The document discusses various water treatment methods including sedimentation, coagulation, filtration, and disinfection. Sedimentation involves the settling of suspended particles in water by gravity. Coagulation uses chemicals to destabilize colloidal particles and form larger particles that are more easily removed. Filtration passes water through sand or other media to remove remaining particles. Disinfection uses chemicals or UV light to inactivate pathogens. The document provides details on the various processes and their applications in water treatment.

1. Chapter 01:
Water Treatment Methods
1
CEN 441 :
ENVIRONMENTAL ENGINEERING -II
Dr AFM Kamal Chowdhury
Assistant Professor, Dept. of Civil Engineering, IUBAT
Office: 423, Cell: 01711- 479153
E-mail: afm.chowdhury@iubat.edu

2. CHAPTER COMPONENTS
 Water Treatment Methods: Plain sedimentation,
sedimentation with coagulation, filtration, disinfection,
treatment of industrial water.
 Waste Water: Estimation of waste water, wastewater
collection system, hydraulics of sewer, design, construction
and maintenance of sanitary sewer and storm sewer,
microbiology of waste water, primary and secondary
treatment of sewage.
 Environmental Sanitation: Introduction, environmental
pollution, environment protection and management, sanitation
practices in Bangladesh.
 Health and Hygiene: Diseases description, transmission and
control, hygiene education.
 Pollution: Introduction to air pollution and noise pollution.
 Solid Waste Management: Solid waste collection, transport,
disposal and management.
2

3. WHY WE SHOULD TREAT WATER
 Natural water often contains impurities that are harmful
for human health
 Common impurities include:
 Impurities of mineral origin – iron, arsenic, lead, heavy metals
 Impurities of organic origin – vegetable dyes
 Living impurities – bacteria, viruses, algae, protozoa, fungi
 Radioactive impurities
 These impurities may be present in suspension or
solution
3

4. WHY WE SHOULD TREAT WATER
 Some impurities might be detected by sight (turbidity,
colour), taste (salty, offensive) and smell (odour)
 Detection of many pathogenic and poisonous impurities
require systematic laboratory tests
 Scopes of water treatment:
 Treatment for drinking water
 Treatment of wastewater before disposing into water-bodies
4

5. BASIC REQUIREMENTS OF DRINKING
WATER
Water should:
 Be completely free of pathogenic micro-organisms that
can cause diseases
 Contain no element or compound in concentrations that
can cause acute or long-term adverse effect on human
health
 Be aesthetically acceptable – free of bad colour, taste
(e.g. salty), and smell
 Not cause corrosion, scale formation, discoloration
 Not have a temperature unacceptable to the consumers
5

6. JUSTIFICATION FOR WASTEWATER TREATMENT
 Pollution from sewage is a primary environmental health
hazard (wastewater effluent).
 The purpose of municipal wastewater treatment is to limit
pollution of the receiving watercourse.
 The receiving watercourse may also be a source of
drinking water.
6

7. GOALS OF WASTEWATER TREATMENT:
 Reduction of organic load of the wastewater effluent to
limit eutrophication (BOD, COD limits),
 Reduction of microbiological contamination that may
transmit infectious disease.
7

8. ASSIGNMENT
 Brief report on water quality parameters (Turbidity, Total
Dissolved Solids (TDS), Alkalinity, Hardness, Nitrate,
Total Coliform and Faecal Coliform, Iron, Arsenic):
 Definition/Causes/Source
 Measurement procedure
 In-Stream Acceptable Limits (WHO and Bangladesh
Standards)
 Drinking Water Acceptable Limits (WHO and Bangladesh
Standards)
 Impacts
8

9. WATER TREATMENT METHODS
9

10. COMMON WATER TREATMENT METHODS
 Clarification - primarily a physical process (e.g. plain
sedimentation), but may be aided by addition of
chemicals (e.g. coagulation).
 Removes suspended and colloidal particles including color
producing substances.
 Filtration - also primarily physical, but chemicals may aid
the process.
 Removes visible impurities.
 Disinfection - typically a chemical process.
 Reduces pathogenic microorganisms.
10

