Jar Test at Laboratory for Modunaghat Water Treatment Plant

Jar testing is a pilot-scale test of the treatment chemicals used in a particular water plant.It simulates the coagulation flocculation process in a water treatment plant and helps operators determine if they are using the right amount of treatment chemicals, and, thus, improves the plant's performance.
The jar test is a laboratory procedure that simulates coagulation flocculation with differing chemical doses. The purpose of the procedure is to estimate the minimum coagulant dose required to achieve certain water quality goals.
The purpose of the laboratory jar test is to select and quantify a treatment program for removal of suspended solids or oil from raw water or a dilute process or waste stream. Jar tests are conducted on a four- or six-place gang stirrer, which can be utilized to simulate mixing and settling conditions in a clarifier.
Basically, coagulation is a process of addition of coagulant to destabilize a stabilized charged particle. Meanwhile, flocculation is a mixing technique that promotes agglomeration and assists in the settling of particles. ... The product of the mixing intensity and mixing time is used to describe flocculation processes.
Turbidity
•
Turbidity
-
particles (sand, silt, clay, bacteria, viruses) in
the initial source water that need to be removed to
improve treatment.
1. Suspended Solids
2. Colloidal Solids (~0.1 to 1 mm)
3. Dissolved Solids (0.02 mm)
3
1
2
7 ntu
Raw water
375 ntu
backwash
0.02
ntu
Treated
3
Why coagulation is needed
Various sizes of particles in raw water
Particle diameter
Type
Settling velocity
mm
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/year
0.000001 (1
nano
)
Small colloids
3 m/million year

Colloids
-
so small, gravity settling not possible

Metal precipitates are usually colloidal
G r a v i t y s e t t l i n g
4
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
H
2
O
Colloid

Impossible to remove colloids by gravity
settling
Two primary destabilization methods

Colloids can be destabilized by charge neutralization :

Positively charged ions (Na
+
, Mg
2
+
, Al
3
+
, Fe
3
+
etc.) neutralize the
colloidal negative charges and thus destabilize them.

With destabilization, colloids aggregate in size and start to settle
Two primary destabilization methods

Colloids can be destabilized by
sweep
flocculation (
Enmeshment
in a precipitate
)

If metal salts, e.g., Al
2
(SO
4
)
3
,
FeCl
3
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.

Most drinking water treatment plants operate using sweep
flocculation

requires
a higher coagulant dose, rather than charge
neutralization.

In
charge neutralization, the positively charged metal
coagulant is attracted to the negatively charged colloids
via electrostatic interaction.

Adding
excess coagulant beyond charge
-
neutralization
results in the formation of metal coagulant precipitates.
These metal hydroxide compounds (e.g., Al(OH)
3
or
Fe(OH)
3
) are heavy, sticky and larger in particle size.
Water Treatment Coagulants
Particles in water are negative; coagulants usually
positively charged.
1. Alum
-
aluminum sulfate
2. Ferric chloride or ferrous sulfate
3. Polymers
Water Treatment Coagulant Alum
Alum
-
(aluminum sulfate)
-
particles suspended in natural,
untreated water normally carry a negative electrical charge. These
particles are attracted to the positive charges created by aluminum
hydroxides. Dosage is generally around 25 mg/L.
1. Trivalent Al
+3
charge attracts
neg
-
particles
2. Forms
flocs
of aluminum hydroxide (AlOH
3
).
3. Impacted by
mixing, alkalinity, turbidity
and temp.
4. Ideal pH range 5.8
-
8.5