The document discusses different types of screens used in wastewater treatment. Coarse screens called racks or bar screens have openings over 50 mm. Medium screens have bar spacings from 6-40 mm set at an angle. Fine screens have perforations from 1.5-3 mm that get clogged and need regular washing. Sedimentation and flocculation processes are also covered briefly.

1. UNIT 2:

2. Types of Screens:
• Coarse screens known as racks or bar screens.
• Opening size 50 mm or more.
• Medium screens has a spacing of bars of about 6-40 mm. screens are set in
a masonry chamber at an angle of 30-600C
• Fine screens have perforations of about 1.5 to 3mm in size. These screens however
get clogged so they have to be washed regularly.

3. Types of Screens:
• Coarse screens known as racks or bar screens.
• Opening size 50 mm or more.
• Medium screens has a spacing of bars of about 6-40 mm. screens are set in
a masonry chamber at an angle of 30-600C
• Fine screens have perforations of about 1.5 to 3mm in size. These screens however
get clogged so they have to be washed regularly.

4. Types of Screens:
• Coarse screens known as racks or bar screens.
• Opening size 50 mm or more.
• Medium screens has a spacing of bars of about 6-40 mm. screens are set in
a masonry chamber at an angle of 30-600C
• Fine screens have perforations of about 1.5 to 3mm in size. These screens however
get clogged so they have to be washed regularly.

5. Sedimentation

6. Type I – Discrete Settling
• The Force balance which is applied to a particle.
Assume spherical particles.
• Terminal settling velocity (vs) is assumed to be
constant.
• Stoke’s Law for laminar flow:
vs = g(ρp – ρw)dp
2/18μ
• Check NR, if not laminar use:
• NR=dpv/μ


D
pwp
s
C
gd
v
)(
3
4 

34.0
324

RR
D
NN
C

7. Grit Chambers – Type I
• Typical configurations are horizontal, aerated, or vortex type.
• The design is based on removal of grit particles (ρp = 2.65 g/cm3)
• Typical dimensions range from 2-5 m in depth, 7.5-20 m in
length, 2.5-7 m in width.
• Width:depth ratios = 2:1
• Detention time = 3 min. peak hourly flow is used for design.

8. Type II – Dilute suspension of
flocculating particles
• Particle size changes due to
flocculation (sticking
together) of particles,
therefore vs changes.
• Empirical data is used for
calculations
• Typical column experiment
and in this context,primary
Settling Basin good example.
Particles are sticky.

10. Higher Flow (Horizontal Velocity) in One Part of Tank
(Detention Time vs. Fall Velocity)
Currents Within the Tank Carry Solids Through the Tank.
Not the Same as Excessive Hydraulic Load
Clarifier Short Circuiting

11. Clarifier Short Circuiting
Missing or poor influent baffling
Other Causes may be

12. UNIT 2:
COAGULATION &
FLOCCULATION

13. Stokes Law and the Idea of Terminal velocity
4
3
 R
3
   g
2   R vt
For Laminar flow, there are three forces that can be calculated to act
On a solid sphere moving vertically through a fluid.
Gravity
Or Buoyancy
Form Drag
Friction Drag
4   R vt
Gravity
Form Drag Friction Drag

14. Gravity
Form Drag Friction Drag
4
3
 R
3
   g 4   R vt  2   R vt
 solid  fluid
Falling sphere with velocity Vt
Motionless Sphere in Moving Fluid
Fluid velocity=Vt
Gravity
What Happens if Vfluid>Vt?
vt
2
9
R
2

g



15. Stokes Law for Turbulent and Laminar Flow
Laminar  Turbulent when Re>0.1 to 1
Re sphere
d V FluidRelativeToSphere 

f1 Resphere  24
Resphere
 when Resphere 1
f2 Resphere  18.5
Resphere
3
5
 when 1 Resphere 500
f3 Resphere  0.44 w hen 500 Resphere 2 10
5

As the pressure exceeds that required for minimum fluidization, a second empirical equation is
necessary to describe the relationship between Gaand Resphereand pore fraction, , given as

4.7
Ga 18 Resphere 2.7 Resphere
1.687

As the increasing fluid velocities are considered, the fluid velocity eventually reaches the terminal
velocity of the particles and blow s the particles out of the bed. This process is referred to as
'elutriation', w here the terminal velocity is described by the familiar equation
vt
4 d  s   g
3 f 






1
2
w heref is the friction factor. The friction factor w ill have three ranges of dependence on the Re.

16. COAGULATION
Definition and Measurement
Many contaminants in water and wastewater contain matter in the
colloidal form.
These colloids result in a stable “suspension” which is stable enough
so that gravity forces will not be able to settle colloidal particles.So
these particles are removed by destabilization of colloids known as
“coagulation”.

