressure sand filter consists of a vertical or horizontal shell which

accommodates a filtering media bed through which the water to be filtered is

passed from top to bottom. The composition and depth of the media bed

varies depending on the rating and/or the manufacturers system that may be

employed. The filtration process is assisted by feeding a coagulant, normally

a solution of alum into the entering water until a gel is formed over the face

of the bed. The filter pressure drop increases as particulate matter is trapped

in the surface layers of the media bed and after a period of time, the bed

requires cleansing by the process of backwashing (backwashing is the

cleansing of the filter by reversing the flow of water through the filter and

flushing the debris with the wash water). The cleansing during the

backwashing process is assisted by the agitation of the bed, either with

compressed air or with mechanical rakes.

A spreader system in the crown of the filter shell and a collection system at

the base are both required ensuring an even distribution of water across the

bed during filtration and backwashing process. Most manufacturers use their

own particular arrangements for this purpose. Filter shells are readily

available in various sizes up to 3000mm in diameter; the capacity of a

particular type of filter depends on the filtration rate selected. The collection

system at the base of the filter shell is required to ensure an even flow of

37

water through all parts of the bed during both the filtration and the backwash

processes and there are various arrangement of pipe work and nozzle systems

for this purpose.

A manhole is normally provided on the top of the shell to gain access for the

sand removal and replacement. This is a considerable task and the operation

may be considerably facilitated by the provision of an additional manhole

level with the bottom of the bed, hence obviating the arduous lifting,

bucketful by bucketful through the top manhole.

Conventional pressure sand filters with media bed depth of between 0.75m

and 1.5m are capable of filtration rates between 10 to 50m3 of water per m2 of

bed surface area per hour [m3/(m2.h-1)]. Generally, filters rated in the range of

10 to 30m3/m2.h-1 are termed medium rate filters and filters rated above

30m3/m2.h-1 are termed high rate filters.

Fig. 3.5: Medium and high rate pressure sand filters

38

For pools which does the public use or have regular periods of high bathing

loads, filtration rates above 30m3/m2.h-1 are not recommended. Filter rates

between 25 and 50m3/m2.h-1 are generally only satisfactory for highly loaded

or residential pools.

Backwashing of Pressure Sand Filters

Backwashing is achieved by reverting the flow of water from its normal path

so that flow through the filter bed is from bottom to top and then to waste.

This reversal of flow is achieved by the manipulation of valves. The

backwash water is withdrawn from the pool and its loss may be made good

by topping up as and when convenient; however the effluent is disposed off at

the rate of flow through a single filter and a comparable capacity in the

drainage system of the building is therefore a necessary requirement.

The need for backwashing is determined by the increase of pressure drop

across the filter bed and a differential pressure gauge is provided for this

purpose. The performance of medium rate pressure sand filters can be

enhanced by the use of a flocculant that forms a gel on top of the media bed,

causing smaller particles to group together and become trapped. This

flocculant is lost during the backwash cleansing and a suitable feed facility is

required in order to introduce the flocculant during the filtration during the

filtration cycle.

39

Fig.3.6: Typical valves to redirect water flow for backwash

Agitation in Pressure Sand Filters

The backwash process for the medium rate filter can be assisted by the

agitation of the media bed. This can be achieved using filters with built-in

mechanical rakes or more usually by the use of compressed air. This

compressed air system is carried out prior to backwashing normally at a rate

of approximately 32m3m2.h-1 and at a rate pressure of 0.35bar, although the

rate and pressure requirements differ from one manufacturer to the other. The

mechanical system is more complex and involves higher capital outlay than

the compressed air system, although it is held by some that, by this means,

disturbance of the filter bed is more positively achieved than with compressed

air, there are, nevertheless, very many air agitated installations which operate

quite satisfactorily.

40

Fig. 3.7:Vertcal rake pressure sand filter

Fig. 3.8: Vertical air scoured pressure sand filter

Pre-coat Filters:

The pre-coat filter differs fundamentally from the pressure sand filter in that

while the former uses a permanent filter bed which is cleaned at intervals as it

becomes fouled, the pre-coat filter uses an expendable medium which is

41

disposed off and renewed each time the filter is cleaned. The normal but

expendable medium used for this purpose is powdered diatomaceous earth,

which is made up into slurry, and for pressure pre-coat filters, is pumped into

the filter shell where it is deposited onto plates or cones (candles). The water

to be filtered is then passed through the plates or cones and the dirt collects

on the medium until a rising pressure indicates that cleansing is necessary.

