smarter, cheaper, ash dewatering · smarter, cheaper, ash pond dewatering. david m. seeger, ......

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Smarter, Cheaper, Ash Pond Dewatering David M. Seeger, PhD, Steven Kosler AECOM, 9400 Amberglen Blvd., Austin, TX 78729 CONFERENCE: 2017 World of Coal Ash – (www.worldofcoalash.org) KEYWORDS: CCR, consolidation, dewatering, closure, pond, vacuum, VCD ABSTRACT Targeted, specialty dewatering does not have to be expensive or complicated. AECOM is demonstrating an ash pond dewatering system that was designed with ash pond closures in mind. AECOM has performed lab-scale and pilot-scale tests for the Vacuum Consolidation Dewatering (VCD) technology that uses a system of horizontal drains installed below the surface of the pond for dewatering. The water entering the drains is drawn out of the ash by a pump system located near the edge of the pond. Lab tests and a pilot-scale demonstration have been performed and some of the results will be discussed. Methods of installation or building the VCD system within existing ash ponds or in new landfill closure sites will also be presented along with a suggested device for installation. 2017 World of Coal Ash (WOCA) Conference in Lexington, KY - May 9-11, 2017 http://www.flyash.info/

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Page 1: Smarter, Cheaper, Ash Dewatering · Smarter, Cheaper, Ash Pond Dewatering. David M. Seeger, ... consolidation, dewatering, closure, pond ... vacuum header in as …

Smarter, Cheaper, Ash Pond Dewatering

David M. Seeger, PhD, Steven Kosler AECOM, 9400 Amberglen Blvd., Austin, TX 78729 CONFERENCE: 2017 World of Coal Ash – (www.worldofcoalash.org) KEYWORDS: CCR, consolidation, dewatering, closure, pond, vacuum, VCD ABSTRACT Targeted, specialty dewatering does not have to be expensive or complicated. AECOM is demonstrating an ash pond dewatering system that was designed with ash pond closures in mind. AECOM has performed lab-scale and pilot-scale tests for the Vacuum Consolidation Dewatering (VCD) technology that uses a system of horizontal drains installed below the surface of the pond for dewatering. The water entering the drains is drawn out of the ash by a pump system located near the edge of the pond. Lab tests and a pilot-scale demonstration have been performed and some of the results will be discussed. Methods of installation or building the VCD system within existing ash ponds or in new landfill closure sites will also be presented along with a suggested device for installation.

2017 World of Coal Ash (WOCA) Conference in Lexington, KY - May 9-11, 2017http://www.flyash.info/

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INTRODUCTION CCR ponds can be dewatered faster and drier using VCD over traditional dewatering methods. VCD is a technology to dewater soft sediment material via horizontal drains installed at one depth or a variety of depths below the surface and connected to a common header for vacuuming water out of the material. Horizontal vacuum dewatering will remove free water and pore water from soft sediment material such as fly ash and scrubber solids or other CCR which in turn reduces volume, increases stability, decreases leachability and limits double handling of material. VCD DESCRIPTION VCD makes use of co-planar horizontal drains installed below the surface of a CCR pond at single or multiple depths and is used for CCR dewatering.1,2 A drawing of drains installed at a single, co-planar depth below the surface is shown in Figure 1. For example, in coal ash ponds a single drain may be installed 10 ft. to 15 ft. below the surface of the ash and used to dewater the pond, or a set of drains may be installed simultaneously at, for example, 3, 6, 9, 12, and 15 ft. depths below the surface to dewater the pond. Figure 2 shows a drawing of a VCD installation device that will install drains simultaneously at 2 depths below the CCR pond surface. This installation device is in the concept stage and is being considered in many different possible configurations. Bench testing of the material to be dewatered will indicate if dewatering can be performed with one drain installed deep or if multiple drains are required, i.e., some material requires additional levels of drains for dewatering due to the particle size and type of CCR material. The full length of each drain should be installed as closely as possible to one elevation, i.e., co-planar. The VCD dewatering of most ponds should be coupled with imparting vibrations to the surface of the pond to further promote both additional dewatering and compaction of the CCR material in the pond. The vibrations could be imparted by driving Low Ground Pressure (LGP) pond closure equipment, such as an amphibious excavator, over the surface of the pond after the VCD has mostly dewatered the CCR; alternately compaction equipment may be used. Dewatering and vibratory compaction activities should continue until the CCR shearing strength is approximately 10 psi, down to the depth of the drain as measured by vane shear so that heavier tracked equipment can be operated on the pond surface for closure activities. The VCD technology can be used to dewater CCR ponds in a few different configurations:

• The drains can be installed beneath the surface of the existing CCR pond or in a sectioned-off, dewatering area within an existing CCR pond. The drains are intended to be installed at a depth of 10 ft. to 15 ft. below the pond surface, and 10 ft. apart across the pond, but placing the drains closer together, e.g., horizontally separated by 5 ft., should reduce the required dewatering time.

