FLOCCULATION ENHANCEMENT TECHNOLOGIES  FOR THE DREDGING CONTRACTOR: 
GUIDELINES FOR THE SELECTION OF A POLYMER SUPPLIER

 E. H. Seagren

ABSTRACT

The use of synthetic organic polymers as flocculation enhancements have been used in the dredging industry for the past 30 years to speed up the release of free water from sediment particles in the dredge slurry in a variety of applications. In fact, there are over twenty-five current dredging applications where polymers have been used. 

The technology was first used by the writer to coagulate digested sewage sludge at The City of Milwaukee WWTP in 1973. A small MUD CATT horizontal auger dredge pumped the digested sludge from a pond mixed with water and the polymers were added about 200 feet into the dredge pipeline from the discharge. The slurry, with polymer added, was pumped into a small geo-container approximately 20 foot (6.09m) square by 7 feet (2.13m) high. Although this was a trial test, the system worked well, and when the bag was cut open (after 24 hours), the sludge had jelled to a cake like consistency. The sludge went from a solids content of 2.1% by weight to a solids content of 9 % by weight. 

Polymer use continued to grow slowly in the dredging industry, and today it is a common practice to investigate the use of polymers in the dredging process, especially when dredged material disposal sites are limited in both number and proper size. Polymer cost ranges will be discussed, as well as dosage rates, and usually, polymer cost is given as a cost per yard of dredge material moved "in-situ". 

Keywords: dredge disposal area, re-cycle, contaminated sediments, mechanical plants, polymers, flocculation. 

INTRODUCTION 

This paper is specifically geared to the use of polymers, and how they are affecting the dredging industry, as we now move to new methods of de-watering sediments and disposing of sediments. The paper will not address the chemistry of polymers and there are many excellent books on the subject. To date, the author has identified over 25 various dredging applications where polymers have been used and this list will be presented later in the paper. The decision to use polymers must begin in the early phases of the dredging project design because once the project is started, the use of polymers may not be permitted. There are dredging projects today that could not happen without the use of polymers, and this is the most promising area where the engineer can become innovative and is most exciting. Their use will grow into the 21st century and the author is convinced that many more innovative uses will be found.

The purpose of this paper is to make the dredging contractor aware of the many advantages of polymers in dredging and then provide guidelines for the selection of a polymer supplier who is familiar with the dredging industry and can supply the proper polymer and delivery system. 

EARLY DREDGING PRIOR TO 1970

Prior to 1970 there was basically no small dredge market, and almost all work was done with large dredges, too large for the project, and in almost all cases it was a simple task of pumping from A to B and not much concern for the quality of the de-cant water. Disposal areas were improperly designed and filled up quickly, and some projects had to be abandoned prior to completion. 

When the environmental movement was in its infancy in early 1970, the hydraulic dredge was depicted by the new class of environmentalists as a horrible monster devouring the earth, and its use was opposed in almost every case. 

Much has changed in the past 30 years as the environmental community now accepts the dredge as a viable clean-up tool. 

Then early in 1970 an event took place that would change the dredging industry forever. A large car rental company located in Minnesota heard of a new small dredge for cleaning silt from Minnesota lakes and thought it such a good idea, that they would build these new horizontal auger dredges and rent them out like they rented cars. They went ahead and built 250 dredges and named it the MUD CATT. The renting of these dredges proved to be a total failure, and in 1972 they decided to sell the entire inventory at auction. Just two weeks before the auction, the Clean Water Act was passed and America's heavy industry heard of the MUD CATT through ads in the Wall Street Journal and saw an immediate need to dredge their settling ponds. In fact, the industrial need was so great that numerous machines were put out on a demo basis and they were a total success, especially in the fine colloidal sludges found in industrial settling ponds.

The auction was cancelled and as of today, over 1,000 of these small dredges have been sold. Following the introduction of the MUD CATT, other dredge manufactures saw the need for smaller cutter-head dredges, and they proved very successful on small projects where there was harder material since the MUD CATT could not excavate harder sediments. The demand for small dredging projects exploded as the small units became known. THE SMALL DREDGE INDUSTRY WAS BORN. Today there are thousands of small dredges working around the world. Polymers can play a critical role in the dredging process, and in some cases a project may not be feasible without their use. 

