Lab 5
Lab 5
Lab 5
Environmental Laboratory
Lab #5
Jar Testing
1
JAR TESTING
The jar test is a common laboratory procedure used to determine the optimum operating
conditions for water or wastewater treatment. This method allows adjustments in pH,
variations in coagulant or polymer dose, alternating mixing speeds, or testing of different
coagulant or polymer types, on a small scale in order to predict the functioning of a large
scale treatment operation. A jar test simulates the coagulation and flocculation processes
that encourage the removal of suspended colloids and organic matter which can lead to
turbidity, odor and taste problems.
The jar testing apparatus (see Figure 1) contains six paddles which stir the contents of six
1 liter containers. One container acts as a control while the operating conditions can be
varied among the remaining five containers. A rpm gage at the top-center of the device
allows for the uniform control of the mixing speed in all of the containers.
2
2. Add the coagulant to each container and stir at approximately 100 rpm for 1
minute. The rapid mix stage helps to disperse the coagulant throughout each
container. Coagulants are chemical additions, such as metallic salts, which help
cause smaller aggregates to form larger particles.
3. Reduce the stirring speed to 25 to 35 rpm and continue mixing for 15 to 20
minutes. This slower mixing speed helps promote floc formation by enhancing
particle collisions which lead to larger flocs. These speeds are slow enough to
prevent sheering of the floc due to turbulence caused by stirring to fast.
4. Turn off the mixers and allow the containers to settle for 30 to 45 minutes. Then
measure the final turbidity in each container. The final turbidity can be evaluated
roughly by sight or more accurately using a nephelometer.
The following animation shows a typical jar test. Varying amounts of coagulant doses
were added to 5 of the 6 containers. The first jar on the left is serving as a control and no
coagulant was added. The coagulant doses increased in the containers from left to right.
For this water, as the dose of coagulant increased the residual turbidity improved. It is
important to note that the optimum coagulant dose is the dose which meets the specified
turbidity required on the regulatory permit. The addition of excess coagulant may reduce
turbidity beyond what is required but also could lead to the production of more sludge
which would require disposal.
The following results were achieved after a series of jar test on two sample waters, A and
B, were treated with two different coagulants, alum and ferric chloride, at varying doses.
Water A had low alkalinity and required less coagulant to achieve good coagulation and
flocculation than the higher alkalinity of Water B. This is seen by the curves of Water A
reaching a minimum turbidity at ~20 and ~30 mg/L of aluminum sulfate and ferric
chloride added, respectively. Plots of turbidity versus coagulant dose for Water A with
alum, Water B with ferric chloride and Water B with alum all showed a continual
decrease in turbidity with an increase in coagulant dose. This trend is a sign that sweep
flocculation is the main coagulation mechanism occurring. Water A, with ferric chloride,
showed a decrease followed by an increase (at ~40 mg/L) in turbidity with a
corresponding increase in coagulant dose. This dictates that adsorption and charge
neutralization is taking place due to the colloids destabilizing and not coagulating.
Although not shown in the graphs, the addition of coagulant to the low alkalinity waters
lead to a drop in the pH of Water A, which enhanced adsorption and charge neutralization.
Therefore, higher coagulant doses are often needed with high alkalinity waters (like
Water B) where pH remains fairly constant and sweep floc is the main coagulation
mechanism. Although slightly less alum than ferric chloride was needed to reach an
3
optimum level, the residual turbidity when using the alum coagulant did not fall below 1
NTU. This means that even though alum may require a slightly smaller dose, it still may
not be able to meet the desired effluent regulations without the additional help of a filter
or polymer.
FeCl3
Al2(SO4)3
FeCl3
Al2(SO4)3
Conclusion
4
PUBLISHED BY THE NATIONAL ENVIRONMENTAL SERVICES CENTER
Jar Testing
By Zane Satterfield, P. E., NESC Engineering Scientist
Photos by Julie Black, courtesy of Morgantown Utility Board
Summary
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.
temperature, pH, turbidity, and alkalinity formed as follows: Operate the stirrers at a
of the raw water before beginning. high RPM for 1 minute to simulate the
static mixer. Then reduce the speed of the
operator returned to his plant and began jar An Equal Opportunity/Affirmative Action Institution
Published by The National Environmental Services Center at West Virginia University, P.O. Box 6064, Morgantown, WV 26506-6064