Water Treatment (Water Supply Engineering)

CHAPTER – VI WATER TREATMENT 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 layout of conventional water treatment is as follows:

6.1 Objectives of Water Treatment a. b. c. d. e. f.

To remove the colour, odour (taste causing substances) To remove the turbidity present in water To remove pathogenic organisms To remove hardness To make water potable To prevent the spread of diseases

6.2 Treatment Processes and Impurity Removal 1. 2. 3. 4. 5. 6. 7.

8. 9.

SCREENING: Bulky and floating suspended matters are removed by the process of screening. PLAIN SEDIMENTATION: Heavy and coarse suspended matters are removed by the process of plain sedimentation. SEDIMENTATION WITH COAGULATION: This process helps to remove fine suspended and colloidal matters. FILTRATION: It is the most important stage in the purification process of water. It removes very fine suspended impurities and micro-organisms. DISINFECTION: It is carried out to eliminate or reduce pathogenic micro-organisms that have remained after the process of filtration. SOFTENING: Removes hardness of water. AERATION: Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due to algae and related organisms. Aeration also oxidize iron and manganese, increases dissolved oxygen content in water, removes CO2 and reduces corrosion and removes methane and other flammable gases. Removal of Fe and Mn. Removal of other harmful constituents.

6.3 Screening 6.3.1 Purpose The function of screening is to remove large floating, suspended and settleable solids. The treatment devices for the purpose of screening include bar racks and screens of various description. 6.3.2 Coarse, Medium and Fine Screens

COARSE SCREENS: Coarse screens are called racks, are usually bar screens, composed of vertical or inclined bars spaced at equal intervals across a channel through which water flows. Bar screens with relatively large openings of 75 to 150 mm are provided ahead of pumps, while those ahead of sedimentation tanks have smaller opening of 50 mm. 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. Mechanically cleaned racks are generally erected almost vertically.

MEDIUM SCREENS: Medium screens have clear opening 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 flow.

FINE SCREENS: 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. They are used to remove smaller suspended impurities at the surface or ground water intakes, sometimes alone or sometimes following a bar screen. In case of surface intakes, fine screens are usually arranged with rotary drum perforated with holes and are called rotary drum strainer. Micro strainer also can be used for this purpose where some device is set up to clean continuously so that fine screens do not get clogged up. Fine screens normally get clogged and are to be cleaned frequently. So they are avoided nowadays for surface intakes and fine particles are separated in sedimentation.

HEAD LOSS: 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:

h = 0.0729 (V2 - v2) 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:

h = b (W/b) 4/3 hv sin q where h = head loss, m b = 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 u/s and d/s face as semi-circular). W = maximum width of bar u/s of flow, m

b = minimum clear spacing between bars, m hv = velocity head of flow approaching rack, m = v2/2g q = angle of inclination of rack with horizontal The head loss through fine screen is given by h = (1/2g) (Q/CA) where, h = head loss, m Q = discharge, m3/s C = coefficient of discharge (typical value 0.6) A = effective submerged open area, m2 6.4 Plain Sedimentation When the impurities are separated from suspending fluid by action of natural forces alone i.e. by gravitation and natural aggregation of the settling particles, the operation is called plain sedimentation. 6.4.1 Purpose The main purpose of plain sedimentation is to remove large amounts of suspended solids present in raw water. It is done after screening and before sedimentation with coagulation and located near the filter unites and in case of variation of demand it can be used as the storage reservoir. 6.4.2 Theory of Settlement Principle of Sedimentation: Suspended solids present in water having specific gravity greater than that of water tend to settle down by gravity as soon as the turbulence is retarded by offering storage, thereby making easy to remove the sediments (called sludge) and floating matters (called scum). Basin in which the flow is retarded is called settling tank or sedimentation tank or settling basin or sedimentation basin. Theoretical average time for which the water is detained in the settling tank is called the detention period/time or retention period/time. The sedimentation is affected by: i. ii. iii. iv. v.

Velocity of flowing water Size, shape and specific gravity of particles Viscosity of water Detention time Effective depth and length of settling zone

vi.

Inlet and outlet arrangements

Types of Settling Type I: Discrete Particle Settling: Particles settle individually without interaction with neighbouring particles. Type II: Flocculent Particles: Flocculation causes the particles to increase in mass and settle at a faster rate. Type III: Hindered or Zone Settling: The mass of particles tends to settle as a unit with individual particles remaining in fixed positions with respect to each other. Type IV: Compression: The concentration of particles is so high that sedimentation can only occur through compaction of the structure. 6.4.2.1 Derivation of Stoke’s Law In Discrete Particle Settling, particles settle individually without interaction with neighbouring particles. Size, shape and specific gravity of the particles do not change with time. Settling velocity remains constant. If a particle is suspended in water, it initially has two forces acting upon it.

If the density of the particle differs from that of the water, a net force is exerted and the particles are accelerated in the direction of the force: (

)

This net force becomes the driving force. Once the motion has been initiated, a third force is created due to viscous friction. This force, called the drag force, is quantified by:

Because the drag force acts in the opposite direction to the driving force and increases as the square of the velocity, acceleration occurs at a decreasing rate until a steady state velocity is reached at a point where the drag force equals the driving force:

(

)

For spherical particles,

Thus, (

√(

)

)

Also, we have, ( ) Hence, (

√(

)

)

The above equation is called Hazen’s Equation and applicable for particles having diameter greater than 0.1 and less than 1 mm and Reynold’s Number ‘Re” greater than 1 and less than 1000. The nature of settling is neither laminar nor turbulent and so the settling is called transition settling. Expressions for CD change with characteristics of different flow regimes. (

)

(

√ (

) )

( Temperature ‘T’ (°C) ‘ ’-kinematic viscosity (mm2/s or centistokes)

0 1.792

5 1.519

10 1.308

) 15 1.141

20 25 1.007 0.897

30 0.804

Hazen further indicated that for particles having diameter ‘d’ ≤ 0.1 mm and Reynold’s number ‘Re’ ≤ 1, Stoke’s Law is applicable. Mathematically,

Now,

Putting

, we get,

Thus, (

)

This is Stoke’s Equation. 6.4.2.2 Temperature Effect on Settlement Since kinematic viscosity of water depends on temperature; the settlement process also depends on temperature. Alternatively, if temperature ‘T’ is introduced in place of ‘ ’ in above formula, it can be expressed as: (

)

These equations are valid for d ≤ 0.1 mm and Re ≤ 1. In this range, settling of particles is laminar and so it is termed as laminar settling of particles. If the nature of settling of particles is turbulent (i.e. 1000 < Re ≤ 10000) and ‘d’ > 1 mm, the value of CD = 0.4. Then, Hazen’s equation becomes: ( ) (

)

√

(

)

This equation is called Newton’s Equation. 6.4.3 Ideal Sedimentation Tank 

Sedimentation tanks may function either intermittently or continuously. The intermittent tanks also called quiescent type tanks are those which store water for a certain period and keep it in complete rest. In a continuous flow type tank, the flow velocity is only reduced and the water is not brought to complete rest as is done in an intermittent type.



