SEDIMENT TRAP PILOT PROJECT: Design and Bench-Testing

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1 StormCon 2017 Paper, Packman and Schmoyer, Q12 James Packman, Aspect Consulting LLC, Seattle Beth Schmoyer, Seattle Public Utilities, Seattle SEDIMENT TRAP PILOT PROJECT: Design and Bench-Testing INTRODUCTION As part of the City of Seattle s (City) pollutant source control efforts in the Lower Duwamish Waterway (LDW), Seattle Public Utilities (SPU) undertook a project to develop a new storm drain suspended solids collection device. Commonly referred to as a sediment trap, the solids collection device is used widely in the LDW for source tracing efforts to find and eliminate sources of stormwater pollution. A small, effective trap was needed to aid and improve sampling efforts that often occur in small pipes. This paper presents the results of the design and bench-testing phases of a pilot project for developing a new sediment trap. The final phase, field-testing, is currently underway and will be reported separately. Work is being funded by a grant from the Washington State Department of Ecology (Ecology). METHODS The process to design the new trap began with a design charrette to review existing devices used for suspended solids collection and to generate ideas for the new trap. Participants in the charrette included individuals with experience using and designing sediment traps from the City of Seattle, King County, City of Tacoma, Ecology, and two consulting firms (Cardno and SoundView). Prototype Design The functional objective for the new trap is to collect representative samples of suspended solids in stormwater runoff from a variety of drainage structures (e.g., pipes, catch basins, vaults, etc.). During the design charrette, the following functional criteria for the new trap were identified to meet the objective: 1. Suitable for installation in pipes as small as 12 inches in diameter. 2. Maximum effective height of mounted trap expected to be 6 inches or less. 3. Easy to extract the sample and transfer to a sample container for submittal to a laboratory for chemical and/or physical analysis. 4. Not susceptible to sample resuspension during backflow or stagnant water conditions. 5. Sample capacity of about one liter to ensure adequate volume of material for physical and chemical analysis. 6. Easy to mount in pipes, vaults, maintenance holes, catch basins, and other storm drain structures. 7. Mounting method needs to work with a variety of pipe materials including concrete, brick, metal, and plastic. 8. Utilize clamps or adjustable screws for assembling and disassembling the trap without the need for power tools or specialty equipment. 9. Fabrication material suitable for long term deployment in urban storm drains. 10. Minimize the potential to disrupt flow in the drain lines and fouling of the trap, which can prevent or bias sample collection. The outcome of the design charrette resulted in identification of two trap types that fit most or all of the criteria and for which design, fabrication, and testing moved forward. The first type was a bowl-style Page 1

2 trap similar to the trap currently used by SPU. Initial flume tests were conducted using an inexpensive plastic prototype trap constructed from a modified storage container with a 4.5-inch-diameter rounded shape and removable lid (Figure 1). An orifice was cut on top of the lid for suspended solids to enter as water flows over the trap. The second type was a flow-through tube-style filter trap, the prototype for which was constructed using a 100 or 200 micrometer (µm) mesh filter sock mounted in a 4-inchdiameter PVC pipe (see Figure 1). Two orifice sizes on the bowl traps (⅜ and ¾-inch) and two lengths of tube traps (6 and 20 inches) were tested. Figure 1. Plastic prototypes: bowl-style trap on left (¾-inch orifice shown) and tube-style traps on right. Based on the initial results of the bench-testing on the plastic prototype traps, two versions of bowl-style traps were fabricated in stainless steel for field testing. Figure 2 shows the two field versions of the bowl-style traps that were developed and tested: Round-dome style fabricated from raw materials based on a design developed by the project team (Figure 2). Flat-dome style designed by metal fabricator and made from stainless steel bowls readily available from restaurant supply stores (Figure 2). Both field versions of the bowl-style trap used a ⅜-inch orifice based on testing results from the plastic prototype. As with the plastic prototype, the field prototypes were designed with a removable lid to accommodate lids with other orifice sizes if desired. Figure 2 also shows the mounting system for the traps. Field traps are equipped with stainless steel Unitstrut components for low-profile mounting in pipes, vaults, and other structures. Figure 2. Field prototypes: round-dome style on left and flat-dome style on right. Page 2