The water flow is then reversed, flushing the medium and dirt from the plates

into the base of the shell to drain.

The main advantage of the pre-coat filter is that they can provide a much

greater filter surface area than a comparably sized sand filter and

consequently need less plant space. In addition, they filter out bacteria and

organic substances of sized down to 1 to 5 microns, which can result in fine

water clarity and polish. The ability to remove bacteria and the oocysts/cysts

of organisms such as cryptosporidium parium and giardia lambia, means that

pre-coat filter are ideal in areas where the quality of the source water is poor

or where these aspects are particularly problematic.

Pressure pre-coat filter should have a filtration rate of approximately

6m3/m2.h-1 and vacuum pre-coat filter approximately 4m3/m2.h-1. Backwash

rates for pressure pre-coat filters should be the same as the filtration rates.

The principle of operation of the pre-coat filter may be summarized as

follows:

Coating: a quantity of slurry is made up in a separate mixing tank of water

and the filter medium, and this is then pumped into the bottom (dirty) section

42

of the shell from where it passes through the cores, depositing on the external

faces there of, up into the top (clean) section and from there back to the

mixing tank. Re-circulation in this manner continues until deposition is

shown to be complete by the water in circulation becoming clear.

Filtering: water to be filtered is pumped into the bottom of the shell, through

the filter medium and up through the cores into the top of the shell and then

through the outlet connection. Dirt collects on the filter medium until a rising

pressure differential between clean and dirty sides indicates that cleaning is

necessary.

Cleaning: this is effected by the reversal of the water flow through the shell

to flush both the dirt and the filter medium from the cores into the lower

section of the shell from where the flushing water with dirt and filter medium

in suspension is dumped to drain until such times as the effluent is observed

to be clean.

Performance of Pre-coat Filters

The filter is susceptible to blockages by quite minor quantities of greasy

material such as may derive from body oils, hair creams or cosmetics which,

collecting on the surface of the filter medium, form an impervious barrier that

may obstruct water flow. This difficulty may be overcome by continuous

slurry feeding through out the time during which the filter is in use. The

effect here is to deposit new filter medium concurrently with any greasy

material so that the latter is prevented from forming a homogenous layer and

the filter bed remains pervious.

43

The degree of filtration achieved by this type of filter is very high indeed,

perhaps beyond that which is absolutely necessary for a swimming pool.

However, it is essential that coating of the cores is complete and that it

remains so, otherwise a complete bypass of the filter bed will exist and, in a

swimming bath application, there is no immediate means of establishing that

such faulty conditions exist. Flow of water through the filter contributes to

the retention of the filter medium on the cores and in order to ensure that the

medium remains in place during periods of disuse (e.g. overnight), the

continuous re-circulation of a minor quantity of water by means of a small

secondary pump is sometimes advocated.

Comparison between Sand and Pre-coat Filters

The principal advantage of the pre-coat filter for the treatment of pool water

is its small size although this is largely nullified by the storage area required

for the consumable coating medium which is of low density and thus bulky.

In comparison to the sand filter, (a rugged piece of equipment that will

withstand a great deal of abuse), the alternative is a delicate piece of

apparatus remarkably efficient when operated correctly. As in most such

comparisons, the deciding factor is the cost in use.

Cartridge filters:

Cartridge or pad filters offer low capital cost filtration. They are normally of

the induced or vacuum flow type and are designed primarily for small, lightly

loaded pools. Some cartridges or pads are dispensable and expensive while

others can be removed, hosed down and reused. There are a number of

44

different types and filtration rates vary between 1 and 25m3/m2.h-1 depending

on the membrane material. They also vary widely in efficiency, filtering

particles sized between 1 and 25 microns. Spa pools operate with cartridge

filters employing polyester material and not paper.

3.5 DISINFECTION AND SANITATION

Disinfection is 100% destruction of all disease- causing bacteria (pathogens)

on the object being disinfected. As with sterilization one cannot obtain

complete destruction in the pool environment. Although improper, the term

disinfection has persisted for long and is now commonly used while

Sanitation, the destruction of microorganisms to levels (usually by 99% or

more) deemed safe by public health standards. This is the proper term to be

used with pool or spa water.

