1,2,3_water supply and distribution.pdf

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KIGALI INSTITUTE OF SCIENCE AND TECHNOLOGY FACULTY OF ENGINEERING

DEPARTMENT OF CIVIL ENGINEERING AND ENVIRONMENTAL TECHNOLOGY

Water Supply and Distribution

by Charlotte UWIMPUHWE

Assistant lecturer CEET department

MODULE CODE AND TITLE: WEE 3313: Water Supply and Distribution

Brief Description of Aims and Content This course intends to provide the students with understanding of the importance of water supply systems in society and with a good understanding of water demand estimation, design of water supply system including storage capacity and appurtenances of water network.

It will cover five chapters Chapter 1: INTRODUCTION Chapter 2: WATER DEMAND AND QUANTITY ESTIMATION Chapter 3: SOURCES AND WATER STORAGE Chapter 4: STEAD FLOW IN PRESSURIZED NETWORK Chapter 5: DESIGN OF WATER TRANSPORT AND DISTRIBUTION SYSTEMS

References: WATER SUPPLY ENGINEERING by PUNMIA et all (1998). KIST library WATER SUPPLY AND WASTEWATER ENGINEERING by Raju B S N (2002). KIST library WATER SUPPLY: WATER DISTRIBUTION by UFC (unified facilities criteria) (2004). WATER SYSTEM DESIGN MANUAL by Department of Health, Olympia, WA. Available at: http://www.doh.wa.gov/ehp/dw.

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Chap 1. INTRODUCTION

1.1 WATER SUPPLY SYSTEM

A water supply system is a system for the collection, transmission, treatment, storage and distribution of water from source to consumers, for example, homes, commercial establishments, industry, irrigation facilities and public agencies for waterrelated activities (firefighting, street flushing and so forth).

1.2 WATER DISTRIBUTION SYSTEMS

Distribution system is used to describe collectively the facilities used to supply water from its source to the point of usage. The water distribution system starts where the main supply conduit from the treatment or source ends. The purpose of distribution system is to deliver water to consumer with appropriate quality, quantity and pressure.

Function of Water Distribution Water Distribution Systems main function is to supply treated water safe for human consumption and complying with increasingly stringent quality regulations.

Requirements of Good Distribution System 1. Water quality should not get deteriorated in the distribution pipes. 2. It should be capable of supplying water at all the intended places with sufficient pressure

head. 3. It should be capable of supplying the requisite amount of water during fire fighting. 4. The layout should be such that no consumer would be without water supply, during the repair

of any section of the system. 5. All the distribution pipes should be preferably laid one metre away or above the sewer lines. 6. It should be fairly water-tight as to keep losses due to leakage to the minimum.

Water distribution usually accounts for 40 to 70 % of the total cost of the water supply scheme. Therefore, it is important to make proper design and layout of the system.

Systems of distribution - Gravitational system

In a gravitational system, water flows from the source to the treatment plant and from treatment plant to the distribution area by gravity. This system is reliable and economical.

- Direct pumping

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In direct pumping, water is pumped from the source to the treatment plant and from treatment plant to the distribution area. The disadvantages are: the system is costly; water supply fails whenever power supply fails and pressure at the consumers end fluctuates a lot.

- Combined

Water flows from the source to the treatment plant and from treatment plant to the distribution area by gravity or pumping. The advantages are: pump can work at convenient schedule, uniform pressure can be maintained during water supply, and water from distribution reservoir can be used when pump fail and the quality of water would be improved due to detention of water in the elevated reservoir.

1.3 SOME DEFINITIONS

Piping

Water service pipe means a pipe on the property that conveys potable water from a water works or private water source to the inside of the building

Water mains Water mains can be divided into three categories:

i. Trunk mains

ii. Secondary mains iii. Service pipes

Trunk mains These carry water from a source of supply (reservoir, pumping station etc.) without supplying consumers directly.

Secondary main These are distribution mains fed from a trunk main and supplying the consumers connections.

Service pipes The branch supplies from the secondary mains that serve individual premises.

1.4 COMPONENTS OF WATER DISTRIBUTION

There are major Components of a Water Distribution System including Pipes, Valves, pumps, Flush Hydrants, fittings, water meter, storage tanks and reservoirs. Each component plays a role in ensuring adequate water service and in maintaining quality water.

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Because the pipes and valves are buried, a detailed map is needed to gain quick access to the system for maintenance and repairs.

Pipes

The systems of pipes that transport water from the source (such as a treatment plant) to the customer are often categorized from largest to smallest as transmission or trunk mains, distribution mains, service lines, and premise plumbing.

Transmission or trunk mains usually convey large amounts of water over long distances such as from a treatment facility to a storage tank within the distribution system.

Distribution mains are typically smaller in diameter than the transmission mains and generally follow the city streets.

Service lines carry water from the distribution main to the building or property being served. Service lines can be of any size depending on how much water is required to serve a particular customer and are sized so that the utilitys design pressure is maintained at the customers property for the desired flows.

Premise plumbing refers to the piping within a building or home that distributes water to the point of use. In premise plumbing the pipe diameters are usually comparatively small, leading to a greater surface-to-volume ratio than in other distribution system pipes.

Valves

The two types of valves generally utilized in a water distribution system are isolation valves (or stop or shutoff valves) and control valves.

