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 in tended for various kinds of activities activ ities 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 world’s 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 healt h risks unlike modern mod ern day pools which use us e 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 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 children’s activity pools designed
2
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 children’s 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
4
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
5
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
6
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. ClO 2 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
7
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:
8
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 today’s 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.
10
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 5
th
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.
11
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 th
10 scientific experiments in the preceding 1,000 years (prior to 14 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.
12
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. Amy’s filters consisted primarily of sponges and sand in a variety of configurations, the smallest of which provided for the
13
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,
15
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
16
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 country’s 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
18
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
20
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 King’s 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 King’s 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,
21
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. th
Modern swimming pool only became popular in the middle of 19 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:
23
- 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
24
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 Bidwell’s 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. Today’s 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 0
where extremes are involved, below freezing and above 40 C- 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 world’s 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 typestrainer 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.
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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 manufacturer’s 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 m 2 of 3
2
1
bed surface area per hour [m /(m .h- )]. Generally, filters rated in the range of 10 to 30m3/m2.h-1 are termed ‘medium rate’ filters and filters rated above 3
2
1
30m /m .h- are termed ‘high rate’ filters.
Fig. 3.5: Medium and high rate pressure sand filters
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For pools which does the public use or have regular periods of high bathing 3
2
1
loads, filtration rates above 30m /m .h- are not recommended. Filter rates 3
2
1
between 25 and 50m /m .h- 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.
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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 3
2
1
of approximately 32m m .h- 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.
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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 3
2
1
3
2
1
6m /m .h- and vacuum pre-coat filter approximately 4m /m .h- . 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.
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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
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3
2
1
different types and filtration rates vary between 1 and 25m /m .h- 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 pool’s 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, ℓ, of the particulate material.
d ∆ N dl
= Al − β
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:
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