REASON FOR USE OF POLYMERS IN DREDGING

Polymers or flocculent agents have taken on a new and critical use in the dredging industry for the following reasons: 

1) Good upland confined disposal areas are becoming harder to find. 

2) Smaller disposal areas must be re-cycled using polymers. 

3) Since the de-cant water in almost all cases is returned to the original body of water, the polymer must be EPA approved for that use.

4) The release of free water in the slurry at an enhanced rate permits more solid material to be placed in a given disposal area, thus saving disposal costs. 

5) An increase in contaminated sediment dredging has shown that the contaminated materials cling, or attach themselves to the larger polymer flocculated particles. This means that the released water contains less contaminated materials. 

6) The use of geo-containers is becoming a very acceptable dredging disposal technique and it has been shown that polymers assist in the settling of the particles inside the containers and also permits earlier removal of dried material if the tubes are cut open. It has also been found that the contaminants remain inside the container and are not released with the container water discharge. 

7) All the new mechanical de-watering plants use polymers in the de-watering process and their use is on the increase. 

These are but some of the uses for polymers in the dredging industry and new applications will be found. Costs are also coming down and some companies are designing polymers specifically for the dredging industry. Initial sediment testing is critical, and the polymer supplier should have a properly equipped geo-technical lab to do this and provide a comprehensive written report to the contractor. Also, the polymer delivery system must be approved by the supplier and in some cases, the EPA also. The EPA will demand the specific polymer test report if the dredge de-cants water is returned to the dredge body of water.

SMALL DREDGING PROJECTS 

Many of these projects were in congested areas such as home-owner lakes or canals with homes on both sides and virtually no available land for properly sized dredge disposal areas. The dredging rule of thumb for a properly sized disposal area is the volume storage in cubic yards shall be 2-1/2 times that of the in-situ cubic yards to be dredged relying on gravity only to settle the sediments. Areas this large were almost impossible to find and smaller volumetric areas had to be used. 

This was the birth of the use of polymers in the dredging industry since smaller disposal areas could now be utilized and still have a de-cant water which met approved turbidity standards. Some early and current dredging projects will be described where polymers solved the problem. 

ADVANTAGES OF POLYMERS 

1) Smaller volume storage disposal areas could now be used.

2) Acceleration of particle settling, up to 100 times faster than that of natural means. 

3) Increased in-situ percent solids by weight of the material in the disposal area. 

4) Thus, greater amount of sediment storage per unit volume of disposal area.

5) Substantially increased clarity of de-cant water, especially when disposal area is greater than 3/4 full. Note: Without polymers the 3/4 full mark is the danger zone for dredge de-cant water without the use of polymers. 

6) Polymers can eliminate the need for a second or third disposal area in series to clarify the de-cant water or smaller ones can be built instead. 

7) Polymers permit the use of carbon filter media for the de-cant water since the de-cant water is clearer and will not clog the carbon.

8) Settling ponds can be re-cycled and cleaned during dredging by having 3-4 ponds to pump into. 

SPECIAL USES FOR POLYMERS 

1) In-lake disposal of sediments where water is very deep polymers prevent excessive dispersion. 

2) Use with geo-containers to improve efficiency and contain contaminated material within the container. 

3) Assists in the creation of wetlands by increasing density of settled material, thus faster growth of vegetation.

4) Increase in contaminated sediment dredging because of ability of polymers to enhance the attachment of the contaminants to the now larger flocculated sediment particles. 

5) Permits use of mechanical de-watering technologies to produce a dry cake for hauling to a landfill. 

POLYMER INJECTION TECHNIQUES 

In dredging, it is most common to inject the liquid polymer directly into the dredge discharge line and this is usually done within 200 feet of the dredge open discharge. This is again a rule of thumb only and the polymer manufacturer can give the exact distance. One of the problems with polymers is that they are sensitive to tearing and if injected too far back from the discharge, can produce tearing caused by turbulent flow in a dredge slurry line. Sometimes the polymer is injected right on the dredge. 

Liquid polymers are preferred to dry polymers which have to be mixed on the job and they are shipped to the job in rectangular stainless steel tanks for ease of storage and for contamination. A polymer feed pump is then used to inject the liquid polymer into the dredge line at about 20 psi higher than the pressure in the pipeline (usually quite low at this point). If the polymer must be diluted, a special intermediary pump is used to inject water prior to injecting into the dredge line. 