A pools filter system does the heavy lifting in keeping the water clean, but it

takes chemistry to do the fine-tuning. The disinfection function is a

complicated process involving rather intricate chemistry. It is important to

carefully manipulate the chemical balance in pools for several reasons:

- Dangerous pathogens such as bacteria thrive in water. A pool filled

with untreated water would be a perfect place for disease carrying

microorganisms to move from one person to another.

- Water with the wrong chemical balance can damage the various parts of

the pool.

- Improperly balanced water can irritate the skin and the eyes

- Improperly balanced water can get very cloudy.

45

A modern re-circulation and purification system or even continuous flow

pool for thermal and mineral waters, holds purity and clarity equally

important for the safety of the bathers. Accidents can go unnoticed in murky

water to the extent that even today a young person may be drowned in a

swimming pool and the body may not be found until the tank is emptied a day

or so later.

To take care of pathogens in the water, a disinfecting agent is introduced; the

most popular pool disinfectant is the element chlorine, in the form of

chemical compound such as calcium hypochlorite (a solid) or sodium

hypochlorite

(a liquid). When the compound is added to the water, the chlorine reacts with

the water to form various chemicals; most notably hypochlorous acid (HOCl).

Hypochlorous acid kills bacteria and other pathogens by attacking the lipids

in the cell walls and destroying the enzymes and structure inside the cell

through an oxidation reaction. Alternative sanitizers such as bromide, do

basically the same thing with slightly different results.

When filtration is adequate and disinfection is properly operated, coliform

and E.coli will not normally be detectable in 100ml samples of water. To

guarantee that no dangerous E.coli (which causes all kinds of nauseating

troubles it is faecal bacteria and is equivalent to saying that one is

swimming in sewage or effluent) can appear in any sample, a very fast-kill

disinfectant such as free-fast-acting chlorine, must exist in the pool water at

all times. A high residual of fast acting hypochlorous acid resulting from

46

super chlorination (i.e. the addition of more chlorine beyond that required to

combine with all the ammonia present in the water), acts rather like the white

corpuscles that destroy bacteria within the blood stream. Ions of low

molecular weight with absence of electrical charge make it relatively easy for

the hypochlorous acid to degenerate cell walls of bacteria to burn them out.

Invading bacteria is overwhelmed and absorbed, but in the process, some of

the residual defence material also gets used up.

Fast and free chlorine (as hypochlorous acid) is easily dissipated by UV light

and requires the support of a slow acting, more stable form of chlorine for

back up. Because chlorine is typically prepared in liquid, powder or tablets

form (though some professionals use gaseous chlorine), it can be added to the

water any where in the cycle. Pool experts generally recommend adding it

just after the filtering process, using a chemical feeder. If it is added directly

into the pool, using tablets in the skimmer boxes for example, the chlorine

tends to be too concentrated in those areas.

A major problem with hypochlorous acid as mentioned earlier is that it is not

particularly stable. It can degrade when exposed to UV light from the sun,

and it may combine with other elements to form new compounds. Pool

chlorinators often include a stabilizing agent, such as cyanuric acid that reacts

with chlorine to form a more stable compound that does not degrade as easily

when exposed to UV light. Even with a stabilizing agent, hypochlorous acid

may combine with other chemicals, forming compounds that are not very

effective sanitizers. For example, hypochlorous acid may combine with

47

ammonia, found in urine, amongst other things, to produce various

chloramines such as monochloramines (NH2Cl) and dichloramines (NHCl2).

These combined residual form of chlorine are relatively slow acting as

sanitizing agents and to this extent are unsatisfactory. Not only are these

chloramines poor sanitizers, they can actually irritate the skin and eyes and

have an unpleasant odour. The distinctive smell and eye irritation associated

with swimming pools are actually due to chloramines, not ordinary

hypochlorous acid- a strong smell usually means that there is too little free

chlorine (hypochlorous acid) rather than too much. To get rid of chloramines,

there is a need to shock treat the pool by adding an unusually strong dose of

chemicals to clear out organic matter and unhelpful chemical compounds.

