DESIGN OF A WATER TREATMENT PLANT

CHAPTER ONE
    INTRODUCTION
Next to the air, the other important requirement for human life to exist is water. Humans can survive for several weeks without food, but for only a few days without water. Pure water is never found in nature, it contains number of impurities in varying amounts. The rainwater which is originally pure also absorbs various gases, dust and other impurities while falling. Surface and ground water have varying amounts of impurities, and these impurities must be properly treated before it is safe for drinking.
The quality of water, whether it is used for drinking, irrigation, or recreational purposes, is significant for health in both developing and developed countries worldwide. As of 2006, waterborne diseases are estimated to cause 1.8 million deaths each year. The World Health Organization (WHO) estimates that 94% of these diarrheal cases are preventable through modifications to the environment, including through access to safe water.  These deaths are attributable to inadequate public sanitation systems and it is clear that proper sewerage (or other options as small-scale water treatment) need to be installed. According to a 2007 World Health Organization report, 1.1 billion people lack access to an improved drinking water supply, 88% of the 4 billion annual cases of diarrheal disease are attributed to unsafe water and inadequate sanitation and hygiene.

To ensure safety of human health, water treatment processes are developed to treat water to its required specification for drinking, irrigation, or recreational purposes. Thus, to ensure the end process of the treatment gives the required specification, water quality standards (guidelines) are established by different regulatory bodies of which the World Health Organization (WHO) is the chief regulatory body.  But, in order to define mandatory limits, it is preferable to consider the guidelines in the context of local or national environmental, social, economic and cultural conditions.
The aim of this project is to produce a comprehensive design of a water treatment plant that incorporates all essential modern process units that would give drinking water that meets WHO quality standards from raw water will be produced for a new urban settlement with a population of about 100,000 people.
The objective of this design is to design a water treatment plant to produce potable water while incorporating effective, less costly and properly controlled modern process units which will be functional and reliable for a long period of time.
The scope of this work is limited to the design of a water treatment plant for an urban settlement with about 100,000 people in Nigeria that meets World Health Organization requirements. 








CHAPTER TWO
2.0    SURVEY OF LITERATURE
2.1    WATER
    Water is a common name applied to the liquid state of the hydrogen-oxygen compound H2O. The ancient philosophers regarded water as a basic element typifying all liquid substances. Scientists did not discard that view until the latter half of the 18th century. In 1781 the British chemist Henry Cavendish synthesized water by detonating a mixture of hydrogen and air. However, the results of his experiments were not clearly interpreted until two years later, when the French chemist Antoine Laurent Lavoisier proved that water was not an element but a compound of oxygen and hydrogen. In a scientific paper presented in 1804, the French chemist Joseph Louis Gay-Lussac and the German naturalist Alexander von Humboldt demonstrated jointly that water consisted of two volumes of hydrogen to one of oxygen, as expressed by the present-day formula H2O( Microsoft Encarta, 2008).
Water is the most abundant compound in nature. It covers 75% of the earth surface. About 97.3% of water is contained in the great oceans that are saline and 2.14% is held in icecaps glaciers in the poles, which are also not useful. Barely the remaining 0.56% found on earth is in useful form for general livelihood.
2.2     HYDROLOGIC CYCLE
To attain a better understanding how water is made available, an understanding of the hydrologic cycle (water cycle) is necessary (see Figure 2.1). The hydrologic cycle is a cycle without a beginning or an end. It transports the earth’s water from one location to another. As shown in Figure 2.1, it consists of precipitation, surface runoff, infiltration, percolation, and evapotranspiration.


Figure 2.1 Natural water cycle. (Spellman, 2001.)
In the hydrologic cycle, water from streams, lakes, and oceans evaporated by the sun, together with evaporation from the earth and transpiration from plants, furnishes the atmosphere with moisture. Masses of warm air laden with moisture are either forced to cooler upper regions or encounter cool air masses, where the masses condense and form clouds. This condensed moisture falls to earth in the form of rain, snow, and sleet. Another part of the precipitation
runs off to streams and lakes, while a third part enters the earth to supply vegetation and rises through the plants to transpire from the leaves, and part seeps or percolates deeply into the ground to supply wells, springs, and the base flow (dry weather flow) of streams. The cycle constantly repeats itself — a cycle without end.
2.3    SOURCES OF WATER
All the sources of water can be broadly divided into
   1. Surface sources and
   2. Sub surface or Groundwater sources
2.3.1    Surface Sources
    The surface sources can be further divided into
  i. Streams
  ii. Rivers
  iii. Ponds
  iv. Lakes
  v. Impounding reservoirs etc.
Natural Ponds and Lakes
In mountains at some places natural basins are formed with impervious bed by springs and streams are known as “lakes”. The quality of water in the natural ponds and lakes depends upon the basin’s capacity, catchment area, annual rainfall, porosity of ground etc. But lakes and ponds situated at higher altitudes contain almost pure water which can be used without any treatment. But ponds formed due to construction of houses, road, and railways contains large amount of impurities and therefore cannot be used for water supply purposes.
Streams and Rivers
Rivers and streams are the main source of surface source of water. In dry season  the quality of river water is better than monsoon because in rainy season the run-off water also carries with it clay, sand, silt, etc which make the water turbid. Therefore, river and stream water require special treatments. Some rivers are snow fed and perennial and have water throughout the year and therefore they do not require any arrangements to hold the water. But some rivers dry up wholly or partially in dry season. So they require special arrangements to meet the water demand during hot weather. Mostly all the cities that are situated near the rivers discharge their used water of sewage in the river, therefore much care should be taken while drawing water from the river.
Impounding Reservoirs
In some rivers the flow becomes very small and cannot meet the requirements of hot weather. In such cases, the water can be stored by constructing a bund, a weir or a dam across the river at such places where minimum area of land is submerged in the water and maximum quantity of water is to be stored. In lakes and reservoirs, suspended impurities settle down in the bottom, but in their beds algae, weeds, vegetable and organic growth takes place which produce bad smell, taste and colour in water. Therefore this water should be used after purification. When water is stored for long time in reservoirs it should be aerated and chlorinated to kill the microscopic organisms which are born in water.


Advantages and Disadvantages of Surface Water
The biggest advantage of using a surface water supply as a water source is that these sources are readily located; finding surface water sources does not demand sophisticated training or equipment. Many surface water sources have been used for decades and even centuries (e.g., in the U.S.), and considerable data are available on the quantity and quality of the existing water supply. Surface water is also generally softer (not mineral-laden), which makes its treatment much simpler.
     The most significant disadvantage of using surface water as a water source is pollution. Surface waters are easily contaminated (polluted) with microorganisms that cause waterborne diseases and chemicals that enter the river or stream from surface runoff and upstream discharges. Another problem with many surface water sources is turbidity, which fluctuates with the amount of precipitation. Increases in turbidity increase treatment cost and operator time.
Surface water temperatures can be a problem because they fluctuate with ambient temperature, making consistent water quality production at a waterworks plant difficult.
Drawing water from a surface water supply might also present problems; intake structures may clog or become damaged from winter ice, or the source may be so shallow that it completely freezes in the winter.
2.3.2    Subsurface Sources
The subsurface sources can be further divided into
    i. Infiltration galleries
   ii. Infiltration wells
   iii. Springs, etc