• The drains can be laid out in a pattern on the surface of a separate area intended for dewatering CCR and the CCR may be pumped on top of the drains for

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dewatering. Before the CCR is pumped into the dewatering area on top of the drains it is best to weigh down the drains to keep them fixed in place as the CCR is spread over the top of them.

• The drains can be installed at multiple depths below the surface rather than at one deep elevation, e.g., a horizontal row of drain maybe installed at 10 ft. below the surface and a second row may be installed above that row at 5 ft. below the surface. The AECOM VCD installation device is expected to be capable of simultaneously installing drains at multiple depths below grade such as at 2, 4, 6, 8, 10 ft. below grade, the drawing shown in Figure 2 depicts simultaneous installation at 2 depths below grade3.

• Almost all VCD operations should include imparting mechanical vibration to the pond surface to compact the CCR and re-liquefy the material to enhance dewatering of the CCR. The mechanical vibration and vacuum dewatering should be performed continuously until the CCR is consolidated and the desired vane shear and CCR density is achieved. Amphibious excavators or other LGP pond closure equipment can be used to traverse the surface of the pond to impart the vibration.

ORDER OF MAGNITUDE VCD SYSTEM COST The ROM cost for a VCD system was developed using the following design assumptions:

– The pond area is dewatered using VCD to a depth of 10 ft. below the surface for a cap and close in place pond closure

– Co-planar drains are laid out in a pattern for maximum efficiency – Only one layer of horizontal, co-planar drains is used at a depth of 10 ft. below

the surface of the pond – Drains are connected to a common header – Common header is attached to two vacuum pumping systems

The estimated cost for purchasing the system for dewatering is approximately $5,000 / acre. The cost of installation of the VCD system was not included in the ROM cost above due to the variability associated with that cost, such as: location, weather, terrain, local labor rates, etc. PILOT DEMONSTRATION DESIGN A VCD pilot demonstration was performed in an area of a large CCR pond that was built specifically for dewatering difficult to handle fly ash. For this particular pilot demonstration test 2 separate drains were tested and they are shown in Figure 3. The VCD drains were installed in 2 groupings to separate the 2 different drains by approximately 50 ft.; the drains were installed within each group at 5 ft. apart horizontally as shown in Figure 4. The end of each drain was attached to 1 in. diameter hose that extended from the drain end to a valve and then to a common vacuum header that was connected to a dewatering pump. The other end of the drain, i.e., the opposite

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end from the one that was attached to the vacuum header was attached to a vacuum gauge so that for this pilot test the vacuum achieved at the far end of the drain could be measured. In this way the vacuum loss across the length of the drain could also be determined. The method for connecting the hoses was straightforward, simple hose clamps were used for the 4 inch wide drain (Figure 3a) and a special adapter and hose clamps were used for the 6 inch wide drain (Figure 3b). The hoses were attached to the vacuum header in as short a length of hose as possible to limit vacuum loss caused by both elevation change and hose length. The vacuum header was made out of 1 in. hose and connected to the pump as near to the edge of the pond as possible. CCR fly ash was dredged onto the top of the drains to a depth of about 4 ft. to 5 ft. over the drain elevation. Piezometers were installed near the drains in the test area and water level was measured throughout the testing. In addition vane shear measurements were made and representative samples were taken to get measurements of moisture content at multiple depths in the pond near the drains. A device similar to a fence posthole digger was used to dig the 4 ft. deep cylindrical sample holes. The vacuum pressure and produced water flow were recorded as often as practical during the first few days of operation. During operation the vacuum was used to lift the water (about 7 to 10 feet vertically from the drain elevation to the pump suction and header elevation) out of the CCR pond from below the surface, transport it through the hoses and discharge it out of the pond. The vacuum was kept low until the produced water stopped flowing to reduce the possibility of the fine CCR solids plugging the filter sock that envelopes the plastic core of the drain. The vacuum level should be minimized so that the vacuum is roughly equivalent to the pressure drop required to lift and transport the produced water through the hoses and pump from the (lowered) phreatic level in the test area of the CCR pond. The VCD system was left operating 24 hours per day for the 3-day test. The operation continued until the water stopped flowing and the piezometer measurements indicated that the phreatic water level was lowered to the drain level. Unfortunately, the phreatic level was at or near the drain level very quickly on the first day of operation, i.e., there wasn’t a significant depth of water to remove from the dredge-filled area. After about 25 hours of operation, LGP equipment (an amphibious excavator) was driven over the surface directly above the installed drain area and also away from the installed drain area. The tracks of the excavator sunk approximately 12 – 24 in. over the area where the drains were not installed and only 4 – 6 in. over the area with drains installed below. The phreatic water level increased with the activity on the surface, as expected, and the drains were pumped again to remove the free water and lower the phreatic level. A hand-held vane shear testing device was used to measure the CCR strength during the VCD demonstration, in addition, representative samples were taken.