11. SOME SPECIFIC WATER TREATMENT METHODS
 Aeration
 Water softening
 Iron removal
 Activated carbon application
 Fluoridation and defluoridation
 Demineralization
 Desalinization
11

12. COMMON APPLICATIONS OF WATER TREATMENT
METHODS
 Surface water is turbid, colored and contaminated by
pathogenic micro-organisms and needs extensive
treatment such as sedimentation, coagulation-
sedimentation, filtration and disinfection.
 Groundwater is usually hard (may require softening) but
free from pathogenic bacteria and can be supplied for
drinking purpose without treatment.
 Some Tube-well water in Bangladesh may contain iron,
arsenic and hardness in excess of acceptable levels,
and may therefore require specific treatment. 12

13. TYPICAL SURFACE WATER TREATMENT SYSTEM
13

14. PLAIN SEDIMENTATION
14

15. PLAIN SEDIMENTATION
Organic or inorganic particles heavier than water (specific
gravity > 1) settle by retaining water in a tank or basin
These particles are generally held in suspension in natural
water by turbulence or current
When the current is retarded, particles heavier than water tend
to move downward by the force of gravity, accelerating until the
frictional resistance ('drag') of the water equals the gravitational
force acting upon the particles.
Thereafter the particle travels with a constant vertical velocity
called the "terminal velocity' or 'settling velocity' of the particle.
15

16. THE SETTLING VELOCITY OF THE PARTICLE
DEPENDS UPON
 Horizontal flow velocity of water
 Shape and size of the particle
 Specific gravity of the particle
 Viscosity of water
 Density of water
 Temperature of water
16

17. 17

18. SETTLING OF DIFFERENT TYPES OF
PARTICLES
 Stoke's Law is valid for computation of settling velocity of
discrete particles
 Discrete particles are those which do not change size,
shape and mass during settling and which do not influence
each other by being too close. Particle settling under this
conditions is called discrete settling
 In case of closely packed particles, the water displaced by
the particles may cause additional friction and the settling
velocity is reduced. This is termed as hindered settling.
 Hindered settling becomes noticeable when the
concentration of suspended solids is greater than 2,000 mg/1.
This situation of high concentration of suspended solids may
happen in river water during high flooding and heavy rainfall18

19.  Sometimes settling particles may adhere to each other
and grow in size and thus deviate from the settling
characteristics represented by Stoke's Law. This my occur
in settling of algae or freshly formed floc by the process of
flocculation with coagulant.
 These particles/flocs tend to stick together and form new
bigger particles which settle at a faster rate. This type of
settling is called flocculent settling.
19
SETTLING OF DIFFERENT TYPES OF
PARTICLES

20. 20

21. DESIGN OF SEDIMENTATION TANKS
 A rectangular sedimentation tank can be subdivided
into four different areas comprised of an inlet,
settling, outlet and sludge accumulation zones
 The inlet zone serves to provide even flow
distribution over the full cross section, the outlet
zone collects the clarified water over the full tank
width
 Sludge is accumulated at the tank bottom where it
is stored and removed periodically
 The settling zone shown in Figure is the most
important area where solid separation takes place
21

22. 22

23. The efficiency of the settling tank in the removal of
suspended particles can be determined using limiting
settling velocity v0 of a particle which will just travel the full
depth (H) of the tank within the detention time (T). Using
the dimensions and notations used in Figure the following
equations can be written:
23

24.  The tank will remove all the particles having settling
velocity vs > vo and the particles with settling velocity vs
< vo will be removed in the proportion vs : vo .
 The above analysis shows that the settling efficiency
depends on the ratio between the influent flow rate Q
and the surface area of the tank BL, which is called the
'surface loading'.
 Hence the efficiency of the settling tank is independent
of the depth of the tank.
 The higher the surface area the greater is the
efficiency. Plate settlers and tube settlers have been
designed to provide a larger surface area and achieve
higher efficiency. 24