Infiltration Galleries
A horizontal nearly horizontal tunnel which is constructed through water bearing strata for tapping underground water near rivers, lakes or streams are called “Infiltration galleries”. The yield from the galleries may be as much as 1.5 x 104 lit/day/meter length of infiltration gallery. For maximum yield the galleries may be placed at full depth of the aquifer. Infiltration galleries may be constructed with masonry or concrete with weep holes of 5cm x 10cm (Venkateswara , 2005).
Infiltration Wells
In order to obtain large quantity of water, the infiltration wells are sunk in series in the blanks of river. The wells are closed at top and open at bottom. They are constructed by brick masonry with open joints. For the purpose of inspection of well, the manholes are provided in the top cover. The water filtrates through the bottom of such wells and as it has to pass through sand bed, it gets purified to some extent. The infiltration well is in turn connected by porous pipes to collecting sump called jack well and there water is pumped to purification plant or treatment.
Springs
     This is natural flow of water from the ground at a single point within a restricted area; when a spring has no visible current, it is called a seep. Springs may emerge at different points on dry land or in the beds of streams, ponds, or lakes. Cold spring waters are usually of meteorological character, that is, rain that has soaked into the ground and emerged as a spring at some other point on a lower level. Hot spring waters may be of igneous origin, or they may represent surface waters heated by contact with underground uncooled igneous rock. This is due to presence of sulphur and useful only for the cure of certain skin disease patients. Sometimes ground water reappears at the ground surface in the form of springs. Springs generally supply small quantity of water and hence suitable for the hill towns.
    THE HISTORY OF DRINKING WATER TREATMENT
Ancient civilizations established themselves around water sources. While the importance of ample water quantity for drinking and other purposes was apparent to our ancestors, an understanding of drinking water quality was not well known or documented. Although historical records have long mentioned aesthetic problems (an unpleasant appearance, taste or smell) with regard to drinking water, it took thousands of years for people to recognize that their senses alone were not accurate judges of water quality (EPA Fact Sheet, 2000).
 Water treatment originally focused on improving the aesthetic qualities of drinking water i.e. if the water was clear and had no smell it was considered clean. Methods to improve the taste and odor of drinking water were recorded as early as 4000 B.C. Ancient Sanskrit and Greek writings recommended water treatment methods such as filtering through charcoal, exposing to sunlight, boiling, and straining. The earliest recorded knowledge of water quality and its treatment are found in Sanskrit literature “Sushuri Sanhita” compiled about 2000 B.C. It deals with storage of drinking water in copper vessels, exposure to sunlight, boiling filtering through charcoal, sand etc. Visible cloudiness (later termed turbidity) was the driving force behind the earliest water treatments, as many source waters contained particles that had an objectionable taste and appearance. To clarify water, the Ancient Egyptians as early as 1500 B.C. treated water by siphoning water out of the top of huge jars after allowing the suspended particles in muddy water from the Nile River to settle by adding chemical alum. Hipocrates, known as the father of medicine, directed people in Greece to boil and strain water before drinking it. In turn, the Romans passed water from aque¬ducts through settling basins to clarify it (remove impurities).  During the 1700s, filtration was established as an effective means of removing particles from water, although the degree of clarity achieved was not measurable at that time. By the early 1800s, slow sand filtration was beginning to be used regularly in Europe. The first water facility to deliver water to an entire town was built in Paisley, Scotland in 1804 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 began operating in Paris. The plant’s filters were made of sand and char¬coal and where renewed every six hours. Pumps were driven by horses working in three shifts. Water was settled for 12 hours before filtration. In 1827 Englishman James Simpson built a sand filter for drinking water purification.
 During the mid to late 1800s, scientists gained a greater understanding of the sources and effects of drinking water contaminants, especially those that were not visible to the naked eye. In 1855, epidemiologist Dr. John Snow proved that cholera was a waterborne disease by linking an outbreak of illness in London to a public well that was contaminated by sewage. This was proven correct when in the 1870s Drs Robert Koch and Joseph Lister demonstrated that microorganisms existing in water supplies can cause disease. In the late 1880s, Louis Pasteur demonstrated the “germ theory” of disease, which explained how microscopic organisms (microbes) could transmit disease through media like water. This therefore gave man reason to treat water not just for aesthetic quality, but for portability (safe and palatable).
During the late nineteenth and early twentieth century’s, concerns regarding drinking water quality continued to focus mostly on disease-causing microbes (pathogens) in public water supplies. Scientists discovered that turbidity was not only an aesthetic problem; particles in source water, such as fecal matter, could harbor pathogens. As a result, the design of most drinking water treatment systems built in the U.S. during the early 1900s was driven by the need to reduce turbidity, thereby removing microbial contaminants that were causing typhoid, dysentery, and cholera epidemics. These diseases are therefore water borne, and there was need to reduce them. A report from Uttarpradesh by W.H.O (World Health Organization) in 1963, shows that the death rate by cholera decreased by 74.1%, Typhoid fever by 63.6% , by dysentery 23.1% and diarrhea by 63.6%. All these were achieved by drinking water treatment (Venkateswara, 2005).
 To reduce turbidity, some water systems in U.S. cities (such as Philadelphia) began to use slow sand filtration. While filtration was a fairly effective treatment method for reducing turbidity, it was disinfectants like chlorine that played the largest role in reducing the number of waterborne disease outbreaks in the early 1900s. In 1908, chlorine was used for the first time as a primary disinfectant of drinking water in Jersey City, New Jersey. The use of other disinfectants such as ozone also began in Europe around this time.
2.5     IMPORTANCE AND NECESSITY FOR PLANNED WATER SUPPLIES
    Water is necessary for all life on Earth. Next to the air, the other important requirement for human life to exist is water. Humans can survive for several weeks without food, but for only a few days without water. A constant supply is needed to replenish the fluids lost through normal physiological activities, such as respiration, perspiration and urination. Water is available in various forms such as rivers, lake, streams etc. The earliest civilizations organized on the banks of major river systems and required water for drinking, bathing, cooking etc. But with the advancement of civilization the utility of water enormously increased and now such a stage has come that without well organized public water supply scheme, it is impossible to run the present civic life and the develop the towns. The importance of water from only a quantity viewpoint was recognized from the earliest days and the importance of quality come to be recognized gradually in the later days.
Large rivers may be polluted with sewage effluent, surface runoff or industrial pollutants from sources far upstream. However even small streams, springs and wells may be contaminated by animal waste and pathogens. The presence of dead animals upstream is not uncommon. In most parts of the world, water may contain bacterial or Protozoa contamination originating from human and animal waste or pathogens which use other organisms as an intermediate host.
Common in developing countries are organisms such as Vibrio cholerae which causes cholera and various strains of Salmonella which cause typhoid and para-typhoid diseases. Pathogenic viruses may also be found in water. The larvae of flukes are particularly dangerous in area frequented by sheep, deer or cattle. If such microscopic larvae are ingested, they can form potentially life threatening cysts in the brain or liver. This risk extends to plants grown in or near water including the commonly eaten watercress.
As of 2006, waterborne diseases are estimated to cause 1.8 million deaths each year. The WHO estimates that 94% of these diarrheal cases are preventable through modifications to the environment, including through access to safe water.  These deaths are attributable to inadequate public sanitation systems and it is clear that proper sewerage (or other options as small-scale water treatment) need to be installed. According to a 2007 World Health Organization report, 1.1 billion people lack access to an improved drinking water supply, 88% of the 4 billion annual cases of diarrheal disease are attributed to unsafe water and inadequate sanitation and hygiene.
Water supply scheme grew in Nigeria with the advent of colonization, more especially for domestic supply to the European quarters called Government Reserved Areas (GRA) established in the regional capitals and divisional headquarters like Lagos, Abeokuta, Ibadan, Enugu, Kaduna, Lokoja, Kano, Zaria, Port-Harcourt, Jos and several other places
2.6    NEED FOR PROTECTED WATER SUPPLY
Protected water supply means the supply of water that is treated to remove the impurities and made safe to public health. Water of sufficient quality to serve as drinking water is termed potable water, whether it is used for drinking or not. Although many sources of water are utilized by humans, some contain disease vectors or pathogens and cause long-term health problems if they do not meet certain water quality standards. Water that is not safe for human consumption, but is not harmful for human use, is sometimes referred to as safe water. Water may be polluted by physical and bacterial agents. Water is also good carrier of disease causing germs. The causes of outbreak of epidemics are traced to pollute water and poor sanitation, and hospitals are continued to be flooded with the sick due to ignorance about health and this continues to be profound. However during the last few decades, there has been an improvement in the public health protection by supplying safe water and sanitation to all the people in the developing countries. In 1977, United Nations declared to launch a movement known as “HEALTH FOR ALL BY THE YEAR 2000 A.D.”  For this to be achieved in a developing country like Nigeria, potable water from protected water supply should be made available to the entire population. Pure and whole some water is to be supplied to the community alone can bring down the morbidity rates.
The objectives of the community water supply system include
1. To provide whole some water to the consumers for drinking purpose.
2. To supply adequate quantity to meet at least the minimum needs of the individuals
3. To make adequate provisions for emergencies like fire fighting, festivals, meeting etc
4. To make provision for future demands due to increase in population, increase in                  standard of living, storage and conveyance
5. To prevent pollution of water at source, storage and conveyance
6. To maintain the treatment units and distribution system in good condition with                           adequate staff and materials
7. To design and maintain the system that is economical and reliable
2.7    WHOLE SOME WATER
Absolutely pure water is never found in nature and it contains only two parts of hydrogen and one part of oxygen by volume. But the water found in nature contains number of impurities in varying amounts. The rainwater which is originally pure also absorbs various gases, dust and other impurities while falling. This water when moves on the ground further carries silt, organic and inorganic impurities. The removal of the turbidity, odour and smell is considered as good and removal of dissolved substances is considered as “chemically pure”. But removal of substances like Calcium, Magnesium, Iron, Zinc, etc completely is not good for health. These minerals are required for tissue growth and some act as propylatic in preventing diseases. Therefore wholesome water is defined as the water which containing the minerals in small quantities at requisite levels and free from harmful impurities chemically pure water is also corrosive but not whole some water. The water that is fit for drinking safe and agreeable is called potable water.
The following are the requirements of wholesome water.
1. It should be free from bacteria.
2. It should be colourless and sparkling.
3. It should be tasty, odour free and cool.
4. It should be free from objectionable matter.
5. It should not corrode pipes.
6. It should have dissolved oxygen and free from carbonic acid so that it may remain                    fresh.