3 A third trap type was also tested, which was the Screened Inline Flow-Through (SIFT) trap previously developed by the City of Portland Bureau of Environmental Services. The SIFT is a flow-through tubestyle trap made of stainless steel and contains a 1,270-µm mesh screen followed by a 228-µm mesh screen mounted in a 6-inch cylinder. Bench-Testing Prototype designs were bench-tested in a small laboratory flume to evaluate performance prior to field installation. Performance was primarily based on particle size distribution (PSD) of material captured in the traps. The flume used for the bench-testing is owned by SPU and is used to calibrate flow monitoring equipment (Figure 3). The flume is smooth-walled PVC, 12 inches in diameter, and 14 feet long. Water is delivered to the flume by two 0.4 HP 70-gpm pumps installed in a 700-gallon intake tank that pump water into a weir box from where it gravity flows through the flume. A recirculation tank collects and drains water back to the intake tank via a 6-inch-diameter return flow pipe. The slope of the flume can be adjusted using a hydraulic jack installed at the downstream end of the pipe; however, during testing it was found that slope adjustment was not sufficient to significantly change the flow velocity. Flow velocity was measured two or more times during each test, and mean velocity from all tests was 2.1 feet per second (fps) with measurements ranging from 0.7 to 4.1 fps. The flume was customized for the testing by cutting away the top of the pipe to provide access for inserting and removing traps. Additional customization included cutting a slit in the top of the pipe at the downstream end to accommodate a flat thick piece of plastic that served as a check dam that could be inserted to increase water depth over the higher-profile traps. The check dam increased water depth by about 0.1 feet, which helped keep traps submerged. Two long-stem industrial mixers were also installed in the intake tank to keep the solids in suspension during testing. Traps were mounted in the flume and testing occurred by running water containing a known total suspended solids (TSS) concentration and particle size distribution (PSD) over the traps for a set period of time. Sediment traps are usually left in the field for 6 to 12 months to capture enough sample material for chemical and/or physical analysis. In order to accelerate the sample collection process, a relatively high TSS concentration (1,000 mg/l) was used during the bench tests. The flume was cleaned to prepare for each test and fresh water was added and dosed with a custom silt and mixture. Testing initially occurred for 4 to 6 hours and was then reduced to 3 hours based on adequate sample collection rates and for budget considerations. Figure 3. Flume used for bench-testing. Page 3

4 The mixers kept water and solids well-mixed in the intake tank, but not all the solids remained in suspension throughout the flume system. During the first test, solids slowly settled in low velocity spots like on the downstream side of the traps and in the corners of the weir box. Therefore, the test procedure was modified (starting during the first test) to include manual remixing of the water and silt/ in the weir box and pipe at 30-minute intervals. Grab samples were collected at the beginning and end of each test to check TSS concentration and ranged from 280 to 633 mg/l with an overall average of 507 mg/l. The TSS results indicates variable suspension of solids during the testing at lower than desired concentration; however, the effective concentration still allowed for sufficient sample collection with all traps in all tests exposed to the same mixing procedure. In total, 15 test runs were performed. During Tests 1 through 4, three traps were mounted in the flume in the upper third, middle, and lower third mounting locations. Starting with Test 5, only one trap at a time was mounted in the flume to minimize potential hydraulic interference on downstream traps. The bowl and tube (100 µm mesh only) traps were selected for Tests 5 through 15 based on their performance in Tests 1 through 4. Three silt/ mixtures were created and used throughout the testing to dose the test water with particle sizes representative of urban stormwater. The silt/ mixtures were created from commercial ground silica, including Granusil 70 and SIL-CO-SIL 106 products. The silica material was sieved into four size classes from 62.5 to 425 µm, which correspond to coarse silt to medium, respectively, on the Wentworth scale. Table 1 summarizes the test runs that were performed, which traps were tested, and which silt/ mixture was used in each test. Table 1. Flume tests summary. Test Traps Tested No. 1 Silt/Sand Flow Rate Test Mixture 2 (cfs) 3 Period (hr) 1 Bowl ¾ Tube 200 short SIFT (1) Bowl ⅜ Tube 100 short SIFT (1) Bowl ⅜ Bowl ¾ Tube 100 short (2) Tube 100 short Bowl ⅜ Bowl ¾ (2) Bowl ⅜ (2) Bowl ¾ (2) Tube 100 long (2) Round dome prototype (2) Flat dome prototype (2) Round dome prototype (2) Flat dome prototype (2) Flat dome prototype (3) Flat dome prototype (3) Round dome prototype (3) Round dome prototype (3) Notes: 1. For Tests 1-4, three traps were tested at a time, and the trap type name is presented in the order of mounting location in the pipe (upper third, middle, or lower third). To avoid potential hydraulic interference, a single trap, which was always mounted at the lower third pipe location, was tested in Tests 5 through 15. The plastic bowl trap names are followed by the size of the orifice (in inches) and the tube trap names include the size of mesh (in micrometers) and if the tube was long (20 inches) or short (6 inches). 2. The silt/ mixture is shown by number followed by the PSD percentages for particle sizes in µm as follows: Page 4

