Preface The issues of water supply and environmental sanitation are tackled by various levels of government without considerable success. Lack of potable water and poor environmental sanitation are largely responsible for the loss of lives associated with spread of cholera in many communities first in 1971 and later in 1991. History has a way of repeating itself in Nigeria.

The problem of wastes has grown throughout the years. Until today the disposal of wastes has become one of the most crucial matters confronting society in general. It is a problem which presents many facets and requires many solutions. Inadequate disposal of wastes must lead inevitably to environmental pollution. Hence there is a need to study public health engineering in the university.

The author is grateful to World Health Organisation for W.H.O. awards on two occasions and the civil engineering departments of University of Ibadan and University of Uyo for providing good environment for the teaching and research facilities in public health engineering.

In the preparation of this book the author had to consult constantly numerous textbooks on public health engineering and the author hereby acknowledges his indebtedness to them.

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CHAPTER 1

INTRODUCTION Public health engineering is the aspect of civil engineering which deals with the planning, design, construction, operation, maintenance, research and development of physical facilities in our surroundings which can affect health.

What is health?

W.H.O. definition;

Health is a state of complete physical, spiritual, mental and social well being, and not the absence of diseases and infirmities. For example drug addicts are not socially healthy.

There are seven branches of public health engineering;

1. Water Supply; This must be adequate, wholesome and safe for drinking.2. Sewage Disposal; This must be safe and appropriate for the community. Sewage is

defined as the water-carried wastes of any community.3. Solid Waste Management; This involves the elimination of unwanted solid materials

of all types from the human environment e.g. from rural/urban areas, factory premises e.t.c.

4. Housing; There must be provision of housing which will promote health i.e. by having all the facilities and equipment needed for healthy living.

5. Vector Control; This aspect is involved in all facets of engineering design and it is to make sure that the environment discourages vectors e.g. mosquitoes, flies e.t.c.

6. Food Sanitation; This aspect deals with activities involved in the manufacturing and preservation of food in such a way that the food will remain wholesome and sound e.g. maintenance of abattoirs, food factories, market design and maintenance.

7. Air Pollution Control; Pollution is caused by substances which should naturally not be there – either for a short time or long duration in a concentration that is above normal. The substances are classified (a) Particulate matter e.g. smoke, dust (b) Chemical e.g. So2, Co (exhaust from petrol engines), oxides of Nitrogen, H2S from

refineries e.t.c. HF and carcinogenic hydrocarbons

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CHAPTER 2

WATER SUPPLY Water is a vital mineral resource for human activities and it should be treated as such despite its special attribute of being renewable after depletion. As a matter of fact, next to air water is the most essential to man’s requirement for life. Air, Water, Food, Heat and Light are the 5 basics for life in this order. By W.H.O. standards the basic physiological need for humans is 2L/capita/day (minimum). For modern living (with laundry, toilets e.t.c.) the minimum is 50L/c/d. The quantity of water required will depend on so many factors.

(i) Standard of living(ii) Cost of water(iii) Quality of water (iv) Pressure of supply

Man’s prime need in his environment is for water and wherever a number of people live together in a community a supply of potable water is required. The liquid and solid wastes from such a community if not disposed of in a satisfactory manner can tender the surroundings unpleasant and unhealthy. Excretal contamination of drinking water leads inevitably to a vicious circle of internal disorders of increasing magnitude. In primitive civilizations the remedy to pollution problems was to move the community to a new unspoiled site, but in more advanced communities this solution becomes impracticable. Water can thus be considered as the most important raw material of civilisation, since without it man cannot live and industry cannot operate. The concept of water as a natural resource is essential demand ever-increasing supplies of water. In 1971, there was an outbreak of cholera in Ibadan. Water supplies became increasingly contaminated by sewage. The Ogunpa River became most objectionable to sight and smell and a series of cholera outbreak eventually demonstrated the connection between diseases such as cholera and typhoid and polluted drinking water. At the present time, World Health Organization (W.H.O.) surveys show that 86% of the population in Africa are without reasonable access to safe water.

In conclusion water is without price in its natural state. It is priceless too in the benefits it can bestow. Best of all things is water.

THE ENGINEERING ROLE The responsibilities of the engineer start with the provision of an ample supply of wholesome water i.e. water free from.

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(i) Visible suspended matter (ii) Colour (iii) Taste(iv) Odour(v) Objectionable dissolved matter(vi) Aggressive constituents(vii) Bacteria indicative of pollution

The water must be fit for human consumption i.e. potable, but it should also be palatable i.e. aesthetically attractive.