2.8    WATER DEMANDS   
2.8.1    Various Types of Water Demands
While designing the water supply scheme for a town or city, it is necessary to determine the total quantity of water required for various purposes by the city. As a matter of fact the first duty of the engineer is to determine the water demand of the town and then to find suitable water sources from where the demand can be met. But as there are so many factors involved in demand of water, it is not possible to accurately determine the actual demand. Certain empirical formulae and thumb rules are employed in determining the water demand, which is very near to the actual demand.
 The following are the various types of water demands of a city or town:
i. Domestic water demand
ii. Industrial, institutional and commercial demand
v. Fire demand
vi. Loses and wastes
2.8.2    Domestic Water Demand
According to the United Nations, a person living in Europe or North America uses between 500 and 1,000 liters (130 to 260 gallons) of water per day. The typical person living in the developing countries of Asia, Latin America, and Africa uses between 50 and 100 liters (13 to 26 gallons) per day. In areas where water is scarce, the figure is even lower (Microsoft Encarta, 2008.).
The quantity of water required in the houses for drinking, bathing, cooking, washing etc is called domestic water demand and mainly depends upon the habits, social status, climatic conditions and customs of the people. The domestic consumption of water in Nigeria is about 120 liters/day/capita. But in developed countries this figure may be 350 litres/day/capita because of the use of air coolers, air conditioners, maintenance of lawns, automatic household appliances. The details of the domestic consumptions are given in the table below.
Table 2.1  Domestic Consumption Rate
   
                        Source: 1980 World Bank standard water requirements
2.8.3    Industrial, institutional and commercial demand
A field survey for Kaduna and Abeokuta representing the North and South of Nigeria respectively were conducted (Ayoade, et al, 1998). From his field survey, he expressed other expected demands (i.e. industrial, institutional and commercial requirements) as a percentage of total domestic demand: hence, he gave the following range of values.
    Industrial    0.1¬¬¬ to 0.3%
    Institutional    1.9 to 2.9%
    Commercial    2.1 to 3.5%
Taking the higher limits of the demands, the total estimate for industrial, institutional and commercial demand is 6.7% which is approximately 7% of the total domestic consumption.


2.8.4    Fire Demand
Fire may take place due to faulty electric wires by short circuiting, fire catching materials, explosions, bad intension of criminal people or any other unforeseen mishaps. If fires are not properly controlled and extinguished in minimum possible time, they lead to serious damage and may burn cities.
All the big cities have full fire-fighting squads. As during the fire breakdown large quantity of water is required for throwing it over the fire to extinguish it, therefore provision is made in the water work to supply sufficient quantity of water or keep as reserve in the water mains for this purpose. In the cities fire hydrants are provided on the water mains at 100 to
150 m apart for fire demand. The quantity of water required for fire- fighting is generally calculated by using different empirical formulae. For Nigeria conditions, assuming the Kuichings formula gives satisfactory results (Venkateswara, 2005), i.e.
             Q=3182√P                                                                                                              2.1        
Where ‘Q’ is quantity of water required in liters/min, and
‘P’ is population of town or city in thousands.
2.8.5    Losses and Wastes
All the water, which goes in the distribution, pipes does not reach the consumers.
The following are the reasons;
1. Losses due to defective pipe joints, cracked and broken pipes, faulty valves and fittings.
2. Losses due to, consumers keep open their taps of public taps even when they are not using the water and allow the continuous wastage of water
3. Losses due to unauthorized and illegal connections
While estimating the total quantity of water of a city; allowance of 10% of total quantity of water is made to compensate for losses in developed countries; but in developing countries like Nigeria, losses fall between 40 to 50%. With careful maintenance and metered system, it may be considered to be about 25% of the total consumption. Over the years the loss has reduced to 20% due to conscientization, public enlightenment and activities of low income earners like water truck pushers. 
2.9    QUALITY AND CHARACTERISTICS OF WATER
Absolutely pure water is never found in nature, it contains number of impurities in varying amounts. The rainwater which is originally pure also absorbs various gases, dust and other impurities while falling. This water when moves on the ground further carries salt, organic and inorganic impurities. So this water before supplying to the public should be treated and purified for the safety of public health, economy and protection of various industrial process, it is most essential for the water work engineer to thoroughly check analyze and do the treatment of the raw water obtained the sources, before its distribution. The water supplied to the public should be strictly according to the standards laid down from time to time.
There are three main classification of characteristics of water; the physical, chemical, and biological/ microscopic characteristics.
2.9.1     Physical Characteristics
    The physical characteristics of water refer to those that are apparent to the senses of smell, taste, sight, and touch. Solids, turbidity, color, taste and odor, and temperature also fall into this category.