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PILOT DEMONSTRATION RESULTS PIEZOMETER MEASUREMENTS The reference elevations of each piezometer were recorded, the water depth in each piezometer was measured and an average phreatic surface elevation was determined at the start of the testing period. The phreatic level varied very little throughout the testing except after LGP equipment was driven across the surface during compaction. After the LGP equipment was driven across the pond surface (called a compaction test), the phreatic surface rose to 2 ft. above the horizontal drains for one piezometer location and 0.2 - 1 ft. above the horizontal drains for the other piezometer locations. As expected, vibration due to the equipment driven on the surface increased the phreatic level in the test area. This water was drawn out of the area using the VCD system, resulting in additional compaction of the fly ash and increased stability. VANE SHEAR MEASUREMENTS Vane shear data were recorded immediately following the compaction testing and indicated generally higher strengths for locations over the horizontal drains as compared to those locations not located over the drains. The average of shear strengths over the drains was 651 PSF and at locations not over the drains, 480 PSF. The average test results are shown in the table below. The average of the various depth measurements at each measurement location were used to calculate the statistics in the Table below. Vane Shear Measurement Location

Average (PSF)

Range (PSF)

No. of Vane Shear Measurement

Locations

No. of Average Vane Shear

below 500 PSF Over the drains 651 353 - 1016 13 2 Outside of drain installation area 480 435 - 566 3 2

Only 2 of the 13 averages for each vane shear location made over the horizontal drains were below 500 PSF, compared with 2 of the 3 averages for each vane shear location made not over a horizontal drain. The vane shear results indicate that the fly ash over the drains has significantly higher strength (+36%) than the fly ash not over the drain area. The average vane shear strengths measured in the drain areas are consistently in the 500 to 700 PSF range. The weakest areas disclosed by the vane shear tests were measured away from the installed drains. Based on this result, we conclude that repeated compaction and VCD operation would increase the vane shear strength of the fly ash. FLY ASH SAMPLE MOISTURE RESULTS Four separate samples were gathered across the 3 days using a hand auger to bore a hole down to a depth of 4 ft. below the surface with portions of the samples taken at 1, 2, 3, and 4 ft. depth. Those samples were sent to a local lab for specific gravity, particle

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size and (gravimetric) moisture analyses. The results for moisture determinationa are summarized in the table below.

Fly Ash Sample Percent Moisture Test Results Sample ID 1 ft. 2 ft. 3 ft. 4 ft. Before Compaction 30.0 28.0 30.8 32.2 After Compaction 29.3 37.0 35.0 34.1 24 hours after compaction 35.2 29.7 30.9 31.9 The moisture results indicate that after compaction the samples gathered at 2, 3, and 4 ft. depth were wetter than the samples gathered before compaction at the same depths. The water is most likely pore water that was released from the fly ash during the compaction testing. The core sample that was gathered approximately 24 hours after compaction showed that the moisture content at 2, 3 and 4 ft. depth decreased to about the same values that existed before compaction, indicating that the ash contracted as water was pumped from the drains (or when the pore water pressures dissipated). The compaction activities had the desired effect of releasing pore water from the fly ash which was subsequently dewatered using the VCD system. FLY ASH SAMPLE PARTICLE SIZE RESULTS The fly ash sample particle size results indicate that the ash comprises 1% to 3% sand, 77% to 85% silt, and 12% to 19% clayb. The grain size data from the three tests indicate that the filter sock wrapped around the plastic core selected for use with VCD should have very small openings so that the small particles do not pass through the sock with the water being discharged from the drain. Both filter socks used for the demonstration had Apparent Opening Sizes (AOS) of 0.21 mm, equivalent to a No. 70 mesh U.S. Standard sieve opening according to the drain manufacturer. CUMULATIVE WATER VOLUME DISCHARGED FROM DRAINS During testing approximately 3050 gallons of water were suctioned out of the horizontal drains as estimated by measurements from inline flow meters; however, the flow meters were not functioning properly for a significant amount of time during the testing. Considering the frequent flow meter problems it is possible that multiple times the measured water was really suctioned from the drains. As expected the clarity of the discharged water from the drains is dependent on the filter sock material enclosing the plastic core of the drain. A visual examination of the drain filter sock material shown in Figure 3b revealed that the openings in the geotextile varied from very small (micrometer) to the size of a hole like that made by a toothpick (1-2 millimeter). The effluent from that drain was very cloudy and apparently high in Total Suspended Solids (TSS), although we did not make that measurement. Therefore care must be taken when choosing the correct filter geotextile to use for the drain, especially if there are specific TSS discharge regulations required for the effluent.