Settling basins may be either long rectangular or circular in plan. Long narrow rectangular tanks with horizontal flow are generally preferred to the circular tanks with radial or spiral flow.

In practice, settling occurs in flowing water. An ideal horizontal flow settling tank has the following characteristics:  At the inlet, the suspension has a uniform composition over the cross-section of the tank.  The horizontal velocity ‘vo’ is the same in all parts of the tank.  A particle that reaches the bottom is definitively removed from the process. 6.4.4 Types of Sedimentation Tank Sedimentation tanks are generally made of RCC and may be rectangular or circular in shape. According to the method of function or operation, they are classified into: i. ii.

Quiescent or fill and draw type Continuous flow type

Quiescent or Fill and Draw Type This tank is normally rectangular in plan. The water is first filled and then allowed for some retention period of 30 to 60 hours (normally 24 hours) for sedimentation of particles. The clear water is drawn from outlet and the tank is then emptied and cleaning of sediments is done. After cleaning, again the filling and emptying process is similarly repeated. These tanks need more detention period, more labour and supervision. More than one tank is required and head loss is high; hence, these tanks are not used nowadays. Continuous Flow Type Raw water is admitted continuously through inlet and allowed to flow slowly in the tank for continuous settlement, cleaning and clear water continuously flows out through outlet. These tanks work under the principle that by reducing the velocity of flow of water, large amounts of particles present in water can be made to settle down. The velocity of flow of water in these tanks is reduced by providing sufficient length of travel for water in the tank. Further, the velocity of flow of water in these tanks is so adjusted that the time taken by particles of water to move from inlet to outlet is slightly more than that required for settling of suspended particles in water. Continuous flow type sedimentation tanks may be rectangular, circular or square in shape. a. Horizontal Flow Type b. Vertical Flow Type

Long Rectangular Settling Basin

 

Long rectangular basins are hydraulically more stable and flow control for large volumes is easier with this configuration. A typical long rectangular tank has length ranging from 2 to 4 times its width. The bottom is slightly sloped to facilitate sludge scraping. A slow moving mechanical sludge scraper continuously pulls the settled material into a sludge hopper from where it is pumped out periodically.

A long rectangular settling tank can be divided into four different functional zones: Inlet Zone: Region in which the flow is uniformly distributed over the cross section such that the flow through settling zone follows horizontal path. Settling Zone: Settling occurs under quiescent conditions. Outlet Zone: Clarified effluent is collected and discharged through outlet weir. Sludge Zone: For collection of sludge below settling zone. Inlet and Outlet Arrangements Inlet Devices: Inlets shall be designed to distribute the water equally and at uniform velocities. A baffle should be constructed across the basin close to the inlet and should project several feet below the water surface to dissipate inlet velocities and provide uniform flow. Outlet Devices: Outlet weirs or submerged orifices shall be designed to maintain velocities suitable for settling in the basin and to minimize short-circuiting. Weirs shall be adjustable, and at least equivalent in length to the perimeter of the tank. However, peripheral weirs are not acceptable as they tend to cause excessive short-circuiting.

Circular Basins 



Circular settling basins have the same functional zones as the long rectangular basin, but the flow regime is different. When the flow enters at the centre and is baffled to flow radially towards the perimeter, the horizontal velocity of the water is continuously decreasing as the distance from the centre increases. Thus, the particle path in a circular basin is a parabola as opposed to the straight line path in the long rectangular tank. Sludge removal mechanisms in circular tanks are simpler and require less maintenance.

Vertical Flow Type Sedimentation Tank

These tanks may be square or circular in shape at the top and have hopper bottom. So it is also called hopper bottom tank. The flow of water in this tank is vertical. Water enters into the tank through centrally placed pipe and by the action of deflector box, it travels vertically downwards. The sludge is collected at the bottom and removed from the sludge pipe with pump. The clear water flows out through a circumferential weir discharging into the draw off channel. 6.4.5 Design of Sedimentation Tank Design of sedimentation tank needs the following: a. Inlet Zone with appropriate Inlet Structure: Suitable inlet structure should be designed. It is kept at the halfway between the surface and the floor of the tank and mid of the width of the water depth. The length of the inlet zone is taken as 0.2 to 1 m according to velocity.

b. Outlet Zone with appropriate Outlet Structure: Suitable outlet structure should be designed. The length of the outlet zone is taken as 0.2 to 1 m according to velocity. c. Sludge Zone: The zone in the bottom of the tank in which sludge is retained before being removed is called sludge zone. The depth of the sludge zone depends upon the quantity of sediments in the raw water and the de-sludging period. Depth of sludge zone is taken as 0.5 to 1.5 m (generally 1 m). d. Free Board: The free space left on the top of the water level on the tank is called free board (FB) and in design FB is taken as 0.1 to 1 m (generally 0.3 to 0.5 m). e. Others such as Baffles, Washout/Drain and Overflow etc.: Baffle walls are provided to improve L/B ratio without increasing tank size. Washout is provided at the bottom of the sloped portion for drain at cleaning. Overflow is provided just below from the inlet in suitable side for overflow. f. Settling Zone or Effective Zone: Actual settlement occurs in this zone. Hence, effective dimensions [effective length (l), width (b) and effective depth (d)] of this zone is very important for design.

Settling Operations 

Particles falling through the settling basin have two components of velocity: 1.

(

)

2. The path of the particle is given by the vector sum of horizontal velocity (vh) and vertical settling velocity (vt).



Assume that a settling column is suspended in the flow of the settling zone and that the column travels with the flow across the settling zone. Consider the particle in the batch analysis for type-1 settling which was initially at the surface and settled through the depth of the column Zo, in the time to. If to also corresponds to the time required for the column to be carried horizontally across the settling zone, then the particle will fall into the sludge zone and be removed rom the suspension at the point at which the column reaches the end of the settling zone. All particles with vt > vo will be removed from suspension at some point along the settling zone.



Now consider the particle with settling velocity < vo. If the initial depth of this particle was such that Zp/vt = to, this particle will also be removed. Therefore, the removal of suspended particles passing through the settling zone will be in proportion to the ratio of the individual settling velocities to the settling velocity vo. The time to corresponds to the retention time in the settling zone.