Solids
Other than gases, all contaminants of water contribute to the solids content. Classified by their size and state, chemical characteristics, and size distribution, solids can be dispersed in water in both suspended and dissolved forms. In regards to size, solids in water can be classified as suspended, settleable, colloidal, or dissolved.
Solids are also characterized as being volatile or nonvolatile. The distribution of solids is determined by computing the percentage of filterable solids by size range. Solids typically include inorganic solids, such as silt, sand, gravel, and clay from riverbanks, and organic matter, such as plant fibers and microorganisms from natural or manmade sources. We use the term siltation to describe the suspension and deposition of small sediment particles in water bodies. In flowing water, many of these contaminants result from the erosive action of water flowing over surfaces.
In water, suspended material is objectionable because it provides adsorption sites for biological and chemical agents. These adsorption sites provide attached microorganisms a protective barrier against the chemical action of chlorine. In addition, suspended solids in water may be degraded biologically resulting in objectionable byproducts. Thus, the removal of these solids is of great concern in the production of clean and safe drinking water.
In water treatment, the most effective means of removing solids from water is by filtration. It should be pointed out, however, that not all solids, such as colloids and other dissolved solids can be removed by filtration.
Turbidity
One of the first things that is noticed about water is its clarity. The clarity of water is usually measured by its turbidity. Turbidity is a measure of the extent to which light is either absorbed or scattered by suspended material in water. Both the size and surface characteristics of the suspended material influence absorption and scattering.
Although algal blooms can make waters turbid, in surface water, most turbidity is related to the smaller inorganic components of the suspended solids burden, primarily the clay particles. Microorganisms and vegetable material may also contribute to turbidity. Wastewaters from industry and households usually contain a wide variety of turbidity-producing materials. Detergents, soaps, and various emulsifying agents contribute to turbidity. In water treatment, turbidity is useful in defining drinking-water quality.
The colloidal material associated with turbidity provides absorption sites for microorganisms and chemicals that may be harmful or cause undesirable tastes and odors.
Moreover, the adsorptive characteristics of many colloids work to provide protection sites for microorganisms from disinfection processes. Turbidity in running waters interferes with light penetration and photosynthetic reactions.
Colour
Colour is another physical characteristic by which the quality of water can be judged. Pure water is colorless. Water takes on colour when foreign substances such as organic matter from soils, vegetation, minerals, and aquatic organisms are present. Colour can also be contributed to water by municipal and industrial wastes.
Colour in water is classified as either true color or apparent colour. Water whose colour is partly due to dissolved solids that remain after removal of suspended matter is known as true colour. Colour contributed by suspended matter is said to have apparent colour. In water treatment, true colour is the most difficult to remove. It should be noted that water has an intrinsic colour, and this colour has a unique origin. Intrinsic colour is easy to discern, as can be seen in Crater Lake, or, which is known for its intense blue colour. The appearance of the lake varies from turquoise to deep navy blue depending on whether the sky is hazy or clear. Pure water and ice have a pale blue colour.
The obvious problem with coloured water is that it is not acceptable to the public. Given a choice, the public prefers clear, uncoloured water. Another problem with coloured water is the effect it has on laundering, papermaking, manufacturing, textiles, and food processing. The colour of water has a profound impact on its marketability for both domestic and industrial use. In water treatment, colour is not usually considered unsafe or unsanitary, but is a treatment problem in regards to exerting a chlorine demand that reduces the effectiveness of chlorine as a disinfectant.
Taste and Odour
Taste and odour are used jointly in the vernacular of water science. In drinking water, taste and odour are not normally a problem until the consumer complains. The problem is that most consumers find taste and odour in water aesthetically displeasing. Taste and odour do not directly present a health hazard, but they can cause the customer to seek water that tastes and smells good, but may not be safe to drink. Most consumers consider water tasteless and odourless. When consumers find that their drinking water has a taste, odour, or both, they automatically associate the drinking water with contamination.
Water contaminants are attributable to contact with nature or human use. Taste and odour in water are caused by a variety of substances such as minerals, metals, and salts from the soil; constituents of wastewater; and end products produced in biological reactions. When water has a taste but no accompanying odour, the cause is usually inorganic contamination. Water that tastes bitter is usually alkaline, while salty water is commonly the result of metallic salts. However, when water has both taste and odour, the likely cause is organic materials. The list of possible organic contaminants is too long to record here, but petroleum-based products lead the list of offenders.
Taste- and odour-producing liquids and gases in water are produced by biological decomposition of organics. A prime example of one of these is hydrogen sulfide; known best for its characteristic rotten-egg taste and odour. Certain species of algae also secrete an oily substance that may produce both taste and odour. When certain substances combine (such as organics and chlorine), the synergistic effect produces taste and odour.
In water treatment, one of the common methods used to remove taste and odour is to oxidize the materials that cause the problem. Oxidants, such as potassium permanganate and chlorine, are used. Another common treatment method is to feed powdered activated carbon before the filter. The activated carbon has numerous small openings that absorb the components that cause the odour and tastes. These contained spaces must then be positively vented to wet-chemical scrubbers to prevent the buildup of toxic concentrations of gas.
Temperature
Heat is added to surface and groundwater in many ways. Some of these are natural, and some are artificial. In regards to human heated water, heat is commonly added to water whenever a raw water source is used for cooling water in industrial operations. The influent to industrial facilities is at normal ambient temperature. When it is used to cool machinery and industrial processes and then discharged back to the receiving body, it is often heated. The problem with heat or temperature increases in surface waters is that it affects the solubility of oxygen in water, the rate of bacterial activity, and the rate at which gases are transferred to and from the water.
Water temperature does partially determine how efficiently certain water treatment processes operate. For example, temperature has an effect on the rate at which chemicals dissolve and react. When water is cold, more chemicals are required for efficient coagulation and flocculation to take place. When water temperature is high, the result may be a higher chlorine demand because of the increased reactivity, and there is often an increased level of algae and other organic matter in raw water. Temperature also has a pronounced effect on the solubility of gases in water.
Ambient temperature (temperature of the surrounding atmosphere) has the most profound and universal effect on temperature of shallow natural water systems. When water is used by industry to dissipate process waste heat, the discharge locations into surface waters may experience localized temperature changes that are quite dramatic. Other sources of increased temperatures in running water systems result because of clear-cutting practices in forests (where protective canopies are removed) and from irrigation flows returned to a body of running water.
2.9.2    Chemical Characteristics
Another category used to define or describe water quality is its chemical characteristics. The most important chemical characteristics are: total dissolved solids (TDS), pH value, alkalinity, hardness, fluoride, metals, organics, biochemical oxygen demand (BOD) and nutrients.
Chemical impurities can be either natural, man-made (industrial), or be deployed in raw water sources by enemy forces.
Total dissolved solids (TDS)
Because of water’s solvent properties, minerals dissolved from rocks and soil as water passes over and through it produce TDS (comprised of any minerals, salts, metals, cations or anions dissolved in water). TDS constitutes a part of total solids in water; it is the material remaining in water after filtration.
Dissolved solids may be organic or inorganic. Water may be exposed to these substances within the soil, on surfaces, and in the atmosphere. The organic dissolved constituents of water come from the decay products of vegetation, from organic chemicals, and from organic gases.
Dissolved solids can be removed from water by distillation, electrodialysis, reverse osmosis, or ion exchange. It is desirable to remove these dissolved minerals, gases, and organic constituents because they may cause psychological effects and produce aesthetically displeasing color, taste, and odors.
While it is desirable to remove many of these dissolved substances from water, it is not prudent to remove them all. This is the case, for example, because pure, distilled water has a flat taste. Further, water has an equilibrium state with respect to dissolved constituents. If water is out of equilibrium or undersaturated, it will aggressively dissolve materials with which it comes into contact. Because of this problem, substances that are readily dissolvable are sometimes added to pure water to reduce its tendency to dissolve plumbing.
pH Value of Water
pH value denotes the concentration of hydrogen ions in the water and it is a measure of acidity or alkalinity of a substance. Depending upon the nature of dissolved salts and minerals, the pH value ranges from 0 to 14. For pure water, pH value is 7 and 0 to 7 acidic and 7 to 14 alkaline ranges. For public water supply pH value may be 6.5 to 8.5. The lower value may cause tuberculation and corrosion; where as high value may produce incrustation, sediment deposits and other bad effects. pH value of water is generally determined by pH papers or by using pH meter. pH can read directly on scale or by digital display using pH meter. Generally, the pH influences the corrosiveness of the water, chemical dosages necessary for proper disinfection, and the ability to detect contaminants.
Alkalinity
Another important characteristic of water is its alkalinity — a measure of water’s ability to neutralize acid or really an expression of buffering capacity. The major chemical constituents of alkalinity in natural water supplies are the bicarbonate, carbonate, and hydroxyl ions. These compounds are mostly the carbonates and bicarbonates of sodium, potassium, magnesium, and calcium. These constituents originate from carbon dioxide (from the atmosphere and as a by-product of microbial decomposition of organic material) and from their mineral origin (primarily from chemical compounds dissolved from rocks and soil).
Highly alkaline waters are unpalatable; this condition has little known significance for human health. The principal problem with alkaline water is the reactions that occur between alkalinity and certain substances in the water. Alkalinity is important for fish and aquatic life because it protects or buffers against rapid pH changes. It is also important because the resultant precipitate can foul water system appurtenances. In addition, alkalinity levels affect the efficiency of certain water treatment processes, especially the coagulation process.
Hardness
Hardness is due to the presence of multivalent metal ions that come from minerals dissolved in water. Hardness is based on the ability of these ions to react with soap to form a precipitate or soap scum. In freshwater, the primary ions are calcium and magnesium; iron and manganese may also contribute.
Hardness is classified as carbonate hardness or noncarbonate hardness. Carbonate hardness is equal to alkalinity but a noncarbonated fraction may include nitrates and chlorides. Hardness is either temporary or permanent. Carbonate hardness (temporary hardness) can be removed by boiling. Non carbonate hardness cannot be removed by boiling and is classified as permanent. Water with a hardness of less than 50 ppm is soft while the one with hardness of greater than 300ppm is very hard.
There are advantages to be gained from usage of hard water. These include:
1. Hard water aids in the growth of teeth and bones.
2. Hard water reduces toxicity to many by poisoning with lead oxide from lead pipelines.
3. Soft waters are suspected to be associated with cardiovascular diseases
Fluoride
Fluoride is seldom found in appreciable quantities in surface waters and appears in groundwater in only a few geographical regions. However, fluoride is sometimes found in a few types of igneous or sedimentary rocks. Fluoride is toxic to humans in large quantities and is also toxic to some animals. If used in small concentrations (about 1.0 mg/L in drinking water), fluoride can be beneficial. Experience has shown that drinking water containing a proper amount of fluoride can reduce tooth decay by 65% in children between ages 12 to 15 (Spellman, 2003)
Metals
Metal ions are dissolved in groundwater and surface water when the water is exposed to rock or soil containing the metals, usually in the form of metal salts. Metals can also enter with discharges from sewage treatment plants, industrial plants, and other sources. The metals most often found in the highest concentrations in natural waters are calcium and magnesium which are non toxic. In natural water systems, other nontoxic metals are generally found in very small quantities. Most of these metals cause taste problems well before they reach toxic levels.
Fortunately, toxic metals are present in only minute quantities in most natural water systems. Even in small quantities, toxic metals in drinking water are harmful to humans and other organisms. Arsenic, barium, cadmium, chromium, lead, mercury, and silver are toxic metals that may be dissolved in water. Arsenic, cadmium, lead, and mercury, all cumulative toxins, are particularly hazardous.
Organics
Generally, the source of organic matter in water is from decaying leaves, weeds, and trees; the amount of these materials present in natural waters is usually low. The general category of “organics” in natural waters includes organic matter whose origins could be from both natural sources and from human activities. It is important to distinguish natural organic compounds from organic compounds that are solely man-made (anthropogenic), such as pesticides and other synthetic organic compounds.
The presence of organic matter in water is troublesome for the following reasons; colour formation,  taste and odour problems, oxygen depletion in streams,  interference with water treatment processes, and  the formation of halogenated compounds when chlorine is added to disinfect water. Many organic compounds are soluble in water, and surface waters are more prone to contamination by natural organic compounds that are groundwater. In water, dissolved organics are usually divided into two categories: biodegradable and non-biodegradable.
Biodegradable (breakdown) material consists of organics that can be utilized for nutrients (food) by naturally occurring microorganisms within a reasonable length of time. These materials usually consist of alcohols, acids, starches, fats, proteins, esters, and aldehydes. They may result from domestic or industrial wastewater discharges, or they may be end products of the initial microbial decomposition of plant or animal tissue. The principle problem associated with biodegradable organics is the effect resulting from the action of microorganisms. Some biodegradable organics can also cause colour, taste, and odour problems.
Non-biodegradable organics are resistant to biological degradation. For example, constituents of woody plants, such as tannin and lignin acids, phenols, and cellulose, are found in natural water systems and are considered refractory (resistant to biodegradation).
The quantity of oxygen-consuming organics in water is usually determined by measuring the biochemical oxygen demand (BOD) (Spellman, 2003).
Biological Oxygen Demand (BOD)
This is the amount of dissolved oxygen needed by aerobic decomposers to break down the organic materials in a given volume of water over a 5-day incubation period at 20ºC (68ºF). Dissolved oxygen is the amount of oxygen dissolved in water. Concentrations of less than 5 ppm can limit aquatic life or cause offensive odors.
BOD directly affects the amount of DO in water bodies. The greater the BOD, the more rapidly oxygen is depleted in the water body, leaving less oxygen available to higher forms of aquatic life. The consequences of high BOD are the same as those for low DO: aquatic organisms become stressed, suffocate, and die. (Spellman, 2003).
Nutrients
Nutrients (biostimulents) are essential building blocks for healthy aquatic communities, but excess nutrients (especially nitrogen and phosphorous compounds) over-stimulate the growth of aquatic weeds and algae. Excessive growth of these organisms can clog navigable waters; interfere with swimming and boating; outcompete native submerged aquatic vegetation; and, with excessive decomposition, lead to oxygen depletion.
Oxygen concentrations can fluctuate daily during algae blooms, rising during the day as algae perform photosynthesis and falling at night as algae continue to respire, which consumes oxygen. Beneficial bacteria also consume oxygen as they decompose the abundant organic food supply in dying algae cells. Carbon, also found as nutrient in water is readily available from a number of natural sources, including alkalinity, decaying products of organic matter, and dissolved carbon dioxide from the atmosphere. Nitrogen in water is commonly found in the form of nitrate (NO3). Nitrate in drinking water can lead to a serious problem. Specifically, nitrate poisoning in infant humans, including animals, can cause serious problems and even death.
   
