Width of Carriage Way, Width of Formation and Right of Way

Width of Carriage Way

Width of the carriage way or the width of the pavement depends on the width of the traffic lane and number of lanes. Width of a traffic lane depends on the width of the vehicle and the clearance. Side clearance improves operating speed and safety. The maximum permissible width of a vehicle is 2.44 m and the desirable side clearance for single lane traffic is 0.68 m. This require minimum of lane width of 3.75 m for a single lane road (Fig.1). However, the side clearance required is about 0.53 m, on either side and 1.06 m in the center. Therefore, a two lane road require minimum of 3.5 meter for each lane (Fig.1). The desirable carriage way width recommended by IRC is given in Table 1.

Fig. 1 Lane Width of Single and Two Lane Roads

Table 1 IRC Specification for Carriage Way Width

Type of Carriage Way	Width (m)
Single lane	3.75
Two lane, no kerbs	7.0
Two lane, raised kerbs	7.5
Intermediate carriage	5.5
Multi-lane	3.5

Fig. 2 Representation of Various Road Width

Width of Formation/Roadway Width

Width of formation or roadway width is the sum of the widths of pavements or carriage way including separators and shoulders. This does not include the extra land in formation/cutting. The values suggested by IRC are given in Table 2.

Table 2 Width of Formation of Various Classification of Roads

Road Classification	Roadway width in m
	Plain and Rolling Terrain	Mountainous and Steep Terrain
NH/SH	12	6.25 - 8.8
MDR	9	4.75
ODR	7.5 - 9.0	4.75
VR	7.5	4.0

Right of Way/ Land Width

Right of way (RoW) or land width is the width of land acquired for the road, along its alignment. It should be adequate to accommodate all the cross-sectional elements of the highway and may reasonably provide for future development. To prevent ribbon development along highways, control lines and building lines may be provided. Control line is a line which represents the nearest limits of future uncontrolled building activity in relation to a road. Building line represents a line on either side of the road, between which and the road no building activity is permitted at all. The right of way width is governed by:

• Width of formation : It depends on the category of the highway and width of roadway and road margins.
• Height of embankment or depth of cutting : It is governed by the topography and the vertical alignment.
• Side slopes of embankment or cutting : It depends on the height of the slope, soil type etc.
• Drainage system and their size which depends on rainfall, topography etc.
• Sight distance considerations : On curves etc. there is restriction to the visibility on the inner side of the curve due to the presence of some obstructions like building structures etc.
• Reserve land for future widening : Some land has to be acquired in advance anticipating future developments like widening of the road.

The importance of reserved land is emphasized by the following. Extra width of land is available for the construction of roadside facilities. Land acquisition is not possible later, because the land may be occupied for various other purposes (buildings, business etc.) The normal RoW requirements for built up and open areas as specified by IRC is given in Table 3.

Table 3 Normal Right of Way for Open Areas

Road Classification	Roadway width in m
	Plain and Rolling Terrain	Mountainous and Steep Terrain
Open Areas
NH/SH	45	24
MDR	25	18
ODR	15	15
VR	12	9
Built-up Areas
NH/SH	30	20
MDR	20	15
ODR	15	12
VR	10	9

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Camber in Road

Camber or cant is the cross slope provided to raise middle of the road surface in the transverse direction to drain off rain water from road surface and keeps cars from sliding and causing damage to the pavement. It has a lot of benefits that make the road surface last longer. Generally, a camber is a crown-shaped part of the road surface that is made by raising the middle of the road compared to its edges. Most of the time, the rate of camber or cross slope of a road is written as “1 in n,” which means that the transverse slope is in the ratio of 1 vertical to n horizontal or it can be written as a percentage. For example, a road with 2.5% camber has a camber of 1 in 40. Too steep slope is undesirable because it will erode the surface. The common types of camber are parabolic, straight or combination of them. The objectives of providing camber are given below.

• Surface protection especially for gravel and bituminous roads
• Sub-grade protection by proper drainage
• Quick drying of pavement which in turn increases safety

Required camber depends on

• Type of pavement
• Amount of rainfall

The values suggested by IRC for various categories of pavement are given in Table 1. 

Table 1 - IRC Recommended Values of Camber in Road for Different Types of Road Surfaces 

Type of Road Surface	Range of Camber in Areas of
	Low Rainfall	Heavy Rainfall
Cement concrete and thick bituminous surface	1 in 60 or 1.7%	1 in 50 or 2.0%
Water bound macadam and gravel pavement	1 in 40 or 2.5%	1 in 33 or 3.0%
Thin bituminous surface	1 in 50 or 2.0%	1 in 40 or 2.5%
Earth Road	1 in 33 or 3.0%	1 in 25 or 4.0%

Types of Camber in Road 

1) Sloped or Straight Camber 

Straight line camber comprises two slopes that come from the edges and meet in the middle of the carriageway. It is the simplest type of camber. It is easy to build and easy to keep in good shape. 

Fig.1 Sloped or Straight Camber 

2) Parabolic or Barrel Camber 

It is a continuous elliptical or parabolic curve. It provides a level roadway in the centre and gradually rises to a steeper grade on the road’s periphery. Greater drainage efficiency results from the sharper edges of this camber type. Faster vehicles prefer this camber, so it is recommended for city streets. Maintaining and building a camber like this is challenging. Barrel camber is less user-friendly due to its sharper corners. Also, extra curbs are needed because the sharper the edge, the faster it will be damaged if it is not protected.

Fig. 2 Parabolic Camber

3) Composite Camber

Composite camber could be part parabola and part straight line or it could be made up of two straight lines with different slopes. Most of the time, the middle of the road is made to be parabolic and the edges are given straight slopes. It helps to lessen the force of the pressure by making the wheel’s contact area bigger.

Fig. 3 Composite Camber

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Pavement Surface Characteristics – Friction, Unevenness, Light Reflection & Drainage

The features of the cross-section of the pavement influences the life of the pavement as well as the riding comfort and safety. Of these, pavement surface characteristics affect both of these. Camber, kerbs and geometry of various cross-sectional elements are important aspects to be considered in this regard. For safe and comfortable driving four aspects of the pavement surface are important; the friction between the wheels and the pavement surface, smoothness of the road surface, light reflection characteristics of the top of pavement surface and drainage to water.