No matter how one sanitizes, a pool in use never becomes that wishful sterile

environment, but is a disinfected one repeatedly polluted. When bacteria

combine with oxygen, they are made harmless, chlorine speed this oxidation

process tremendously. In properly run pools, polluted water and infection is

rare- almost impossible, but if treatment is below par, chlorine resistant

organisms will develop, super chlorination will therefore always be necessary

to cope in heavier bathing pools and higher water temperature.

Disinfection may also be accomplished with bromine and the chemistry

involved is much the same. The chemical reaction produces a mild acid with

germ killing properties approximately equal to those of hypochlorous acid.

Regardless of the disinfecting or sanitizing agent used, the primary goal is the

same: to provide uniformly distributed sanitization and oxidation residual of

48

sufficient strength to rapidly destroy disease-producing organisms in pool

water.

3.5.1 pH of pool water

The pH is a chemical abbreviation used to describe the presence of the

hydrogen ion in water. It is often explained as a measure of the relative

acidity or basicity of water (alkalinity of water). There are many factors

affecting the efficacy of a purification system and adding up to the

disinfection demand. A major aspect controlling the most efficient kill rate is

this acidity or alkalinity of water. The indicative pH factor must be balanced

with the addition of acids or bases to neutralize extreme conditions, not only

for the comfort of the bathers but for the optimum activity from the residual

disinfectant.

pH is measured on a scale of 0 to 14. The midpoint 7 is the neutral point;

above which alkalinity becomes progressively greater. In swimming pool

water, it is important to maintain a slightly alkaline condition between 7.2

and 7.8. Problems develop when this range is exceeded on either side. A high

pH, for example, can cause precipitation of dissolved minerals such as

calcium and iron with resulting discolouration and turbidity. Low pH can

cause serious corrosive damage to metals in the re-circulation system. Both

high and low pH will cause eye irritation. The most recommended pH

position for the most active result from most disinfectant is 7.5 also the pH

of the tear duct and the most compatible level for the bathers skin. Whenever

complaints are made about the chlorine it is almost certain that the pH is out

49

or there is insufficient free chlorine available in the pool water to burn out all

by-product compounds and all contaminants. An active swimmer or bather

can perspire one litre per hour; and when the average contribution of urine

per bather is in the region of 25 to 50 ml, or almost 2litres for every class full

of children, the purifying method chosen for the water for the swimming pool

must work well.

3.6 OTHER DISINFECTION TECHNIQUES

3.6.1 Electro-disinfection Techniques

The processes stem from the principle of electrolytic corrosion, where

dissimilar metals in pool water conduct an electrical current between them.

The pool is really a vast battery where dissimilar metals can actually be

transferred back and forth in electrolysis. This is highly dependent upon the

amount of dirt in the water; the pH, the dissolved metals accelerating

corrosion or staining electro-plating elsewhere.

When a pool is charged with 4000ppm common salt solution, electrolytic

equipment can disassociate constituent elements. Nascent and fast acting

chlorine is one of them. These electro-chemical systems work best with

bathing loads not subject to sudden change and with balanced water but can

donate by product such as hydrogen, sodium etc that must be dealt with.

Molecular chlorine is produced at the positive anode while hydroxyl ion plus

water at the negative cathode.

By inserting other metallic plates to carry current, different water treatment

action can also be provided. Copper and aluminium plates will flocculate fine

50

materials for the water to trap. Platinum and silver will purify and oxidise

microbes (silver is highly bactericidal at level ten times lower than marginal

chlorination). Ion exchange systems can be very successful. They suit small

pools admirably but regrettably a little neglect goes along way in limiting

their very convenient advantages.

3.6.2 Ozonators

Ozonators are used on swimming pools and spas to reduce traditional

chlorine or bromine levels. Ozone water purification systems can be installed

in new pool or spa or retrofitted for existing systems. The ozone system

attaches to the water circulation system quickly and easily. It generates ozone

and injects it into the return, where it instantly oxidized and purifies the

water. In the process, ozone destroys bacteria, virus and algae and oxidizes

metals, which bond together for easy removal by the filter. It holds distinct

advantages in that rapid and total oxidizing of organic matter with purer

agents cuts down the side effect problems, which in turn allows a far more

comfortable swimming environment, plus the increased chance of operating

very successful total heat recovery and re-circulation system.

Below is a simplified drawing showing the basic configuration.