Chemical Oxygen Demand (COD)
    This is the amount of chemically oxidizable materials present in the wastewater. This is a measure of the amount of oxidizable matter present in the sample. The COD is normally in the range of 200 to 500 mg/L. The presence of industrial wastes can increase this significantly. It is not actually a characteristic of water, but it is one of the important characteristics of the raw water to be treated.
2.9.3     Biological/Microbial Characteristics
    The presence or absence of certain biological organisms is of primary importance to the water or wastewater specialist. These are the pathogens. Pathogens are organisms that are capable of infecting or transmitting diseases in humans and animals. It should be pointed out that these organisms are not native to aquatic systems and usually require an animal host for growth and reproduction. They can, however, be transported by natural water systems. These waterborne pathogens include species of bacteria, viruses, protozoa, and parasitic worms (helminthes).
The examination of water for the presence of bacteria is important for the water supply engineer from the viewpoint of public health. The bacteria may be harmless to mankind or harmful to mankind. The former category is known as non-pathogenic bacteria and the latter category is known as pathogenic bacteria. Many of the bacteria found in water are derived from air, soil and vegetation. Some of these are able to multiply and continue their existence while the remaining dies out in due course of time. The selective medium that promote the growth of particular bacteria and inbuilt the growth of other organisms is used in the lab to detect the presence of the required bacteria, usually Coliform bacteria.
A virus is an entity that carries the information needed for its replication but does not possess the machinery for such replication. They are obligate parasites that require a host in which to live. Viruses are the smallest biological structures known, so they can only be seen with the aid of an electron microscope. Waterborne viral infections are usually indicated by disorders with the nervous system rather than of the gastrointestinal tract. Viruses that are excreted by human beings may become a major health hazard to public health. Waterborne viral pathogens are known to cause poliomyelitis and infectious hepatitis.
Protozoa (singular: protozoan) are mobile, single-celled, complete, self-contained organisms that can be free-living or parasitic, pathogenic or nonpathogenic, or microscopic or macroscopic. Most protozoa are harmless, only a few cause illness in humans — Entamoeba histolytica (amebiasis) and Giardia lamblia (giardiasis) being two of the exceptions (Spellman, 2003).
Worms are the normal inhabitants in organic mud and organic slime. They have aerobic requirements, but can metabolize solid organic matter not readily degraded by other microorganisms. Worms pose hazards primarily to those persons who come into direct contact with untreated water.
2.9.4     Water Quality Standards
    Various regulatory bodies have come up with drinking (potable) water standards. World Health Organization (WHO) has control over the drinking water standards of all countries.  In United States of America (USA), the Clean Water Act (1972), the Safe Drinking Water Act (1974), all control water quality by using the Environmental protection Agency (EPA), American Water works Association (AWWA), e.t.c.
In Nigeria, apart from WHO, various regulatory bodies are responsible for safe water. The following provide the regulatory framework for drinking water quality in Nigeria: Consumer Protection Council Act 66 (1992), Council for Regulation of Engineering in Nigeria Act 55 (1972), Federal Environmental Protection Agency- Retained as Cap 131, Food and Drug Retained as Cap 150, Food and Drugs (1999-No.19) Changed to NAFDAC Act 11, Institute of Chartered Chemist of Nigeria Act No.91 (1993), Institute of Public Analyst of Nigeria Act No.100 (1992), National Water Resources Institute Act- Retained as Cap 284,  Public Health Act (1958), Standards Organization of Nigeria (SON) - Retained as Cap 412, Water Resources Act No. 101 (1993), International Organization for Standardization (ISO) – Service activities relating to drinking water and wastewater – Guidelines for the management of drinking water utilities and for the assessment of drinking water services, National Guidelines and standards for Water Quality in Nigeria, Nigerian Industrial Standards for Natural Mineral Water (NIS 345: 2003) and Potable Water (NIS 306: 2004) (Nigerian Standard for Drinking Water Quality, NIS 554: 2007)
In the publication of Nigerian Standard for Drinking Water Quality (NIS 554: 2007), it was concluded that all drinking water shall at any time meet the minimum requirements set out in Table 2.2, Table 2.3, Table 2.4, Table 2.5, Table 2.6, and Table 2.7 below.

