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  2. TREATMENT OF WATER SUPPLIES  Available raw water must be treated and purified before they are supplied to the public for their uses.  The extend of treatment depends upon the quality and characteristics of raw water and also upon the quality requirement for the intended use.  The available water must be made safe, good in appearance and attractive to human taste and tongue.
  3. Methods of Purification of Water Various methods adopted for purifying the public water supplies are: 1. Screening 2. Plain sedimentation 3. Sedimentation aided with coagulation 4. Filtration 5. Disinfection 6. Aeration 7. Softening 8. Miscellaneous treatment such as fluoridation, recarbonation liming, desalination, etc.
  4.  Screening: Removes big and visible objects such as trees, branches, sticks, vegetation, fish, animal life, etc.  Aeration: Removes unpleasant taste and odour  Plain Sedimentation: Removes coarser suspended materials  Sedimentation aided with coagulation: Effectiveness of sedimentation is increased by adding some chemicals  Filtration: Removes finer particles in suspension which remains even after sedimentation  Disinfection: Removes pathogenic bacteria
  5.  Softening: Removes Hardness of water  DeFluoridation: Removal of fluoride  Liming: Addition of lime for controlling acidity and reduce corrosive action  Recarbonation: Addition of carbon dioxide to prevent deposition of calcium carbonate scale  Desalination: Removal of excess salt
  6. SCREENING  Screens are provided in front of intake structures to exclude large sized particles  Coarse screen: Placed in front of fine screen.  It consists of parallel iron rods/ bars placed vertically or inclined at 45o – 60o  It removes bigger floating bodies  Fine screen: Consist of woven wire mesh.  It removes fine materials.  Fine screens get clogged easily and are to be cleaned frequently.
  7. AERATION OBJECTIVES  Aeration removes odour and tastes due to volatile gases like hydrogen sulphide and due to algae and related organisms.  Aeration also oxidise iron and manganese,  Increases dissolved oxygen content in water,  Removes CO2 and reduces corrosion and  Removes methane and other flammable gases.  Bacteria may be killed to some extend  Also used for mixing chemicals with water
  8. Types of Aerators  Gravity aerators  Fountain aerators  Diffused aerators  Mechanical aerators.  Gravity Aerators (Cascades): 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: These are also known as spray aerators with special nozzles to produce a fine spray  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.  Mechanical Aerators: Mixing paddles as in flocculation are used. Paddles may be either submerged or at the surface.
  9. SEDIMENTATION  It is the removal of suspended particles by gravitational settling  Sedimentation tanks are designed to reduce the velocity of flow of water so as to permit suspended solids to settle out of water by garvity PLAIN SEDIMENTATION  When the impurities are separated from water by action of natural forces alone ie, by gravitation & natural aggregation of the particles is called plain sedimentation SEDIMENTATION WITH COAGULATION/ CLARIFICATION  When chemicals or other substances are added to induce seggregation & settling of colloidal suspended particles, the operation is called clarification
  10. Chemical Precipitation  When chemicals are added to separate dissolved impurities the operation is called chemical precipitation Discrete Particles  A particle that doesnot alter its shape, size & weight while settling or rising in water is known as discrete particle
  11. Types of Settling Particles settle out of suspension in the following 4 ways,  Type I sedimentation (Discrete settling)  Type II (Flocculent settling)  Type III ( Hindered /Zone settling)  Type IV ( Compression settling)
  12. Type I: Discrete particle settling –  Sedimentation of discrete particles  Particles settle individually without interaction with neighbouring particles.  Also know as free settling Type II: Flocculent settling–  This refers to dilute suspension  Particles flocculate during sedimentation process  Flocculation causes the particles to increase in mass and settle at a faster rate.