USES OF WATER1. Domestica) For satisfying the basic physiological needs b) Sanitation (clothes, toilets, e.t.c.)c) For comfort and recreation (central air conditioning, watering lawns and swimming

pools)2. Agricultural

The amount of water required for agriculture depends on several factors;

a) Whether irrigation is used and if so what type of irrigation technique is used b) Whether livestock are kept e.g. cows if kept for milk require about 150L/C/D,

sheep require about 10L/C/D, goats require about 7.5L/C/D, poultry require 5L/C/D

IRRIGATION Different people will think about this title in different ways. A man born in Benin, notorious for its rainfall, would laughingly say that they get too much irrigation. A man born in Kano however would immediately think of his land turning from a dry sandy colour to a mass of green. Thus the first man’s thought of rain are of it spoiling his day out to watch football, and the second man’s thought would be of food. Irrigation is not just a question of watering one’s garden plants with a hose pipe. A better definition of irrigation would be the science of economical utilization of water to supplement natural rainfall for the production of food. Economy plays an important part, for you may not be aware that it takes about 200L of water to produce 1 slice of bread, for wheat will not grow without water, 215 litres are required to grow the food necessary to produce one egg. Each 100Kg of steel needs a quarter of million litres in its manufacture. One bottle of beer needs nine bottles of water for it to be brewed. Life could not even exist without irrigation.

Irrigation is one of the most effective technical means of raising agricultural production. Even in the areas where rainfall is sometimes assumed to be adequate, supplementation with irrigation has shown an increase in yield.

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Water is applied to the soil either by surface irrigation or by sprinkler irrigation. Surface irrigation depends on gravity. A river is dammed and the water is either allowed to flow over the land as in flood irrigation or is diverted to canals and from there to laterals on the farm. Surface irrigation is gradually being replaced by the labour-saving but capital intensive methods of sprinkler irrigation. This transition is encouraged by further advantages which the sprinkler method has to offer particularly the fact that it is independent of the type of terrain. Vegetables, citrus fruit and even rice are watered by this method as well as much irrigated fodder such as grass, Lucerne and oats. These enable dairy cattle and fat lambs to be reared, a very different type of animal from the rather thin ones which formerly grazed on the natural grassland.

3. INDUSTRIAL

The quality and quantity of water required depends on the type of industry. Water corporations tend to be self-supporting because the industries are big water consumers and they pay back substantially e.g. breweries, steel mills, paper mills, laundry and canning factories. 1 tonne of steel requires about 100 tonnes of water which is used mainly for cooling. In the brewery 1-5 litres of water is required to produce 1 litre of beer or soft drinks and the water is required mainly for mixing and washing. Paper industry uses about 10-100 tonnes of water to produce 1 tonne of paper and the water is used for washing at the different stages.

4. Water is used for producing hydraulic and steam power.

5. Water is used for protecting life and property against fire (Fire hydrants are provided at strategic points).

6. Water is also used for removing offensive and possibly dangerous wastes from household (sewage) and industry (industrial waste water).

7. Finally solid waste is water flushed through pipes more than 200mm diameter into underground tanks. This is a convenient method for refuse collection in districts with multi-storey blocks of flats.

QUANTITY OF WATER The supply of water must be satisfactory in quality and adequate in quantity, on tap day and night, readily available to the user. Relatively cheap, and easily disposed of after it has served its purpose. Before any design processes are carried out, the following factors which affect the quantity of water required must be considered.

(i) Domestic uses

(ii) Agricultural

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(iii) Industrial

(iv) Size of population and the rate of growth

We must project the requirements of the future i.e. agricultural, industrial and domestic uses in the future. Also amount of money available will determine the quantity which can be provided.

P= population t= time in years or months

If the growth rate is arithmetic;

P2 – P1 = (t2 –t1) K

If the growth rate is geometric;

Log P2 P1 = K (t2 –t1)

The population of Moniya was 10,000 in 1970. In 1980 it was 15,000. Calculate the population in 2000 if;

a) The growth rate is arithmetic

b) The growth rate is geometric

SOLUTIONa) 15,000 - 10,000 = 10K

Thus K = 500

Let population in 2000 be P2

P2 – 10,000 = 30K

Thus P2 = (30 500) + 10,000 = 25,000

b) Log 15,000 10,000 = 10K

10K = 1.5

K = 0.1761 10 = 0.01761

Log P2 10,000 = 3.375

P2 = 33,750

QUALITY OF WATER Potable water; this refers to water that is safe and attractive to use. Potable water is characterised by the following;