Thus, the depth of the basin is not a factor in determining the size of particles that can be removed completely in the settling zone. The determining factor is the quantity Q/As, which has the units of velocity and is referred to as the overflow rate (SOR – Surface Overflow Rate or Surface Loading Rate) qo. This overflow rate is the design factor for settling basins and corresponds to the terminal settling velocity of the particle that is 100% removed. As = effective surface area of tank Removal Efficiency of Sedimentation Tank Let, is the settling velocity of smaller particles less than SOR (i.e. ( )) and if out of xo particles, x particles settle down and are removed, the ratio of removal of these particles (x/xo) is called removal efficiency of sedimentation tank for discrete particles of same size and is given by,

(

)

Where, is SOR and represents the settling velocity of the slowest particles, which are 100% removed.

Design Criteria of Sedimentation Tank/Design Details 1. 2. 3.

4. 5. 6.

Detention Period: For plain sedimentation: 3 to 4 hours, and for coagulate sedimentation: 2 to 2.5 hours Velocity of Flow: Not greater than 30 cm/min (horizontal flow) Tank Dimensions: L:B = 3 to 5:1. Generally L = 30 m (common); maximum 100 m. Breadth = 6 to 10 m. Circular: Diameter not greater than 60 m. Generally 20 to 40 m. Depth 2.5 to 5.0 m (3 m). SOR: For plain sedimentation: 12000 to 18000 L/d/m2 tank area; for thoroughly flocculated water: 24000 to 30000 L/d/m2 tank area. Slopes: Rectangular 1% towards inlet and circular 8%.

6.5 Sedimentation with Coagulation/Clarification General Properties of Colloids 1. Colloidal particles are so small that their surface area in relation to mass is very large. 2. Electrical Properties: All colloidal particles are electrically charged. If electrodes from a D.C. source are placed in a colloidal dispersion, the particles migrate towards the pole of opposite charge. 3. Colloidal particles are in constant motion because of bombardment by molecules of dispersion medium. This motion is called Brownian motion (named after Robert Brown who first noticed it). 4. Tyndall Effect: The Tyndall effect, also known as Tyndall scattering, is light scattering by particles in a colloid or particles in a fine suspension. 5. Adsorption: Colloids have high surface area and hence have a lot of active surface for adsorption to occur. The stability of colloids is mainly due to preferential adsorption of ions. There are two types of colloids: i. ii.

Lyophobic Colloids: that is solvent hating. Lyophilic Colloids: that is solvent loving.

6.5.1 Purpose  

Colloidal particles are difficult to separate from water because they do not settle by gravity and are so small that they pass through the pores of filtration media. To be removed, the individual colloids must aggregate and grow in size.

The settling down and removal of such fine suspended particles and colloidal matters can be achieved by chemically assisted sedimentation called sedimentation with coagulation or clarification. The chemicals added are called coagulants; the formed insoluble gelatinous precipitate is called floc; the process of adding coagulants to raw water and mixing it thoroughly is known as coagulation and the process of formation of floc is called flocculation. If the content of suspended solids in raw water is greater than 50 mg/l, the sedimentation with coagulation is used to effect more complete removal of the suspended matters. 6.5.2 Coagulants (types and their chemical reactions) The following chemicals are used as coagulants:

1. 2. 3. 4.

Aluminium sulphates or alum Iron salts Chlorinated copperas Sodium aluminate

The dose of coagulants depends upon turbidity, colour, pH, temperature and the time of the settlement. 1. Aluminium Sulphates or Alum [Al2(SO4)3.18H2O] It is the commonly used coagulant for coagulation in water in which alum is added and for alum water shall contain some alkalinity. If bicarbonate alkalinity is present in water, the floc formed is given by: (

)

(

)

(

) (

)

If raw water contains little or no alkalinity, then either lime (hydrated lime) or soda ash is added for alkalinity. Then, ( (

)

(

)

)

( (

) (

) (

) )

Amount of alum required depends upon turbidity and colour of raw water. Usual dose is 5 mg/l for relatively clear water to 30 mg/l for highly turbid water. Average dose for normal water is 14 mg/l but amount to be added is determined by jar test. Advantages: i. ii. iii. iv.

It forms excellent floc which is better than that formed by any other coagulant. The floc formed is stable and not broken easily. It is relatively cheap and removes colour, odour and taste. It doesn’t require skilled supervision and produces clear and crystal free water.

Disadvantages: i. ii. iii.

It requires alkalinity ranging pH from 6.5 to 8.5 in water for effective use. The product calcium sulphate may cause permanent hardness and carbon dioxide may cause corrosion. Difficult to dewater the heavy sludge formed because it is not suitable for filling in the low levels.

2. Iron Salts The various iron salts used as coagulants are ferrous sulphates, ferric sulphates and ferric chloride. 1.

Ferrous Sulphates [FeSO4.7H2O]

It is also known as copperas and used as coagulant in conjunction with lime.

When ferrous sulphates is added first (with bicarbonate alkalinity) ( (

)

) (

(

)

)

(

)

When lime is added first (

)

(

)

In above equation, Fe(OH)2 is unstable and absorbs dissolved oxygen and forms the stable floc. (

)

(

) (

)

The effective range of pH value for coagulation with ferrous sulphates and lime is 8.5 and above. 2. Ferric Sulphates [Fe2(SO4)3] It is also used as a coagulant in conjunction with lime and the reaction is: (

)

(

)

(

) (

)

The effective range of pH for coagulation with ferric sulphates is 4 to 7. 3. Ferric Chloride [FeCl3] It is used as a coagulant in conjunction with lime or without lime. Reactions: When used without lime: (

)

When used with lime: (

)

(

)

The effective range of pH for coagulation with ferric chloride is 3.5 to 6.5. Advantages of Iron Salts: 1.

Produces fast forming, denser, quick settling and less breakable floc than alum at low temperature. 2. Can be used in the wider range of pH and ferric chlorides and/or ferric sulphates may remove manganese at > 9 pH. 3. Ferric chloride is effective in removing H2S, taste and colour. Disadvantages of Iron Salts:

1.

Additional lime increases the treatment cost and iron salts impart more corrosiveness to water than alum. 2. Iron salts are difficult to handle due to corrosiveness and require skilled supervision on application. 3. Promotes the growth of bacteria in distribution system. 4. It is suitable for sewage treatment.

