The chemical salesman wants
to prove his product is better than what you're now using. He will try to favor the results to that end. The
most elaborate jar test apparatus and the best laboratory technique with the select reagents are all organized
to sell his product. If his product is better (be open minded), it will prove itself without augmentation. Because
of his vists to many different companies/facilities, the chemical salesman can be very helpful by sharing the solutions
others have found.
Probably
the single most valuable tool in operating and controlling a chemical treatment process is the variable speed,
multiple station jar test unit. Various types of chemicals or different doses of a single chemical are added to
sample portions of wastewater and all portions of the sample are rapidly mixed. After rapid mixing, the samples
are slowly mixed to approximate the conditions in the plant. Mixing is then stopped and the floc formed is allowed
to settle. The appearance of the floc, the time required to form a floc, and the settling conditions are recorded.
The supernatant is analyzed for turbidity, suspended solids and pH. With this information the operator selects
the best chemical or best dosage to feed on the basis of clarity of effluent and minimum cost of chemicals.
One invariable requirement in all jar
test procedures is that the tests only have meaning if the tested wastewater exactly resembles the flow stream
that will be ultimately treated by the chemicals being evaluated.
Simply running a single grab batch
of tests will rarely provide information of sufficient reliability to operate a treatment process.
Jar tests are effective in indicating
the relative quantity of chemicals required, but should be followed by on-site dewatering experiments to more accurately
determine the required dosage. Jar tests should be followed by pilot or full-scale tests to determine the exact
chemical requirements.
The jar tests are very good indicators
of the concentration of polymers that produces optimum floc formation.
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. Samples of water to be treated are
placed in several jars, various amounts of chemicals are added to each jar, stirred and the settling of solids
is observed. The lowest dose of chemicals that provides satisfactory settling is the dose used to treat the water.
You must realize that it is 'almost'
impossible to exactly duplicate in the jar test the flow-through conditions that are occurring in your treatment
plant. The jar test attempts to duplicate in the laboratory what is occurring in the plant in the relation between
detention times, mixing conditions, and settling conditions. By watching the jar test floc form and settle, you
can get a good idea of what should happen in your plant for the test chemical dose. The jar test should be used
as an indication of what you can expect in your water treatment plant. By closely watching the floc form in the
flocculators and settle out in the sedimentation basin of your plant,
Jar tests are tests designed to show
the effectiveness of chemical treatment in a water treatment facility. Many of the chemicals we add to water can
be evaluated on a small laboratory scale by the use of a jar test. The most important of these chemicals are those
used for coagulation, such as alum and polymers. Using the jar test, the operator can approximate the correct coagulant
dosage for plant use when varying amounts of turbidity, color, or other factors indicate raw water quality changes.
The jar test is also a very useful tool in evaluating new coagulants or polymers being considered for use on a
plant scale.
The optimum pH level can be determined
by a series of jar tests or bench tests on the wastestream.
In my experience operating an industrial
pretreatment facility since 1993, I've used the jar test and learned some unique aspects of this procedure.
It seems to me that the theory of jar testing is meaningful when
the wastestream character is consistent. Batching and jar testing work. Or, when variables are made constant
then the lab procedure gives reliable results. Jar testing is useful even in chaotic
situations. Time is a major factor. I've grabbed samples one after another within minutes and seen major changes.
Continuous processes require constant testing. A duplicate pretreatment facility used only for testing would
give real data but that's out of the question - costs too much. A pilot plant (scaled down version) sometimes
is used but even that cannot duplicate the synergistic actions. However, the jar test does point the way to go.
Here's what I do:
At the point just prior to flowing into the DAF, I grab 3 samples (one after another) discarding the first two. Those two flush the sample pipe
so that I'm sure that I get the closest real sample. Also, I grab a small amount of dissolved air for the third
sample in the same jar. This is as near to simulating what is happening in the DAF as I can get. Within the next
10 to 20 seconds, I observe the floc formation and the relative percent loading. The time frame of this protocol
is one minute. I step through this protocol each and every time. I'm sure that if I skip a step, what I see will not
be 'what's happening'.
I perform this operation at least once each hour. Recording the
percent loading (relative), the flow rate and the floc blanket. I could detail the dosage of each polymer using
the drawdown tubes and knowing the current percent concentration of mixed polymers but time is not available for
these details except on rare occasions. Experience has shown that the important parameters are the ones that I
now track.
10 minutes to do a lab type jar test is far too long when the wastestream
is 200 gpm and needs a correction. While there are occasional opportunities for batch processing, continuous operation
is the normal routine and that requires rapid response to stay within the ballpark of desired goal.
Was
this enough info? Too much? Shall I add more info?