18. Colloidal definition:
Particles just big enough to have a surface which is
microscopically observable or capable of adsorption of
another phase.
- Size (arbitrary): 0.001 to 1 micron.
-Surface area: ~ 1 sq yd to 1 acre/ gram.

19. 1. Hydrophobic ( water-hating):
• Colloids are normally negatively charged.
• Dispersion is stabilized by electrostatic repulsion. Generally the
negative charge is of physical/chemical origin (e.g. clays, metal oxides,
sulfides). Such kind of colloids are classified as thermodynamically
unstable or “irreversible”
COLLOID CLASSIFICATION:

20. 2. Hydrophilic ( water-loving):
This colloids have a great affinity for the solvent (usually water in our
case).
• Colloids usually possess a slight charge (-ve), but dispersion is
stabilized by hydration (attraction for particles of water). In general,
these colloids are the particles which are biological in origin: e.g.,
gelatin,proteins, starch etc..

22. As a result of this EDL there is a net electrostatic repulsion/attraction developed
between colloids. This net force is shown below:

23. The net resultant force is a result of:
1. attractive potential energy (mostly van der Waals forces), Va.
These forces are very strong at short separation distances
2. repulsion potential energy (electrostatic forces), VR. (by Coulomb’s
law).
a 6
1V
r

R 2
1V
r


24. Methods to Destabilize Colloids ( Coagulation Processes):
• 1. Double layer compression: This can be accomplished by addition of an indifferent electrolyte (charged ions with no specific attraction for colloid
primary surface). Adding indifferent electrolyte increases the ionic strength of solution which has the effect of compressing the EDL. As the
counterions are pushed closer to the surface the repulsion forces becomes easier to negate by van der Waals forces.
• 2. Adsorption and Charge Neutralization: If charged (+) counterions have a specific affinity for the surface of the colloid (not merely electrostatic
attraction) then adsorption of the counter-ion will reduce the primary charge of the colloid. This will reduce the net potential, y{r}, at any particular r
thus making the attraction forces more effective. ZP is likely to be reduced also.
• Counter-ions can be adsorbed by ion exchange, coordination bonding, van der Waals forces, repulsion of the coagulant by the aqueous phase
(surface-active coagulant).
• 3. Adsorption and Interparticle bridging: In this case polymers, metal salt or synthetic organic types, specifically adsorb to surface, often charge
neutralization occurs but further, other parts of the polymer adsorb to other colloids
• 4. Enmeshment in a precipitate (sweep floc): If metal salts, e.g., Al2(SO4)3 , FeCl3 are added in sufficient quantities to exceed the solubility products of
the metal hydroxide, oxide or, sometimes carbonates a “sweep floc” will form. Colloids will become enmeshed in the settling sweep floc and be
removed from the suspension.

25. Coagulant aids:
• Activated silica
• Bentonite clays
• Chlorine
• Ozone
• Polyelectrolytes
• Potassium permanganate

26. Coagulation/Flocculation Tanks
Coagulation and flocculation processes are carried out in coagulation and flocculation tanks. Coagulation is
performed in a rapid-mix tank:
In order to destabilize the colloids.
Flocculation:
Rapid mix (coagulation ) is followed by flocculation where there is slow, enhanced contact between destabilized
(coagulated) particles. Flocculation basins are used to promote growth of the destabilized floc by promoting
particle-particle contact.
There are two modes of particle-particle contact.

27. Jar Test:
 The jar test – Is a laboratory test to determine the optimum pH
and the optimum coagulant dose
 Jar test simulates the coagulation as well as flocculation processes
• Determination of optimum pH
 Fill the jars with raw water sample
(500 or 1000 mL) – 6 jars
 Adjust pH of the jars while mixing
using H2SO4 or we can take NaOH/lime
(pH: 5.0; 5.5; 6.0; 6.5; 7.0; 7.5)
 After that add same dose of the selected
coagulant (alum or iron) to each jar
(Coagulant dose: 5 or 10 mg/L)

28.  Rapid mix each jar at 100 to 150 rpm for 1 minute. The rapid mix
helps to disperse the coagulant throughout each container
 Reduce the stirring speed to 25 to 30 rpm
• and continue mixing for 15 to 20 mins
• The slow mixing speed results floc formation by enhancing particle collisions,
which lead to larger flocs and they settle under the action of gravity.
Mixers should be turned off and flocs are allowed to settle for 30 to 45 mins
 Final residual turbidity is measured.
 Residual turbidity against pH is plotted.