Is Polymer Effective Or Needed When
And Pumping Lake Sediments To
By Don Searles
In 2004, Earth
Tech had a contract to remove, dewater, and dispose of PCB contaminated
sediments from White Lake in Michigan. Earth Tech subcontracted the
dredging portion of the work to the Faust Corporation. Faust
commissioned Dredging Specialists to design, build, and operate a
slurrying and pumping system to pump the dredged material into Geo-Bags.
was as follows. Faust dredged the material from the lake with a
Cable-Arm clam bucket. The material was placed in a hopper [mud]
barge. When a mud barge was full, it was towed to a landing barge tied
at the shore. On the landing barge, there was an elevated hopper and a
hydraulic excavator with a two cubic yard clam bucket. The excavator
removed mud from the mud barge and placed it in the hopper. Twenty
cubic yard dump trailers, pulled by highway tractors, were loaded from
the elevated hopper. The material was hauled to, and dumped into, the
Dredging Specialists slurrying and pumping system tank. An eight inch
dredge pump was used to pump the material through a 1,000 foot pipeline
that went under a highway, and into Geo-Bags. In the slurrying and
pumping system, water was added, if needed, and mixed into the mud by
circulating the slurry through the dredge pump and back into the tank.
When the consistency of the mud was satisfactory, it was pumped to the
are large cylindrical tubes made from Geo-Textile fabric which is woven
from heavy plastic threads. A common Geo-Bag size is 45 feet in
diameter and 200 feet long. This size bag will hold 700 cubic yards of
material when it is full. When this size bag is full, the cross section
is shaped like an egg, with a height of six feet and a width of twenty
feet. The fabric has small openings of 50 to 100 microns between the
threads. The mud and water slurry enters the bag, which retains the
solids and allows water to pass through the small openings. Over a one
to three month period, the retained material dewaters sufficiently to
pass the paint filter test and be hauled to disposal in dump trucks.
Earth Tech injected polymer into the pipeline that ran from the dredge
pump to the Geo-Bags in an effort to improve the dewatering of the
slurry in the Geo-Bags.
chemicals that are added to a solids water slurry, in this case, lake
mud, and cause the very small silt particles to be attracted to other
particles, which cause them to come together and form a larger particle
much like large curd cottage cheese. The large particles quickly
settle, and clear water soon rises to the surface. This process is
called flocculation. The purpose of flocculation is to quickly separate
the solids from the water. Floc is short for flocculation.
How floc is
The dosing rate [amount of polymer added] is critical to making a good
floc. The dosing rate is determined by the flow rate and percent solids
in a pipeline. There is not much room for error. If the dosing rate is
too low, a floc will not be made. If the dosing rate is too high, the
slurry will become sticky and blanket on the sides of the Geo-Bags and
prevent the passage of water. Any change in the flow rate or percent
solids requires an instantaneous change in the polymer dosing rate.
polymer in a pipeline, the polymer must be gently and completely mixed
in with the solids and water to make a floc. The floc particles are
fragile, and unless the mixing is gentle, the floc will be destroyed.
There are two
types of flow in a pipeline, laminar and turbid flow. With laminar flow,
the slurry moves forward in a straight line. The flow is laminar when
the velocity is low and the percent solids are high. [Thick mud]
Laminar flow will not provide the mixing needed to mix the polymer with
With turbid flow,
the water and solids are tumbling as they move forward. The flow is
turbid when the velocity is high and the percent solids are low. [Thin
mud] Turbid flow can cause mixing that can destroy a floc.
In the pipeline at
White Lake, most of the time, the slurry was thick enough to provide
laminar flow. However, it could, and did, change from thick mud to
nearly clear water very quickly. At White Lake, the velocity in the
pipeline ranged from 2 feet per second to 12 feet per second. The
percent solids ranged for 0 to 40 percent. These vast changes in flow
rate and percent solids, coupled to changes from laminar flow with no
mixing, to turbid flow with violent mixing, made it virtually impossible
to make an effective floc.
