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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Sources of Water

Sources from which water is available for water supply schemes can conveniently be classified into the following two categories according to their proximity to the ground surface.

1. Surface Sources

2. Sub-surface Sources or Underground Sources

Surface Water

Water that gets collected on the surface of the ground or top layer of a body of water is called surface water. In this type of source, the surface runoff is available for water supply schemes. Usual forms of surface sources are as follows.

1) Lakes and Streams

A natural lake represents a large body of water within land with impervious bed. Hence, it may be used as source of water supply scheme for nearby localities. The quantity of runoff that goes to the lake should be accurately determined and it should be seen that it is at least equal to the expected demand of locality. Similar is the case with streams which are formed by the surface runoff. It is found that the flow of water in streams is quite ample in rainy season. But it becomes less and less in hot season and sometimes the stream may even become absolutely dry.

The catchment area of lakes and streams is very small and hence, the quantity of water available from them is also very low. Hence, lakes and streams are not considered as principal sources of water supply schemes for the large cities. But they can be adopted as sources of water supply schemes for hilly areas and small towns. The water which is available from lakes and streams is generally free from undesirable impurities and can therefore be safely used for drinking purposes.

2) Ponds

A natural large sized depression formed within the surface of the earth, when gets filled with water is known as pond. A pond is a man-made body of standing water smaller than a lake. Thus ponds are formed due to excessive digging of ground for the construction of roads, houses, etc. and they are filled up with water in rainy season. The quantity of water in pond is very small and it contains many impurities. A pond cannot be adopted as a source of water supply and its water can only be used for washing of clothes or animals only.

3) Rivers

Since the dawn of civilization, the ancient man settled on the banks of river, drank river water and ate fish caught from river water and sailed down rivers to find out unknown lands. Large rivers constitute the principal source of water supply schemes for many cities. Some rivers are perennial while others are non-perennial. The former rivers are snow fed and hence, water flows in such rivers for all the seasons. The latter type of rivers dries in summer either wholly or partly and in monsoon, heavy flood visits them. For such types of rivers, it is desirable to store the excess water of flood in monsoons by constructing dams across such rivers. This stored water may then be used in summer.

In order to ascertain the quantity of water available from the river, the discharges at various periods of the year are taken and recorded. The observations over a number of years serve as a good guide for estimating the quantity of water available from the river in any particular period of the year. Generally, the quantity of water available from non-perennial rivers is variable throughout the year and it is likely to fall down in hot season when demand of water is maximum. It becomes therefore essential to augment such source of water supply by some other sources so as to make the water supply scheme successful.

The quality of surface water obtained from rivers is not reliable. It contains silt and suspended impurities. When completely or partly treated sewage is being discharged into the river at some upstream point, the river water is to be suspected for high contamination. The river water requires to be properly analyzed as regards to the contents of disease bacteria, harmful impurities, etc. The presence of all such undesirable elements in river water requires an exhaustive treatment of water before it can be make fit for drinking purposes. It should however be noted that the quality of river water is subject to the widest variations because it depends on various uncertain factors such as character of the catchment area, the discharges of sewage and industrial wastes, climatic conditions, season of the year, etc. The character of the water differs not only with each individual river, but also at many points along the course of the same river. It is usually found that the quality of river water at its head is good, but it goes on deteriorating as the river proceeds along its course. The chief use points to be considered in investigating a river supply of water are as follows.

• Adequacy of storage of purified water so as not to disturb the distribution system during periods of fold when the river water is turbid
• Efficiency of the subsequent stages of purification system adopted
• General nature of river, the rate of flow and the distance between the sources of pollution and the intake of the water
• Relative proportions of the polluting matter and the flow of river when at its minimum.

4) Storage Reservoirs

An artificial lake formed by the construction of dam across a valley is termed as a storage reservoir. It essentially consists of the following three parts

• A dam to hold water
• A spillway to allow the excess water to flow and
• A gate chamber containing necessary valves for regulating the flow of water

At present, this is rather the chief source of water supply schemes for very big cities. The multi-purpose reservoirs also make provisions for other uses in addition to water supply such as irrigation and power generation.

5) Oceans

Normally, it is not used as water supply source.