Table 2.2 Physical / Organoleptic Parameters
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Colour    TCU    15    None   
Odour    -    Unobjectionable    None   
Taste    -    Unobjectionable    None   
Temperature    0Celcius    Ambient    None   
Turbidity    NTU    5    None   

Table 2.3 Inorganic Constituents
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Aluminum (Al)    mg/L    0.2    Potential Neuro-degenerative
disorders


















    Note 1
Arsenic (As)    mg/L    0.01    Cancer   
Barium (Ba)    mg/L    0.7    Hypertension   
Cadmium (Cd)    mg/L    0.003    Toxic to the kidney   
Chloride    mg/L    250    None   
Chromium (Cr6+)    mg/L    0.05    Cancer   
Conductivity    μS/cm    1000    None   
Copper (Cu2+)    mg/L    1    Gastrointestinal disorder   
Cyanide (CN-)    mg/L    0.01    Very toxic to the thyroid and
nervous system   
Fluoride (F-)    mg/L    1.5    Fluorosis, skeletal tissue (bones and teeth)  morbidity   
Hardness (as CaCO3)    mg/L    150    None   
Hydrogen Sulphide (H2S)    mg/L    0.05    None   
Iron (Fe2+)    mg/L    0.3    None   
Lead (Pb)    mg/L    0.01    Cancer, interference with vitamin D metabolism, affect mental development in infants, toxic to the central and peripheral nervous systems   
Magnesium (Mg2+)    mg/L    0.2    Consumer acceptability   
Manganese (Mn2+)    mg/L    0.2    Neurological disorder   
Mercury (Hg)    mg/L    0.001    Affects the kidney and central nervous system   
Nickel    mg/L    0.02    Possibly carcinogenic   
Nitrate (NO3)    mg/L    50    Cyanosis, and asphyxia(blue-baby syndrome) in infants under 3 months   
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Nitrite (NO2)    mg/L    0.2    Cyanosis, and asphyxia(blue-baby syndrome) in infants under 3 months   


pH       -    6.5 – 8.5    None   
Sodium (Na)    mg/L    200    None   
Sulphate (SO4)    mg/L    100    None   
Total Dissolved Solids    mg/L    500    None   
Zinc (Zn)    mg/L    3    None   
Note 1: Parameter to be monitored only if aluminum chemicals are used for water
Table 2.4 Organic Constituents
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Detergents    mg/L    0.01    Possibly carcinogenic   
Mineral oil    mg/L    0.003    Possibly carcinogenic   
Pesticides    mg/L    0.01    Possibly carcinogenic   
Phenols    mg/L    0.001    Possibly carcinogenic   
 Poly Aromatic       Hydrocarbon    mg/L    0.007    Possibly carcinogenic   
Total Organic Carbon or
Oxidisability    mg/L    5    Cancer   

 Table 2.5 Disinfectants and their by-products
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Free residual chlorine    mg/L    0.2 – 0.25    None    Note 2
Trihalomethanes Total    mg/L    0.001    Cancer    Note 2
2,4,6- trichlorophenol    mg/L    0.02    Cancer    Note 2
Note 2: For chlorinated water only
Table 2.6 Radioactive limits
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Radionuclides    Bq/L    0.1    Cancer   


Table 2.7 Microbiological Limits
Parameter    Unit    Maximum Permitted Levels    Health Impact    Note
Total Coliform count    cfu/mL    10    Indication of faecal contamination   
Thermo tolerant Coliform or E coli    cfu/mL    0    Urinary tract infections bacteraemia, meningitis, diarrhea, (one of the main cause of morbidity and mortality among children), acute renal failure and hemolytic anaemia   
Faecal streptocous    cfu/mL    0    Indication of recent faecal contamination   
Clostridium perfringens spore    cfu/mL    0    Index of intermittent faecal contamination   

2.10    WATER TREATMENT OPERATIONS AND UNIT PROCESSES
     Municipal water treatment operations and associated treatment unit processes are designed to provide reliable, high quality water service for customers, and to preserve and protect the environment for future generations. The purpose of water treatment is to condition, modify and/or remove undesirable impurities, to provide water that is safe, palatable, and acceptable to users. While this is the obvious, expected purpose of treating water, various regulations also require water treatment.
    The surface sources generally contains large amount of impurities therefore they requires sedimentation, filtration and chlorination as treatment. If the water contains algae or other micro organisms, pre chlorination has to be done tastes and odours; dissolved gases like CO2, H2S are removed by aeration. During the flood season, the turbidity of the surface water may be high and flocculation may become necessary to remove turbidity.