3. Chlorinated copperas [FeCl3.Fe2(SO4)3] The mixture of ferric chloride and ferric sulphate is called chlorinated copperas and prepared by adding 1 part chlorine to 7.8 part ferrous sulphate. [

]

[

(

) ]

This chlorinated copperas when added to water forms a tough floc which is removed in sedimentation. The effective range of chlorinated copperas is from pH of 6 to 8. It is effective to remove colour but very corrosive and common in sewage treatment. 4. Sodium Aluminate [Na2Al2O4] It is sometimes used as coagulant. This chemical when dissolved and mixed with water reacts with salts of calcium and magnesium and forms the precipitate of calcium and magnesium aluminate. (

)

Similar chemical reactions take place in case of Mg salts. The effective range of pH for coagulation with sodium aluminate is 6 to 8.5. This removes temporary and permanent hardness but very costly and not used in public water supplies and used to treat boiler water. 6.5.3 Mixing Devices (Purpose and Types) Following operations are involved in sedimentation with coagulation: 1. 2. 3. 4.

Feeding the coagulant Mixing of coagulant Flocculation Sedimentation

1. Feeding the Coagulant

The coagulant may be fed to raw water either in powder form (called dry feeding) or in solution form (called wet feeding). Coagulant feeding is done using dry feeding or wet feeding devices. The choice between wet and dry feeding depends on: a. Characteristics of coagulant and convenience of its application. b. Dosages of coagulants: High dose-dry feed and vice versa. c. Size of the treatment plant: Large size-wet feeder is used and vice versa. 2. Mixing of Coagulant After the addition of coagulants to raw water, they are thoroughly and vigorously mixed so that the coagulants get fully dispersed into the entire mass of water. Various mixing devices are as follows but the first two types are common: I. II. III. IV. V. VI.

Mixing basin with baffle walls Mixing basin with mechanical means Mixing channels Hydraulic jump method Compressed air method Centrifugal pumping method

a. Mixing Basin with Baffle Walls

These are the rectangular basins or tanks which are provided with baffle walls. The disturbance created by the presence of baffle walls in the path of following water cause vigorous agitation of water which resolves in through mixing of water with coagulant. Such basins are of two types: i. Horizontal or Round End Type The mixture and coagulants after entering the basin through an inlet provided at one end of the basin, flows horizontally for short distance and due to the presence of baffle walls; it takes turn and moves further as shown by the arrows and comes out through an outlet provided at the other end of the basin to the flocculator. ii. Vertical or Over and Under Type The mixture of water and coagulant after entering the basin through an inlet provided at one end of the basin, flows up and down as shown by the arrow due to the presence of vertical walls projecting alternatively from the roof and the floor of the basin. Ultimately it flows out through outlet at the other end of the basin to the flocculator. Mixing basin should be properly designed to get the desired effect. The various considerations for design are: i. ii. iii. iv.

Velocity of flow in the channel should be between 0.15 m/s to 0.45 m/s (between baffle walls). Detention period kept is 20 to 50 minutes. Distance between successive baffle walls should be at least 0.45 m. End opening between end baffle walls and basin walls should be about 1.5 times the distance between the successive baffle walls subjected to minimum 0.6 m.

b. Mixing Basin with Mechanical Means

Mixing basin with baffle walls are used only for small treatment plant but for large plant mixing basins with mechanically driven paddles is used. It is also called flash mixture and consists of a deep circular or square tank which is provided with a propeller type impeller fixed at the lower end of a vertical shaft which is driven by electric motor. Diameter of impeller provided is 0.2 to 0.4 times the tank diameter and impeller rotation speed is of 100 rpm. 3. Flocculation (6.5.4 Flocculation tanks)

Longitudinal Flow Flocculator

Vertical Flow Flocculator

From the mixing basin, water is taken to the flocculator for flocculation. In a flocculator, slow stirring of water is brought about to permit build up of the floc particles. There are various types of flocculators but the mechanical flocculators are most commonly used. Mechanical flocculator consists of a tank provided with paddles for stirring of water; hence, it is called paddle flocculator. Depending upon the direction of flow of water in the tank, the mechanical flocculator may be longitudinal or vertical flow flocculator. A longitudinal flow flocculator consists of a rectangular tank provided with paddles revolving on a horizontal shaft as shown in figure. A vertical flow flocculator consists of a circular tank provided with paddles revolving on a vertical shaft as shown in second figure above. The paddles are moved by electric motor. The water coming from mixing basins enters the flocculator through an inlet and leaves through outlet to sedimentation tank. In longitudinal flow flocculator, inlet and outlet are provided near the top of tank in opposite end but in vertical type, inlet is provided at the bottom and outlet is provided near the top in opposite end. Design Criteria: A. B. C. D. E.

Depth of tank = 3 to 4.5 m Detention period = 10 to 45 minutes (30 min common) Velocity of flow = 0.2 to 0.8 m/min (0.4 m/min common) Total area of paddles = 10 to 25 % (15% common) of X-section of area of tank Outflow velocity = 0.15 to 0.25 m/s to prevent settling or breaking of floc.

4. Sedimentation (6.5.5 Clarifier) The water from the flocculator is taken to the sedimentation tank also called the sedimentation tank or clarifier. It consists of floc chamber and sedimentation tank. The detention period for floc chamber is about 15 to 40 minutes and that for sedimentation tank is about 3 to 4 hours. The surface overflow rate is from 20 to 40 meter cube/meter square/day. The depth of floc chamber is usually kept about half of the depth of the sedimentation tank. The cleaning of this tank is usually carried out at an interval of 3 to 6 months. 6.5.6 Jar Test The jar test is a common laboratory procedure used to determine the optimum operating conditions for water or wastewater treatment. This method allows adjustments in pH, variations in coagulant or polymer dose, alternating mixing speeds, or testing of different coagulant or polymer types, on a small scale in order to predict the functioning of a large scale treatment operation. Jar Testing Apparatus The jar testing apparatus consists of six paddles which stir the contents of six 1 litre containers. One container acts as a control while the operating conditions can be varied among the

remaining five containers. An rpm gage at the top centre of the device allows for the uniform control of the mixing speed in all of the containers.

The jar test procedures involve the following steps: 

  



Fill the jar testing apparatus containers with sample water. One container will be used as a control while the other 5 containers can be adjusted depending on what conditions are being tested. For example, the pH of the jars can be adjusted or variations of coagulant dosages can be added to determine optimum operating conditions. Add the coagulant to each container and stir at approximately 100 rpm for 1 minute. The rapid mix stage helps to disperse the coagulant throughout each container. Turn off the mixers and allow the containers to settle for 30 to 45 minutes. Then measure the final turbidity in each container. Reduce the stirring speed to 25 to 35 rpm and continue mixing for 15 to 20 minutes. This slower mixing speed helps promote floc formation by enhancing particle collisions which lead to larger flocs. Residual turbidity vs. coagulant dose is then plotted and optimal conditions are determined. The values that are obtained through the experiment are correlated and adjusted in order to account for the actual treatment system.