25.  The settling velocity of different fractions of discrete
particles can be computed by Stoke's Law if the
particle size distribution and specific gravity of
particle are determined by suitable methods.
 The settling velocity of different fractions of particles
in water can be conveniently determined by a settling
column test of a representative sample in the
laboratory.
 In the absence of column test data, the design
guideline given below may generally be followed for
good results
25

26. COAGULATION-FLOCCULATION
26

27. COAGULATION-FLOCCULATION PROCESS
 A chemical-aided clarification/sedimentation
process
 Removes colloids and very fine particles having
very low or no settling velocity, which cannot be
removed by plain-sedimentation
 Coagulation involves:
 addition of a salt that produces positive ions in water
 application of rapid mixing (hydraulic or mechanical)
 destabilization of colloids
 promotion of frequent contact among the particles 27

28.  Common Coagulants:
 Aluminum sulphate – Al2(SO4)3.nH2O
 Ferric sulphate – Fe2(SO4)3.9H2O
 Ferric chloride – Fe2Cl3.6H2O
 Ferrous sulphate – FeSO4
 The Aluminium and Iron Salts react with natural alkalinity
of water and produce Aluminium and Iron Hydroxides –
Al(OH) 3 and Fe(OH) 3
 The Al(OH) 3 and Fe(OH) are gelatinous (sticky) which
entrap the colloidal particles and form micro-flocs
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COAGULATION-FLOCCULATION PROCESS

29.  Flocculation – Sedimentation:
 Gentle and continuous stirring for agglomeration of micro-flocs
to produce larger flocs
 The larger focs gain sufficient settling characteristics and
finally removed by sedimentation
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COAGULATION-FLOCCULATION PROCESS

30. 30
EFFICIENCY OF COAGULATION
 Eeach coagulant has optimum pH for best coagulation
 Iron salt is very effective over a wider range of pH
 Aluminium salt is most effective at a pH slightly higher than 7

31. 31
EFFICIENCY OF COAGULATION
 If required alkalinity is not naturally present in water,
alkalinity is added as Ca(OH)3 or Na2CO3
 Mixing should be rapid for immediate dispersal of
coagulant throughout the raw water
 Two types of mixing:
• Hydraulic rapid mixing (e.g. by producing turbulent
condition in baffled channels, or by feeding coagulants
at the suction side of the pump)
• Mechanical rapid mixing (e.g. paddles, propellers,
turbines etc. which require continuous power supply)

32. 32

33. 33

34. 34
PROBLEM ON COAGULATION-FLOCCULATION
 Example 4.4 and Example 4.5 from Peavy’s Book

35. FILTRATION
35

36. FILTRATION
 Water is allowed to pass through a bed of filtering media
usually sand and gravel.
 Common filtration methods:
 Mechanical staining
 Sedimentation and adsorption
 Microbial action
 Electrostatic attraction
36

37. MECHANICAL STRAINING
 Large particles that cannot pass through the thin openings
between the sand grains are retained in the top layer of the
filter media.
 Accumulated material in the top layer of the bed increases
the straining efficiency but decreases the downward flow of
water.
 Cannot remove bacteria and colloidal matter.
37

38. SEDIMENTATION AND ADSORPTION
Pores in the sand bed act as a minute
sedimentation basin.
Curved flow paths around grains bring the fine
particles and bacteria in contact with sand
surfaces.
Sticky gelatinous coatings are formed on the
sand grains which retain the colloids, small
particles and bacteria.
38

39. MICROBIAL ACTION
 A part of the organic material present in raw water is
transformed into cell materials for microbial growth.
 A coating of micro-organisms is formed around the sand
grains which retain the organic matters and bacteria.
39

40. ELECTROSTATIC ATTRACTION
 Sand particles with negative surface charges
cannot attract negatively charged bacteria and
colloids
 With continuous adsorption of positively charged
particles and ions, the negatively charged sand
surface turned into positively charged surface.
 Overall charge of filter bed becomes positive which
attract and retain the negatively charged bacteria
and colloids.
40

