1. Coagulation and Flocculation LOKESH SAINI M.Tech-Ist
Semester Student ID 2015PCE5271 3/31/2016 1
2. 3/31/2016 2 Why coagulation and flocculation? Various sizes
of particles in raw water Particle diameter (mm) Type Settling
velocity 10 Pebble 0.73 m/s 1 Course sand 0.23 m/s 0.1 Fine sand
0.6 m/min 0.01 Silt 8.6 m/d 0.0001 (10 micron) Large colloids 0.3
m/y 0.000001 (1 nano) Small colloids 3 m/million y Colloids so
small: gravity settling not possible GravItysettlIng
3. COAGULATION AND FLOCCULATION 3/31/2016 3 2TYPE SETTLING
4. 3/31/2016 4 Typical layout of a water treatment plant
5. 3/31/2016 5 What is Coagulation? Coagulation is the
destabilization of colloids by addition of chemicals that
neutralize the negative charges by rapid mixing. The chemicals are
known as coagulants, usually higher valence cationic salts (Al3+,
Fe3+ etc.) Coagulation is essentially a chemical process -- - - ---
-- - ---- -- - ---- -- - ----
6. The net resultant force is a result of: 1. attractive
potential energy (mostly vander Waals forces), Va. These forces are
very strong at short separation distances 2. repulsion potential
energy (electrostatic forces), VR. (by Coulombs law). a 6 1V r R 2
1V r
7. 3/31/2016 8 Colloid Stability ------ ------ Repulsion
Colloid - A Colloid - B Colloids have a net negative surface charge
Electrostatic force prevents them from agglomeration Brownian
motion keeps the colloids in suspension H2O Colloid Impossible to
remove colloids by gravity settling
11. 3/31/2016 12 Jar Tests Determination of optimum pH The jar
test a laboratory procedure to determine the optimum pH and the
optimum coagulant dose A jar test simulates the coagulation and
flocculation processes Fill the jars with raw water sample (500 or
1000 mL) usually 6 jars Adjust pH of the jars while mixing using
H2SO4 or NaOH/lime (pH: 5.0; 5.5; 6.0; 6.5; 7.0; 7.5) Add same dose
of the selected coagulant (alum or iron) to each jar (Coagulant
dose: 5 or 10 mg/L) Jar Test
12. 3/31/2016 13 Jar Test set-up 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 This slower mixing speed
helps promote floc formation by enhancing particle collisions,
which lead to larger flocs Turn off the mixers and allow flocs to
settle for 30 to 45 mins Measure the final residual turbidity in
each jar Plot residual turbidity against pH Jar Tests determining
optimum pH
13. 3/31/2016 water treatment 14 Optimum pH: 6.3 Jar Tests
optimum pH
14. 3/31/2016 15 Optimum coagulant dose Repeat all the previous
steps This time adjust pH of all jars at optimum (6.3 found from
first test) while mixing using H2SO4 or NaOH/lime Add different
doses of the selected coagulant (alum or iron) to each jar
(Coagulant dose: 5; 7; 10; 12; 15; 20 mg/L) 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 for 15 to 20 mins
15. 3/31/2016 16 Turn off the mixers and allow flocs to settle
for 30 to 45 mins Then measure the final residual turbidity in each
jar Plot residual turbidity against coagulant dose Coagulant Dose
mg/L Optimum coagulant dose: 12.5 mg/L The coagulant dose with the
lowest residual turbidity will be the optimum coagulant dose
Optimum coagulant dose
16. 3/31/2016 17 Hydraulic Jump: Hydraulic Jump creates
turbulence and thus help better mixing. Mechanical mixing In-line
flash mixing Inflow Chemical feeding Chemical feeding Inflow Back
mix impeller flat-blade impeller Coagulant RAPID MIXING RAPID
MIXING
17. 3/31/2016 18 What is Flocculation? Flocculation is the
agglomeration of destabilized particles into a large size particles
known as flocs by slow mixing which can be effectively removed by
sedimentation or flotation.
18. Design of Coagulant Chamber Detention Time 't' = Volume of
Tank in sec Discharge t is taken 30 to 60 sec IF THE CIRCULAR TANK
IS CONSIDERED H/D may be taken 1.5 IMPELLER DIA/ TANK DIA = 0.2-0.4
VELOCITY OF TIP OF THE IMPELLER>3m/sec Free Board= 0.3m
3/31/2016 19
19. 3/31/2016 20
20. 3/31/2016 21 DESIGN OF FLOCCULATION CHAMBER The constant
G.t = velocity gradient X detention time G= 20 to 75sec Where Gt=2
to 60000 = 1 to 15000 t = 10 to 30 min For Al coagulant For Fe
coagulant
21. 3/31/2016 22 MECHANICAL FLOCCULATOR DESIGN Inlet pipe &
Tank Sizing Depth of the tank = 3 to 4.5m Detention time t = 20 to
40 min Total area of paddles = 10 to 25% of the cross sectional
area of the tank Velocity of flow = 0.2-0.6m/s Peripheral velocity
of blades = 0.2 to 0.6m/s Outlet velocity = 0.15 to 0.25m/s Water
loss in de sledging = 2% Velocity in inlet pipe = 1m/s Free board=
0.5m Paddle Sizing Power input P=G. X vol. of tank= .Cd..Ap.(V-v)
Where, Cd= Drag coefficient, 1.8 = Density of water at 25 c,
997Kg/m V = Velocity at the tip blades= 0.4m/s v = Velocity of the
water at tip of blades is 25% of V V=2.r.n/60 where r is the paddle
length
22. 3/31/2016 23 Clarifier sizing SoR= 40m/m/day /4{(Dia of
clf) - (Dia of flocculator)}= Design flow/SoR Length of Weir= .Dia
of clf< 300 m/day
30. 3/31/2016 31 References 1. Manual on water supply and
treatment, CPHEEO, Ministry of MOUD, New Delhi,
1999;201-232:621-625 2. Peavy S. Howard, Rowe, Tchobanoglous,
Environmental Engineering, 2014; 120-150 3. Weikipedia on
coagulation and flocculation 4. Water treatment: Principlea and
design, MWH(2005), (ISBN 04710110183) 5. Unit process in drinking
water treatment W. Masschelein(1992), (ISBN 082478678 5)(635 pgs)
6. IS 3025