6.6 Filtration 6.6.1 Purpose The resultant water after sedimentation will not be pure and may contain some very fine suspended particles and bacteria in it. To remove or to reduce the remaining impurities still further, the water is filtered through the beds of fine granular materials such as sand, etc. The process of passing the water through the beds of such granular materials is known as filtration. The main purpose of filtration is to remove colour, odour, taste, bacteria and colloidal impurities.

6.6.2 Theory of Filtration There are four basic filtration mechanisms: a. Mechanical Straining: Suspended matters larger than the size of interstices (voids between sand grains) cannot pass through them and are trapped or arrested and then removed which is called mechanical straining. Being smaller in size, colloidal matters or bacteria may not be strained. b. Sedimentation and Adsorption: The interstices between the sand grains act as very small sedimentation tanks where the suspended matters settle. Due to physical attraction between the suspended particles and sand grains and presence of gelatinous coating forded due to these matters, other suspended particles, colloidal matters and bacteria adhere there and are removed. c. Biological Metabolism: Organic matters such as algae, plankton also caught by voids between sand grains and these matters are used by bacteria for survival and convert them into harmless compounds from biological metabolism. These harmless compounds formed are deposited at the surface of sand and contains a zoological jelly called ‘dirty skin’ which further enhances in adsorbing and straining activities. Bacteria not only break organic impurities into harmless compounds but they destroy each other and make a balanced life in the filter. d. Electrolytic Action: As per ionic theory, when two substances of opposite charges come into contact, the charge is neutralized and in doing so, new chemical substances are formed. Sand particles in filter media also have charges of some polarity which attracts the suspended, colloidal and dissolved matters of opposing polarity in neutralizing and changes the chemical characteristics of water. After a long use, charges in the sand grains get exhausted and it becomes necessary to clean filter for regeneration of charges. 6.6.3 Types of Filter A tank or device with filter media used for the filtration is called filter. Sand, either fine or coarse, is generally used as filter media. Based on the filtration rate and driving force to overcome friction by water flowing to filter, filters are classified as: a) Slow Sand Filter (SSF) b) Rapid Sand Filter (RSF) c) Pressure Filter (PF) 6.6.3.1 Slow Sand Filter SSF consists of fine sand, supported by gravel. This earliest type of gravity filter has a slow rate of filtration (≤ 1/20th of that of RSF or PF). It captures particles near the surface of the bed and is usually cleaned by scraping away the top layer of sand that contains the particles. Pretreatment of water is not needed for < 20 NTU turbidity and can be directly fed to filter but if water has high turbidity, plain sedimentation or sometimes sedimentation with coagulation is done before feeding into SSF.

SSF consists of following: I.

II.

III.

IV.

V.

Enclosure Tank: It is open and water tight rectangular tank made of masonry or concrete. The depth is 2.5 to 3.5 m, surface area is 10 to 20 m2 or more (depends upon filtration rate), filtration rate is 100 to 200 lph/m2. The floor is provided at cross slope of 1 in 100 to 1 in 200 towards central drain. Filter Media: Consists of 90 to 110 cm thick sand layer with effective size (D10) of sand 0.25 to 0.35 mm (0.3 mm is common) and coefficient of uniformity (C u) of 3-5. Finer the sand better will be the removal of turbidity and bacterial removal efficiency but lowers the filtration rate. The sand should not contain >2% of Ca and Mg. Base Material: The sand layer is supported on base material of 30 to 75 cm thick gravel bed. The gravel beds are provided four layers of each about 15 cm thicknesses with size 3 to 6 mm, 6 to 20 mm, 20 to 40 mm and 40 to 65 mm from the top. Mid layers between top layer and bottom layer are called intermediate layers. Under Drainage System: It consists of central main drain and lateral drains 2 to 3 m apart and starts from 50 to 80 cm distance of walls of tank. Lateral drain consists of earthenware or perforated pipes laid with open joint or patented drain in slope. Appurtenances: For efficient working, vertical air pipes, depth controlling device, head loss measuring device, rate maintaining devices etc. are installed.

Principles of Slow Sand Filtration  

In a slow sand filter, impurities in the water are removed by a combination of processes: sedimentation, straining, adsorption, and chemical and bacteriological action. During the first few days, water is purified mainly by mechanical and physical-chemical processes. The resulting accumulation of sediment and organic matter forms a thin layer on the sand surface, which remains permeable and retains particles even smaller than the spaces between the sand grains.



As this layer (referred to as “Schmutzdecke”) develops, it becomes living quarters of vast numbers of micro-organisms which break down organic material retained from the water, converting it into water, carbon dioxide and other oxides. Most impurities, including bacteria and viruses, are removed from the raw water as it passes through the filter skin and the layer of filter bed sand just below. The purification mechanisms extend from the filter skin to approx. 0.3-0.4 m below the surface of the filter bed, gradually decreasing in activity at lower levels as the water becomes purified and contains less organic material. When the micro-organisms become well established, the filter will work efficiently and produce high quality effluent which is virtually free of disease carrying organisms and biodegradable organic matter.





They are suitable for treating waters with low colors, low turbidities and low bacterial contents. 6.6.3.2 Rapid Sand Filter

RSFs are the most commonly used gravity filters in large water supply system. They consist of larger sand grains supported by gravel and capture particles throughout the bed. They are cleaned by backwashing water through the bed to ‘lift out’ the particles. RSF consists of following: I.

Enclosure Tank: It is open and water tight rectangular tank made of masonry or concrete. The depth is 2.5 to 3.5 m, surface area is 10 to 50 m2, and filtration rate is 3000 to 6000 lph/m2. Various number of filter units in series may be provided. Minimum

II.

III.

IV.

V.

four units should be provided for large water supplies and minimum two for small supplies. The length to width ratio is 1.25 – 1.35. Filter Media: It should be free from dirt and clay. Consists of 60 to 90 cm thick sand layer with effective size (D10) of sand 0.35 to 0.60 mm and coefficient of uniformity (Cu) of 1.3-1.7. Base Material: The sand layer is supported on base material of 45 to 60 cm thick gravel bed. The gravel beds are provided four layers of each about 15 cm thicknesses with size 2 to 6 mm, 6 to 12 mm, 12 to 20 mm and 20 to 50 mm from the top. Under Drainage System: The under drainage system is provided to collect the filtered material and to provide uniform distribution for back water wash. Commonly used drainage systems are (i) Perforated Pipe System and (ii) Pipe and Strainer System. Appurtenances: Wash water trough, air compressor, rate control device, head loss indicator meters, valves, etc. are used.