Fig. 3.9: A typical ozone system

51

CHAPTER FOUR

4.0 METHODOLOGY OF DESIGN

Design Consideration

Swimming and bathing pools vary considerably in size, shape and in the

intensity and pattern-of-use. The design and operational management brief is

usually considered by relevant professionals (civil, mechanical, electrical and

chemical) with no aspect determined in isolation. The choice of water

treatment system is dependent on a variety of factors, including:

(a) nature of incoming water supply

(b) the size and shape of the pool and variety of features to be incorporated in

the scheme.

(c) The anticipated bathing loads and pattern of use

(d) The finances available.

Also, basic assumptions are made at the initial design stage and these will be

the limiting factors for operation duration, and schedules and maximum

numbers of bathers.

4.1 WATER TREATMENT SYSTEM OBJECTIVE

The objective of a pool water treatment system is to provide a hygienic, safe,

comfortable and aesthetically pleasing environment for bathing. These are to

be achieved irrespective of the loading within the predetermined parameters.

The water treatment system should be capable of:

(a) Providing clear, colourless and bright water by removing suspended and

colloidal matter.

52

(b) Removing organic matter, which may provide a source of food for

bacterial and cause a cloudy, dull appearance.

(c) Destroying and removing bacteria and ensuring that the water is

bactericidal.

(d) Maintaining the pH of the water at an optimum for disinfection and bather

comfort.

(e) Maintaining the water at a comfortable temperature for bathers.

The primary functions of the system are to filter, circulate, disinfect and heat

the recirculating pool water so as to achieve the above.

4.2 METHODOLOGY OF FILTRATION

Filtration of pools is carried out with the use of filters. These filters strain out

suspended solids down to sub-micron size in order to retain clarity.

The heterogeneous particulate suspension commonly found in water is often

characterised by size distribution function known as the power law. The law

states that the number of particulates N per size category is an inverse

power function of the size, , of the particulate material.

= Aldl

Nd 4.1

The slope of the power law function is a useful parameter to characterise the

type of suspension being treated. Depending on the value of the power law

coefficient P, the major portion of the surface area or volume fraction of a

suspension will be found in certain size range.

This is summarised in the table below:

53

Table 4.1: Influence of power law coefficient on distribution of surface area volume of particulates by size

Power Law Coefficient,

% of Surface area in Fraction > 2cm

% of Volume in Fraction > 2cm

1. 99.95 99.995 2. 98.3 99.95 3. 73.3 98.3 4. 25 73.3

Fig.4.1: Particulate size frequency distribution

4.2.1 Process Selection

The appropriate solids-liquid separation process is initiated by a

preliminary screening of processes that may be suitable for the particular

design problem. Several filtering devices are available as have been

discussed previously.

The methodology for process selection is based on physical characteristics of

the particulates. Expected regions are defined in which various processes are

54

likely to be appropriate for the removal of particulate materials depending on

initial number and mass concentration of particulate materials and the average

size characterising the distribution. For particulate suspension with an

average size greater than 100mm and suspended solids greater than 50mg/L,

gravity sedimentation is the most cost effective solid liquid separation

process. This is usually the case with the land of particulates found in

swimming pools.

4.2.2 Quantitative Predictions of Particulate Removal

Particulate removal in filter media occurs by straining or by attachment to the

media itself. In addition, material already deposited can be retained or

detached due to sharing forces that increase as the filter clogs. The relative

importance of different mechanisms will depend on physiochemical

variables.

4.2.3 Collection Efficiency of Filter Media

Straining: Straining becomes an important removal mechanisms when the

ratio of the particle size to the media size in porous media is greater than

0.2(Herzig, 1970; Boller, 1980). This ratio at which straining becomes

important depends to some extent on the number flux of particles

approaching the media (Flux is defined as the superficial velocity times the

particle number concentration).

For particle sizes greater than 100m, straining in porous media becomes a

dominant removal mechanism (Maroudes, 1965; Tien, 1979). In case of

granular media filtration straining is undesirable because head loss will

55

increase rapidly due to the formation of a surface mat.

Consequently, in the design of grander filters (found in swimming pools) the

size of the filter media is selected to minimise this straining phenomena.

Non-Straining Mechanisms: The rate of particulate capture in granular filter

media due to non-straining mechanisms is made from knowledge of

particulate mechanisms in porous media under the influence of hydrodynamic

and physiochemical forces. The solution of the governing equations for

particulate motion in porous media require selection of a geometric model of

the porous media and the quantitative description of all forces acting on the

particulates as they pass through the granular media.