Underground Sources

In this type of source, the water that has percolated into the ground is brought on the surface. The entrance of rain water or melted snow into the ground is referred to as infiltration. The movement of water after entrance is called percolation. It is observed that the surface of earth consists of alternate courses of pervious and impervious strata. The pervious layers are those through which water can easily pass while it is not possible for water to go through an impervious layer. The pervious layers are known as aquifers or water-bearing strata. If aquifer consists of sand and gravel strata, it gives good supply of drinking water. The aquifer of limestone strata can supply good amount of drinking water, provided there is presence of cracks or fissures in it.

Forms of Underground Sources

Following are the four forms in which underground sources are found.

1) Infiltration Galleries

An infiltration gallery is a horizontal or nearly horizontal tunnel which is constructed through water bearing strata. It is sometimes referred to as horizontal well. Horizontal tunnels constructed at shallow depth (3-5m). The gallery is usually constructed of brick walls with slab roof. The gallery obtains its water from water bearing strata by various porous drain pipes. These pipes are covered with gravel, pebble, etc. so as to prevent the entry of very fine material into the pipe.

The gallery is laid at a slope and the water collected in the gallery is led to a sump from where it is pumped and supplied to consumers after proper treatment. The manholes are provided along the infiltration gallery for the purposes of cleaning and inspection. The infiltration galleries are useful as sources of water supply when ground water is available in sufficient quantity just below ground level or so. The galleries are usually constructed at depth of about 5 to 10 metres from the ground level.

Fig. 1 Infiltration Gallery

2) Infiltration Wells

In order to collect large quantities of water, infiltration wells are sunk in series in the banks of river. The wells are closed at top and open at bottom. They are constructed of brick masonry with open joints. Various infiltration wells are connected by porous pipes to a sum well, called jack well.

3) Springs

Natural outflow of ground water at the earth’s surface forms a spring. A pervious layer sandwiched between two impervious layers form a spring. Springs are capable of supplying very small amount of water and is not recommended for water supplies. Good developed springs can sometimes use for water supply source for small towns, especially in hilly areas. The two types of springs are

a) Gravity Springs – when the ground water raises high and water overflows through the sides of a natural valley or depression.

b) Surface Springs – sometimes an impervious obstruction (or) stratum supporting the underground storages become inclined causing water table to go up and get exposed to the ground surfaces.

c) Artesian Springs – when water comes out of pressure it’s called artesian springs. Since water comes out of pressure; they are able to provide higher yields and may be considered as source of water supply.

Fig. 2 Spring

4) Open Well

Well is a vertical structure dug in ground for the purpose of bringing ground water to the earth’s surface. Open wells have comparatively large diameters and lower discharges. Usually they have discharge of 20 m³/hr but if constructed by efficient planning it gives discharge of 200-300 m³/hr. They are constructed of diameter of about 1-10 m and have depth of about 2-20m. They are constructed by digging therefore they are also known as dug wells. It can put 8 to 10 cm diameter bore hole in the center of the well, to extract additional water. Types of open wells are given below.

a) Shallow Open Well - These are the wells resting on the water bearing strata and gets their supplies from the surrounding materials.

b) Deep Open Well - These are the wells resting on the impervious layer known as mota layer beneath which lies water bearing pervious layer and gets their supply from this layer.

c) Kachha wells - These type of wells is only constructed when water table is high as these type of wells sometimes collapses.

d) Driven Well - This is a shallow well, constructed by driving a casting pipe of 2.5cm – 15cm in diameter. The bottom end of casting pipe is pointed known as well point or drive point. The discharge of these wells is very small and these are suitable for domestic purpose only.

5) Tube Well

A tube well is a long pipe sunk in ground intercepting one or more water bearing strata. As compared to open well their diameter is less about 80-600 mm. It is deep as 70 to 300m and tap more than one aquifer. Types of tube wells are given below.

a) Shallow Tube Well - These are the tube which has depth limited to 30 meters and maximum have discharge of 20 m³/hr.

b) Deep Tube Well - These are the tube wells which have maximum depth of about 600 m and may give discharge more than 800 m³/hr.

c) Strainer type Tube Well - These is most commonly used tube such that in general a tube well means strainer tube well. In this type of well, a strainer which a wire mesh with small openings is wrapped around the main pipe which also has large openings such that area of opening in strainer and main pipe remains same. Annual space is left between two strainers so that the open area of pipe perforations is not reduced. The type of flow is radial.