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(a) Absence of disease carrying organisms

(i) Protozoan e.g. amoeba

(ii) Bacteria e.g. cholera or typhoid causing organisms

(iii) Worms e.g. guinea worms

(iv) Virus e.g. infective hepatitis

(v) Fungus e.g. ringworm of the foot common among swimmers

(b) Absence of poisonous chemicals

Poisonous chemicals may be introduced artificially or be present naturally. The natural chemicals, nitrates and fluorides are the most common. Fluorides in low concentration are beneficial (may even be introduced). Up to 1.5 mg/l of fluoride is beneficial because it promotes good teeth formation and prevents dental decay or caries. Higher amounts cause dental problems and bone malformation, (physiological problem). Above 45 mg/l of nitrates affect babies causing blue babies. These chemicals are present in underground waters (fluorides) if such chemicals are present in the geological formation of the area. Some areas in Ethiopia have high fluoride concentration water. For lead, safe concentration is below 0.001mg/L. Higher concentrations than this are a result of human activity i.e. from industries. Others include mercury and cyanides.

(c) Absence of excessive organic and inorganic substances

Presence of such substances gives rise to odour, colour and tastes. Ammonia and hydrogen sulphide are common odour causing gases in water formed from decaying substances. Any water must satisfy these three conditions before it can be called potable.

Points (a) and (b) ensure safety and (c) ensures attractiveness

CASE STUDY; Health hazards arising from water shortage on our campus in 1980’s

The health situation in our campus on Tuesday April 27, 1982 was similar to the situation described by Boyd on Saturday January 24, 1970. History has a way of repeating itself. According to Boyd (1970) the water shortage in the university constituted a menace to the health of the community, there were two different situations existing on the campus.

1. Students living in large numbers in halls of residence and

2. Individual households functioning as multiple small units

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In both situations water was required for drinking, cooking, personal washing, dish washing, general scrubbing and cleaning purposes, laundry and lavatory flushing. The normal water requirement at the university was 2.25 million litres per day. It was estimated that the minimum demand for these purely domestic functions for the halls of residence and staff housing and off-campus workers’ canteens and office lavatories was 1.14 million litres per day. It would be seen therefore just how serious the position was when the total delivery of water to the university was less than 0.9 million litres per day for all purposes including laboratory use.

The main danger to health was in the kitchens and in the lavatories. The situation was different in the residences as opposed to the houses; the halls of residence had roof storage tanks but having once received their daily quota of water which was almost used up immediately in the desire of the various subsections of the hall to store some for their own purposes e.g. the kitchens for the cooking of three main meals, drinking water, dish washing e.t.c. was limited, hygiene consequently suffered. Also the toilets for the cooks and stewards, who were food handlers, had only one flush daily and there was a real risk of infection being transmitted to the students. In the houses of the staffs the same sort of conditions obtained. Some houses received no water at all as they were too high in relation to the head of head of water in the main receptor tanks. Householders had to store water for the next 24 hours consumption for all purposes.

SOURCES OF WATER(a) Rain water

(b) Underground

(c) Surface water

(a) Rain water is used in rural areas but we make sure that the collecting surface does not make the water non-potable

(b) Underground sources include

(i) Water holes

(ii) Springs

(iii) Wells

(iv) Boreholes

(c) Surface sources include

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(i) Rivers

(ii) Lakes

(iii) Seas and oceans. The Sea of Galilee contains fresh water. Desalination processes are used in Iraq and Libya.

We can use (a) only for small communities and where there are no other sources. The nature of the water source commonly determines the planning, design and operation of the collection, purification, transmission and distribution works.

ENGINEERING WORKS INVOLVED WITH WATER SUPPLY SCHEME These include the following;

(i) Water collection

(ii) Water treatment and purification

(iii) Water transmission

(iv) Water distribution

(i) Collection works – tap a source e.g. dam

(ii) Purification or treatment works – render the incoming water suitable for the purposes they are expected to serve

(iii) Transmission works – conveyed the collected and treated water from the source to the community

(iv) Distribution works – dispense the collected, treated and transmitted water to consumers in wanted volume at adequate pressure through systems of pipes and reservoirs.