Isolation valves (typically either gate valves or butterfly valves) are used to isolate sections for maintenance and repair and are located so that the areas isolated will cause a minimum of inconvenience to other service areas. Maintenance of the valves is one of the major activities carried out by a utility.

Many utilities have a regular valve-turning program in which a percentage of the valves are opened and closed on a regular basis. It is desirable to turn each valve in the system at least once per year. The implementation of such a program ensures that water can be shut off or diverted when needed, especially during an emergency, and that valves have not been inadvertently closed.

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Control valves are used to control the flow or pressure in a distribution system. They are normally sized based on the desired maximum and minimum flow rates, the upstream and downstream pressure differentials, and the flow velocities.

Typical types of control valves include pressure-reducing, pressure-sustaining, and pressure-relief valves; flow-control valves; throttling valves; float valves; and check valves. Most valves are either steel or cast iron, although those found in premise plumbing to allow for easy shut-off in the event of repairs are usually brass. They exist throughout the distribution system and are more widely spaced in the transmission mains compared to the smaller-diameter pipes.

Other appurtenances in a water system include blow-off and air-release/vacuum valves, which are used to flush water mains and release entrained air. On transmission mains, blow-off valves are typically located at every low point, and an air release/vacuum valve at every high point on the main. Blow-off valves are sometimes located near dead ends where water can stagnate or where rust and other debris can accumulate. Care must be taken at these locations to prevent unprotected connections to sanitary or storm sewers.

Pumps

Pumps are used to impart energy to the water in order to boost it to higher elevations or to increase pressure. Pumps are typically made from steel or cast iron. Most pumps used in distribution systems are centrifugal in nature, in that water from an intake pipe enters the pump through the action of a spinning impeller where it is discharged outward between vanes and into the discharge piping. The cost of power for pumping constitutes one of the major operating costs for a water supply.

Flush hydrants Flush hydrants are the most visible part of the water distribution system. They must be at the end of all lines to remove accumulated corrosion products from dead-ends. Flush hydrants should also be installed throughout the system to provide for periodic flushing to maintain high water quality.

Hydrants are also primarily part of the fire fighting aspect of a water system. Proper design, spacing, and maintenance are needed to insure an adequate flow to satisfy fire-fighting requirements. Fire hydrants are typically exercised and tested annually by water utility or fire department personnel.

Fire flow tests are conducted periodically to satisfy the requirements of the Insurance Services Office or as part of a water distribution system calibration program (ISO, 1980). Fire hydrants are installed in areas that are easily accessible by fire fighters and are not obstacles to pedestrians

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and vehicles. In addition to being used for fire fighting, hydrants are also for routine flushing programs, emergency flushing, preventive flushing, testing and corrective action, and for street cleaning and construction projects (AWWA, 1986).

Fittings

Pipes come in all sorts of sizes, sometimes they need to connect with each other. While the connections are sometimes straight, at other times they may take a turn in direction. For those situations, pipe fittings help to connect pipes of different sizes and diameters and help lines to change direction. Pipe fittings, like pipes themselves, come in different types of material. They can be made of strainless steel, copper, or PVC.

Meter A water meter A water meter records the amount of water being used in your home for billing purpose. Your water company checks your water meter to calculate how much to charge you.

A velocity-type meter A velocity-type meter measures the velocity of flow through a meter of a known internal capacity. The speed of the flow can then be converted into volume of flow for usage. There are several types of meters:

jet meters (single-jet and multi-jet), turbine meters, propeller meters, and mag meters.

Storage Tanks and Reservoirs

Storage tanks and reservoirs are used to provide storage capacity to meet fluctuations in demand (or shave off peaks), to provide reserve supply for fire-fighting use and emergency needs, to stabilize pressures in the distribution system, to increase operating convenience and provide flexibility in pumping, to provide water during source or pump failures, and to blend different water sources.

The recommended location of a storage tank is just beyond the center of demand in the service area (AWWA, 1998). Elevated tanks are used most frequently, but other types of tanks and reservoirs include in-ground tanks and open or closed reservoirs.

Common tank materials include concrete and steel. An issue that has drawn a great deal of interest is the problem of low water turnover in these facilities resulting in long detention times.

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Much of the water volume in storage tanks is dedicated to fire protection, and unless utilities properly manage their tanks to control water quality, there can be problems attributable to both water aging and inadequate water mixing. Excessive water age can be conducive to depletion of the disinfectant residual, leading to biofilm growth, other biological changes in the water including nitrification, and the emergence of taste and odor problems. Improper mixing can lead to stratification and large stagnant (dead) zones within the bulk water volume that has depleted disinfectant residual.

1.5 TYPES OF DISTRIBUTION SCHEMES There are basically two types of distribution system:

i. Branched system ii. Looped network system

i. Branched systems: these are only used for small capacity community supplies delivering the mostly through public standpipes and having few house connections, if any

Branched systems have the advantage that their design is straight-forward. The direction of the water flow in all pipes and the flow rate can be readily determined

ii. A looped network: This usually has a ring of mains to which the secondary pipes are connected.

In large (urban) distribution systems, the secondary pipes are usually all inter-connected which requires many valves and special parts. For small distribution systems, over-crossing secondary pipes that are not inter-connected may be advantageous with a considerable cost saving.

For larger distribution systems looped network grids are more common. The figure1.1 below shows typical branched systems ad looped networks.