Fig. 3 Strainer type Tube Well

d) Cavity type Tube Well - A cavity type tube well consists of a pipe sunk in ground up to the hard clay layer. It draws water from the bottom of well. In initial stages fine sand is also pumped with water and in such manner a cavity is formed at the bottom so the water enters from the aquifer into the well through this cavity.

Fig. 4 Cavity type Tube Well

e) Slotted type Tube Well - If the geological formation of the earth strata is such that; sufficient number of water bearing stratum are not available for the construction of strainer tube well even up to the depth of 100m, slotted type tube wells are constructed.

Fig. 5 Slotted type Tube Well

f) Perforated type Tube Well - When the water table is very near to the ground or the tube wells are required for obtaining water for short duration only, these types of tube wells are used. In these tube wells the pipes are made perforated by drilling holes in them.

Fig. 6 Perforated type Tube Well

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Chemical Oxygen Demand (COD)

One problem with the BOD test is that it takes 5 days to run. If the organic compounds were oxidized chemically instead of biologically, the test could be shortened considerably. Such oxidation can be accomplished with the chemical oxygen demand (COD) test. Because nearly all organic compounds are oxidized in the COD test, while only some are decomposed during the BOD test, COD results are always higher than BOD results. The common compounds which cause COD to be higher than BOD include sulphides, sulphites, thiosulfates and chlorides. One example of this is wood pulping waste, in which compounds such as cellulose are easily oxidized chemically (high COD) but are very slow to decompose biologically (low BOD).

This test is carried out on the sewage to determine the extent of readily oxidizable organic matter, which is of two types.

1. Organic matter which can be biologically oxidized is called biologically active.
2. Organic matter which cannot be oxidized biologically is called biologically inactive.

COD gives the oxygen required for the complete oxidation of both biodegradable and non-biodegradable matter. COD is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. It is an indirect method to measure the amount of organic compounds in water. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution.

The standard COD test uses a mixture of potassium dichromate and sulphuric acid to oxidize the organic matter (HCOH), with silver (Ag+) added as a catalyst. A simplified example of this reaction is illustrated below, using dichromate (Cr₂0₇^2-) and hydrogen ions (H+). A sample is refluxed in strongly acidic solution with a known excess of potassium dichromate (K₂Cr₂O₇) for 2-3 hours. After digestion, the remaining unreduced K₂Cr₂O₇ is titrated with ferrous ammonium sulphate to determine the amount of K₂Cr₂O₇ consumed. Then, the oxidizable matter is calculated in terms of oxygen equivalent. This procedure is applicable to COD values between 40 and 400 mg/L.

A known amount of a solution of K₂Cr₂O₇ in moderately concentrated sulphuric acid is added to a measured amount of sample and the mixture is boiled in air. In this reaction, the oxidizing agent, hexavalent chromium (Cr^VI), is reduced to trivalent chromium (Cr^III). After boiling, the remaining Cr^VI is titrated against a reducing agent, usually ferrous ammonium sulphate. The difference between the initial amount of Cr^VI added to the sample and the Cr^VI remaining after the organic matter has been oxidized is proportional to the chemical oxygen demand.

Total Organic Carbon

Since the ultimate oxidation of organic carbon is to CO₂, the total combustion of a sample yields some information about the potential oxygen demand in an effluent sample. A common application of total organic carbon testing is to assess the potential for creating disinfection by-products. Disinfection by-products are the result of halogens (e.g., bromine, chlorine) or ozone interacting with naturally occurring organic carbon compounds during the drinking water disinfection process. For example, trihalomethane, a carcinogen, is created when halogens displace three hydrogen ions on methane. Water that is high in total organic carbon has a greater potential to develop disinfection by-products. Some of the organics can be removed by adding levels of treatment specific for organic carbon absorption. It is usually not economically feasible to remove all naturally occurring organics from finished drinking water.

Total organic carbon is measured by oxidizing the organic carbon to CO₂ and H₂0 and measuring the CO₂ gas using an infrared carbon analyser. The oxidation is done by direct injection of the sample into a high-temperature (680-950°C) combustion chamber or by placing a sample into a vial containing an oxidizing agent such as potassium persulfate, sealing and heating the sample to complete the oxidation, then measuring the C02 using the carbon analyser.