a Moisture contents are based on the weight of water to the oven-dry weight of each test specimen. b All grain size limits are based on the Unified Soil Classification System.

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EVALUATION OF TEST AREA AFTER TESTING Approximately 2 weeks after the testing was complete, a long-reach excavator was used to dig evaluation holes in the ash at places above the drains and at locations not above the drains to determine if any differences in the samples could be observed. Primarily the team was investigating the thickness of the top crust of the test area, the ash stability, and wetness. In general the ash over the drains was dry and stable down to 4-5 ft. below the surface and the ash not over the drains was not as dry nor as stable, and the crust at those locations was only 0.5 – 2 ft. thick. Some pictures were taken of the holes that were dug by the long reach excavator. The locations where the excavator dug the holes are identified by the blue circles in Figure 4. Shown in Figure 5 is a photo of a hole that was dug in an area not over a drain, point C (point C in Figure 4). The hole that was dug at this location shows unstable fly ash that is moister compared to the ash shown in Figure 6, which was taken of the hole dug between the drain test sites at point A (in Figure 4). Figure 7 shows that the hole dug at point A is very stable and dry down to 4-5 ft. below the surface; the hole dug at point B indicates a similar result as shown in Figure 6. The holes that were dug by the long reach excavator indicate that the use of VCD will result in drier ash at deeper depths in a CCR pond in a faster more efficient manner than compared to other dewatering methods. SUMMARY VCD can be used to dewater CCR ponds using co-planar, horizontal drains installed at a single or multiple depths below the pond surface; the choice of installing at multiple levels is based on lab test results. The drains can be installed using the installation unit as shown in Figures 1 and 2 or by horizontal drilling. A pilot demonstration showed that the CCR material, fly ash in this case, above the drains was drier and showed more strength than the fly ash that was not in the test area. The VCD system is a cost-effective method to dewater CCR ponds.

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Figure 1: Diagram of a VCD system and one possible VCD installation device that is expected to be used to install VCD drains below the surface of a CCR pond. A second option for installation is horizontal drilling, which is depicted on the far left side of the drawing.

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Figure 2: Diagram of one possible VCD installation device that is expected to be used to install VCD drains below the surface of a CCR pond having the option of installing drains at multiple depths below the surface,

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3a 3b Figure 3a and 2b: Shown in 3a is a drain made of a corrugated plasitc core approximately 1/8 inch thick and covered with an AOS 70 nonwoven filter sock, approximately 4 inches wide. Shown in Figure 3b is a drain made of a 6-inch wide plastic core with dimples approximately 1 inch thick and covered with an AOS 70 (reported by manufacturer) nonwoven filter sock. (Actual openings in the 3b drain were visibly larger than a 70-mesh sieve).

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Figure 4: The black lines in the photo are the drains lying on the surface of the pond prior to the demonstration test, prior to fly ash being dredged to the test area in the pond and added on top of the drains to a depth of approximately 4.5 ft. The blue dots indicate where a long-reach excavator dug holes to determine crust thickness, evaluate the moisture and stability of the ash in that immediate area approximately 2 weeks after the completion of the demonstration.

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Figure 5: Hole dug near point C that is shown in the overall view in Figure 4. The crust is 1-2 ft. thick and the ash is wet at the bottom of the hole.

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Figure 6: Hole dug near point A in between the two sets of installed drains as is shown in the overall view in Figure 4. The ash is dry all the way to the bottom of the 4-5 ft. deep hole, and the ash appears very stable.

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Figure 7: Hole dug near point B over one set of the drains as is shown in the overall view in Figure 4. The ash is dry all the way to the bottom of the 4-5 ft. deep hole, and the ash appears very stable.

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[1] Hwang, D., U.S. Patent No.: 8,926,221, January 6, 2015. [2] Seeger, D. M., Kosler, S., Bird, G.R., U.S. Provisional Patent Application Filed July

28, 2016. [3] Seeger, D.M., Kosler, S., Morett, D., U.S. Provisional Patent Application Filed July

28, 2016.