1) Friction

Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe operating speed. Further, it also affects the acceleration and deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles. Skidding happens when the path travelled along the road surface is more than the circumferential movement of the wheels due to friction. Slip occurs when the wheel revolves more than the corresponding longitudinal movement along the road. Various factors that affect friction are given below.

• Type of the pavement (like bituminous, concrete or gravel)
• Condition of the pavement (dry or wet, hot or cold etc.)
• Condition of the tyre (new or old)
• Speed and load of the vehicle

The frictional force that develops between the wheel and the pavement is the load acting multiplied by a factor called the coefficient of friction and denoted as “f”. The choice of the value of this is a very complicated issue since it depends on many variables. IRC suggests the coefficient of longitudinal friction as 0.35 - 0.4 depending on the speed and coefficient of lateral friction as 0.15. The former is useful in sight distance calculation and the latter in horizontal curve design.

2) Unevenness

It is always desirable to have an even surface, but it is seldom possible to have such a one. Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures. Unevenness affect the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of tyres.

Unevenness index

It is a measure of unevenness which is the cumulative measure of vertical undulations of the pavement surface recorded per unit horizontal length of the road. An unevenness index value less than 1500 mm/km is considered as good, a value less than 2500 mm/km is satisfactory up to speed of 100 kmph and values greater than 3200 mm/km is considered as uncomfortable even for 55 kmph.

3) Light Reflection

White roads have good visibility at night, but caused glare during day time. Black roads have no glare during day, but has poor visibility at night. Concrete roads has better visibility and less glare. It is necessary that the road surface should be visible at night.

4) Drainage

The pavement surface should be absolutely impermeable to prevent seepage of water into the pavement layers. Further, both the geometry and texture of pavement surface should help in draining out the water from the surface in less time.

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Aeration Process

Aeration is the process by which the area of contact between water and air is increased either by natural methods or by mechanical devices. In other words, it is the method of increasing the oxygen saturation of the water. Aeration is usually effective against several pollutants like carbon dioxide, some taste and odour producing compounds like methane, hydrogen sulphide, volatile organic compounds like industrial solvents etc. Principle of treatment underlines on the fact that volatile gases in water escape into atmosphere from the air-water interface and atmospheric oxygen takes their place in water, provided the water body can expose itself over a vast surface to the atmosphere. This process continues until an equilibrium is reached depending on the partial pressure of each specific gas in the atmosphere.

Aeration brings water and air in close contact in order to remove dissolved gases (such as carbon dioxide) and oxidizes dissolved metals such as iron, hydrogen sulphide and volatile organic chemicals (VOCs). Aeration is often the first major process at the treatment plant. During aeration, constituents are removed or modified before they can interfere with the treatment processes.

Aeration brings water and air in close contact by exposing drops or thin sheets of water to the air or by introducing small bubbles of air (the smaller the bubble, the better) and letting them rise through the water. The scrubbing process caused by the turbulence of aeration physically removes dissolved gases from solution and allows them to escape into the surrounding air. Aeration also helps to remove dissolved metals through oxidation, the chemical combination of oxygen from the air with certain undesirable metals in the water. Once oxidized, these chemicals fall out of solution and become particles in the water and can be removed by filtration or flotation.

Oxygen is added to water through aeration and can increase the palpability of water by removing the flat taste. The amount of oxygen in which the water can hold depends primarily on the temperature of the water. (The colder the water, more oxygen the water can hold). Water that contains excessive amounts of oxygen can become very corrosive. Excessive oxygen can also cause problems in the treatment plant i.e. air binding of filters.

Efficiency

The efficiency of aeration depends on the amount of surface contact between air and water, which is controlled primarily by the size of the water drop or air bubble. This contact is controlled primarily by the size of the water droplet or air bubble. The goal of an aerator is to increase the surface area of water coming in contact with air so that more air can react with the water. As air or water is broken up into smaller drops/bubbles or into thin sheets, the same volume of either substance has a larger surface area.

Aeration Process

Aeration removes or modifies the constituents of water using two methods - scrubbing action and oxidation. Scrubbing action is caused by turbulence which results when the water and air mix together. The scrubbing action physically removes gases from solution in the water, allowing them to escape into the surrounding air. Scrubbing action will remove tastes and odours from water if the problem is caused by relatively volatile gases and organic compounds. Oxidation is the other process through which aeration purifies water. Oxidation is the addition of oxygen, the removal of hydrogen or the removal of electrons from an element or compound. When air is mixed with water, some impurities in the water, such as iron and manganese, become oxidized. Once oxidized, these chemicals fall out of solution and become suspended in the water. The suspended material can then be removed later in the treatment process through filtration.

Problems with Aeration

Aeration typically raises the dissolved oxygen content of the raw water. In most cases, this is beneficial since a greater concentration of dissolved oxygen in the water can remove a flat taste. However, too much oxygen in the water can cause a variety of problems resulting from the water becoming supersaturated. Supersaturated water can cause corrosion (the gradual decomposition of metal surfaces) and sedimentation problems. In addition, air binding occurs when excess oxygen comes out of solution in the filter, resulting in air bubbles which harm both the filtration and backwash process. Aeration can also cause other problems unrelated to the supersaturated water. Aeration can be a very energy-intensive treatment method which can result in over use of energy. In addition, aeration of water can promote algae growth in the water and can clog filters.

Types of Aerators

Aerators fall into two categories. They either introduce air to water or water to air. The water-in-air method is designed to produce small drops of water that fall through the air. The air-in-water method creates small bubbles of air that are injected into the water stream. All aerators are designed to create a greater amount of contact between air and water to enhance the transfer of gases and increase oxidation. Pumping water through air is much more energy efficient than pumping air through water.