5 <62.5 (coarse silt) (very fine ) (fine ) (medium ) 3. Flow rate was determined from the established rating curve for the weir based on depth readings of the weir staff plate. When activating the pumps, flow in the flume almost instantly reached and stayed at a steady-state. After each test, the samples were collected by rinsing the traps with tap water to dislodge and remove the accumulated sample material from the trap into a stainless steel mixing bowl. The contents of the bowl were then transferred to a sample container for submittal to the laboratory for physical analysis of particle sizes. The particle size analysis procedure was specified to be one appropriate for solids/sediment (ASTM method D422), which uses sieving for particle sizes of 75 µm and larger and hydrometer for sizes smaller than 75 µm. The method, which shakes dry solids through a series of sieves, was modified for this project due to the wet nature of the samples and to accommodate the particle size range of interest. Modifications to the particle size analysis method included: Using only sieves, no hydrometer Extending the sieving range down to 62.5 µm (sieve no. 230) Washing the samples with water through a series of nested sieves Drying and weighing each size fraction. In addition to the trap samples, dry samples of the source silt/ mixtures that were used in the test water were also submitted for particle size analysis. The source silt/ samples were analyzed by dry sieving (per Method D422) and were compared to trap samples to determine particle size capture performance. The particle size results for both the trap samples and source samples were determined by the mass of material retained within each size fraction of interest. RESULTS Table 2 provides the mass per particle size fraction by test for the traps and source silt/ samples. The particle size results were used to create PSD distribution graphs, which are presented by trap style in Figures 4 through 8. The PSD graphs are semi-log cumulative distributions of percent retained by mass and include results for both the traps and the source silt/. The results for only the best performing traps are presented here (e.g., Bowl ⅜, Bowl ¾, round-dome field prototype, flat-dome field prototype, and Tube 100), but results are not provided for the poorer performing traps (e.g., Tube 200 and SIFT). In each PSD graph, the trap results are shown by a solid line and the source silt/ results are shown by the same color dotted line. The graphs were created this way to facilitate visual interpretation by comparing the distributions of the same line color to see how closely the trap samples match the source samples. Table 2. Particle Size Results (in grams). Test Trap (µm) > < 62.5 Total Coarse Medium Fine Very fine Coarse silt T1 1,4 Bowl ¾ Tube 200 short SIFT T2 1,4 Bowl ⅜ Tube 100 short SIFT T3 Bowl ¾ Page 5

6 Test Trap (µm) > < 62.5 Total Coarse Medium Fine Very fine Coarse silt Bowl ⅜ Tube 100 short Source silt/ T4 Bowl ¾ Bowl ⅜ Tube 100 short Source silt/ T5 Bowl ⅜ Source silt/ T6 Bowl ¾ Source silt/ T7 Tube 100 long Source silt/ T8 4 Round dome prototype Source silt/ T9 4 Flat dome prototype Source silt/ T10 Round dome prototype Source silt/ T11 Flat dome prototype Source silt/ T12 Flat dome prototype Source silt/ T13 Flat dome prototype Source silt/ T14 Round dome prototype Source silt/ T15 Round dome prototype Source silt/ Notes: 1. Mass data not available for source silt/ for Tests 1 and µm for Tests 1, 2, 8, and µm for Tests 3 through 7 and 10 through µm for Tests 1,2,8, and µm for Tests 3 through 7 and 10 through Results for Tests 1, 2, 8, and 9 were of limited use due to the lab inadvertently using the wrong PSD method. Page 6

7 Figure 4. PSD graph for Bowl ⅜ trap. Figure 5. PSD graph for Bowl ⅜ trap. Page 7

8 Figure 6. PSD graph for round dome field prototype. Figure 7. PSD graph for flat dome field prototype. Page 8

9 Figure 8. PSD graph for Tube 100 trap. Results for Tests 1, 2, 8, and 9 were affected by problems with the analytical method used by the lab. For Tests 1 and 2, the lab inadvertently analyzed the samples by laser diffraction rather than the specified sieving method. For Tests 8 and 9, the lab again inadvertently used laser diffraction. As can be seen in the PSD graphs, the results for these tests show the poorest performance in terms of distribution match among the trap and source material. Because of this issue, additional testing was done on the field prototypes to obtain a larger set of comparable test results. Ultimately, results from only the tests that used the preferred sieving particle size procedure (tests 3 through 7 and 10 through 15) were used to identify the best performing trap. The PSD graphs provide a quick visual comparison of the distributions since differences between the trap samples and source material samples are readily apparent for most samples. However, some lines on the PSD graphs are so close together that visual comparison is not precise enough to see significant differences. Thus, in addition to visual comparison of distributions, the particle size results were also quantitatively compared with a residuals analysis. The residuals analysis assessed the differences between the sample and source material distribution for each size class at the points measured (i.e. 62.5, 106, 250, and 425µm). The mean and variance of the residuals from the test results for each trap (grouped by size fraction) are provided in Table 3. The particle size data and the residuals are log-normally distributed; therefore, the means and variances represent the anti-log values calculated from log-transformed particle size mass data. Residuals and variances are provided for just the five traps that performed the best in the flume tests, including the plastic bowl traps (with both ⅜-inch-diameter and ¾-inch-diameter orifices), the stainless steel field prototypes, and the tube trap with 100 µm mesh. Due to the issue noted above with the wrong PSD analytical procedure used on some samples, the results for Tests 1, 2, 8, and 9 are not included in the values in Table 3. Page 9