WATER TREATMENT UNIT OPERATIONS – UNIVERSITY OF IBADAN AS A CASE STUDY The raw water is pumped from the waterworks by suction pump through 250mm diameter pipe into 5,000 m3 raw water tank where it is aerated. In this process oxygen of the air is added to the water to increase the proportion of oxygen – carbon dioxide ratio thus reducing the corrosion effect of the water on the water main. The aerated water is then pumped by the low lift pump into sedimentation tank. However alum is added to the aerated water prior to the expulsion into the sedimentation tank. In the sedimentation tank

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water undergoes sedimentation which is the unit operation by which particles heavier than the liquid in which they exist are removed by gravitational settling. The process is very important in clarifying water. Sedimentation can be affected in any of the following ways; the discrete sedimentation, the hindered sedimentation and the blanket sedimentation. In whatever ways the sedimentation process is brought about, four different zones have been identified in any liquid undergoing sedimentation. In each zone a combination of different modes of sedimentation exist. The liquid molecules should have piston or plug flow pattern through the sedimentation tank. The flow pattern is complex for this reason the detention time of different elements of the liquid and particle varies. The numbers of particles which are eliminated from the liquid in continuous sedimentation process are dependent on time, and the different types of particles which are allowed to remain in the tank. There are two types of sedimentation tanks when considered from the flow pattern of view. These are the vertical flow sedimentation tanks and the horizontal flow sedimentation tanks, which are modified by structural devices to suit various conditions, thus giving rise to different types of vertical and horizontal flow sedimentation tanks. The commonest types of sedimentation employed in water treatment include candy vertical flow sedimentation tanks, horizontal flow sedimentation tanks, centrifloc sedimentation tank and accentrifloc sedimentation tanks. Centrifloc sedimentation tank is provided with automatic sludge extractor to enable dislodging to be kept in step with the volume of sludge. It is also equipped with mechanical flocculators.

After sedimentation, water leaves sedimentation tank for the filters where it undergoes filtration which is the process which renders water attractive and reduces pathogenic bacteria. Different types of filter media are in use; slow sand filter, rapid sand filter and reverse osmosis.

Rapid sand filter is commonly used. Water to be filtered through rapid sand filter is normally precoagulated and often settled in sedimentation tank. After a rapid filter is operated for a period normally between 24 – 72 hours depending on the characteristics of water, filter and the rate of filtration, the pores of filter become blocked with the dirty particles which the filter removes from water.

A different and more traditional method of filtration is the slow sand filter; it has the advantage of being of a simpler technology in both construction and operation. Its ability to bring about substantial improvements in the physical, chemical and bacteriological quality of water makes it an appropriate treatment method, especially for small communities in rural areas of Africa.

Slow sand filtration is a process by which contaminated water is passed through a sand bed, of relatively small sand size (0.15 – 0.3 mm); at very low velocities (80 – 150 l/m2/h).

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Another and fairly recent type of filter uses a method known as reverse osmosis to clean water. These filters are however rather expensive and can cope only with small outputs. Their big advantage is that they are capable of extracting up to 95% of dissolved, salts from brackish waters. Owing to the rather delicate nature of these filters, the raw water is usually passed through a simple sand filter first. The water is then pressurized to 40 – 50 atm and passed over a special membrane of extremely fine texture. The process of osmosis, or the tendency for chemical intermixing, is now reversed, so that pure water passes through the membrane, leaving the dissolved salts to be separated. The membrane is also capable of retaining micro – organisms and organic matter.

It can be seen; therefore that sand filtration is not just a simple method of removing foreign particles, but quite a complex biochemical process, enabling bacteria to be removed as well.

The filtered water then passes through 200mm pipe into 500m3 clean water tank. This is disinfected before the passage into the elevated tank. Disinfection is very important in water supply for some reasons. It safeguards against the pathogenic organisms which may escape other processes. Chlorine is often used as a disinfectant and the process of disinfection is followed by the addition of lime for the correction of pH which should be between the ranges of 6.5 and 9.2

The water is then expelled into clean water testing unit from 500m3 clean water tank. At this unit, water is then examined and tested to determine the physical, chemical, bacteriological and other microscopic characteristics of water. This is followed by the use of high lift pump to pump water to the service reservoir.

The service reservoir is the storage tank into which treated water is pumped and from it water flows under gravity to customers. It is normally sited so that a minimum pressure of 30m is provided for water going into the distribution mains. The main functions of the service reservoir are;

1. For storing water so that water may be available at relatively uniform pressure at all time

2. It can be used to provide reserve water for fire fighting

3. It enables water supply systems to have economic sizes for distribution mains and for pumps

4. It is used to control pressure in the distribution mains

5. It can be used to improve water qualities

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A service reservoir must be designed and constructed so that it can perform the functions for which it is intended adequately and safely. To enable it to do this, the following features must be considered in design and construction.