(Consult a book for working of RSF.)

Backwashing of Rapid Sand Filter 

 

For a filter to operate efficiently, it must be cleaned before the next filter run. If the water applied to a filter is of very good quality, the filter runs can be very long. Some filters can operate longer than one week before needing to be backwashed. However, this is not recommended as long filter runs can cause the filter media to pack down so that it is difficult to expand the bed during the backwash. Treated water from storage is used for the backwash cycle. This treated water is generally taken from elevated storage tanks or pumped in from the clear well. The filter backwash rate has to be great enough to expand and agitate the filter media and suspend the floc in the water for removal. However, if the filter backwash rate is too high, media will be washed from the filter into the troughs and out of the filter.

When is Backwashing Needed? The filter should be backwashed when the following conditions have been met:   

The head loss is so high that the filter no longer produces water at the desired rate; and/or Floc starts to break through the filter and the turbidity in the filter effluent increases; and/or A filter run reaches a given hour of operation.

Operational Troubles in Rapid Gravity Filters Air Binding: 



When the filter is newly commissioned, the loss of head of water percolating through the filter is generally very small. However, the loss of head goes on increasing as more and more impurities get trapped into it. A stage is finally reached when the frictional resistance offered by the filter media exceeds the static head of water above the bed. Most of this resistance is offered by the





top 10 to 15 cm sand layer. The bottom sand acts like a vacuum, and water is sucked through the filter media rather than getting filtered through it. The negative pressure so developed, tends to release the dissolved air and other gases present in water. The formation of bubbles takes place which stick to the sand grains. This phenomenon is known as Air Binding as the air binds the filter and stops its functioning. To avoid such troubles, the filters are cleaned as soon as the head loss exceeds the optimum allowable value.

Formation of Mud Balls: 

The mud from the atmosphere usually accumulates on the sand surface to form a dense mat. During inadequate washing this mud may sink down into the sand bed and stick to the sand grains and other arrested impurities, thereby forming mud balls.

Cracking of Filters: 

The fine sand contained in the top layers of the filter bed shrinks and causes the development of shrinkage cracks in the sand bed. With the use of filter, the loss of head and, therefore, pressure on the sand bed goes on increasing, which further goes on widening these cracks.

Remedial Measures to Prevent Cracking of Filters and Formation of Mud Balls   

Breaking the top fine mud layer with rakes and washing off the particles. Washing the filter with a solution of caustic soda. Removing, cleaning and replacing the damaged filter sand.

Sand Filters vs. Rapid Sand Filters 



   



Base material: In SSF it varies from 3 to 65 mm in size and 30 to 75 cm in depth while in RSF it varies from 3 to 40 mm in size and its depth is slightly more, i.e. about 60 to 90 cm. Filter sand: In SSF the effective size ranges between 0.2 to 0.4 mm and uniformity coefficient between 1.8 to 2.5 or 3.0. In RSF the effective size ranges between 0.35 to 0.55 and uniformity coefficient between 1.2 to 1.8. Rate of filtration: In SSF it is small, such as 100 to 200 L/h/sq.m. of filter area while in RSF it is large, such as 3000 to 6000 L/h/sq.m. of filter area. Flexibility: SSF are not flexible for meeting variation in demand whereas RSF are quite flexible for meeting reasonable variations in demand. Post treatment required: Almost pure water is obtained from SSF. However, water may be disinfected slightly to make it completely safe. Disinfection is a must after RSF. Method of cleaning: Scrapping and removing of the top 1.5 to 3 cm thick layer is done to clean SSF. To clean RSF, sand is agitated and backwashed with or without compressed air. Loss of head: In case of SSF approx. 10 cm is the initial loss, and 0.8 to 1.2 m is the final limit when cleaning is required. For RSF 0.3 m is the initial loss, and 2.5 to 3.5 m is the final limit when cleaning is required.

6.6.3.3 Pressure Filter

(Consult any standard textbook for details.) 6.7 Disinfection 6.7.1 Purpose The filtered water may normally contain some harmful disease producing bacteria in it. These bacteria must be killed in order to make the water safe for drinking. The process of killing the pathogenic bacteria using chemicals called disinfectants is known as disinfection. Disinfection Kinetics: When a single unit of microorganisms is exposed to a single unit of disinfectant, the reduction in microorganisms follows a first order reaction.

This equation is known as Chick’s Law. N = number of microorganisms (No is initial number) K = disinfection constant T = contact time

6.7.2 Methods of Disinfection (introduction only) 1.

2.

3.

4.

5. 6. 7.

8. 9.

Chlorination: The germicidal action of chlorine is explained by the recent theory of enzymatic hypothesis, according to which the chlorine enters the cell walls of bacteria and kill the enzymes which are essential for the metabolic processes of living organisms. Boiling: The bacteria present in water can be destroyed by boiling it for a long time. However, it is not practically possible to boil huge amounts of water. Moreover, it cannot take care of future possible contaminations. Treatment with Excess Lime: Lime is used in water treatment plant for softening. But if excess lime is added to the water, it can, in addition, kill the bacteria also. Lime when added raises the pH value of water making it extremely alkaline. This extreme alkalinity has been found detrimental to the survival of bacteria. This method needs the removal of excess lime from the water before it can be supplied to the general public. Treatment like recarbonation for lime removal should be used after disinfection. Ozone Treatment: Ozone readily breaks down into normal oxygen and releases nascent oxygen. This nascent oxygen is a powerful oxidizing agent and removes the organic matter as well as the bacteria from the water. Iodine Treatment Bromine Treatment Potassium Permanganate Treatment: This is a common method of disinfection in rural areas, where mostly the water supplies are from wells which contain lesser amount of bacteria. Potassium permanganate is dissolved in a bucket of well water and the bucket full of this water is mixed with the well water thoroughly. Potassium permanganate not only kills the bacteria but it also helps in oxidizing the taste producing organic matter. Silver Treatment: Silver when immersed in water has been observed to exert an inhibiting action on bacterial life. UV Ray Treatment: Light is effective in killing both the active bacteria as well as spores.