Figure 1.1: Typical branched distribution system and looped distribution system

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Chapter 2 WATER DEMAND AND QUANTITY ESTIMATION 2.1 WATER DEMAND

This is the volume of water, which has to be put into a supply and distribution system to satisfy the requirements of consumers plus leakage and other waste, which may be incurred in the process. The total demand in each category of uses; domestic, commercial and public/institutional, is made up of several water use activities such as drinking, washing, gardening, etc.

Major Determinants of Water Demand

SN Categories Determinants

1

A. Domestic Demand

1. Number and size of households

2. Family income and income distribution

3. Costs of water presently used

4. Cost of future water used

5. Connection charges

6. Availability and quality of service

7. Cost and availability of water using devices

8. Availability of alternative water sources

9. Present water consumption

10. Legal requirements

11. Population density

12. Cultural influences

B. Commercial Demand

1. Sales or value added of non-subsistence commercial

sector

2. Costs and volume of water presently used

3. Price of future water used

4. Connection charges

5. Costs of water using appliances

6. Quality and reliability of service

7. Working hours of various types of commercial

establishments


C. Industrial Demand

1. Present and future costs of water

2. Type of industry and water use intensity

3. Relative price of alternative sources

4. Quality and reliability of supply

5. Costs of treatment and disposal of waste water


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D. Agricultural Demand (for [non]

piped water supply)


2. Availability of other sources

3. Quality and reliability of supply

4. Supply cost of alternative water systems

5. Number of cattle


E. Public Services Demand


2. Per capita revenue of local governments

3. Number and size of public schools, hospitals etc.


2.2 URBAN AND RURAL WATER DEMAND

The factors which determine domestic demand may differ between the urban and the rural sector. In the rural sector, special attention needs to be given to such things as the availability of alternative water sources, the income and ability to pay for or contribute to the project facilities and their management, the choice of technology and the use of water for other purposes like agriculture (e.g. livestock or vegetable growing) and, the ability to operate and maintain facilities.

In the rural context, the assessment of effective demand will have to be carried out in close consultation with the local population, and attention needs to be given to issues such as community participation and hygiene education.

Forecasting Demand in Urban areas

There are a number of factors that affect urban water demand and that should be taken into account when forecasting urban water demand and use. These include:

Demographic factors (e.g. population growth, migration, overspill from larger cities) Socio-economic factors (e.g. standard of housing and living , employment opportunities); Climate; Type of sanitation; Extent of distribution leakage which is dependent upon the age of and pressure of the

system; Consumer wastage; Ratio of institutional and business demand and use to domestic consumption Availability of non-piped supplies; Tariff levels

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The factors which determine demand will, to a large extent, define the need for information. The project analyst will have to determine the key factors which need to be considered into the analysis and design of the project.

2.3 EFFECTIVE WATER DEMAND

The effective demand for water is the quantity of water demanded of a given quality at a specified price. The analysis of demand for water, including realistically forecasting future levels of demand, is an important and critical step in the economic analysis of water supply projects.

The results of demand analysis will enable the project team to: (i) determine the service level(s) to be provided; (ii) determine the size and timing of investments; (iii) estimate the financial and economic benefits of the project; and (iv) assess the ability and willingness to pay of the project beneficiaries.

Furthermore, the surveys carried out during the demand assessment will provide data on cost savings, willingness to pay, income and other data needed for economic analysis.

It is useful to note the difference between effective demand for water and actual consumption of water. Water consumption is the actual quantity of water consumed whereas effective demand relates that quantity to the price of water.

2.4 WATER QUANTITY ESTIMATION

The quantity of water required for municipal uses for which the water supply scheme has to be designed requires following data:

1. Water consumption rate (Per Capita Demand in litres per day per head) 2. Population to be served.

Quantity = per capita demand x Population

2.4.1 Water Consumption Rate

It is very difficult to precisely assess the quantity of water demanded by the public, since there are many variable factors affecting water consumption. The various types of water demands, which a city may have, may be broken into following classes:

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Water Consumption for Various Purposes: Types of Consumption Normal Range

(lit/capita/day) Average %

1 Domestic Consumption 65-300 160 35 2 Industrial and Commercial

Demand 45-450 135 30

3 Public Uses including Fire Demand 20-90 45 10

4 Losses and Waste 45-150 62 25

Fire Fighting Demand:

The per capita fire demand is very less on an average basis but the rate at which the water is required is very large. The rate of fire demand is sometimes treated as a function of population.

Factors affecting per capita demand:

a. Size of the city: Per capita demand for big cities is generally large as compared to that for smaller towns as big cities have sewered houses.

b. Presence of industries. c. Climatic conditions. d. Habits of people and their economic status. e. Quality of water: If water is aesthetically $ medically safe, the consumption will increase

as people will not resort to private wells, etc. f. Pressure in the distribution system. g. Efficiency of water works administration: Leaks in water mains and services; and

unauthorized use of water can be kept to a minimum by surveys. h. Cost of water. i. Policy of metering and charging method: Water tax is charged in two different ways: on

the basis of meter reading and on the basis of certain fixed monthly rate.

Fluctuations in Rate of Demand

Average Daily Per Capita Demand = Quantity Required in 12 Months/ (365 x Population)

If this average demand is supplied at all the times, it will not be sufficient to meet the fluctuations.

Seasonal variation: The demand peaks during summer. Firebreak outs are generally more in summer, increasing demand. So, there is seasonal variation.