I) Water-Into-Air Aerators

1) Cascade Aerators

A cascade aerator consists of a series of steps that the water flows over similar to a flowing stream. Cascade aerators allow water to flow in a thin layer down steps. In all cascade aerators, aeration is accomplished in the splash zones. Splash zones are created by placing blocks across the incline. They are the oldest and most common type of aerators. Cascade aerators can be used to oxidize iron and to partially reduce dissolved gases.

Fig. 1 Cascade Aerator

2) Cone Aerators/Cone Tray Aerator

Cone aerators are used primarily to oxidize iron and manganese from the ferrous state to the ferric state prior to filtration. The design of the aerator is similar to the cascade type, with the water being pumped to the top of the cones and then being allowed to cascade down through the aerator. The cone tray aerator consists of several cones in which water flows through the cone and over the rim of the cone.

Fig. 2 Cone Tray Aerator

3) Slat and Coke Aerators

Slat and coke trays are similar to the cascade and cone aerators. They usually consist of three-to-five stacked trays, which have spaced wooden slats in them. The trays are then filled with fist-sized pieces of coke, rock, ceramic balls, limestone or other materials. The primary purpose of the materials is providing additional surface contact area between the air and water. Coke tray aerators also pass water through air in small streams. A coke tray aerator is comprised of a series of activated carbon trays, one above another, with a distributing pan above the top tray and a collecting pan below the bottom tray. The distributing pan breaks the water up into small streams or drops. The holes in the trays should be designed to develop some head loss to provide for equal distribution to the lower tray.

Fig. 3 Coke Tray Aerator

4) Draft Aerators

Draft aerators are similar to other water-into-air aerators, except that the air is induced by a blower. There are two basic type of draft aerators. One has external blowers mounted at the bottom of the tower to induce air from the bottom of the tower. Water is pumped to the top and allowed to cascade down through the rising air. The other, an induced-draft aerator, has a top-mounted blower forcing air from bottom vents up through the unit to the top. Both types are effective in oxidizing iron and manganese before filtration. This type of aerator is most effective in the reduction of hydrogen sulphide and carbon dioxide.

Fig. 4 Forced Draft Aerator

5) Spray Aerators

Spray aerators have one or more spray nozzles connected to a pipe manifold. Water moves through the pipe under pressure and leaves each nozzle in a fine spray and falls through the surrounding air, creating a fountain affect. Spray aeration is successful in oxidizing iron and manganese and increases the dissolved oxygen in the water.

Fig. 5 Spray Aerator

II) Air-Into-Water Aerators

1) Pressure Aerators

There are two basic types of pressure aerators. One uses a pressure vessel; where water to be treated is sprayed into high-pressure air, allowing the water to quickly pick up dissolved oxygen. The other is a pressure aerator commonly used in pressure filtration. Air is injected into the raw water piping and allowed to stream into the water as a fine bubble, causing the iron to be readily oxidized. The higher the pressure, the more readily the transfer of the oxygen to the water. The more oxygen that is available, the more readily the oxidation of the iron or manganese.

2) Centrifugal Aerators

Centrifugal aerators create enhanced conditions for dissolving gas into liquid phase, including bubble size, bubble size distribution and duration of interaction with liquid. Centrifugal aerators combine several elements like high turbulence swirling flow of liquid, orthogonal flow of liquid and gas, constant pressure inside the vessel, optimum flow velocity generating centrifugal forces thereby extending diffusion rate within the vessel and very small pores, through which gas permeates into the liquid and is sheared off into liquid phase, thereby forming small bubbles.

3) Air Diffusion Aerator

Air diffusion is a type of aerator in which air is blown through a trough of water. As water runs through the trough, compressed air is blown upward through porous plates on the bottom. This method is not very efficient due to limited air transfer.

Fig. 6 Air Diffusion Aerator

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Screening

Screening is the first unit operation used at wastewater treatment plants. A screen is a device with openings for removing bigger suspended or floating matter in sewage which would otherwise damage equipment or interfere with satisfactory operation of treatment units. Screening removes objects such as rags, paper, plastics and metals to prevent damage and clogging of downstream equipment, piping and appurtenances. Some modern wastewater treatment plants use both coarse screens and fine screens.

Fig. 1 Screen

The primary treatment incorporates unit operations for removal of floating and suspended solids from the wastewater. They are also referred as the physical unit operations. Screen is used to remove larger particles of floating and suspended matter by coarse screening. This is accomplished by a set of inclined parallel bars, fixed at certain distance apart in a channel. The screen can be of circular or rectangular opening. Industrial wastewater treatment plant may or may not need the screens. When packing of the product and cleaning of packing bottles/ containers is carried out, it is necessary to provide screens even for industrial wastewater treatment plant to separate labels, stopper, cardboard and other packing materials. The cross section of the screen chamber is always greater (about 200 to 300 %) than the incoming sewer. The length of this channel should be sufficiently long to prevent eddies around the screen.

Fig. 2 Fixed Bar Screen (Coarse or Medium)

Advantages

Manually cleaned screens require little or no equipment maintenance and provide a good alternative for smaller plants with few screenings. Mechanically cleaned screens tend to have lower labour costs than manually cleaned screens and offer the advantages of improved flow conditions and screening capture over manually cleaned screens.

Disadvantages

Manually cleaned screens require frequent raking to avoid clogging and high backwater levels that cause build-up of a solids mat on the screen. The increased raking frequency increases labour costs. Removal of this mat during cleaning may also cause flow surges that can reduce the solids-capture efficiency of downstream units. Mechanically cleaned screens are not subject to this problem, but they have high equipment maintenance costs.

Types of Screens

Screens can be broadly classified depending upon the opening size provided as coarse screen (bar screens) and fine screens. Based on the cleaning operation they are classified as manually cleaned screens or mechanically cleaned screens. Due to need of more and more compact treatment facilities many advancements in the screen design are coming up.

1) Coarse Screens

It is used primarily as a protective device and hence used as first treatment unit. Coarse screens also called racks, are usually bar screens, composed of vertical or inclined bars spaced at equal intervals across a channel through which sewage flows. Common type of these screens are bar racks (or bar screen), coarse woven-wire screens and comminutors. Bar screens are used ahead of the pumps and grit removal facility. Bar screens with relatively large openings of 75 to 150 mm are provided ahead of pumps, while those ahead of sedimentation tanks have smaller openings of 50 mm. Types of coarse screens include mechanically and manually cleaned bar screens, including trash racks.

