TREATMENT OF WATER SUPPLIES

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.

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.

 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

 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

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.

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

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.

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

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

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)

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.

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

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

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

 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

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

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

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

 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.

 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

 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

 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

Design Elements

 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)

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

Central Feed Tank

Peripheral tank

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.

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 =