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Biochemical Oxygen Demand (BOD)

The rate of oxygen use is commonly referred to as Biochemical Oxygen Demand (BOD). Biochemical Oxygen Demand is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it is considered as an indication of the quality of a water source. Biochemical oxygen demand is not a specific pollutant, but rather a measure of the amount of oxygen required by bacteria and other microorganisms engaged in stabilizing decomposable organic matter over a specified period of time. The BOD test is often used to estimate the impacts of effluents that contain large amounts of biodegradable organics such as that from food processing plants and feedlots, municipal wastewater treatment facilities and pulp mills. A high oxygen demand indicates the potential for developing a dissolved oxygen sag as the microbiota oxidize the organic matter in the effluent. A very low oxygen demand indicates either clean water or the presence of a toxic or non-degradable matter.

The BOD test was first used in the late 1800s by the Royal Commission on Sewage Disposal as a measure of the amount of organic pollution in British rivers. At that time, the test was standardized to run for 5 days at 18.3⁰C. These numbers were chosen because none of the British rivers had headwater-to-sea travel times greater than 5 days and the average summer temperature for the rivers was 18.3⁰C. Accordingly, this should reveal the "worst case" oxygen demand in any British river. The BOD incubation temperature was later rounded to 20⁰C, but the 5-day test period remains the current as standard.

In its simplest version, the 5-day BOD test begins by placing water or effluent samples into two standard 60 - or 300 - mL BOD bottles (Fig.1). One sample is analysed immediately to measure the initial dissolved oxygen concentration in the effluent, often using a Winkler titration. The second BOD bottle is sealed and stored at 20°C in the dark. (The samples are stored in the dark to avoid photosynthetic oxygen generation). After 5 days the amount of dissolved oxygen remaining in the sample is measured. The difference between the initial and ending oxygen concentrations is the BOD.

Fig.1 Biochemical Oxygen Demand (BOD) Bottle

BOD is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it is considered as an indication of the quality of a water source. It is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20°C or 3 days of incubation at 27°C. The BOD test must be inhibited to prevent oxidation of ammonia. If the inhibitor is not added, the BOD will be between 10% and 40% higher than can be accounted for by carbonaceous oxidation.

Stages of Decomposition in the BOD test

There are two stages of decomposition in the BOD test: a carbonaceous stage and a nitrogenous stage. The carbonaceous stage represents oxygen demand involved in the conversion of organic carbon to carbon dioxide. The second stage or the nitrogenous stage represents a combined carbonaceous plus nitrogenous demand, when organic nitrogen, ammonia and nitrite are converted to nitrate. Nitrogenous oxygen demand generally begins after about 6 days. Under some conditions, if ammonia, nitrite and nitrifying bacteria are present, nitrification can occur in less than 5 days. In this case, a chemical compound that prevents nitrification is added to the sample if the intent is to measure only the carbonaceous demand. The results are reported as carbonaceous BOD (CBOD) or as CBOD5 when a nitrification inhibitor is used.

BOD – Dilution Method

BOD is the amount of oxygen (Dissolved Oxygen (DO)) required for the biological decomposition of organic matter. The oxygen consumed is related to the amount of biodegradable organics. When organic substances are broken down in water, oxygen is consumed

              Organic Carbon + O₂ → CO2

Where, organic carbon in human waste includes protein, carbohydrates, fats, etc. Measure of BOD = Initial oxygen- Final Oxygen after (5 days at 20 °C) or (3 days at 27 °C). Two standard 300 mL BOD bottles are filled completely with wastewater. The bottles are sealed. Oxygen content (DO) of one bottle is determined immediately. The other bottle is incubated at 20°C for 5 days or (or at 27 °C for 3 days) in total darkness to prevent algal growth. After which its oxygen content is again measured. The difference between the two DO values is the amount of oxygen consumed by micro-organisms during 5 days and is reported as BOD5.

Where, DOi and DOf are initial and final DO concentrations of the diluted sample, respectively. P is called as dilution factor and it is the ratio of sample volume (volume of wastewater) to total volume (wastewater plus dilution water). 

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