Bar screens are usually hand cleaned and sometimes provided with mechanical devices. These cleaning devices are rakes which periodically sweep the entire screen removing the solids for further processing or disposal. Hand cleaned racks are set usually at an angle of 45° to the horizontal to increase the effective cleaning surface and also facilitate the raking operations. Mechanical cleaned racks are generally erected almost vertically. Such bar screens have openings 25% in excess of the cross section of the sewage channel.

Grinder or Comminutor

It is used in conjunction with coarse screens to grind or cut the screenings. They utilize cutting teeth (or shredding device) on a rotating or oscillating drum that passes through stationary combs (or disks). Object of large size are shredded when it will pass through the thin opening of size 0.6 to 1.0 cm. Provision of bye pass to this device should always be made.

2) Medium Screens

Medium screens have clear openings of 20 to 50 mm. Bar are usually 10 mm thick on the upstream side and taper slightly to the downstream side. The bars used for screens are rectangular in cross section usually about 10 x 50 mm, placed with larger dimension parallel to the flow.

3) Fine Screens

Fine screens are typically used to remove material that may create operation and maintenance problems in downstream processes, particularly in systems that lack primary treatment. Fine screens are mechanically cleaned devices using perforated plates, woven wire cloth or very closely spaced bars with clear openings of less than 20 mm. Typical opening sizes for fine screens are 1.5 to 6 mm (0.06 to 0.25 in). Very fine screens with openings of 0.2 to 1.5 mm (0.01 to 0.06 in) placed after coarse or fine screens can reduce suspended solids to levels near those achieved by primary clarification. Fine screens are not normally suitable for sewage because of clogging possibilities.

Fine screens are also used to remove solids from primary effluent to reduce clogging problem of trickling filters. Various types of micro screens have been developed that are used to upgrade effluent quality from secondary treatment plant. Fine screen can be fixed or static wedge-wire type, drum type, step type and centrifugal screens. Fixed or static screens are permanently set in vertical, inclined or horizontal position and must be cleaned by rakes, teeth or brushes. Movable screens are cleaned continuously while in operation. Centrifugal screens utilize the rotating screens that separate effluent and solids are concentrated.

Velocity

The velocity of flow ahead of and through the screen varies and affects its operation. The lower the velocity through the screen, the greater is the amount of screenings that would be removed from sewage. However, the lower the velocity, the greater would be the amount of solids deposited in the channel. Hence, the design velocity should be such as to permit 100% removal of material of certain size without undue depositions. Velocities of 0.6 to 1.2 m/s through the open area for the peak flows have been used satisfactorily. Further, the velocity at low flows in the approach channel should not be less than 0.3 m/s to avoid deposition of solids.

Head loss

Head loss varies with the quantity and nature of screenings allowed to accumulate between cleanings. The head loss created by a clean screen may be calculated by considering the flow and the effective areas of screen openings, the latter being the sum of the vertical projections of the openings. The head loss through clean flat bar screens is calculated from the following formula.

where, 

         h = head loss in m

        V = velocity through the screen in m/s

        v = velocity before the screen in m/s

Another formula often used to determine the head loss through a bar rack is Kirschmer's equation.

where

         h = head loss, m

        K = bar shape factor (2.42 for sharp edge rectangular bar, 1.83 for rectangular bar with semicircle upstream, 1.79 for circular bar and 1.67 for rectangular bar with both upstream and downstream face as semi-circular).

        W = maximum width of bar upstream of flow, m

        b = minimum clear spacing between bars, m

       hv = velocity head of flow approaching rack, m = v²/2g

        θ = angle of inclination of rack with horizontal

The head loss through fine screen is given by

where, 

       h = head loss, m

      Q = discharge, m³/s

      C = coefficient of discharge (typical value 0.6)

      A = effective submerged open area, m2

The quantity of screenings depends on the nature of the wastewater and the screen openings.

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Introduction to Water Treatment

Water treatment is required for surface waters and some ground waters for drinking purposes. Water treatments involves the removal of pollutants generated from different sources and to produce water that is pure and suitable for human consumption without causing any long term or short-term adverse health effects. Many aquifers and isolated surface waters are high in water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes or fire control. However, clean water sources are the exception in many parts of the world, particularly regions where the population is dense or where there is heavy agricultural use. In these places, the water supply must receive varying degrees of treatment before distribution.

The available raw water must be treated and purified before they can be supplied to the public for their domestic, industrial or any other uses. The extent of treatment required to be given to the particular water depends upon the characteristics and quality of the available water and also upon the quality requirements for the intended use. Raw water may contain suspended, colloidal and dissolved impurities. The purpose of water treatments is to remove all those impurities which are objectionable either from taste and odour perspective or from public health perspective.

Impurities enter water as it moves through the atmosphere, across the earth’s surface and between soil particles in the ground. These background levels of impurities are often supplemented by human activities. Chemicals from industrial discharges and pathogenic organisms of human origin, if allowed to enter the water distribution system, may cause health problems. Excessive silt and other solids may make water aesthetically unpleasant and unsightly. Heavy metal pollution, including lead, zinc and copper may be caused by corrosion of the very pipes that carry water from its source to the consumer.

The method and degree of water treatment are important considerations for environmental engineers. Generally, the characteristics of raw water determine the treatment method. Most public water systems are relied on for drinking water as well as for industrial consumption and firefighting, so that human consumption, the highest use of the water, defines the degree of treatment. The flow chart of the process occurring in conventional water treatment plant is shown below in Fig.1.

Fig.1 Process of Water Treatment

A typical water treatment plant is diagrammed in Fig.2. It is designed to remove odour, colour, and turbidity as well as bacteria and other contaminants. Raw water entering a treatment plant usually has significant turbidity caused by colloidal clay and silt particles. These particles carry an electrostatic charge that keeps them in continual motion and prevents them from colliding and sticking together. Chemicals like alum (aluminium sulphate) are added to the water both to neutralize the particles electrically and to aid in making them “sticky” so that they can coalesce and form large particles called flocs. This process is called coagulation.

