Geotextile Tube Case Studies Disclose Effectiveness In Dewatering Contaminated Materials
Where the dredged material is highly contaminated, the laydown area is enclosed with earthen berms and lined with high-density polyethylene fabric to collect the filtrate water for further processing.
A polymer system injects measured amounts into the dredged material.
Clear water flows from geotextile tubes during a dredging operation.
Their experience has demonstrated that precise specifications in the construction of the tube laydown area, tube layout, and chemical conditioning of the dredge slurry must all be combined to deliver a successful project execution. Combining these key elements prior to dredge slurry entering the geotextile tubes enhances sediment dewatering and reduces the level of contamination in the filtrate water leaving the geotextile tubes. Filtrate water usually requires minimal, if any, treatment prior to discharge back to the waterway being dredged. In these types of applications, geotextile tube units are used to dewater the sediments and to store the dewatered material on-site prior to ultimate disposal or reuse of the material.
CASE STUDY 1: CONNER CREEK COMBINED SEWER OVERFLOW PROJECT
The Conner Creek Combined Sewer Overflow Project used a geotextile tube system to dewater more than 180,000 cubic yards of hydraulically dredged contaminated sediments from a one-mile stretch of Conner Creek. The project was conducted as part of the construction of a one billion gallon per day combined sewer overflow (CSO) treatment facility for the City of Detroit, Michigan. The contaminated sediments had accumulated in the waterway, which provides a conduit for treated effluent from the CSO facility to Lake Erie, through municipal storm sewer outlets and combined sewer overflows.
Project schedules required that the dredging operation be performed during the construction of the CSO facility. These schedules were managed closely by Senior Project Manager Paul Stage, and Project Manager Aaron Wright. To minimize impact of the dewatering operation on construction activities, the geotextile tube containers were staged in three separate lay-down areas. Each lay-down area was filled with three layers of stacked geotextile tubes and turned over three times during dredging operations to complete the project.
The dredged material was conditioned with a cationic polymer to aid the dewatering process and produce a high quality filtrate prior to discharge back into Conner Creek. In order to treat a wide range of dredge slurry concentrations, the polymer solution was injected into the flow of dredge slurry using a flow- and density-paced chemical feed system.
Field testing determined that 60-foot-circumference tubes, each 150 feet in length and constructed of GT 500 geotextile fabric were the best choice for the project. The geotextile tubes used in the Conner Creek project were also fabricated with circumferential seaming techniques to maximize the strength and capacity of each tube.
As tubes reached maximum capacity, measured by a final fill height of six feet, they were taken offline and allowed to rest, while the dredged material contained within continued to consolidate. After approximately 21 days, the dredged material consolidated, on average, to a concentration of 50 percent total solids by weight. On average, a final fill volume of approximately five and a half dewatered yards of material per lineal foot of geotextile tube was realized. The dosage rate of cationic polymer was about three and a half pounds of dry chemical per dry ton of sediment treated, averaged over the course of the entire project.
Overall, hydraulic dredging and geotextile tube dewatering proved to be an effective means to complete this project.
CASE STUDY 2: BLACK RIVER CONTAMINATED SEDIMENT REMEDIATION
The Black River Contaminated Sediment Remediation project used hydraulic dredging with geotextile tube dewatering to remove and dewater 25,000 cubic yards of sediments contaminated with PCBs, mercury, chromium and other heavy metals from a mill pond segment of the Black River, located near Kalamazoo, Michigan, and again was led by Infrastructure Alternatives’ Senior Project Manager Paul Stage.
To accommodate this volume of in-situ material, 4,600 lineal feet of 60 foot circumference geotextile tubes were required. After careful design phase testing, it was determined that the tubes used in the Black River project would also be constructed of GT 500 geotextile fabric.
There were two special design considerations for the Black River project. First, restrictive site conditions limited the size and shape of the geotextile tube lay-down area. Second, the sediment to be dredged from the river was contaminated with PCBs, chromium and mercury. If these contaminants passed through the geotextile tubes in the filtrate water, they would have to be removed before the filtrate could be discharged back into the river. The filtrate had to be collected, monitored and treated as needed to meet established limits prior to discharge.