6.7.3 Chlorination Chlorine is added to the water supply in two ways. It is most often added as a gas, Cl 2 (g). However, it also can be added as a salt, such as sodium hypochlorite (NaOCl) or bleach. Chlorine gas dissolves in water following Henry’s Law. ( )

(

)

Once dissolved, the following reaction occurs forming hypochlorous acid (HOCl): (

)

Hypochlorous acid is a weak acid that dissociates to form hypochlorite ion (OCl-).

All forms of chlorine are measured as mg/L of Cl2 (MW = 2 x 35.45 = 70.9 g/mol).

Hypochlorous acid and hypochlorite ion compose what is called the free chlorine residual. These free chlorine compounds can react with many organic and inorganic compounds to form chlorinated compounds. If the products of these reactions posses oxidizing potential, they are considered the combined chlorine residual. A common compound in drinking water systems that reacts with chlorine to form combined residual is ammonia. Reactions between ammonia and chlorine form chloramines, which is mainly monochloramine (NH2Cl), although some dichloramine (NHCl2) and trichloramine (NCl3) also can form. Many drinking water utilities use monochloramine as a disinfectant. If excess free chlorine exits once all ammonia nitrogen has been converted to monochloramine, chloramine species are oxidized through what is termed the breakpoint reactions. The overall reactions of free chlorine and nitrogen can be represented by two simplified reactions as follows: Monochloramine Formation Reaction: This reaction occurs rapidly when ammonia nitrogen is combined with free chlorine up to a molar ration of 1:1.

Breakpoint Reaction: When excess free chlorine is added beyond the 1:1 initial molar ratio, monochloramine is removed as follows: ( )

The formation of chloramines and the breakpoint reaction create a unique relationship between chlorine dose and the amount and form of chlorine as illustrated below:

Chlorine Demand

Free chlorine and chloramines readily react with a variety of compounds, including organic substances, and inorganic substances like iron and manganese. The stoichiometry of chlorine reactions with organics can be represented as shown below: HOCl:

OCl-:

NH2Cl:

The amount of free chlorine consumed in such oxidation is called chlorine demand of water. Chlorine Demand = Total amount of chlorine added – Amount of residual chlorine required after a specified contact period

Chlorine Dose The quantity of chlorine required to be added to water to leave 0.2 mg/l or ppm of freely available residual chlorine after 10 minutes of contact period is called optimum dose of chlorine. This is generally of about 1 ppm. 6.7.4 Types of Chlorine a. Bleaching Powder Bleaching powder of calcium hypochlorite Ca (OCl2) is a chlorinated lime and when mixed with water it dissociates and reacts with hydrogen ions in water.

This process of chlorination is called hypochloration. Bleaching powder contains 30 – 35 % of available chlorine and it is unstable and goes on losing when exposed to atmosphere. So it requires careful storing. Dose: For normal water, 0.5 to 2.5 kg/million litres of water. It is not adopted for large public water supply but can be used in small colonies and swimming pools. b. Chloramines

Chloramines are compounds formed by reaction between ammonia and chlorine. Ammonia, which is quickly soluble in water, is added just before chlorine (1 part ammonia + 4.5 part Cl).

(

) ( (

) )

The compounds are quite stable in water and remains in water as residual for a sufficient time so are more effective than chlorine alone. The reaction is slow hence water treated can only be supplied to consumer after 20 minutes to 1 hour of application. It does not cause bad taste and odour when left at residual but is weaker disinfectant than free chlorine disinfectant. c. Chlorine Gas or Liquid Gas or Free Chlorine Chlorine gas is fed directly to the point of application to the water supply in a pressure of 7 kg/cm2 or first dissolved in small flow of water than free chlorine disinfectant. Application of gas chlorine is less expensive but less satisfactory because of poor diffusion of chlorine and at lower temperature crystalline hydrates of Cl are formed and possibility of choking of pipes. Undissolved Cl may corrode pipes and valves. Hence it is not generally used. Chlorine gas or liquid chlorine dissolved in small quantities of water is normally used. d. Chlorine-dioxide In this method, chlorine dioxide gas if produced by passing chlorine gas through sodium chloride solution. The Cl is unstable and reacts as follows:

ClO2 has oxidizing capacity of 2.5 times than that of chlorine and most effective for removal of taste and odour. 6.7.5 Forms of Chlorination A. Plain Chlorination: Application of chlorine to plain or raw water is called plain chlorination. Dose of plain chlorination is 0.5 to 1 ppm. B. Pre Chlorination: Application of chlorine to water before the treatment is called prechlorination. Especially, we use before filtration. Dose adjusted for residual chlorine is 0.1 to 0.5 ppm. Advantages of pre-chlorination are as follows: i. ii. iii. iv. v.

Reduces the quantity of coagulants required. Reduces the bacterial load in the filters. Helps in maintaining and running filters longer. Controls the algae and planktons in the filter. Eliminates taste and odour.

C. Post Chlorination: Application of chlorine after the treatment is called post chlorination. It is applied after filtration and before entering into the distribution pipe. Doses are adjusted so that it leaves residual chlorine of 0.1 to 0.2 ppm. D. Double or Multiple Chlorination: When raw water contains large number of bacteria, chlorine is applied at two or more points in purification process; it is called double or multiple chlorination. It consists of pre-chlorination just before raw water enters the sedimentation tank and post-chlorination as water leaves the filter and before entering the distribution system. E. Breakpoint Chlorination: (Already described) Breakpoint chlorination has the following effects: i. ii. iii. iv.

It removes taste, colour and manganese. It has adequate bacterial effect. It has the desired residual chlorine. It completes the oxidation of ammonia and other compounds.

F. Super Chlorination: Application of chlorine beyond the breakpoint chlorination is called super chlorination. Generally 2 to 3 ppm beyond the breakpoint is applied for super chlorination. Super chlorination is done during epidemics in a certain locality due to water borne diseases. G. De-chlorination: The process of removing excessive chlorine from water before distribution to the consumers to avoid chlorine taste is known as de-chlorination. It is done either by aeration or adding sodium thiosulphate, sodium metabisulphate, sodium sulphite, sodium bisulphate, ammonia and sulphur dioxide. 6.7.6 Factors affecting efficiency of Chlorination The killing efficiency of bacteria due to chlorination is called bacterial efficiency of chlorine, which depends upon the following factors: 1. 2. 3. 4. 5. 6. 7.