2.10.1    Preliminary Treatment
     Simply stated, water pretreatment (also called preliminary treatment) is any physical, chemical, or mechanical process used before main water treatment processes. It can include screening, presedimentation, and chemical addition. Pretreatment of surface water supplies accomplishes the removal of certain constituents and materials that interfere with or place an unnecessary burden on conventional water treatment facilities.
    Typical pretreatment processes include screening for removal of debris from water from rivers and reservoirs that would clog pumping equipment, aeration to remove dissolved odor-causing gases, such as hydrogen sulfide and other dissolved gases or volatile constituents, and to aid in the oxidation of iron and manganese, chemical treatment of reservoirs to control the growth of algae and other aquatic growths that could result in taste and odor problems,  and presedimentation to remove excessively heavy silt loads prior to the treatment processes
Aeration
    Aeration is commonly used to treat water that contains trapped gases (such as hydrogen sulfide) that can impart an unpleasant taste and odor to the water. Just allowing the water to rest in a vented tank will (sometimes) drive off much of the gas, but usually some form of forced aeration is needed. Aeration works well (about 85 percent of the sulfides may be removed) whenever the pH of the water is less than 6.5.
Aeration may also be useful in oxidizing iron and manganese, oxidizing humic substances that might form trihalomethanes when chlorinated, eliminating other sources of taste and odor, or imparting oxygen to oxygen deficient water.
Three types of aerators are commonly employed. These are: waterfall aerators exemplified by spray nozzle, cascade, and multipletray units; diffusion or bubble aerators which involve passage of bubbles of compressed air through the water; and mechanical aerators employing motor-driven impellers alone or in combination with air injection devices. Of the three types, waterfall aerators, employing multiply trays, are the most frequently used in water treatment. The efficiency of multiple-tray aerators can be increased by the use of enclosures and blowers to provide counter flow ventilation.
Screening
    Surface waters contain fish and debris which can clog or damage pumps, clog pipes and cause problems in water treatment. Streams can contain high concentrations of suspended sediment. Screening is usually the first major step in the water pretreatment process. It is defined as the process whereby relatively large and suspended debris is removed from the water before it enters the plant. The most important criteria used in the selection of a particular screening system for water treatment technology are the screen opening size and flow rate. There are basically two types of screens
    Coarse screens or racks. Coarse screens, often termed bar screens or racks, must be provided to intercept large, suspended or floating material. Such screens or racks are made of l/2-inch to 3/4-inch metal bars spaced to provide 1- to 3-inch openings.
    Fine screens. Surface waters require screens or strainers for removal of material too small to be intercepted by the coarse rack; these may be basket-type, in-line strainers, manually or hydraulically cleaned by backwashing, or of the travelling type, which are cleaned by water jets. Fine-screen, clear openings should be approximately 3/8 inch. The velocity of the water in the screen openings should be less than 2 feet per second at maximum design flow through the screen and minimum screen submergence.

Chemical Addition
Two of the major chemical pretreatment processes used in treating water for potable use are iron and manganese and hardness removal. When chemicals are used in the pretreatment process, they must be the proper ones, fed in the proper concentration and introduced to the water at the proper locations. Chemicals are normally fed with dry chemical feeders or solution (metering) pumps.
Iron and manganese are frequently found in groundwater and in some surface waters. Iron and magnesium can be removed from water using precipitation (pH adjustment), oxidation, ion exchange and aeration processes. Chemical precipitation treatments for iron and manganese removal are called deferrization and demanganization. Air, chlorine, or potassium permanganate can oxidize iron and magnesium from water.
The most common of these hardness-causing ions in water are calcium and magnesium; others include iron, strontium, and barium. Two common methods are used to reduce hardness: ion exchange and cation exchange. The ion exchange process is the most frequently used process for softening water. Accomplished by charging a resin with sodium ions, the resin exchanges the sodium ions for calcium and magnesium ions. Naturally occurring and synthetic cation exchange resins are available. Natural exchange resins include such substances as aluminum silicate, zeolite clays (Zeolites are hydrous silicates found naturally in the cavities of lavas [greensand]; glauconite zeolites. The cation exchange process takes place with little or no intervention from the treatment plant operator. Water containing hardness-causing cations (Ca2+, Mg2+, and Fe3+) is passed through a bed of cation exchange resin (Spellman, 2003).


Presedimentation
     Plain sedimentation, another name for presedimentation is accomplished without the use of coagulating chemicals. Whether plain sedimentation is essential is a judgment decision influenced by the experience of plants treating water from the same source. Water derived from lakes or impounding reservoirs rarely requires presedimentation treatment. On the other hand, water obtained from notably sediment-laden streams, such as those found in parts of the Middle West, requires presedimentation facilities for removal of gross sediment load prior to additional treatment. Presedimentation treatment should receive serious consideration for water obtained from rivers whose turbidity value frequently exceeds 1,000 NTU. Presedimentation takes place at the sedimentation basins (Technical Manual TM 5-813-3/AFM 88-10, 1985).
2.10.2    Softening
Whether water softening is provided will depend entirely on the type of project and the uses to be made of the water. Two general types of processes are used for softening: The “lime-soda ash” process and the “cation ion exchange” or “zeolite” process. Since water is to be used for drinking,  lime - soda ash process will be discussed.
The principal chemicals used to effect softening are lime, either hydrated lime Ca(OH)2 or quick lime (CaO), and soda – ash (Na2CO3) to be softened and react with the calcium carbonate and magnesium in the ater to form insoluble compounds of calcium carbonate and magnesium hydroxide. If quicklime is used, it is usually converted to slurry of hydrated lime by slaking with water prior to application. The chemistry of the process can be illustrated by the following equations:
CO2 + Ca(OH)2 → CaCO3↓ + H2O
Ca(HCO3)2(aq)+ Ca(OH)2(aq) → 2CaCO3(c)↓ + 2H2O(l)
Mg(HCO3)2(aq)+ 2Ca(OH)2(aq) → 2CaCO3(c)↓ +Mg(OH)2 + 2H2O(l)
MgSO4(aq) + Ca(OH)2(aq) → CaSO4(aq) + Mg(OH)2(c)↓
CaSO4(aq) + Na2CO3(aq) → CaCO3(c)↓ + Na2SO4(aq)

