Geotextile tubes with a new weave design were used for sludge dewatering of the Ashburton Washwater settling basin in Baltimore Maryland.
While geotextile tubes in the form of bags or tubes have been successfully deployed for dewatering sediments and sludge since the 1980’s there have been few advances in the performance of this passive dewatering technique. During the past 25-30 years a variety of geotextile tubes have been used to cost-effectively dewater solids from water and sewage treatment plants animal waste water lagoons and a large array of industrial process sludge. Although improvements in tube seam strength and port design have been made rates of dewatering and the holding capacity per unit volume of tube have not changed.
The most recent innovation includes a new geotextile weave design recently introduced to the market place. The objective of the present study was to evaluate this advanced weave design for its effect on active and passive dewatering rates as well as the holding capacity of dried sludge processed under typical field conditions.
A dewatering project at the Ashburton Water Filtration Treatment Plant Baltimore Maryland was selected as a project that would be representative of a typical industrial sludge dewatering application. The project concerned dredging and dewatering alum sludge from the Ashburton Washwater Settling Basin that serves as a backwash reservoir/settling basin for the Ashburton Water Filtration Plant. The project consisted of dredging and dewatering approximately 15000 tons of alum sludge from the settling basin. The contractor awarded the dredging/dewatering project selected the geotextile tubes with the new weave design as the dewatering device for the project. Figure 1 illustrates the project layout.
Figure 2 is an example of a standard type fabric weave for the geotextile tube. Figure 3 is an example of the new weave for the tubes used in the project. Because the weave in Figure 3 has more depth and a staggered weave design dewatering is not as readily cut off from the bottom of the tube. The manufacturer speculates higher dewatering rates are due to the three-dimensional nature of the weave therefore having more dewatering channels per unit of tube surface area.
An area measuring approximately 200 by 230 feet adjoining the Ashburton Washwater Lake was selected as the laydown area for the tubes. In order to accommodate the dewatering of approximately 15000 tons of alum sludge eight geotextile tubes measuring 150 feet in length with a 60 foot circumference were selected for the project. Some of the tubes were stacked two deep to facilitate fitting the eight tubes in the laydown area.
Included in the experiment were methods for determining the volume per linear foot of the new design tube as well as an active dewatering rate and a passive dewatering rate. Other observations of the experiments were: Dewatering in real-time for the duration of the project therefore no downtime having to wait for tubes to dewater; No downtime due to tube failures; i.e. rips blowouts or seam failures; Tubes routinely filled to over seven feet day after day for over six months without seam failures
To project how many tubes are required for a dewatering project there is a need for a more accurate method for calculating volumes of the new design tubes. Cross sectional areas were measured as shown in Figure 4 and empirical equations developed for each curve. Definite integral calculus was then applied to develop the area under each curve. The area under a curve is then multiplied by the length of the tube to obtain volumes. The calculus method is more accurate than making estimations with ellipses or rectangles.
CALCULATED VOLUME VS VOLUME FROM HISTORICAL DATA
Based on historical data the manufacturer claims the 60 foot circumference tube by 200 foot long tube holds 1100 cubic yards of material when the tube is six to 6 ½ feet tall in the middle. The definite integral calculus method determines 953 cubic yards at six feet and 1046 cubic yards at 6.5 feet. The calculation method shows a reasonable comparison to historical data and provides the benefit of knowing the volume of the tube at any height up to eight feet.
METHOD OF MEASUREMENT OF CROSS SECTIONAL AREA
Figure 4 shows the geo-textile tube with the curves marked out on the tube at different elevations above the deck. One half of each curve was generated using a six foot long pole and measuring in from the front edge of the tube at 2 3 4 and 5 feet respectively. The pole used for measurement was set parallel to the long axis. To get the two-dimensional cross section the same pole was laid on each of the lines and extended past the front edge of the tube. A level device was placed on the pole and the pole moved up or down until the level device showed a level reading. The distance from the pole to the deck was measured at distances of 0 1 3 5 7 9 11 and 13.5 feet. The 0 position is the lowest point on the tube and 13.5 feet is the highest point in the middle of the tube.
EXAMPLE: WASTE ACTIVATED SOLIDS AT VARIOUS TUBE HEIGHTS
The authors developed a table by which users can calculate bone dry tons of waste-activated bio-solids per linear foot of tube on the long axis for various tube heights and percent solids. For example if the tube were six feet tall and the bio-solids 15 percent the tube would have 0.664 bone dry tons of solids per linear foot of tube. In this example the tube that is 150 feet long would contain 97 bone dry tons of bio-solids (0.664 x 150). Once geotechnical properties for in-situ sediment or sludge to be dewatered is known tables for any type of material can be generated using geotechnical calculations.
PASSIVE AND ACTIVE DEWATERING RATES TRIAL
Actual data from the Ashburton basin project is provided in Table 2.
Table 2: Data for determining active and passive dewatering rates
Dredge time: 6 hours
Dredge pumping rate: 1142 gallons per minute
Dredge feed solids concentration: 3.85%
Dredge feed solids wet bulk density: 1.013 grams/ml
Dredge feed solids wet bulk density: 8.44 pounds/gallon
Total slurry pumped over 6 hour period: 1120 gallons
Concentration of solids in geo-textile tube after 17 hours of passive dewatering: 18.5%
Solids in geo-textile tube after 17 hours of dewatering wet bulk density: 1.064 grams/ml
Solids in geo-textile tube after 17 hours of dewatering wet bulk density: 8.86 pounds/gallon
Geotextile Tube total length: 150 feet
• The active dewatering rate was determined by calculating the volume of the tube as discussed under “Volume Determination” and subtracting the quantity from the amount of slurry pumped by the dredge. The active dewatering rate over a six-hour dredging period was 780 gallons per minute.
• The passive dewatering rate was determined by subtracting the volume of the tube 17 hours after dredging cessation from the volume of the tube taken immediately after dredging stopped. The passive dewatering rate was 57 gallons per minute.
Based on the average of 3.85% solids (Table 2) the dredge pumped slurry solids to the geotextile tubes at the rate of 11.13 bone dry tons of alum sludge per hour. At the end of the dredging period solids had consolidated to 13 percent. This demonstrates the speed of sludge delivery and dewatering rates and therefore the efficacy of using geotextile tube technology for dewatering projects.
There are several reasons the technology is considered a low cost option:
• Less manpower and a lower cost per man-hour requirement
• Less infrastructure such as frag tanks and belt-presses
• Lower mobilization and demobilization costs
• Less downtime
• Project gets completed faster
A project cost can be more precisely predicted because: The number of tubes required for a project can now be more accurately determined because of the ability to calculate tube volumes over a continuum of tube heights; Dewatering takes place in real time therefore dredging or sludge pumping downtime due to dewatering bottlenecks is practically eliminated; Downtime due to tube failures is minimized or eliminated; the manufacturer claims no tube failures have been reported. The Ashburton project did not have any downtime due to tube failures.
Since real time inline measurement of dredge pumping rates and precise collection of composite samples was used for the project it was possible to determine both the passive and active dewatering rates.
THE AUTHORS: Lowell Sieck Ph.D. is former professor of Biochemistry at University of Delaware and is now CEO of Industrial Technology New York. Co-author Russell Pickett B.S. Chemistry is president of Liquid Solids Separations.
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