Transcript Water Chemistry & Microbiology On completion of this

Water Treatment
On completion of this segment you should be able to:
• Be aware of the objectives of water treatment
• Describe the processes involved in water treatment
• Discuss the types of separation processes
1
Basic Methods for Correcting Water
Quality Deficiencies
• The processes and extent of required treatment are dependent
on the nature and degree of quality deficiencies to be corrected.
• There is virtually no water that cannot be treated to potable
standards. Cost effectiveness is one of the guiding principles
• The basic methods are physical and chemical processes and to
a lesser extent, biological
2
Basic Treatment
Depending on the source, the following unit processes
are likely
• Screening
• Aeration
• Coagulation and flocculation
• Sedimentation and filtration
• Disinfection
3
A belt screen
4
Coagulation and Flocculation
• Coagulants are chemicals that react with colloidal matter to
form absorbent bulky precipitates (flocs)
• Destabilisation of colloidal particles (10-3 - 1 m)
• Salts of aluminium and iron form insoluble hydroxides
• Reaction is pH dependent (6 - 7 optimum range)
• Flocculation is gentle stirring that increases the size of the
flocs
5
Aluminium salts are commonly used
• Aluminium sulfate
• Al2(SO4)3 + 3Ca(HCO3)2
2Al(OH)3 + 3CaSO4 + 6CO2
• Natural or added alkalinity is required
• Reaction is sensitive to pH
• May revert to soluble for if pH increases/decreases
6
Iron salts are more difficult to control
• Ferric chloride/iron(III) chloride
•
2FeCl3 + 3Ca(HCO3)2
2Fe(OH)3 + 3CaCl2 + 6CO2
• Natural or added alkalinity is required
• Wider operating pH range
• Cheaper material and forms heavier floc
• Iron salts cake and are dirty to handle, difficult
sludge to dispose
7
Flash/Rapid Mixer
8
Flocculation
Gentle stirring following rapid mixing so that floc
particles can coalesce and agglomerate
• Two phases are involved; initial periknetic, orthokinetic > 1 m
• Detention time, t = 20 - 60 minutes
• Mechanical flocculation power input
• Tapered flocculation as floc size increase
9
A Mechanical Flocculator
10
Sedimentation
Removal of suspended particles in an aqueous medium
through gravity settling
• Class I Unhindered settling of discrete particles
• Class II Setlling of dilute suspension of flocculent particles
• Class III Hindered settling and zone settling
• Class IV Compressive settling (compaction)
11
Class I settling – unhindered discrete
For discrete particles settling freely, the terminal velocity
is reached when gravitational force is balanced by
frictional drag forcve
• vs = g d2 (1 - )/(18 )
• As particle size increases, vs increases
• Detention time, t = Volume/Q
• Depth of tank is not relevant, vs = Q/surface area
• Performance is influenced by overflow rate and t
12
Idealised unhindered discrete settling
13
Drag coefficient
14
Settlement in horizontal flow tanks
• Overflow rates varies from 18 to 54 m/d
• Typically 28 m/d for a 3.5 m depth and 3 h HRT
• In tropical countries with more turbid water, 18 m/d with 4 h HRT
is appropriate with depths of 3 - 3.5 m
• In practice, particles are not wholly discrete and there is merit in
depth
15
A typical horizontal flow sedimentation tank
16
Vertical flow tank
17
Solids contact clarifier
18
Filtration
A process of passing water through a sand bed or other
suitable medium at low speed to remove suspended
solids
• Removal of non-settleable flocs after coagulation and
sedimentation
• Properties of the medium (effective size, hardness etc)
• More than a mechanism of straining
19
Mechanisms of filtration
• Straining
• Sedimentation
• Interception
• Adhesion
• Flocculation
• Adsorption
20
Rapid sand filter
A process of depth filtration as solids are removed
within the granular medium
• Sand bed 0.6 - 0.75 m deep of 0.4 - 0.7 mm effective size and a
uniformity coefficient  1.7
• Supporting gravel layer 0.3 - 0.5 m (graded 2 - 60 mm)
• Underdrain system to collect filtered water and to discharge air
scour and backwash water uniformly
• Filtration rate varies from 4 - 15 m/h
21
Rapid sand filter (cont)
• Backwash when head loss  2 m
• Application of backwash water assumes practical importance in
the design of filters
• Some problems associated with rapid sand filters are mud balls,
air-binding, surface cracks and shrinkage
• Other forms are direct filtration, and up-flow filtration
22
clogs up readily
ideal but unattainable
Arrangement of filter media
23
Typical rapid sand filter
24
Typical rapid sand filter (Droste 1997, p. 418)
25
Slow sand filters
These are the oldest and effective method for removing
pathogenic microorganisms in water. Cake filtration
when solids are removed on entering the face of the
granular material
• No pre-treatment or chemicals are required but turbidity < 30 JTU
• Filter media 0.7 - 1.2 m layer of 0.2 - 0.4 mm effective size with a
uniformity coefficient  3
• Supported on gravel layer 0.1 m (graded 5 - 25 mm)
• Relies on surface straining and microbial action (schmutzdecke)
• Slow filtration rates of 350 - 700 L/s.ha (3 - 6 m/d)
26
Slow sand filters (cont)
• 1 - 3 months filter run or when head loss  1 m
• Surface renewal by removing 12 - 25 mm of surface layer each
time until 600 mm of sand layer is left
• Requires large land area
• Labour intensive to remove and clean the sand
• Suitable for reservoir-fed supply and small communities
requiring no technical supervision but not effective for turbidity >
40 JTU
• Does not remove colour but is able to deliver bacteriologically
superior water
27
Slow sand filter
28
Pressure filtration
• No different from rapid sand filters
• Filter lies on the HGL
29
Chlorine disinfection
It is presently the most cost-effective disinfection
method but it has some adverse effects
• Properties of chlorine
• Reaction is highly pH dependent
• Cl2 + H2O
HOCl + HCL
• As pH increase the hypochlorous acid (HOCl) will further
dissociate to H+ and OCl- (hypochlorite ions)
• HOCL and OCl represent the free available chlorine
30
Chlorine disinfection (cont)
At 20o C
pH
6
7
8
9
%HOCL
97
79
21
4
%OCl
3
21
73
96
 Chlorine:ammonia reaction
 Breakpoint chlorination
 Superchlorination
31
Chlorine - ammonia reaction
Formation of monochloramine (NH2Cl)
HOCl + NH3
H2O + NH2Cl
Cl2:NH3 < 5:1; pH  7
Formation of dichloramine (NHCl2)
NH2Cl + HOCl
H2O + NHCl2 Cl2:NH3 < 10:1
Formation of trichloramine (NCl3)
NHCl + HOCl
H O + NCl3
Cl2:NH3 < 20:1; pH < 4
Monochloramine and dichloramine represent the
combined available chlorine, with less disinfecting
power compared with the free available chlorine
32
Breakpoint chlorination
• Oxidation of chloramines until appearance of free
available chlorine
• At this point the free available chlorine residual is
lowest
• Taste, odour are reduced through oxidation
• Some colour may also be removed
• Good control required to ascertain that breakpoint is
reached
33
Breakpoint chlorination
34
Superchlorination
• How is it used?
• When is it applied?
• Need to de-chlorinate using sulfur dioxide, sodium
bisulfite, sodium thiosulfate or granular activated
carbon
35
Other forms of disinfection
• Ozone
• Ultra violet light (UVL)
• Halogens (bromine, iodine)
• Metal ions (silver)
• Ultra-filtration
• Simple retention time
36
End of Water Treatment segment
37