  13. Type III: Zone settling –  This refers to intermediate concentration  Inter particle forces hold the particles together & hence the mass of the particles subside as a whole  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 –  This refers to high concentration  Particles come in contact with each other resulting in the formation of a structure  Further settling occur only by compression of the structure due to weight of particles which are added to the structure
  14. PLAIN SEDIMENTATION (Type I Settling)  Theory of sedimentation  Suspended impurities with specific gravity greater than that of water settle under gravity if their flow is retarded.  The following factors affect sedimentation process: 1. Velocity of flow 2. Viscosity of water 3. Size, shape and specific gravity of particles
  15. 1. Velocity of flow:  Horizontal velocity carries the particles horizontally  Greater the flow area, the lesser is the velocity & hence more easily the particle will settle down 2. Viscosity of water:  The viscosity varies inversely with temperature  Warm water is less viscous therefore offer less resistance to settlement 3. Size, shape and specific gravity of particles:  Large sized particles settle more than fine particles
  16.  The settling velocity of particles is given by Stock’s law  Where vs – settling velocity in cm/s  g- acceleration due to gravity in cm2/s (981 cm2/s)  μ- viscosity of water = 0.01 poise  G – Specific gravity of particle  d- diameter of particle in cm
  17. Sedimentation Tank  Types of sedimentation tank-Depending upon the methods of operation, 1. The Quiescent or Fill & Draw type 2. The continous flow type 1. The Quiescent or Fill & Draw type  The tank is filled with incoming water & is allowed to rest for a certain time  During this period the suspended particles settle down at the bottom  Detention time- 24 hrs  Clear water is drawn out through outlet  The tank is then cleaned & filled again  Cleaning process take 6 – 12 hrs  Cycle of operation- 30 – 36 hrs
  18. 2. The continuous flow type  In this type water continuously keeps on moving in the tank, with a very small velocity during which the particles settle at the bottom before they reach the outlet  Two types- a) Horizontal flow tank b) Vertical flow tank  In horizontal flow type, tank is generally rectangular in plan having length equal to atleast twice the width  Water flows in horizontal direction, with max permissible velocity of 0.3 m/s  Vertical flow type are generally deep, circular or rectangular basins with hopper bottom
  19. Horizontal flow tank  Direction of flow is horizontal  Aim is to achieve equal velocity at all points in the settling zone  Design is based on the following assumptions, 1. Particles settle exactly in the same manner as in the quiescent tank of equal depth 2. Flow is horizontal & steady & the velocity is uniform 3. The concentration of suspended particles of each size is same at all points at the inlet end 4. A particle is removed when it reaches the bottom of the settling zone
  20.  The basin can be divided into 4 zones, 1. The inlet zone 2. The settling zone 3. The bottom/ sludge zone 4. The outlet zone  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 discharge through outlet weir.  Sludge zone: For collection of sludge below settling zone.
  21.  Let L & H be the length & depth of the settling zone  Let B be the width  Q be the discharge rate  Vd is the horizontal discharge velocity  Time of horizontal flow/ detention period for a particle
  22.  Equation 3 defines the surface over flow rate (SOR) or overflow rate, which is equal to flow divided by the plan area of the basin
  23.  The equation states that for discrete particles & unhindered settling, basin efficiency is a function of settling velocity of particles & the surface area of the basin
  24. Design Elements
  25.  L/B = 3:1 to 5:1  D= 2.5 to 5 m (prefered value- 3m)  Horizontal flow velocity- 0.2 to 0.4 m/mt (prefered - 0.3 m/mt)  Bottom slopes range from 1% in rectangular tanks to about 8% in circular tanks.  Slope of sludge hoppers range from 1.2 : 1 to 2: 1 ( vert: Horz)
  26. Circular tank with radial flow with central feed  Water enters at the center  Water flows radially outward from the center  It has low velocity and hence particles get settled  Clear water is taken out at the periphery  Mechanical scrapper is provided for sludge removal Circular tank with peripheral feed with radial flow  Raw water is fed from the periphery and the pure water is taken out at the center
  27. Central Feed Tank
  28. Peripheral tank
  29. Problems  Qt. 1. A settling tank is designed for an overflow rate of 4000 l/m2/hr. What percentage of particles of diameter a) 0.05mm & b) 0.02 mm will be removed in the tank at 10 degree celsius.
  30. Qt. 3. The maximum daily demand at a water purification plant has been estimated as 12 million liters per day. Design the dimension of a suitable sedimentation tank (fitted with mechanical sludge removal) for the raw supplies, assuming a detention period of 6 hours and velocity flow as 20 cm/minutes  Solution: Quantity of water to be treated in 24 hours = 12 x 106 liters Quantity of water to be treated during the detention period of 6 hours =