1. SITE To reduce the cost of construction, it must be judiciously sited. It must be located on one of the highest points in the locality.

2. SIZEThis will depend on many factors like the hours of pumping into the reservoir and the urgency with which interruption to supply can be removed. If pumping into it takes place continuously over 24 hours daily, the storage to be provided should be between 12 to 20 percent of the total daily consumption.

3. OTHER FEATURESSince the service reservoir contains treated water it must be covered. Provision must be made to ensure that dust free air can have access.

CHLORINE DIOXIDE FOR WATER TREATMENT The interest in using chlorine dioxide for water treatment has been increasing over the past twenty years.

It is successfully applied around the world for many water treatment problems. The fields of application are;

(i) Drinking water

(ii) Waste water

(iii) Industrial water

As regards drinking water, chlorine dioxide is widely applied for the following uses, where it is more efficient than chlorine.

(i) Organic oxidation

(ii) Disinfection

(iii) Taste and odour control

(iv) Colour removal

(v) Oxidation of iron and manganese

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In waste water treatment, chlorine dioxide is used for disinfection. In industrial water treatment, it is a very efficient biocide.

Always used in aqueous solutions, chlorine dioxide is made on site by means of a generator. There are two ways to provide it from sodium chlorite.

(1) Aqueous sodium chlorite solution + chlorine (either as a gas or in solution)

(2) Aqueous sodium chlorite solution and hydrochloric acid

The generators available on the market produce chloride dioxide safely. They are also very flexible and can be monitored to take care of a wide range of treatment requirements.

DRINKING WATERPRETREATMENT; Organic Oxidation

Even at high pH, removal of organic pollution and algae is achieved with chlorine dioxide. But unlike chlorine, it does not form trihalomethane compounds (THMs) believed to be carcinogenic.

The preoxidation of raw water with chlorine dioxide gives a good control of the THMs. In addition, even in combination with free chlorine, THMs are reduced. The prior action of chlorine dioxide removes the THMs precursors (humic substances). So, chlorine applied at the following stage does not form THMs.

Besides, chlorine dioxide does not react with ammonia and its derivatives as chlorine does. It does not form chloramines.

There is no over – consumption of oxidising agents. The use of chlorine dioxide for preoxidation enhances coagulation – flocculation efficiency.

DISINFECTION; Chlorine dioxide is well known as a very strong bactericide and viricide agent. As a bactericide it is effective against E Coli and B – anthracoides within a pH range from 6 to 10 whereas chlorine is inactive in alkaline solutions.

As a viricide, it is more effective than chlorine against enterovirus, adenovirus, and reovirus e.t.c. The sterilization with chlorine dioxide gives a longer persistent protection than chlorine.

TASTE AND ODOUR CONTROL; Chlorine dioxide does not react with phenolic compounds. Chlorine in presence of phenolics gives chlorophenols which are responsible for odours and unpleasant taste. For control of musty, fishy or earthy taste and odours, chlorine dioxide demonstrates the greatest removal

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IRON AND MANGANESE REMOVAL; Oxidation of manganese; Chlorine dioxide oxidises soluble manganese to insoluble manganese dioxide. It acts more quickly than chlorine or permanganate. Oxidation of iron; Chlorine dioxide rapidly oxidises iron (II) to iron (III) which precipitates as iron hydroxides.

COLOUR REMOVAL; besides all the applications described above, chlorine dioxide can solve the problems of coloured water.

WASTE WATERDISINFECTION

As a bactericide, the contact time for a complete treatment is five times lower with chlorine dioxide than with chlorine. It is mainly applied in protected areas like lagoons, bays and lakes.

INDUSTRIAL WATER For this application, chlorine dioxide is used a biocide. It has two main advantages.

(1) It allows for a very large and non – specific action against micro – organisms. It is non – selective.

(2) Its efficiency remains the same at alkaline pHs

Its applications for industrial water are;

(1) In cooling water systems (anti – fouling treatment)

(2) In the paper industry (anti – slime treatment)

(3) In oil field water floods (injection water treatment)

WATER ECONOMY OR FINANCIAL ARRANGEMENTS

What is the various money matters concerning water supply?

(a) Capital cost needed to start a water project

These could be;

(i) Government grant

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(ii) Bank loan

(iii) Community contributions

In Nigeria and China (i) is the most common while in France and Germany (ii) and (iii) are the most common.