Fig.2 Diagram of a Typical Water Treatment Facility

For surface water, following are the treatment processes that are generally adopted.

1) Screening

This is adopted to remove all the floating matter from surface waters. It is generally provided at the intake point

2) Aeration

This is adopted to remove objectionable tastes and colour and also to remove the dissolved gases such as carbon-dioxide, hydrogen sulphide etc. The iron and manganese present in water also oxidized to some extent. This process is optional and is not adopted in cases where water does not contain objectionable taste and odour.

3) Sedimentation with or without Coagulants

The purpose of sedimentation is to remove the suspended impurities. With the help of plain sedimentation, silt, sand etc. can be removed. However, with the help of sedimentation with coagulants, very fine suspended particles and some bacteria can be removed.

4) Filtration

The process of filtration forms the most important stage in the purification of water. Filtration removes very fine suspended impurities and colloidal impurities that may have escaped the sedimentation tanks. In addition to this, the micro-organisms present in the water are largely removed.

5) Disinfection

It is carried out to eliminate or reduce to a safe minimum limit, the remaining microorganisms and to prevent the contamination of water during its transit from the treatment plant to the place of its consumption

6) Miscellaneous Processes

These include water softening, desalination, removal of iron, manganese and other harmful constituents.

Objectives of Water Treatment

• To remove the dissolved gases and colour of water.
• To remove the unpleasant and objectionable tastes and odours from the water.
• To kill all the pathogenic organisms which are harmful to the human health.
• To make water fit for domestic use such as cooking, washing and various industrial purposes as dyeing, steam generation etc.
• To eradicate the contaminants that are contained in water as found in nature.
• To control the impurities from scale formation.
• Pure water quality is required to minimize the corrosion, radiation levels and fouling of heat transfer surfaces in reactor facility systems.
• To prove safe potable water to the public.
• To reduce the physical, chemical and biological contaminants in water.
• To eliminate the tuberculation and corrosive properties of water which affects the conduits and pipes.

Location of Treatment Plant

The water treatment plants should be located as near to the towns as possible. If the source of water supply is tube well the treatment plant should be located in the central part of the town, so that purified water may reach the public as early as possible. If the city is very large to which water cannot be supplied from one tube well, the city should be divided into zones and a separate tube well with necessary treatment plants should be provided for each zone. If the source of water is river or reservoir, the treatment plant should be located as near the town as possible preferably in the central place. The following points should be kept in mind while giving the layout of any treatment plant.

• All the plants should be located in order of sequence, so that water from one process should directly go into next process.
• If possible, all the plants should be located at such elevations that water should flow from one plant to next under its force of gravity only.
• All the treatment units should be arranged in such a way that minimum area is required, it will also insure economy in its cost.
• Sufficient area should be occupied for future extension in the beginning.
• Staff quarters and office should also be provided near the treatment plants, so that operators can watch the plants easily.
• The site of treatment plant should be very neat and give very good aesthetic appearance.

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Transportation of Water

The term conveyance or transportation of water refers to taking of water from source to purification plants and from treatment plant to consumers. Water supply system broadly involves transportation of water from the sources to the area of consumption, through free flow channels or conduits or pressure mains. Depending on the topography of the land, conveyance may be in free flow and/or pressure conduits. Transmission of water accounts for an appreciable part of the capital outlay and hence careful consideration for the economics is called for before deciding on the best mode of conveyance. Care should be taken so that there is no possibility of pollution from surrounding areas.

If the source is at higher level than the treatment plant, the water can flow under gravity, automatically. Similarly, after necessary purification of water, it has to be conveyed to the consumers. Therefore, for conveyance of water some sort of devices or structures is required. The arrangement may be in the form of open channels, aqueducts, tunnels or pipes.

1) Open Channels

In any water supply systems, raw water from source to treatment plants may be carried in open channels. These can be constructed by cutting in high grounds and banking low grounds. Economical sections of open channels are generally trapezoidal while rectangular sections prove economical when rock cutting is involved. The channels are to be properly lined to prevent seepage. These kind of channels need to be taken along the gradient and therefore the initial cost and maintenance cost may be high. While open channels are not recommended for conveyance of treated water, they may be adopted for conveying raw water. If these kind of channels are unlined, they have to be run with limited velocity of flow so that it does not affect scouring. As water flows only due to gravitational force a longitudinal uniform slope should be given. The velocity of water in channels should not exceed the permissible limit, otherwise scouring will start in the bed and water will get dirty. In channels there is always loss of water by seepage and evaporation.

2) Aqueducts

The term aqueduct is usually restricted to closed conduits made up of masonry. These can be used for conveyance of water from source to treatment plant or for distribution. Aqueducts normally run half to two-third full at required capacity of supply in most circumstances. In ancient times, rectangular aqueducts were most commonly used, but these days circular or horse-shoe shaped ones are more common. Masonry aqueducts unless reinforced with steel, are usually constructed in horse-shoe cross-section. This cross-section has good hydraulic properties and resists earth pressure well. It is economical and easy to build.

3) Tunnels

Tunnels are also like aqueducts. This is also a gravity conduit, in which water flows under gravitational force. But sometimes, water flows under pressure and in such cases, these are called pressure tunnels. Tunnels which are not under pressure are usually constructed in horse-shoe shape. But if they convey water under pressure, circular cross section is the best. In pressure tunnels, the depth of cover is generally such that the weight of overlying material overcomes the bursting pressure. Tunnels are used to convey water into the cities from outside sources. Tunnels should be water- tight and there should be no loss of water.

4) Pipes

Pipe is a circular closed conduit used to convey water from one point to another, under gravity or under pressure. Usually pipes follow the profile of the ground surface closely. If pipes do not run full, they are called to flowing under gravity. But flow under gravity is possible only if the pipe is given a definite longitudinal slope. Pipes running full will be said to be running under pressure. Water is under pressure always and hence the pipe material and the fixture should withstand stresses due to the internal pressure, vacuum pressure, when the pipes are empty it has to withstand water hammer, when the valves are closed it has to withstand temperature stresses. Pipes are mostly made up of materials like cast iron, wrought iron, RCC, asbestos cement, plastic, timber etc.