Isolated Single-Sphere Model: A schematic of isolated spherical collector is

shown below:

Fig. 4.2: Modes of action of the basic transport mechanism A, interception; B, sedimentation; C, diffusion

56

The efficiency of particular collection is defined as the number of successful

collisions for all particulates in the cross-sectional area of the collector

divided by the total possible number of collisions between the particulates

and collector.

Efficiency = successful number of collisionstotal no. of possible collisions in cross-sectional area per particulate to the isolated collector

area = dm2 4 dm = media size or diameter

Therefore, the collection efficiency throughout the depth of the granular

media is the summation of the efficiency of individual collector in the filter

bed.

The change in particulate concentration N with depth then becomes

-mi d

LxN )1( 0=

4.2

where,

n = media depth

x = shape factor (defined as the ratio of area and volume shape

factors for granular media, = 6 for spherical media).

o = initial pore volume or porosity of the granular media.

Assuming state removal, integrating Eq. 4.2 above gives

= LdN

Nm

)1(exp 00

4.3

57

L = total depth of the media

This model provides a framework for understanding the effects of various

design variables on the efficiency of filtration.

Fig. 4.3: Effect of media depth (L), media size (dm), and individual collector

efficiency () on particulate capture on granular media.

4.3 HYDRAULICS OF FLOW THROUGH POROUS MEDIA

When water or any fluid passes through porous materials, either granular or

consolidated, energy losses occur due to both form and drag fraction at the

surface of the media material. In addition, losses occur due to continuous

contraction and expansion experienced by the fluid as it passes through pre-

openings in the media.

Flow patterns through porous media are quite complex, and thus the

prediction of head loss requires different strategies than used for pipes. Head

loss will depend on a wide range of systems variables, including the

fractional void or porosity, the particle shape, roughness, size and size

58

distribution of the granular media, manner of packing, and type of fluid flow,

that is, whether it is laminar, transitional, or turbulent.

4.3.1 Laminar Flow

Laminar or viscous flow is characterised by viscous forces dominating inertia

forces. For the Reynolds number in porous media defined as

0 )1( 0

= Lme VdR 4.4 Flow is observed to be laminar for Re < 10

In a classic study by Darcy (1856), he discovered that the hydraulic gradient,

p/L, under laminar conditions in porous media was given by

p = Vo 4.5 L k

p = pressure drop L = depth of porous media k = hydraulic permeability (determined by experiment)

The Kozeny-Carman equation predicts that the hydraulic permeability is

180)1( 20

230

= md 4.6

for spherical particles. Combining both equations and converting to head

loss.

gdV

LH

Lm

230

02

0 )1(180 = 4.7

H = head loss in units of length

g = gravitational constant

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The table below gives the sphericity, shape and porosity factors for granular

material.

Table 4.2: Typical sphericity, shape and porosity factors of Granular material

Description Sphericity, Shape

Factor, S

Typical

Porosity, o

Spherical 1.00 6.0 0.38

Rounded 0.98 6.1 0.38

Worn 0.94 6.4 0.39

Sharp 0.81 7.4 0.40

Angular 0.78 7.7 0.43

Crushed 0.70 8.5 0.48

4.3.2 Transition Flow

For larger sized media, deeper filtration beds, and higher filtration rates with

appropriate pretreatment, Reynolds number as high as 50 are not uncommon.

For Reynolds numbers > 1000, the hydraulic gradient (by Burker-Plumber) is

30

02

0 )1(75.1

m

L

dV

Lp = 4.8

Combining Eqns.4.7 and 4.8, a general expression is obtained, valid over the

complete range of Reynolds number expected in granular media filtration,

assuming spherical media.

30

0030

2

200 )1(75.1)1(180

gdV

gdV

LH

mmL

+= 4.9

The effect of velocity on head loss per unit depth of media for various media

sizes at 20oC is shown below:

60

Fig. 4.4: Effect of velocity on head loss, T=200C, o=0.4, spherical media,

single size 4.3.3 Non Uniform Beds

In practice, the media used in filtration are not uniform or spherical and

consist of a range of particle sizes. Prediction of head loss through such clean

poly-dispersed media can be