(b) Operation or running cost

What are the sources of generating these funds?

(c) Revenue from water

(i) Adopt the flat rate method. This is adopted where people collect water from public taps.

(ii) Fixed monthly charges are used for areas with pipe borne water per flat or house

(iii) Using meters – the most efficient method

Advantages

(1) It is more economical in water consumption

(2) It facilitates water boards to get revenues commensurate with amount consumed

Disadvantages

(1) Economy in domestic uses may lead to health disadvantages. It is better to meter industrial/institutional consumers – not private homes

(2) Meters in Africa are imported and require skilful maintenance

(iv) Fixed property rating method is a very efficient method because it ensures that people will pay according to the value of their properties

In Ibadan we use the flat rate method – N20 per head whereas property rating is used mainly in Lagos.

Financing a Water Project

CASE STUDYThe provision of potable water to a community or a city is a money consuming proposition. It requires the identification and development of the source of supply, the provision and location of adequate storage, and the reticulation. It does not end with these capital investments. The various units have to be maintained, indicating an ever changing recurrent

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expenditure. The case study is an urban water project for a city with a population of 100,000 people at the time of construction of the project. The

Table 3Chemical Dosages

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Item Description Dosage

1 Alum 60mg/l

2 Hydrated Lime I 30mg/l

3 Activated Silicate 8mg/l

4 Hydrated Lime II 5mg/l

5 Chlorine 0.2mg/l

Table 4Cost of Water Chemicals Daily

Item Chemical Dosage

(tonnes)

Rate

(N1 tonne)

Daily Cost

(N)

1 Alum 3.46 1250 4,325

2 Ca(OH)2 2.02 1000 2,020

3 Activated Silicate 0.46 2500 1,150

4 Chlorine of Lime 0.05 3300 165

Total Cost per day 7,660

Table 5Recurrent Monthly Expenditure

Item Description Cost (N)

1 Water treatment chemicals 230,000

2 Diesel and engine oil 80,000

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3 Personnel 75,000

4 Incidental expenses

Total

5,000

390,000

CHAPTER 3

SEWAGE AND SEWRAGE SYSTEMS Sewage is defined as the water carried wastes of any community. Sewage can also be defined as the water supply of a city after it has been used. Most municipal sewage is about 99.9% water and 0.1% impurities. The sewage flows in sewers to the treatment plant or outfall. There are three types;

(i) Domestic sewage

(ii) Industrial sewage

(iii) Storm sewage

Domestic sewage is that part of community waste – water which arises from human houses and it is derived from the following sources.

(i) Faeces

(ii) Bathroom waste water

(iii) Washand basin and lavatory waste water

(iv) Kitchen waste water

(v) Laundry waste water

Industrial or trade sewage consists of all waste – water from industries. It may contain very toxic substances depending on the type of industries.

Storm water sewage; this is the portion of waste – water arising as a result of surface run – off from rain. In Africa the quantity of surface run – off is a considerable part of sewage to dispose of during the wet season.

Quality, Characteristics and Composition of Sewage

The main characteristics of sewage are;

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(i) Quantity

(ii) B.O.D.

(iii) C.O.D. and dissolved oxygen

(iv) Suspended solids

(v) Dissolved solids

(vi) Synthetic detergents

(vii) pH

(viii) Ammonia

(ix) Copper

(x) Phenol

(xi) Zinc

The characteristics of sewage vary so frequently that representative sample can only be obtained through composite sample. These characteristics vary so frequently with time that an attempt to present a single sample obtained once at a point in time from a source, as a representative sample often leads to misleading results.

The actual representative sample can only be obtained through composite sampling. This composition is obtained by taking the samples at various intervals of time during the day and pooling the quantities together in volume proportions to the flow.

A civil engineering consultant is trying to obtain a composite sample of waste water from a factory. Grab samples are obtained as shown in Table 6

TABLE 6Time (hours) 9.00 12.00 15.00 18.00

Waste water flow ml/s

Quantity of grab samples

(litres)

2 x 106

2

1 x 106

2

0.5 x 106

1

1.5 x 106

1.5

(i) Prepare a composite sample of one litre for laboratory

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(ii) If 1 in 50 dilution of the sample is to be prepared, what is the quantity of distilled water to be added to 10ml of the composite sample?

Solution;

Sum of waste water flow = (2 + 1.5 + 1 + 0.5) x 106 = 5 x 106 ml/s

For a composite sample of 1 litre we have the following proportions of waste water for each time;

At 9.00 = 2 x 106 ÷ 5 x 106

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