Requirements of Pipe Material

• It should be capable of withstanding internal and external pressure
• It should facilitate easy joints
• It should be available in all sizes, transport and erection should be easy
• It should be durable
• It should not react with water to alter its quality
• Cost of pipes should be less
• Frictional head loss should be minimum
• The damaged units should be replaced easily

a) Cast Iron Pipes

Cast iron pipes are used in majority of water conveyance mains because of centuries of satisfactory experience with it. Cast iron pipe is resistant to corrosion and accordingly long lived; its life may be over 100 years.

Advantages

• Cast iron pipes are of moderate cost
• Their jointing is easier
• They are resistant to corrosion
• They have long life

Disadvantages

• They are heavier and hence uneconomical when their diameter is more than 120 cm
• They cannot be used for pressures greater than 7 kg/cm².
• They are fragile

b) Wrought Iron Pipes

Wrought iron pipes are manufactured by rolling flat plates of the wrought iron to the proper diameter and welding the edges. Such pipes are much lighter than the cast iron pipes and can be more easily cut, threaded and worked. These pipes are stronger than cast iron pipes and it can withstand higher pressure. They look much neater, but are much costlier. They corrode quickly and hence are used principally for installation within buildings. These pipes are usually protected by coating them with a thin film of molten zinc. Such coated pipes are known as galvanized iron pipes and they are commonly jointed by screwed and socketed joints.

c) Galvanised Iron (GI) Pipes

Gl pipes are highly suitable for distribution system. They are available in light (yellow colour code), medium (blue colour code) and heavy grades (red colour code) depending on the thickness of pipe used. Normally, medium grade pipes (wall thickness 2.6-4.8 mm) are used for water supply system. It is cheap in cost, light in weight and easy to join. It is usually affected by acidic or alkaline water. Gl pipes can be used in non-corrosive water with pH value greater than 6.5. Gl pipes are normally joined with lead putty on threaded end.

Fig. 1 GI Pipes

d) Mild Steel Pipes

Mild steel pipes are durable and can resist high internal water pressure and highly suitable for long distance high pressure piping. It is flexible to lay in certain curves. The number of joints are less as they are available in longer length. It is light in weight, easy to transport and the damage in transportation is minimum. These pipes are prone to rust and require higher maintenance. It requires more time for repairs and not very suitable for distribution piping. These are available in diameter of 150-250 mm for water supply and cut lengths of 4 - 7 m (2.6-4.5 mm wall thickness).

Fig. 2 Mild Steel Pipes

e) Cement Concrete Pipes

Cement concrete pipes may be either plain or reinforced and are best made by the spinning process. They may be either precast or may be cast-in-situ. Transportation costs are much reduced if pipes are cast in situ. Concrete pipes have low maintenance in resistant to corrosion and particularly suitable to soft and acidic waters. They however can withstand high pressure if reinforced. The plain cement concrete pipes are used for heads up to 7 m while reinforced cement concrete pipes are normally used for head upto 60 m.

Advantages

• They are more suitable to resist the external loads and loads due to backfilling.
• The maintenance cost is low.
• The inside surface of pipes can be made smooth, thus reducing the frictional losses.
• The problem of corrosion is not there.
• Pipes can be cast at site and hence the transportation problems are reduced.
• Due to their heavy weight, the problem of floatation is not there when they are empty.

Disadvantages

• Unreinforced pipes are liable to tensile cracks and they cannot withstand high pressure.
• The tendency of leakage is not ruled out as a result of its porosity and shrinkage cracks.
• It is very difficult to repair them.
• Precast pipes are very heavy and it is difficult to transport them.

f) Poly Vinyl Chloride (PVC Un-plasticised) Pipes

These are cheap in cost and light in weight. It is economical in laying and jointing and are rigid pipes. It is highly durable and suitable for distribution network. These pipes are free from corrosion, tough against chemical attack and good electric insulation. It is highly suitable for distribution piping and branch pipes. The disadvantage is that it is less resistance to heat and direct exposure to sun. Hence, not very suitable for piping above the ground. PVC pipes weigh only 1/5^th of steel pipes of same diameter.

Fig. 3 PVC Pipes

g) HDPE (High Density Polyethylene) Pipes

These are light in weight and flexible than PVC pipes. HDPE pipes are black in colour. These are suitable for underground piping and can withstand movement of heavy traffic. It allows free flowing of water and highly durable and suitable for distribution network. These are free from corrosion and has good electric insulation. It is useful for water conveyance as they do not constitute toxic hazard and does not support microbial growth.

Fig. 4 HDPE Pipes

h) Ductile Iron Pipes

Ductile Iron pipes are better version of cast iron pipes with better tensile strength. These pipes are prepared using centrifugal cast process. DI pipes have high impact resistance, high wear and tear resistance, high tensile strength, ductility and good internal and external corrosion resistance. These pipes are provided with cement mortar lining on inside surface which provides smooth surface and is suitable for providing chemical and physical barriers to water. Such pipes reduce water contamination. The outer coating of such pipes is done with bituminous or Zinc paint. DI pressure pipes are available in range from 80-1000 mm diameter in lengths from 5.5-6 m. They are about 30 percent lighter than conventional cast iron pipes. DI pipes have lower pumping cost due to lower frictional resistance.

Fig. 5 DI Pipes

i) Asbestos Cement Pipe

These pipes are composed of asbestos fibre and Portland cement combined under pressure into dense homogenous structure. These are available in large variation from 50 to 600mm.

j) Wood Pipe

These pipes are built of staves of wood held together by steel bands. Wood is less durable for pipe material and pipe must be constantly full of water to prevent crackdown due to alternate wet and dry conditions.