Turbidity: If turbidity is present in water, bacterial efficiency is decreased. Hence, for effective chlorination, water should be turbidity free. Presence of metallic compound: More chlorine is utilized to oxidize metallic ions; therefore, bacterial efficiency is decreased. Ammonia compound: Efficiency is decreased due to formation of combined available chlorine but it lasts for longer time. pH value of water: If pH is high in water, efficiency is low because HOCl is formed at pH of 5 to 7. Temperature: If temperature decreases, amount of free available chlorine is decreased so the efficiency of chlorine is decreased. Time of contact: For effective chlorination, time of contact should be at least 30 minutes. Type, condition and concentration of micro-organism: For bacteria, efficiency is high for viruses, more concentration is required and efficiency is low. Efficiency becomes low if the favourable condition for bacteria is available and concentration of bacteria is high.

6.8 Softening 6.8.1 Purpose The main purpose of softening is to remove hardness in water. 6.8.2 Removal of Temporary Hardness 6.8.2.1 Boiling Method It is costlier and not used in public water supply. (

)

(

)

The insoluble precipitates

and

are removed by sedimentation.

6.8.2.2 Lime Treatment Method ( (

) )

The insoluble precipitates

( (

)

)

and

are removed by sedimentation.

6.8.3 Removal of Permanent Hardness 6.8.3.1 Lime Soda Method In this process, lime and sodium carbonate is added to water either separately or together and allow them for 30 – 60 minutes for reaction. It removes permanent as well as temporary hardness. ( ( (

) )

) (

(

)

(

)

)

(

)

(

)

(

)

(

)

(

)

( ) are insoluble and removed by sedimentation. Other products are soluble and and do not impart hardness. This process is economical but a large quantity of sludge is formed and requires skilled supervisor. 6.8.3.2 Zeolite Method It is also called Base Exchange or Ion Exchange process. It also removes temporary hardness and is a commonly used process. Zeolite is a natural or artificial granular substance. Natural zeolite is green in colour and artificial is white and commonly used is also called Permutit. The commonly used Permutit is sodium aluminium silicate ( ) and Permutit is manufactured from feldspar, kaolin and soda.

If we denote Permutit as

(

)

(

)

(Z is anionic component of the exchanger), then, ( {

) (

)

{

Here, the calcium and magnesium are replaced by sodium and water and the sodium salts formed are soluble in water but do not impart hardness and water is softened. The product and remains in zeolite.

Regeneration of Zeolite: Due to continuous use of zeolite, it exhausts and zeolite doesn’t work. It can be checked by soap solution test. If it gets exhausted, it needs regeneration by passing NaCl in it.

It can be written as: (

)

(

)

Advantages: 1. 2. 3. 4. 5.

No sludge is formed. Compact and small space is required. Because of simplicity in operation, no skilled persons are required. Hardness may reduce to zero. No other chemicals are required.

Disadvantages: 1. Not suitable for turbid water and water containing Fe and Mn. 2. Growth of bacteria occurs in zeolite if water contains organic matter. 6.8.3.3 Ionization Process/Demineralization Process It is a costlier process and used in industries. It is very similar to zeolite process but metallic ions are exchanged for hydrogen ions and substances like zeolite is used. The substances may be: 1. Acidic Resin (removes alkali substances) e.g. zero karb, organolite, catex etc. 2. Base Resin (removes acidic substances) If we denote these resins as H2R (H means hydrogen and R means organic part of the substance), then,

(

(

)

)

(

)

(

)

In above process, Ca and Mg are replaced by H and water gets softened.

Disadvantages: The disadvantage of this process is that acids are formed. If the quantity of acid is high, it is removed by:

i. ii. iii.

Diluting treated water with raw water. Neutralizing treated water with alkaline substance. Absorbing excessive acids by de-acidic substance ‘D’ regenerated with sodium or caustic soda. ( (

)

) ( (

)

)

6.9 Miscellaneous Treatments 6.9.1 Aeration 6.9.1.1 Purpose   

Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due to algae and related organisms. Aeration also oxidizes iron and manganese, increases dissolved oxygen content in water, removes carbon dioxide and reduces corrosion and removes methane and other flammable gases. 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.

6.9.1.2 Types of Aeration I. II. III. IV.

Free Fall or Gravity Aerators Spray Aerators Diffused Aerators Mechanical Aerators

Gravity Aerators In gravity aerators, water is allowed to fall by gravity such that a large area of water is exposed to atmosphere, sometimes aided by turbulence.

Fountain Aerators/Spray Aerators These are also known as spray aerators with special nozzles to produce a fine spray. Each nozzle is 2.5 to 4 cm diameter discharging about 18 to 36 l/h. Nozzle spacing should be such that each m3 of water has aerator area of 0.03 to 0.09 m2 for one hour.

Injection or Diffused Aerators It consists of a tank with perforated pipes, tubes or diffuser plates, fixed at the bottom to release fine air bubbles from compressor unit. The tank depth is kept as 3 to 4 m and tank width is within 1.5 times its depth. If depth is more, the diffusers must be placed at 3 to 4 m depth below water surface. Time of aeration is 10 to 30 min and 0.2 to 0.4 litres of air is required for 1 litre of water.

Mechanical Aerators Mixing paddles as in flocculation are used. Paddles may be either submerged or at the surface. 6.9.2 Removal of Iron and Manganese Iron and manganese mix into water through soils either in suspension, hydrated oxides, soluble or insoluble form. Ferric hydroxide is insoluble and ferrous bicarbonate is soluble in water. When iron and manganese > 0.3 ppm, they become objectionable. Insoluble ferric hydroxide is removed in sedimentation. Effects: i. ii.

Produces taste, odour and brown red colour. Stains on clothes, corrosion and clogging of pipes by accumulation of precipitates.

iii.

Causes difficulty in various industrial processes.

Methods: (a) By aeration: In this case, aeration is done before sedimentation. Fe: (

)

(

)

Fe (HCO3)2: (

)

Mn:

(b) By adding lime (c) Passing over manganese zeolite 6.9.3 Removal of Colour, Odour and Taste Colour, odour and taste are due to organic and vegetable matters, industrial waste, domestic sewage, dissolved gases and minerals, microorganisms, etc. Methods: The colour, odour and taste are removed to some extent in sedimentation with coagulation followed by filtration, pre chlorination, etc. For effective removal of these, following methods are used: a. Aeration b. Activated carbon treatment: It is the commonly used process for removal of colour, odour and taste. Activated carbon is manufactured by heating saw dust, paper mill waste etc. at 500°C in a closed vessel in controlled condition of burning at 800°C. It is readily available in market in powder or granular form. The powder is mixed with water before filtration and granular is used as filter materials. It absorbs organic matters and removes colour, odour and taste. c. Using Copper Sulphate: It is available in powder or crystal form and easily soluble in water. It is applied in the distribution system or reservoirs to 0.3 to 0.65 ppm. Is application is common in swimming pools.