Polymer injection rates are always given in parts per million (PPM) of total liquid polymer injected into the slurry line and this rate is determined by the manufacturer. Fine tuning is almost always done on site to obtain the clearest de-cant water, the size of the flocculated particles, and speed of free water release. 

POLYMER COSTING 

This is one of the most critical aspects when considering the use of polymers and must be determined prior to bidding a project. Once the polymer manufacturer has sampled the sediments at the job site and then done in house lab testing, a fairly good cost estimate can be made. Costs are generally given as cents or dollars per cubic yard of in-situ material to be dredged. I have seen polymer costs vary from a low of $0.10 per cubic yard up to $1.20 per cubic yard. Generally, the higher the organic content of the material, the higher the polymer dosage and hence higher costs. In many cases the cost of the polymer can make or break a dredging project. 

Also, polymer costs as stated above usually include all costs, polymer in the tanks, delivery, and injection pumps, and any other ancillary equipment. As one can see, it is incumbent on the customer and engineer to determine at the inception of a project if polymers are required, and their costs, as compared to the total project. It is quite embarrassing to find out that polymers are needed on a project that has begun, and no funds have been provided for them. Therefore, the dredging contractor must have enough polymer knowledge to educate the customer early on if he/she feels that polymers are required. 

POLYMER DREDGING PROJECTS 

A partial list of polymer dredging projects is listed in Table 1. The projects range from industrial ponds, fish farming, settling ponds, to dredged material disposal sites. 

Table 1. Partial list of polymer dredging projects.

                  

POLYMER DREDGING PROJECTS COMPLETED (A SAMPLING) 

The following projects could not have been accomplished without the use of polymers and this is becoming the case for an increasing number of projects in this century: 

1) Evergreen, CO., Water supply lake, 400,000 cubic yards-1986 

2) Crane Creek, City of Melbourne, FL,1998 

3) Inlet dredging, Clinton, IA., 2001

 4) Badger Army Ammunition Depot, Grubers Grove Bay, Wisconsin River, 2001

FACTORS TO BE CONSIDERED WHEN SELECTING A POLYMER SUPPLIER 

1) Is the supplier the primary manufacturer of the polymer or is the supplier purchasing polymer from a primary and re-selling the product under a different name?

2) Does supplier have a specific polymer designed for the dredging industry?

3) Are there local sales people who can visit the potential dredging site and perform initial tests?

4) Does the supplier have an in-house lab for testing sediment samples and will the supplier provide a written report to the dredging contractor?

5) Is there good technical support after the project has begun and can changes be made quickly?

6) Does the supplier have their own injection equipment and provide it to the contractor, included in the polymer cost?

7) Will the supplier provide written guarantees as to performance such as settling times, cake dryness, etc?

8) Are the polymers approved by the EPA or responsible State Agency for compatibility with potable water and return of de-cant water to the body of water where the dredge is operating? Necessary documentation available? 

CONCLUSIONS 

The author predicts that there will be a substantial increase in the use or polymers in the dredging industry, and will not only be limited to small projects but large projects as well. Many projects will not be possible without the use of polymers and their additional costs must be dealt with up front. The owner must be flexible, and if polymers are found to be needed, after a project has begun, necessary change orders must be authorized so as not to delay the project. 

The cost of polymers will continue to come down, and today, polymers can vary from 5% of a dredging project up to 50%. One must look at polymers as another tool available to the contractor to not only obtain more projects but to increase efficiency on projects not using polymers in the past. 

REFERENCES 

Dredging and Port Construction (DPC) (1998). "Florida Manatees Appreciate Dredging Spring Clean," Nov. Issue. Herbich, J. B. Handbook of Dredging Engineering, Chapter 5, Second Edition, McGraw Hill: New York. International Dredging Review (IDR) (1986), "Rocky Mountain High: Dredging at Elevation 7071 Feet," Sept./ Oct. Public Works, (2002), "City Solves Complicated Inlet Dredging Challenge," Sept. Issue. 
Seagren, E. H., (2003) "Small Dredges," Texas A&M Dredging Engineering Shortcourse Lecture Notes, Center for Dredging Studies, Texas A&M University, College Station, Texas. TEXAS A&M SHORT COURSE, SMALL DREDGES, January 10-13, 2003. 
World Dredging, Mining and Construction (WDM&C), (2001), "Innovative De-watering, USA. Badger Army Ammunition Depot," Dec. Issue.