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Daily variation depends on the activity. People draw out more water on Sundays and Festival days, thus increasing demand on these days.

Hourly variations are very important as they have a wide range. During active household working hours i.e. from six to ten in the morning and four to eight in the evening, the bulk of the daily requirement is taken. During other hours the requirement is negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be supplied during short duration, necessitating the need for a maximum rate of hourly supply.

So, an adequate quantity of water must be available to meet the peak demand. To meet all the fluctuations, the supply pipes, service reservoirs and distribution pipes must be properly proportioned. The water is supplied by pumping directly and the pumps and distribution system must be designed to meet the peak demand.

The effect of monthly variation influences the design of storage reservoirs and the hourly variations influences the design of pumps and service reservoirs. As the population decreases, the fluctuation rate increases.

Maximum daily demand = 1.8 x average daily demand Maximum hourly demand of maximum day i.e. Peak demand = 1.5 x average hourly demand = 1.5 x Maximum daily demand/24 = 1.5 x (1.8 x average daily demand)/24 = 2.7 x average daily demand/24 = 2.7 x annual average hourly demand

2.4.2 Design Periods

This quantity should be worked out with due provision for the estimated requirements of the future. The future period for which a provision is made in the water supply scheme is known as the design period.

Design period is estimated based on the following:

Useful life of the component; considering obsolescence, wear, tear, etc. Expandability aspect. Anticipated rate of growth of population, including industrial, commercial developments

& migration-immigration. Available resources. Performance of the system during initial period.

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Generally water supply project are designed for a design period of 20 to 40 years, after their completion. The time lay between the design and completion should not be more than 2 years. In some specific components of the project, the design period may be modified. Different segments of water treatment and distribution systems may be approximately designed for differing periods of time using differing capacity criteria, so that expenditure far ahead of utility is avoided. The table 3.1 gives the design periods far various components of a water supply project. Design period for project components

SN Components Design period (years)

1 Storage by dams 50 2 Infiltration works 30 3 Pump sets

(i) All prime movers except electric motors 30 (ii) Electric motors and pumps 15

4 Water treatment units 15 5 Pipe connections to the several treatment

units and other small appurtenances 30

6 Raw water and clear water conveying mains

30

7 Clear water reservoirs at the head works, balancing tanks and service reservoirs(over head of ground level)

15

8 Distribution system 30

2.5 POPULATION FORECASTING METHODS

The various methods adopted for estimating future populations are given below. The particular method to be adopted for a particular case or for a particular city depends largely on the factors discussed in the methods, and the selection is left to the discrection and intelligence of the designer.

1. Arithmetic Increase Method 2. Geometric Increase Method 3. Incremental Increase Method 4. Decreasing Rate of Growth Method 5. Simple Graphical Method 6. Comparative Graphical Method 7. Ratio Method 8. Logistic Curve Method

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Arithmetic Increase Method

This method is based on the assumption that the population increases at a constant rate; i.e. dP/dt=constant=k; Pt= P0+kt. Where Pt = Future population at the end of t time decade Po = Present population k = Average increment for a decade This method is most applicable to large and established cities.

Geometric Increase Method

This method is based on the assumption that percentage growth rate is constant i.e. dP/dt=kP; lnP= lnP0+kt. This method must be used with caution, for when applied it may produce too large results for rapidly grown cities in comparatively short time. This would apply to cities with unlimited scope of expansion. As cities grow large, there is a tendency to decrease in the rate of growth.

Incremental Increase Method

Growth rate is assumed to be progressively increasing or decreasing, depending upon whether the average of the incremental increases in the past is positive or negative. The population for a future decade is worked out by adding the mean arithmetic increase to the last known population as in the arithmetic increase method and to this is added the average of incremental increases, once for first decade, twice for second and so on.

Decreasing Rate of Growth Method

In this method, the average decrease in the percentage increase is worked out, and is then subtracted from the latest percentage increase to get the percentage increase of next decade.

Simple Graphical Method

In this method, a graph is plotted from the available data, between time and population. The curve is then smoothly extended up to the desired year. This method gives very approximate results and should be used along with other forecasting methods.

Comparative Graphical Method

In this method, the cities having conditions and characteristics similar to the city whose future population is to be estimated are selected. It is then assumed that the city under consideration will develop, as the selected similar cities have developed in the past.

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Ratio Method

In this method, the local population and the country's population for the last four to five decades is obtained from the census records. The ratios of the local population to national population are then worked out for these decades. A graph is then plotted between time and these ratios, and extended up to the design period to extrapolate the ratio corresponding to future design year. This ratio is then multiplied by the expected national population at the end of the design period, so as to obtain the required city's future population. Drawbacks:

1. Depends on accuracy of national population estimate. 2. Does not consider the abnormal or special conditions which can lead to population shifts

from one city to another.

Logistic Curve Method The three factors responsible for changes in population are : (i) Births, (ii) Deaths and (iii) Migrations.

Logistic curve method is based on the hypothesis that when these varying influences do not produce extraordinary changes, the population would probably follow the growth curve characteristics of living things within limited space and with limited economic opportunity. The curve is S-shaped and is known as logistic curve.

2.6 ESTIMATING PRESENT AND FUTURE POPULATION

A starting point in demand forecasting is determining the size and future growth of the population in the project area.

(i) The first step is to estimate the size of the existing population. In most cases, different estimates are available from different secondary sources. Often, the survey team will have to make its own estimate based on the different figures obtained.