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Intake Structure for Water Supply

Intakes are the structures used for admitting water from the surface sources (i.e., river, reservoir or lake) and conveying it further to the treatment plant. The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant. It essentially consists of opening, grating or strainer through which the raw water from river, canal or reservoir and is carried to a sump well by means of conduits. Water from the sump well is pumped through the rising mains to the treatment plants. Generally, an intake is a masonry or concrete structure with an aim of providing relatively clean water, free from pollution, sand and objectionable floating material. Its main purpose is to provide calm and still water conditions, so that comparatively purer water may be collected from the source. If intake well has to withstand the effects of severe forces which may be due to striking of high water currents, it may be made from reinforced cement concrete. Intake consists of the following.

• Conduit with protective works
• Screens at open ends
• Gates and Valves to regulate flow

Site for Location of Intake

While selecting a site for location of intakes, the following points should be taken into account.

• As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less.
• The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant.
• The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater.
• The site should be such as to permit greater withdrawal of water, if required at a future date.
• The intake must be located at a place from where it can draw water even during the driest period of the year.
• The intake site should remain easily accessible during floods and should not get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.
• Heavy water currents should not strike the intake directly.
• Site should be well connected by good type or roads
• Site should not be located in navigation channels, the reason being water in such channels are generally polluted.
• As far as possible, the site should be located on the upstream side of the watercourse.
• The intake should be so located that good foundation conditions are prevalent and the possibility of scouring is minimal.
• The site should be selected in such a manner that there is ample scope for further expansion.

Design of Intake

An intake should be designed keeping in mind the following considerations.

• Intake should be sufficiently heavy so that it may not start floating due to up thrust of water.
• All the forces which are expected to work on intake should be carefully analysed and intake should be designed to withstand all these forces such as heavy currents, floating materials, submerged bodies, ice pressure etc.
• The foundation of the intake should be taken sufficiently deep to avoid overturning.
• It should have sufficient self-weight so that it does not float by upthrust of water.
• Strainers in the form of wire mesh should be provided on all the intake inlets to avoid entry of large floating objects.
• Intake should be of such size and so located that sufficient quantity of water can be obtained from the intake in all circumstances.

Types of Intakes

1) Submerged Intake

Submerged intake is the one which is constructed entirely under water. Such an intake is commonly used to obtain supply from a lake. An exposed intake is in the form of a well or tower constructed near the bank of a river or in some cases even away from the river banks. Exposed intakes are more common due to ease in its operation. A wet intake is that type of intake tower in which the water level is practically the same as the water level of the sources of supply. Such an intake is sometimes known as jack well and is most commonly used. In the case of dry intake, there is no water in the water tower. Water enters through entry point directly into the conveying pipes. The dry tower is simply used for the operation of valves etc.

2) Reservoir Intake

There are large variations in discharge of all the rivers during monsoon and winter. The discharge of some rivers in summer remains sufficient to meet up the demand, but some rivers dry up partly or fully and cannot meet the hot weather demand. In such cases reservoirs are constructed by constructing weirs or dams across the rivers. Reservoir intakes is mostly used to draw the water from earthen dam reservoir. When the flow in the river is not guaranteed throughout the year a dam is constructed across it to store water in the reservoir so formed. The reservoir intakes are practically similar to the river intake, except that these are located near the upstream face of the dam where maximum depth of water is available.

It essentially consists of an intake tower constructed on the slope of the dam at such place from where intake can draw sufficient quantity of water even in the driest period. Intake pipes are fixed at different levels, so as to draw water near the surface in all variations of water level. These all inlet pipes are connected to one vertical pipe inside the intake well. Screens are provided at the mouth of all intakes to prevent the entrance of floating and suspended matter in them. The water which enters the vertical pipe is taken to the other side of the dam by means of an outlet pipe. At the top of the intake tower, sluice valves are provided to control the flow of water.

Fig. 1 Reservoir Intake

3) River Intake

Water from the river is always drawn from the upstream side, because it is free from the contamination caused by the disposal of sewage or industrial waste water disposal in it. It is circular masonry tower of 4 to 7 m in diameter constructed along the bank of the river at such place from where required quantity of water can be obtained even in the dry period. They are either located sufficiently inside the river so that demands of water are met with in all the seasons of the year or they may be located near the river bank where a sufficient depth of water is available. The water enters in the lower portion of the intake known as sump-well from penstocks.

The penstocks are fitted with screens to check the entry of floating solids and are placed on the downstream side so that water free from most of the suspended solids may only enter the jack well. Number of penstock openings are provided in the intake tower to admit water at different levels. The opening and closing of penstock valves is done with the help of wheels provided at the pump house floor. Sometimes, an approach channel is constructed and water is led to the intake tower. If the water level in the river is low, a weir may be constructed across it to raise the water level and divert it to the intake tower.

Fig. 2 River Intake

4) Lake Intake

Lake intakes are similar to reservoir intakes if the depth of the water near the banks is reasonable. For obtaining water from lakes mostly submersible intakes are used. These intakes are constructed in the bed of the lake below the slow water level so as to draw water in dry season also. It essentially consists of a pipe laid in the bed of the river. One end of which is in the middle of the lake is fitted with bell mouth opening covered with a mesh and protected by timber or concrete crib. The water enters in the pipe through the bell mouth opening and flows under gravity to the bank where it is collected in a sump-well and then pumped to the treatment plants for necessary treatment. If one pipe is not sufficient two or more pipes may be laid to get the required quantity of water.

Fig. 3 Lake Intake

Advantages of lake intake

• No obstruction to the navigation.
• No danger from floating bodies.
• No trouble due to ice.

5) Canal Intake

Canal intake is a very simple structure constructed on the bank. Sometimes, the source of water supply to a small town may be an irrigation canal passing near the town. The canal intake essentially consists of concrete or masonry intake chamber of rectangular shape, admitting water through a coarse screen. A fine screen is provided over the bell mouth entry of the outlet pipe. The bell mouth entry is located below the expected low water level in the canal. Water may flow from outlet pipe under gravity if the filter house is situated at a lower elevation. Otherwise, the outlet pipe may serve as suction pipe and the pump house may be located on or near the canal bank. The outlet pipe carries the water to the other side of the canal bank from where it is taken to the treatment plants. One sluice valve which is operated by a wheel from the top of the masonry chamber is provided to control the flow of water in the pipe. The intake chamber is so constructed that is does not offer any appreciable resistance to normal flow in the canal. Otherwise, the intake chamber is located inside the canal bank.