To change the
dosing rate to match the flow rate and percent solids, would require a
sensing system that would measure the flow rate and the present solids,
and instantly change the speed of the dosing pump. With a pipeline
velocity of, say 6 feet per second, the time delay between sensing and
the actual change in dosing rate, would permit the material that needed
the new dosing rate to have moved down the pipeline before the new
dosing rate hit the pipeline.
From the above
analysis, and my experience in dredging and working with polymer and
belt presses, it was my opinion, that due to the critical polymer dosing
rate needed to make a floc, and the vastly varying flow rate and percent
solids in a dredge pipeline, a polymer would not make an effective floc
in a dredge pipeline, and thus would not improve dewatering of the
material in the Geo-Bags.
To test my
opinion, I conducted the following tests:
Determine If Polymer will make a Floc In A Dredge Pipeline.
Near the end of
the pipeline, before the slurry entered the Geo-Bags, there was a
sampling valve. I went to the sampling valve and took over 50 samples,
at various times, when we were pumping and polymers were being injected
into the pipeline. I only observed one sample that showed any signs of
a floc. Thus, it was determined that polymer did not make a floc in the
pipeline, and thus, did not improve dewatering in the Geo-Bags. Even
without the aid of polymer, the solids in the Geo-Bags dewatered as
expected and in about two months of dewatering, the material was hauled
Determine If Polymers Make Lake Sediments Dewater Faster or Better Than
Not Using Polymers.
A one gallon sample of mud [just pourable] was recovered from our
tank. The mud was mixed with a drill and a paint mixer to make an even
The mud was poured into two glass Mason jars until the jars were
both two inches from full.
I gave one sample to the Earth Tech polymer person and told him
to add polymer to make a good floc. He added polymer, and it made a
The other sample had no polymer added.
The two samples were covered and left standing, undisturbed for
18 hours. The clear water was carefully poured off each sample, without
any solids, into separate one quart glass Mason jars.
The volume of water in each jar was exactly the same.
The next step of
the test was to procure paint strainers from a Sherwin Williams store.
Inside each paint strainer, a coffee filter was placed.
On the open top of each Mason jar that received the decant water,
was placed a paint strainer with a coffee filter inside it.
The jars with the mud in them were suspended, upside down, over
the paint strainers. Some mud came out of each jar and fell in the paint
strainers, and some mud remained in the jars. However, all the water
that came out of the mud of both samples went through the coffee filter,
the paint strainer, and entered the glass jars that retained the water.
Each test sample was covered with a plastic bag and wrapped with
duct tape to prevent evaporation.
Three days after the start of the test, there were no drops of
water coming from the paint strainer after 30 minutes of observation,
and the test was terminated.
The jars that caught the water were set side by side, to measure
the difference in the volume of water that came out of each mud sample.
The level of water in each jar was exactly the same.
This test showed
that the addition of polymer did not cause any more water to be released
than gravity settling over a three day period. The results of this test
showed that when dredging lake sediments, a polymer has no beneficial
effect on the dewatering process. However, the chemical companies that
sell polymer, the salesmen, and the polymer equipment suppliers, all
benefit. Could that be the reason they recommend polymer?
Polymer Is Not Needed In A Geo-Bag Dewatering Application.
In a belt press
application, polymer is needed to instantly release the free water
because the material is quickly dispersed onto the gravity dewatering
belt of the belt press. In a Geo-Bag dewatering application, there is
plenty of time for the material to dewater by gravity. Thus, in a
Geo-Bag dewatering application, time replaces polymer and the same
dewatering results are attained. The way to prove if this is
true, in your, or any application, is to run the same jar test that I
ran. One sample with polymer and one sample without polymer, and
measure the water released over time. The results may be different than
at White Lake, but the time spent to run the tests could Save A
Lot of Money.