(ii) The second step is to determine the service or project area (the area which will be covered by the project) and the number of people living there. The most important consideration in this respect is the expressed interest from potential customers. Furthermore, the service area will have to be determined in consultation with the project engineer, the municipal authorities and/or the water enterprise. Technical, economic and political considerations will play a role.

(iii)The third step is to estimate future population growth in the project area. This estimate will be based on available data about national, provincial or local population growth.

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It should also take into account the effects of urban and/or regional development plans and the effects of migration from rural to urban areas.

(iv)Finally, the project has to determine which level of coverage it intends to achieve. Often, project objectives contain statements such as: provide safe water supply to 75 percent of the population of town x. In this statement, it is assumed that the town area and service or project area are the same.

2.7 UNACCOUNTED FOR WATER

Normally a certain percentage of the water supplied to consumers is lost due to technical losses (physical leakages) and/or nontechnical losses (unmetered consumption, illegal connections). This so-called Unaccounted For Water (UFW) is normally expressed as a percentage of the volume of distributed water. In 1995, the average percentage of UFW in 50 Asian cities was 35 percent of water distributed (Water Utilities Data Book for the Asian and Pacific Region, 1997). This high level of UFW illustrates the inefficient use of existing water resources and is of great concern to the management of water utilities. A reduction of the UFW rate is therefore normally a specific objective in the formulation of new WSPs.

It will be necessary to include a realistic estimate of UFW in a demand estimate for a WSP. This percentage will naturally relate to the existing UFW rate and should be based on realistic targets for UFW reduction.

It is also necessary to estimate the proportion of technical and nontechnical losses in UFW because, in economic analysis, nontechnical losses (which add to the welfare of the population served) are included in the assessment of economic benefits. This assessment is often difficult and the project analyst will have to make a reasonable estimate in consultation with water enterprise staff. The percentage reduction in UFW should be set realistically in consultation with the project engineers (for technical losses) and utility managers (for nontechnical losses). A reduction in UFW will normally require a sizable portion of the project investment cost.

2.8 PEAK FACTOR

The demand for water will very seldom be a constant flow. Demand for water may vary from one season to another and throughout the day. Daily demand will show variations and there will be peak hours during the day, depending on local conditions. These seasonal and daily peak factors will influence the size of the total installed capacity. These are technical parameters and will be determined by project engineers.

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The demand for water is seldom constant. Rather it varies, albeit seasonally, daily and/or based on other predictable demand characteristics. At different times of the year the demand for water may be higher than others due to factors such as heat which may increase the demand for water for hygiene, drinking and other purposes.

At different times of the day the demand for water may be higher than others, based on peoples and industries needs and patterns of consumption. At other periods, the stock and flow requirements of the system may be impacted by other predictable events, such as an industrial activity. These seasonal, daily and other predictable demand factors are known as peak factors.

In determining the total installed capacity of a planned project, the technical staff needs to consider both these peak demand factors and the projected growth in demand. Failure to do so could result in the project becoming supply constrained and unable to fully meet the demand requirements of its targeted beneficiaries from its outset.

Data about daily and seasonal water consumption patterns will normally be available from secondary data or may be collected in the household survey.

2.9 WATER CONSERVATION

Water conservation is a critical component of meeting existing and future water needs, including instream and out-of-stream uses. Water conservation measures include anything that reduces the amount of water needed to meet water supply uses. Conservation measures entail changing practices and improving system efficiencies to reduce water demand, preserve natural resources and inchoate rights, and accommodate future development opportunities.

Water conservation best management practices that can reduce demand include reducing irrigation, changing landscaping materials, minimizing leaks and systems inefficiencies, and reusing or recycling water. An important finding of this Technical Assessment is that the use of water for irrigation, including commercial and residential landscaping, far exceeds water used for other purposes. Therefore, conservation measures targeted to reducing water for landscaping and irrigation are likely to produce significant water savings.

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Y3WEE & Y4 CE, S1-2011

Chap3 SOURCES AND WATER STORAGE

3.1 SOURCES OF WATER

Water may be obtained from natural sources. Water sources relate to the basic phases of the hydrological cycle of rain, surface and groundwater.

The various sources of water can be classified into four categories: 1. Surface sources, such as

a. Ponds and lakes; b. Streams and rivers; c. Storage reservoirs;

2. Rain water 3. Sub-surface sources or underground sources, such as

a. Springs; b. Infiltration wells ; and c. Wells and Tube-wells.

4. Water obtaining from reclamation

Each source of water has a unique set of contaminants; groundwater stores pesticide chemicals and nitrate while surface water contains most bacteria and other microorganisms. Because of the interconnection of groundwater and surface water, these contaminants may be shared between the two sources.

The treatment processes may be simply done by boiling the water before use for household supplies. However, municipal supplies require one or more treatment processes depending upon the impurities found in the water.

3.1.1 Surface water

Streams, rivers and lakes are the major sources of surface water. Water in these sources originates partly from groundwater outflows and partly from rainwater which flows over the terrestrial areas into the surface water bodies.

Usually in surface water bodies, the dissolved mineral particles will remain unchanged while the organic impurities are degraded by chemical and microbial action. In slow-flowing or impounded surface waters sedimentation of suspended solids occurs naturally.

Although clear water from rivers and lakes requires no treatment, on taking into account the risk of incidental contamination, it is better to practice chlorination.