41. ROUGHING FILTER
 It is used for pretreatment of very turbid water
 Consists of different sizes of gravels or stone chips
 Gravel layers of different sizes are installed with gravel
size decreasing in the direction of flow
 Three common types:
 Down-flow roughing filters
 Up-flow roughing filters
 Horizontal-flow roughing fitters
41

42. 42

43. 43

44. EFFICIENCY OF ROUGHING FILTRATION
 Suspended solids removal of up to 95%
 Turbidity removal between 50 and 90%
 Colour removal between 20 to 50%
 Faecal coliform reduction between 0.65 and 2.5
log units
 Around 50% removal of iron and manganese from
groundwater
44

45. SLOW SAND FILTRATION (SSF)
 Water is allowed to pass through a bed of fine sand
 Retains most of the impurities including fine organic and
inorganic solid matters, dissolved (i.e. oxidized) organic
compounds and micro-organisms
 Suitable for development of a surface water-based water
supply system in developing countries
45

46. CHARACTERISTICS OF SSF
 Rate of filtration is low, 0.1-0.3 m3 per m2 per hr
 Very high removal of turbidity and colour
(80-85%) and bacteria (95-99.9 %)
 Cleaning of filter bed by scraping and removal of a
top layer of sand
 Not suitable for water having turbidity greater than
30 NTU
 Low-cost of operation and maintenance
46

47. DESIGN
 An open tank of around 2m height containing:
 a sand bed of approximately 0.5-0.7 m thickness
 around 1 m depth of water and
 0.1 m freeboard
 underdrain system of gravel with 0.3 to 0.5 m height to
collect clean water
 Water flows by gravity through the filter bed
 Filtration rate should be between 0.1 to 0.2 m/hr
 Sand bed should have an effective size, d10
between 0.1 and 0.3 mm and a uniformity
coefficient d60/d10 below 3
47

48. 48

49. COMBINED ROUGHING AND SLOW SAND FILTERS
 Slow sand filters do not work when the turbidity
exceeds 30 NTU.
 SSF in Bangladesh require frequent washing for
high turbidities.
 Pre-treatment by roughing filters can reduce the
load on SSFs
 Filter can operate for a longer period of time
between cleaning
49

50. 50

51.  High filtration rate of 5-15 m3/m2/hr
 High filtration rate is achieved by using coarse sand
with an effective size of 0.4-1.2 mm
 Filter bed: a coarse sand layer of 1m laid on top of a
gravel layer of 0.5m
 Can be both gravity type and pressure type
 Cleaning by backwashing - water is directed in the
reverse direction at a high rate of flow
 High removal of turbidity and colour (80-85%) and
bacteria (85-95%)
 Pre-treatment is required
 Higher cost of operation and maintenance
51
RAPID SAND FILTER

52.  Filter size is determined by the required capacity
of the plant
 Number of unit of the plant is determined by the
empirical equation:
N = 0.04 √Q
where, N = Number of Units
Q = Plant Capacity in m3/day
 Solve Example 2 of Chapter 18 from Ahmed and
Rahman
52
RAPID SAND FILTER

53. DISINFECTION
53

54. DISINFECTION
 Processes of destruction or at least complete
inactivation of pathogens present in water.
54

55. PHYSICAL DISINFECTION
 Boiling
 Ultraviolet Rays
 Sunlight
55

56. CHEMICAL DISINFECTION
 Quick and effective in killing pathogenic micro-organisms
present in water
 Rapidly soluble in water in concentrations required for
disinfection and capable of providing a residual for
subsequent protection of water
 Not imparting taste, odour, colour or turbidity to water
 Not toxic to human and animal life
 Easy to detect and measure in water
 Easy to handle, transport, apply and control
 Readily available at moderate cost
56

57. 57

58. IMPORTANT TERMINOLOGY
 Combined Available Chlorine
 Free Available Chlorine
 Chlorine Demand
 Break point chlorination
58