Fig. 4 Canal Intake

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Batching of Concrete

The measurement of materials for making concrete is known as batching. It is the process of measurement of specified quantities of cement, aggregates, water and admixture, i.e., ingredients of concrete in correct proportion. There are two methods of batching.

1) Volume Batching

Volume batching is not a good method for proportioning the material because of the difficulty it offers to measure granular material in terms of volume. Volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. The amount of solid granular material in a cubic metre is an indefinite quantity. Because of this, for quality concrete material have to be measured by weight only. However, for unimportant concrete or for any small job, concrete may be batched by volume.

Cement is always measured by weight. It is never measured in volume. Generally, for each batch mix, one bag of cement is used. The volume of one bag of cement is taken as thirty-five (35) litres. Gauge boxes are used for measuring the fine and coarse aggregates. The typical sketch of a gauge box is shown in Fig.1. The volume of the box is made equal to the volume of one bag of cement i.e., 35 litres or multiple thereof. The gauge boxes are made comparatively deeper with narrow surface rather than shallow with wider surface to facilitate easy estimation of top level. Sometimes bottomless gauge-boxes are used. This should be avoided. Correction to the effect of bulking should be made to cater for bulking of fine aggregate, when the fine aggregate is moist and volume batching is adopted.

Fig.1 Gauge Box

Gauge boxes are generally called ‘farmas’. They can be made of timber or steel plates. In a major site it is recommended to have the following gauge boxes at site to cater for change in mix design or bulking of sand. The volume of each gauge box is clearly marked with paint on the external surface.

Water is measured either in kg or litres as may be convenient. In this case, the two units are same, as the density of water is one kg per litre. The quantity of water required is a product of water/cement ratio and the weight of cement; consider an example, if the water/cement ratio of 0.5 is specified, the quantity of mixing water required per bag of cement is 0.5 x 50.00 = 25 kg or 25 litres. The quantity is inclusive of any surface moisture present in the aggregate.

2) Weigh Batching

Weigh batching is the correct method of measuring the materials. For important concrete, weigh batching system should be adopted. Use of weight system in batching, facilitates accuracy, flexibility and simplicity. Different types of weigh batchers are available and the particular type to be used depends upon the nature of the job. Large weigh batching plants have automatic weighing equipment. The use of this automatic equipment for batching is one of sophistication and requires qualified and experienced engineers. In this, further complication will come to adjust water content to cater for the moisture content in the aggregate. In smaller works, the weighing arrangement consists of two weighing buckets, each connected through a system of levers to spring-loaded dials which indicate the load. The weighing buckets are mounted on a central spindle about which they rotate. Thus one can be loaded while the other is being discharged into the mixer skip. A simple spring balance or the common platform weighing machines also can be used for small jobs.

On large work sites, the weigh bucket type of weighing equipment is used. This fed from a large overhead storage hopper and it discharges by gravity, straight into the mixer. The weighing is done through a lever-arm system and two interlinked beams and jockey weights. The required quantity of coarse aggregate is weighed, having only the lower beam in operation. After balancing, by turning the smaller lever, to the left of the beam, the two beams are interlinked and the fine aggregate is added until they both balance. The final balance is indicated by the pointer on the scale to the right of the beams. Discharge is through the swivel gate at the bottom.

Fig. 2 Weigh Batcher

Automatic batching plants are available in small or large capacity. In this, the operator has only to press one or two buttons to put into motion the weighing of all the different materials, the flow of each being cut off when the correct weight is reached. In their most advanced forms, automatic plants are electrically operated on a punched card system. This type of plant is particularly only suitable for the production of ready-mixed concrete in which very frequent changes in mix proportion have to be made to meet the varying requirements of different customers. In some of the recent automatic weigh batching equipment, recorders are fitted which record graphically the weight of each material, delivered to each batch. They are meant to record and check the actual and designed proportions.

Aggregate weighing machines require regular attention if they are to maintain their accuracy. Check calibrations should always be made by adding weights in the hopper equal to the full weight of the aggregate in the batch. The error found is adjusted from time to time. In small jobs, cement is often not weighed; it is added in bags assuming the weight of the bag as 50 kg. In reality, though the cement bag is made of 50 kg at the factory, due to transportation, handling at a number of places, it loses some cement, particularly, when jute bags are used. In fact, the weight of a cement bag at the site is considerably less. Sometimes, the loss of weight becomes more than 5 kg. This is one of the sources of error in volume batching and also in weigh batching, when the cement is not actually weighed. But in important major concreting jobs, cement is also actually weighed and the exact proportion as designed is maintained.

Components of a Batching Plant

• Aggregate bins for various types of aggregates
• Feeding mechanisms such as scrappers, conveyors or hoists etc. to transfer aggregate to scales (balances)
• Balance and measuring system
• Cement silos and a conveyor screw or bucket conveyor
• The storage tank for water and water measuring system
• Dispenser for chemical (liquid) admixture

Measurement of Water

When weigh batching is adopted, the measurement of water must be done accurately. Addition of water by graduated bucket in terms of litres will not be accurate enough for the reason of spillage of water etc. It is usual to have the water measured in a horizontal tank or vertical tank fitted to the mixer. These tanks are filled up after every batch. The filling is so designed to have a control to admit any desired quantity of water. Sometimes, water meters are fitted in the main water supply to the mixer from which the exact quantity of water can be let into the mixer.

In modern batching plants sophisticated automatic microprocessor controlled weigh batching arrangements which not only accurately measures the constituent materials, but also the moisture content of aggregates. Moisture content is automatically measured by sensor probes and corrective action is taken to deduct that much quantity of water contained in sand from the total quantity of water.

Fig. 3 Cans for Measuring Water

(Ref : Concrete Technology – M S Shetty)

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