To address the limited availability of space for dewatering, the geotextile tubes used in the project were just 60 feet in length and were stacked in two layers. In response to the need for filtrate collection and treatment, the lay-down area was enclosed with earthen berms and lined with 40-millimeter high-density polyethylene (HDPE) fabric, which prevented filtrate water from leaving the dewatering area. A ditch around the perimeter of the lay-down area collected the filtrate water and directed it into an adjacent lagoon, which was also enclosed with berms and lined with HDPE fabric. From the lagoon, the filtrate water was pumped to bag filters and granular activated carbon (GAC) units which further polished the filtrate water prior to discharge into the river.
Additionally, design phase testing had shown a direct correlation between total suspended solids and contaminant (PCBs, chromium and mercury) levels in the geotextile tube filtrate water. It was determined through bench testing that with the use of cationic polymers to condition the sediment, contaminant levels in the geotextile tube filtrate remained below established effluent limits when the total suspended solids concentrations in the filtrate remained below 30 milligrams per liter (parts per million). This was incorporated as a key process control indicator during full scale dredging operations.
The sediment was dredged from the river by an eight inch diameter horizontal auger hydraulic dredge. The dredged sediment was delivered to the geotextile tubes at an average flow rate of 800 gallons per minute with an average slurry concentration of eight percent total solids by weight. Each geotextile tube was filled with dredged sediment approximately six times until maximum capacity, determined by final fill height of six feet, was realized.
The material in the geotextile tubes dewatered and consolidated to a concentration of 50 percent total solids by weight within 30 days. At that time, the geotextile tubes were opened and the dewatered material was removed with heavy equipment and transported to a licensed landfill for disposal.
The use of geotextile tube containers in conjunction with hydraulic dredging proved to be a particularly effective method for accomplishing the stated goals of the Black River Remediation project. Geotextile tubes provided an excellent means of dewatering, and together with chemical conditioning of the dredged slurry, reduced the level of contaminants in the filtrate below permitted discharge limits and diminished the need for further filtrate treatment, which significantly lowered associated costs. The ability to determine a quantitative relationship between the level of total suspended solids and contaminants in the geotextile tube filtrate provided operators with a simple process control parameter that was utilized very successfully during full scale production.
When considering the application of geotextile tube technology, one must take into account the following characteristics of the project: first, the size and configuration of the available staging or lay-down area for the tubes; second, chemical conditioning of the material to be dewatered; third, the ability of the selected lay-down area to drain filtrate water away from the tubes; and finally, when geotextile tube containers will be stacked, there are additional safety considerations.
In comparison to mechanical dewatering methods, geotextile tubes have a much larger footprint. There must be enough space available to accommodate the lineal footage of geotextile tubes needed to dewater the volume of material.
The evaluation and selection of conditioning chemicals (polymers) is critically important to maximize the dewatering potential of the material in the geotextile tubes and produce the highest quality filtrate. Filtrate quality is especially significant when the material to be dewatered is contaminated.
The selected staging or lay-down area for the tubes must be able to quickly drain the large volume of filtrate water generated during active filling cycles of the geotextile tubes. Filtrate water pooling around geotextile tubes negatively impacts the rate of dewatering in the tubes. Examples of how the drainage ability of a selected area can be enhanced include grading and the installation of a gravel base beneath the geotextile tubes.
When geotextile tube units are stacked, one must consider the possibility of tubes moving or rolling and incorporate measures to prevent this from happening. Obviously this is a serious safety concern and can also be a recurring problem; unstable geotextile tubes in the base layer of a stack will reduce the overall stability of the entire stack.
The success of these and several other recent projects with similar scope and level of contamination has proven that the use of geotextile tube technology in conjunction with hydraulic dredging is an effective means of remediation. Geotextile tubes can accommodate large volumes of sediments and relatively high dredge production rates: dredged material flows of 3000 to 5000 gallons per minute, at concentrations of up to 20 percent total solids are not uncommon.
In addition to remedial effectiveness, geotextile tube dewatering delivers the significant advantage of requiring few mechanical parts, making it much less susceptible to the impact of debris when compared to other mechanical means of dewatering, such as filter presses. Also, with adequate chemical conditioning of dredged material, geotextile tube technology produces filtrate water with exceptionally low levels of total suspended solids, reducing the need for further downstream treatment of filtrate water.
Bill Cretens is the founder and president of Infrastructure Alternatives. A graduate of Central Michigan University, in biology with 25 years of experience in global water and wastewater management, he formed Infrastructure Alternatives in 2000 to serve the environmental dredging, water and wastewater needs of both public and private entities. Cretens and his team of engineers and specialists have applied this remediation process to more than 40 projects throughout the U.S.