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Unpolluted surface water of low turbidity may be purified by slow sand filtration alone. Alternatively, rapid sand filtration followed by chlorination can be practiced. When the turbidity of water is too high, in addition to slow sand filtration a pre-treatment like sedimentation, rapid sand filtration, or both can be used. When colloidal particles are present sedimentation can be improved by chemical coagulation and flocculation. Thus, depending upon the impurities present in surface waters the required treatment process may vary.

3.1.2 Ground water

Groundwater refers to any subsurface water that occurs beneath the water table in soil (Rail, 2000). Scientists estimate that groundwater makes up 95% of all freshwater available for drinking.

The ground water can be tapped from different sources including natural springs, wells and bore holes, infiltration galleries etc. Wells and springs constitute groundwater supplies. Groundwater mostly originates from infiltrated rainwater which after reaching the aquifer flows through the underground.

Groundwater is generally stored in aqueducts, underground layers of porous rocks that are saturated with water.

These aqueducts receive water as soil becomes saturated with precipitation or through stream and river runoff. As the aqueducts exceed their capacity for water storage, they will bleed water back into streams or rivers.

A little contamination of groundwater occurs from organic and inorganic soil particles, animal and plant debris, fertilizers, pesticides, microorganisms, etc. as it flows through the soil layers. In spite of this contamination, infiltration causes partial removal of suspended particles including microorganisms. Organic substances are also degraded by oxidation. Partial removal of microorganisms occurs by the death of cells due to lack of nutrients.

Thus, properly withdrawn groundwater will be free from turbidity and pathogenic microorganisms. It is important to select the location of groundwater supply at a safe distance from other sources of contamination like septic tanks. If done so, groundwater will be of high quality and can be used directly without any treatment.

Sand filtration is practiced for the removal of fine particles including microorganisms. By sand filtration about 99 percent removal of bacteria can be achieved. Chlorination is employed as the final treatment process before distribution.

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3.1.3 Rainwater

Rainwater run off from roofs can be collected and stored for domestic use. Rainwater will be of high quality and the only possible source of contamination is airborne microorganisms that too will be present in very low numbers.

During collection the first flush of water of a day has to be diverted away from the storage tank since it may contain dust, bird droppings etc. It is important to protect the stored rainwater from contamination and from the entry of mosquitoes. This method is mainly used for household supplies and hence boiling the water is the only treatment.

Rain water may be collected:

i. From roofs of houses and dwellings: Water is stored in small underground tank or cistern, for small individual supplies (see figure 2.2).

ii. From prepared catchments: The surface of catchments is made impervious by suitable lining material, and suitable slope is given so that water is stored in moderate size reservoirs. This water is used for communal supplies, mostly for drinking purposes.

Figure 2.1: (a)Rainwater collected from roof tops (b)From prepared catchments 3.1.4 Water obtaining from reclamation

Oceans and wastewater may be treated suitably and be reused.

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3.2 WATER STORAGE

3.2.1 TYPES, FUNCTIONS AND USES OF WATER STORAGE

A reservoir is a basin filled with water that is often used by humans for different purposes and uses. They are built extensively in regions of water scarcity. The main need of storage reservoirs is for greater quantity of continuously available and easily accessible quantity of water of good quality primarily for its value for meeting the water needs of the population, agriculture, industry and etc. Reservoirs are made by constructing a dam across the rivers and streams, and reservoirs are behind the dams. Dams and reservoirs are the most important elements multipurpose river basin development. They require (dams and reservoirs) very careful planning, design and operation. A number of problems arise in design, construction and operation ie selection of site, the relative merits of different types of dams, storage capacity and optimum yield and co-ordinated use of storage for different purposes.

Major function and uses of storage reservoirs

Drinking water Irrigation Flood control Fish production and production of other useful organisms (Aquatic production,

fisheries and aquaculture) Mining Fire & ice ponds Energy (hydropower generation) Industry Low energy purifiers Recreational Conservation and biodiversity Training and education, etc

Types of reservoirs Depending on purpose served, reservoirs may be classified as follows:

Storage or conservation reservoirs Flood protection reservoir Multipurpose reservoirs dams Intakes Distribution reservoirs ( will be more discussed in this chapter)

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i. Storage or conservation reservoirs: Are primarily used for irrigation, hydroelectric development, domestic and industrial supplies. A river does not carry the same quantity of water throughout the year. A storage reservoir is constructed to store the excess water during the period of large supplies, and release it gradually as and when it is needed.

ii. Flood protection reservoirs: Are those which store water during flood and release it gradually at a safe rate when the flood reduces.

iii. Multipurpose reservoir: Is the one which serves more than one purpose.

iv. Dams. A dam is a hydraulic structure constructed across a river to store water on its upstream side. It is an impervious or fairly impervious barrier put across a natural stream so that a reservoir is formed.

Due to the construction of the dam, water level in the river at its upstream side is very much increased, and a large area may be submerged depending upon the water spread of the reservoir so formed.

Dams may be classified into different categories, depending upon the purpose or basis of the classification. The following table gives a summary of various types of dams.

Types of dams

A storage dam: Is the most common type of dam, and is constructed to impound water to its upstream side during periods of excess supply in the river.