2.10.3    Coagulation
    The primary purpose in surface-water treatment is chemical clarification by coagulation and mixing, flocculation, sedimentation, and filtration. These units, processes, along with disinfection, work to remove particles, naturally occurring organic matter (NOM [i.e., bacteria, algae, zooplankton, and organic compounds]), and microbes from water. Following screening and the other pretreatment processes, the next unit process in a conventional water treatment system is a mixer where chemicals are added in what is known as coagulation.
 Materials present in raw water may vary in size, concentration, and type. Dispersed substances in the water may be classified as suspended, colloidal, or solution. Suspended solids will settle out of water over time, though this may be so slow that it is impractical to merely allow the particles to settle out in a water treatment plant. Colloidal solids include bacteria, fine clays, and silts and do not dissolve in water although they are electrically charged; The particles are so small that they will not settle out of the water. Colloidal solids range between 1 millimicron (10-6 mm) and 1 micron (10-3 mm) in size and can be seen only with a high-powered microscope. Chemicals in solution or solutions are completely dissolved in the water, e.g. sugar in water. Coagulation means a reduction in the forces which tend to keep suspended particles apart. The term coagulation generally refers to the series of chemical and mechanical operations by which coagulants are applied and made effective.
    The coagulant must be added to the raw water and perfectly distributed into the liquid; such uniformity of chemical treatment is reached through rapid agitation or mixing. Coagulation results from adding salts of iron or aluminum to the water. Common coagulants (salts) are as follows: Alum (aluminum sulfate), sodium aluminate, ferric sulfate, ferrous sulfate, ferric chloride, and polymers. Recently the use of moringa olifera extract as a coagulant has gained wide acceptance (Gbebremichael, 2004).
Alum   
  A12(SO4)3 + 3 Ca(HCO3)2                                  2 Al(OH)3 + 3CaSO4 + 6 CO2
Ferric Sulphate
              Fe2(SO4)3 + 3 Ca(HCO3)2                                            2 Fe(OH)3 + 3CaSO4 + 6 CO2
Ferric Chloride
              2 Fe Cl3 + 3 Ca(HCO3)2                                                2 Fe(OH)3 + 3CaCl2 + 6CO2
Ferrous Sulphate
              FeS04 + Ca(HCO3)2                                               Fe(OH)2 + CaS04 + 2CO2
Sodium Aluminate
              2 Na2A12O4 + Ca(HCO3)2                                  8 Al(OH)3 + 3 Na2CO3 + 6 H20
              Na2Al2O4 + CO2                                                               2 Al(OH)3 + Na2CO3
              Na2Al2O4 + MgCO3                                                  MgAl2O4 + Na2CO3
    When alum is placed in water, a chemical reaction occurs that produces positively charged aluminum ions. The overall result is the reduction of electrical charges and the formation of a sticky substance — the formation of floc, which when properly formed, will settle. These two destabilizing factors are the major contributions that coagulation makes to the removal of turbidity, color, and microorganisms.
    The formation of floc is the first step of coagulation; for greatest efficiency, rapid, intimate mixing of the raw water and the coagulant must occur. After mixing, the water should be slowly stirred so that the very small, newly formed particles can attract and enmesh colloidal particles, holding them together to form larger floc. This slow mixing is the second stage of the process (flocculation) and is covered next
Coagulation is the reaction between one of these salts and water. The raw water conditions, optimum pH for coagulation, turbidity, temperature, and mixing condition must be considered before deciding which chemical is to be fed and at what levels.
2.10.4    Flocculation
    Flocculation follows coagulation in the conventional water treatment process. Flocculation is the physical process of slowly mixing the coagulated water to increase the probability of particle collision — unstable particles collide and stick together to form fewer larger flocs. Through experience, we see that effective mixing reduces the required amount of chemicals and greatly improves the sedimentation process, which results in longer filter runs and higher quality finished water.
Flocculation’s goal is to form a uniform, feather-like material similar to snowflakes — a dense, tenacious floc that entraps the fine, suspended, and colloidal particles and carries them down rapidly in the settling basin.
Proper flocculation requires from 15 to 45 min. The time is based on water chemistry, water temperature, and mixing intensity. Temperature is the key component in determining the amount of time required for floc formation. To increase the speed of floc formation and the strength and weight of the floc, polymers are often added.
Theory and Chemistry of Coagulation and Flocculation
     The chemistry of coagulation and flocculation is primarily based on the electrical properties of the particles. Like charges repel each other while opposite charges attract.  Most particles present in water have a negative charge, so they tend to repel each other. As a result, they stay dispersed in the water (http://ocw.kfupm.edu.sa/user062/CE370001/Coagulation%20
and%20Floculation_062_Part%201.pdf)






Figure 2.2 Particles in Water.
The purpose of most coagulant chemicals is to neutralize the negative charges on the colloidal particles to prevent those particles from repelling each other. The amount of coagulant which should be added to the water will depend on the zeta potential, a measurement of the magnitude of electrical charge surrounding the colloidal particles. Zeta potential can be seen as the amount of repulsive force which keeps the particles in the suspension. If the zeta potential is large, then more coagulants will be needed.
Coagulants tend to be positively charged. Due to their positive charge, they are attracted to the negative particles in the water, as shown below.

   





Figure 2.3 Coagulant Particles in Water.
The combination of positive and negative charge results in a neutral, or lack of charge. As a result, the particles no longer repel each other. The next force which will affect the particles is known as van der Waal's forces. Van der Waal's forces refer to the tendency of particles in nature to attract each other if they come close enough.






Figure 2.4 Formation of Neutral Charge
Once the particles in water are not repelling each other and due to their motion in water, they will come close to each other (collide) so that Van der Waal's forces of attraction can make the particles stick to each other. When enough particles have joined together, they become floc and will settle out of the water (http://ocw.kfupm.edu.sa/user062/CE370001/Coagulation%20 and%20Floculation_062_Part%201.pdf).
.





Figure 2.5 Floc Formation
2.10.5    Sedimentation
    After raw water and chemicals have been mixed and the floc formed, the water containing the floc (because it has a higher specific gravity than water) flows to the sedimentation or settling basin. Sedimentation is also called clarification.
Sedimentation removes settleable solids by gravity. Water moves slowly though the sedimentation tank or basin with a minimum of turbulence at entry and exit points with minimum short-circuiting. Sludge accumulates at bottom of tank or basin. Typical tanks or basins used in sedimentation include conventional rectangular basins, conventional center-feed basins, peripheral-feed basins, and spiral-flow basins. In conventional treatment plants, the amount of detention time required for settling can vary from 2 to 6 hours.
Detention time should be based on the total filter capacity when the filters are passing 2 gal/min/ft2 of superficial sand area. For plants with higher filter rates, the detention time is based on a filter rate of 3 to 4 gal/min/ft2 of sand area. The time requirement is dependent on the weight of the floc, the temperature of the water, and how quiescent (still) the basin.
            A number of conditions affect sedimentation (Spellman, 2003):
1. Uniformity of flow of water through the basin.
2. Stratification of water due to difference in temperature between water entering an          water already in the basin.
3. Release of gases that may collect in small bubbles on suspended solids, causing them to rise and float as scum rather than settle as sludge.
4. Disintegration of previously formed floc.
5. Size and density of the floc.

2.10.6    Filtration
Filtration of water is a physical process defined as the separation of colloidal and larger particles from water by passage through a porous medium, usually sand, granular coal, or granular activated carbon. The process of filtration involves straining, settling, and adsorption.
The following are the mechanisms of filtration (Gbebremichael, 2004);
1. Mechanical straining – Mechanical straining of suspended particles in the sand pores.
2. Sedimentation – Absorption of colloidal and dissolved inorganic matter in the
    surface of sand grains in a thin film.
3. Electrolytic action – The electrolytic charges on the surface of the sand particles,
    which opposite to that of charges of the impurities are responsible for binding
    them to sand particles.
4. Biological Action – Biological action due to the development of a film of
    micro-organisms layer on the top of filter media, which absorb organic impurities.
    As floc passes into the filter, the spaces between the filter grains become clogged, reducing this opening and increasing removal. Some material is removed merely because it settles on a media grain. One of the most important processes is adsorption of the floc onto the surface of individual filter grains. This helps collect the floc and reduces the size of the openings between the filter media grains. In addition to removing silt and sediment, floc, algae, insect larvae, and any other large elements, filtration also contributes to the removal of bacteria and protozoans such as Giardia lamblia and cryptosporidium. Some filtration processes are also used for iron and manganese removal (Spellman, 2003).