CE 326 Principles of Environmental Engineering
Water Treatment Plant 1
Typical Flow Diagrams
Treatment of Groundwater
Treatment of Surface Water
1
Dr. S.K. Ong
Coagulation and Flocculation
Colloidal and Fine Suspended Solids
- are too small to settle out in a reasonable time, do not agglomerate naturally
- have large surface area to volume ratio
- are usually negatively charged and surrounded by ions of opposite charge
- Examples of settling velocities
Particles dia. (mm)
1
0.01
0.0001
Size typical of
coarse sand
silt
large colloid
Settling velocity (m/s)
Removal Approach – Coagulation
- destabilize the fine particles by addition of a chemical so that large particles or precipitates are formed.
Destabilization and coagulation are accomplished by the following:
1. ____________________ or _________________________
- coagulation of the particles as a result of reduction in energy barrier by adding chemicals with
multivalent cations such as Fe2+, Fe3+ , Al3+ or polyvalent hydro-metallic complexes
eg., in natural systems – highly turbid water flowing into ocean (high ion strength) – result in
coagulation, settling and form deposits – delta
2. _____________________________________________
- addition of coagulants resulting in production of precipitate, ex., Al(OH)3 or Fe(OH)3,
- concentrations of 2 or 3 orders of magnitude beyond the solubility of the metal is used
- colloidal particles get caught in these precipitates as they are formed
- “___________________” conditions
3. _______________________________
- use of long chain polymers (cationic, anionic, nonionic high molecular weight polymers)
- segments of a polymer chain absorb on more than one particle thereby linking the particle
Chemicals Used (see attached Table)
2
Coagulation can be viewed as consisting of two phases:
Coagulation
- ___________________________________________________________________
_____________________________________________________________________
Flocculation
- ___________________________________________________________________
_____________________________________________________________________
Factors affecting coagulation
__________________
- changes in water quality such as turbidity and suspended solids
- need adjustment in chemical dosage rates and chemicals used
__________________
- low temperature affects the efficiency of coagulation process
___________________
- the nature of suspended solids affects the coagulation efficiency
___________________
- changes the pH of the water and eventually affects coagulation and
precipitation
Design of Rapid Mix (Coagulation) and Slow Mix Tanks (Flocculation)
Rapid mix
- need to rapidly disperse the coagulant
- detention time usually less than 1 min but can be as high as 2 minutes
- high turbulence is required
- a measure of the extent of shearing and mixing within a tank is given by ________________
G can be visualized as the measure of the relative velocity of 2 particles at a given
distance apart
eg., two particles moving 1 m/s relative to each other and at a distance 0.1 m
apart will have a G of 1/0.1 = 10 s-1
A general equation (equation 3-75) which relates the power input needed for mixing:
where
G
P
V

= velocity gradient (s-1)
= power input (watts = Nm/s)
= volume of mixing basin (m3)
= dynamic viscosity (Pa.s or Ns/m2)
Values for G and td___________________________________________________________________
td (s)
G (s-1)
_________________________________________________________________________________
Use of mechanical mixers
- impeller driven mixers are the most efficient devices
- relationship relating power input and the diameter of impellers and speed is given by:
(equation 3-76)
3
Valid for turbulence mixing with Re =  nD2/ > 10,000
Where P = Power (Nm/s)
KT = power number or impeller constant
 = density of the liquid (kg/m3)
n = rotational speed (rev/s)
D = diameter of impeller
Points to note – impeller diameter should be within 30 to 50% of the width of the tank, if not will move the
water and will not get efficient mixing. See Figures 3-22, 3-23
Example- Problem 3-55, 3-57:
3-55
What is the volume required for a rapid mix basin that is to be used to treat 0.05 m3/s of water if
the detention time is 60 seconds? Basin dimensions if the tank is square with the depth equal to
1.5 times the width.
3-57
Determine the power input required for the tank designed if the water temperature is 20 o C and the
velocity gradient is 700 s-1.
Estimate the diameter if the rotational speed of a mechanical impeller is 100 rpm.
1.
2.
Volume = 0.05 m3/s x 60 s
= ______________________ m3
Dimensions
= ______________________
= _______________________ m
= _______________________ m
w x w x 1.5 w
w
H = 1.5w
Power required
P = G2V
= ________________________
= ____________________ Nm/s (W)
Power of motor = 1473/(746 x 0.70) = ________ hp
3.
(1/s)2 x (Ns/m2) x (m3)
assume 70% efficiency
(1 hp = 746 watts)
Impeller diameter – assume turbine 6 curved blades, KT = 4.80
P = KT  n3 D5
= ____________________
= _____________________
= ______________ m
D/w
= 0.57/1.25
= 0.46 or 46%
within 30 to 50% of the width (ok)
Slow Mix or Flocculation
 gentle mixing to promote collision and agglomeration of particles
 if too much mixing (high G values) - flocs will be sheared, use G = _______________________ s-1,
detention time of _____________ minutes, typical ________minutes
or use Gt values ranging from ___________________________
 most plants would vary the G values over the length of the basin. Tapered effect. ______________
Reasons == > allows for formation of strong flocs
high G throughout = > _____________________
low G throughout => ______________________
Tapered effect => _________________________
See Figure 3-26.
4
Coagulants
Name
Formula
Al2(SO4)3.14H2O
Supplied as
Dry or liquid (48.8%)
Reaction
Al2(SO4)3.14H2O + 6HCO3- < == >
2 Al(OH)3 + 6CO2 + 14H2O + 3SO42-
FeSO4.7H2O
FeCl3.xH2O
Crystals or liquid form
Ca(OH)2
Powder
Ca(OH)2 + HCO3- + Ca2+ ==>
2CaCO3 (s) + 2H2O
Natural and synthetic
Polymers (MW > 10,000)
such as polyacrylamide
-grouped as nonionic, anionic
or cationic polymers
Liquid or dry form
--
Sodium silicate activated with
sulfuric acid
Liquid form
--
FeCl3 + 3HCO3- < == > Fe(OH)3 (s)
+ 3CO2 + 3 Cl-
5
Comments
- optimum pH range 6.0 – 6.5
- form white gelatinous flocs [Al(OH)3
or
(Al8(OH)20.28 H2O)4+] which enmeshes
the particles
- coagulation over a wide pH range 4.5 –
9.0
- heavier settling floc than alum
- stains tanks
- removes calcium, used for softening
water
- used for precipitation of metals in
water
-assist in coagulation by chemical
briding
- low concentrations needed usually 1
mg/L
- usually used in conjunction with other
coagulants
- white gelatinous flocs formed
- lighter and weaker than aluminum floc
- proper mixing is needed with close
operational control