TYPES OF DAMS

Basis of classification Types Common examples

Classification according to use Storage dam Gravity dam, earth dam, rockfill

dam, arch dam, etc

Diversion dam Weir, barrage

Detention dam Dike, water spreading dam,

debris dam

Classification by hydraulic design Overflow dam Spillway

Non-overflow type Gravity dam, earth dam, rockfill

dam

Classification by materials Rigid dam Gravity dam, arch dam, buttress

dam, steel dam, timber dam

Non-rigid dam Earth dam, rockfill

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A diversion dam: It simply raises water level slightly in the river, and thus provides head for carrying or diverting water into ditches, canals, or other conveyance systems to the place of use.

A detention dam: Is constructed to sore water during floods and release it gradually at a safe rate, when the flood recedes.

A non-overflow dam: Is the one in which the top of the dam is kept at a higher elevation than the maximum expected high water level.

An overflow dam: However, is designed to carry surplus discharge (including floods) over its crest.

Rigid dams: Are those which are constructed of rigid material such as masonry, concrete, steel or timber.

Non-rigid dams: are those which are constructed of non-rigid materials such as earth and rockfill.

v. Intakes: are the structures used for admitting water from the surface sources and conveying it further to the treatment plant. (More information will be discussed in chapter 5)

vi. Distribution reservoir: is a small storage reservoir used for water supply in a city or industrial area. It account for a varying rate of water during the day. Such a distribution reservoir permits the pumping plants and water treatment works to operate at a constant rate. The varying demand rate, exceeding the constant pumping rate is met from the distribution reservoir.

3.2.2 DISTRIBUTION RESERVOIRS

Distribution reservoirs, also called service reservoirs, are the storage reservoirs, which store the treated water for supplying water during emergencies (such as during fires, repairs, etc.) and also to help in absorbing the hourly fluctuations in the normal water demand.

Functions of Distribution Reservoirs: To equalize the variation in hourly demand of water by the consumers to a uniform rate of

supply from the source either by gravity or pumping, To maintain the desired minimum residual pressure in the distribution system, To provide the required contact time for the disinfectant added in order to achieve

effective disinfection, and To facilitate carrying out repairs either to the pumping main or to pump-set without

interruption to the supply of water.

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Types of distribution reservoirs Surface reservoirs Elevated reservoirs Stand pipes

i. Surface reservoirs: are made mostly of masonry or concrete. Common practice is to line surface reservoir with concrete, gunite, asphalt or asphalt membrane to check the leakage of water. It is usual practice to construct surface reservoir in two or more compartments so that one unit can be cleaned or repaired while other units are in operation. It is advisable to construct the reservoir at high points so that gravity supply can be done directly.

ii. Elevated reservoirs: are commonly known as overhead tanks. They may be constructed of stone masonry, reinforced concrete or steel. All elevated reservoirs are invariably provided with top cover, ladder, and man-holes for inspection and cleaning purposes.

iii. Standpipes: are normally employed where the construction of a surface reservoir would not provide sufficient head. A standpipe is essentially a tall cylindrical tank whose storage volume includes an upper portion (the useful storage), which is above the entrance to the discharge pipe and a lower portion (supporting storage) which acts only to support the useful storage and provide the required head.

3.2.3 DESIGN OF DISTRIBUTION/STORAGE RESERVOIRS

1) Number of Storage Reservoirs Urban water systems should have at least one elevated tank for each of the areas. Two tanks (or a tank with two compartments) are desirable to improve reliability and pump control during times when one tank is out of service for inspection, cleaning, painting, or other maintenance. The height of an elevated tank determines the maximum water pressure available in the part of the distribution system connected to the tank. The town is divided into number of zones with independent storage reservoirs to facilitate effective and equitable water distribution.

2) Location of the Reservoirs The location of the storage reservoir is of importance for regulating the pressure in the water distribution system. The storage reservoir is generally located at the highest point and as far as possible at the centre of the distribution area.

The topography of a water distribution area is an important consideration in system design and type of storage facilities to be incorporated into a water supply system. In some cases, ground-level storage systems can be sited at higher elevations (on hills), allowing for gravity supply to all or portions of a distribution area or pressure zone.

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Storage tanks at higher elevation can also take advantage of topographic features to reduce height requirements of ERs and provide wider pressure zone coverage.

3) Storage Capacity of Distribution Reservoirs

The total storage capacity of a distribution reservoir is the summation of:

i. Balancing Storage: The quantity of water required to be stored in the reservoir for equalizing or balancing fluctuating demand against constant supply is known as the balancing storage (or worked out by mass curve method

ii. Breakdown Storage: storage preserved in order to tide over the emergencies posed by the failure of pumps, electricity, or any other mechanism driving the pumps. A value of about 25% of the total storage capacity of reservoirs, or 1.5 to 2 times of the averageconsidered as enough provision for accounting this storage.

iii. Fire Storage: The third component of the total reservoir storage is the fire storage. This provision takes care of the requirements of water for extinguishing fires. A provto 4 l per person per day is sufficient to meet the requirement.

The total reservoir storage can finally be worked out by adding all the three storages.

R= aD + bD + 10/24 (D + F Where R = total storage capacity (million litres)D = average domestic demand for maximum month (m.l.d)

Storage tanks at higher elevation can also take advantage of topographic features to reduce height requirements of ERs and provide wider pressure zone coverage.

Distribution Reservoirs


The quantity of water required to be stored in the reservoir for or balancing fluctuating demand against constant supply is known as the

balancing storage (or equalizing or operating storage). The balance storage can be worked out by mass curve method.

