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Is Polymer Effective Or Needed When Dredging

And Pumping Lake Sediments To Geo-Bags?

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.

The process 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 Geo-Bags.

Geo-Bags 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. 

Polymers: 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.

Polymers are 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 made: 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. 

When injecting 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 the solids.

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:

Test To 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 to disposal.

Test To Determine If Polymers Make Lake Sediments Dewater Faster or Better Than Not Using Polymers. 

  1. 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 consistency.

  2. The mud was poured into two glass Mason jars until the jars were both two inches from full.

  3. 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 good floc.

  4. The other sample had no polymer added.

  5. 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.

  6. The volume of water in each jar was exactly the same.

  7. 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.

  8. On the open top of each Mason jar that received the decant water, was placed a paint strainer with a coffee filter inside it.

  9. 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. 

  10. Each test sample was covered with a plastic bag and wrapped with duct tape to prevent evaporation.

  11. 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. 

  12. 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?

The Reason 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.