The breakdown storage or often called emergencystorage preserved in order to tide over the emergencies posed by the failure of pumps, electricity, or any other mechanism driving the pumps. A value of about 25% of the total storage capacity of reservoirs, or 1.5 to 2 times of the average hourly supply, may be considered as enough provision for accounting this storage.

The third component of the total reservoir storage is the fire storage. This provision takes care of the requirements of water for extinguishing fires. A provto 4 l per person per day is sufficient to meet the requirement.


R= aD + bD + 10/24 (D + F - P)

R = total storage capacity (million litres) average domestic demand for maximum month (m.l.d)

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Storage tanks at higher elevation can also take advantage of topographic features to reduce


The quantity of water required to be stored in the reservoir for or balancing fluctuating demand against constant supply is known as the

or operating storage). The balance storage can be

The breakdown storage or often called emergency storage is the storage preserved in order to tide over the emergencies posed by the failure of pumps, electricity, or any other mechanism driving the pumps. A value of about 25% of the total

hourly supply, may be

The third component of the total reservoir storage is the fire storage. This provision takes care of the requirements of water for extinguishing fires. A provision of 1


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F = fire demand (m.l.d) P = capacity of pump (m.l.d) a,b coefficients which may be taken as 0.2 and 0.1 respectively.

When a storage or distribution reservoir is to be designed for the purpose of balancing or equalizing the flow, its storage capacity can be determined by two methods:

Hydrograph method Mass curve method.

For more information refer to page 512-522 and pages 36-46: WATER SUPPLY ENGINEERING book BY B.C PUNMIA, 1995.

The volume of water storage needed depends upon the following:

Maximum rate of peak hourly demand, Maximum rate of pumping, and Duration and actual schedule of pumping and distribution in a day.

Volume of storage to be provided in the reservoirs and the rate of pumping are so fixed to permit the pumping at average rate during the period of maximum demand. For electrically operated pumps, the maximum duration of pumping is limited to 20 hours in a day leaving 4 hours rest during lean demand period. However, two shifts of 8 hours each totalling 16 hours pumping is commonly adopted. In very large water supply systems three shifts of 8 hours pumping, totalling 24 hours with dedicated electrical feeder line for power supply is practiced considering the economics of the transmission main. Diesel generators are also provided at the pumping station as redundant power to facilitate uninterrupted operation of pumps.

The general norms for volume of storage required with reference to duration of supply from the source are listed in Table below

Volume of Storage Required Duration of Supply or Pumping Volume of Storage as percentage of

daily requirement Above 16 to 24 hours 20 to 25% Above 12 to 16 hours 33.33% Above 8 to 12 hours 50% Less than 8 hours 100%

The optimum volume of water treatment storage in the reservoirs can be determined from the duration of supply and the actual demand during different time period of distribution using the mass diagram method.

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The day is divided into number of periods of different rate of demands. For each of the durations the demand, the supply, cumulative demand, cumulative supply and cumulative deficits are worked out. The volume of water treatment storage required is the absolute sum of the maximum positive and negative cumulative deficits. This volume of water treatment storage should be provided in the service reservoir (as shown in Illustrative example 3.1).

Illustrative example 3.1 Given hourly demand for the maximum day and assuming a 24 hour pumping/supply at uniform rate (Column1, 2 and 3). Determine the storage capacity of balancing or equalizing reservoir.

Time

Hourly demand

(l)

Hourly supply

(l)

Cumulative demand

(l)

Cumulative supply

(l)

Deficits (-) Excess demand (l)

Deficits(+) Excess supply (l)

1 2 3 4 5 6 =4 - 5 7= 5-4 12 0

1:00 AM 480000 896250 480000 896250 416250 2 540000 896250 1020000 1792500 772500 3 630000 896250 1650000 2688750 1038750 4 660000 896250 2310000 3585000 1275000 5 720000 896250 3030000 4481250 1451250 6 900000 896250 3930000 5377500 1447500 7 1020000 896250 4950000 6273750 1323750 8 1230000 896250 6180000 7170000 990000 9 1320000 896250 7500000 8066250 566250

10 1290000 896250 8790000 8962500 172500 11 1200000 896250 9990000 9858750 131250 12 1080000 896250 11070000 10755000 315000

1:00 PM 960000 896250 12030000 11651250 378750 2 900000 896250 12930000 12547500 382500 3 900000 896250 13830000 13443750 386250 4 930000 896250 14760000 14340000 420000 5 960000 896250 15720000 15236250 483750 6 990000 896250 16710000 16132500 577500 7 1020000 896250 17730000 17028750 701250 8 1020000 896250 18750000 17925000 825000 9 900000 896250 19650000 18821250 828750

10 720000 896250 20370000 19717500 652500 11 600000 896250 20970000 20613750 356250 12 540000 896250 21510000 21510000

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Max excess demand Ed = 828750 l Max excess pumping/supply Ep = 1451250 Storage S = Ed +Ep = 828750 + 1451250 = 2280000 liters.

Illustrative example 3.2

Design service reservoir for 30000 people with 150 per capita water demand.

Illustrative example 3.3

Population of X town in 2010 was 50000. The records show that before 0ne, two and three decades the population was 47100, 43500 and 41000 respectively. Estimate the population in 2010, 2030 and 2040.

1,2,3_water supply and distribution.pdf

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