10 For each particle size fraction, the trap with the best overall performance is shown in bold in Table 3. The best performance was determined by the smallest mean residual paired with the smallest variance. For three size fractions, the best performing trap was clear with one trap per particle size fraction having the smallest residual and variance. The round dome field prototype had the lowest mean and variance in the 62.5 to 106 µm and 250 to 425 µm size classes, the flat dome prototype had the lowest mean and variance in the greater than 425 µm size class, and the Bowl ¾ had the smallest mean and variance in the 106 to 250 µm size class. In the 250 to 425 µm particle size class, the round dome trap had the lowest mean residual but the second lowest variance. The flat dome trap, however, had the opposite with a lower variance and higher mean residual. Thus, these two traps performed about equally in this size class and both are bolded in Table 3 as being the best performing in that size class. For the smallest size fraction of less than 62.5 µm, the Tube 100 had the smallest residual but the Bowl ⅜ had the smallest variance. Because of the concerns about potential sample resuspension in the tubestyle traps, the Bowl ⅜ was ranked as the best performing in this size class. Table 3. Residuals from trap and source material samples (absolute values, in grams). Tests 3-7 and Size Range Mean of Variance of (µm) Trap Residuals Residuals Less than 62.5 Tube Bowl 3/ Bowl 3/ Flat dome Round dome to 106 Round dome Flat dome Bowl 3/ Bowl 3/ Tube to 250 Bowl 3/ Round dome Bowl 3/ Tube Flat dome to 425 Round dome Flat dome Bowl 3/ Bowl 3/ Tube Greater than 425 Flat dome Bowl 3/ Bowl 3/ Round dome Tube Page 10

11 CONCLUSIONS The results of this pilot project indicate that a domed bowl-style trap performs well in capturing representative particle sizes of suspended solids. The testing was conducted on bowl-style flow-over traps and tube-style flow-through traps. The stainless steel field prototypes (flat dome and round dome bowl-style traps) exhibited the best performance for three of five size classes measured, and the plastic bowl-style traps showed the best performance for two size classes. The main performance criterion was representativeness of particle sizes collected by the traps. This was evaluated by visual assessment of PSDs and a residuals analysis between the samples and the source material. Trap performance was also evaluated by the functional design criteria identified during the design phase. Among the trap types tested, the round dome bowl-style field prototype was judged to perform the best overall closely followed by the flat dome bowl-style field prototype. The tube-style traps were noted for high volume of solids capture and performed well in the smallest size fraction but performed poorly in the other four size fractions. Also, concern about trap fouling and sample resuspension from backwater situations was a significant shortcoming of the tube-style flow-through traps. Bench-testing proved to be a very important component of the trap design. It saved considerable time in evaluating performance of the prototype traps, which could not have been conducted efficiently by field testing. Although the flume was not capable of being adjusted to subject the traps to varying flow velocities, it did allow comparison of differences in PSD capture effectiveness between the different trap styles tested. Problems with the analytical laboratory procedures created issues in the quality of some test results and required running additional tests. The results provided a good basis for field testing the stainless steel domed bowl-style traps, which began in 2016 and will continue into AUTHOR BIOGRAPHIES James Packman is a Senior Hydrologist at Aspect Consulting, LLC with 22 years of experience in surface water ecology, engineering, and management. He has successfully managed and collaborated on dozens of projects for local governments and utilities, universities and research agencies, and industrial and private clients throughout Washington. James background includes: degrees in Forest Engineering, Geology, and English; riparian, forest, wetland, and montane ecology; water and sediment monitoring and sampling; fluvial sediment research; soil erosion processes and stabilization BMPs; hydrologic data analysis; stormwater source control and BMP evaluation; NPDES permit effectiveness evaluation; and environmental education. Beth Schmoyer, P.E., is a Senior Civil Engineer with Seattle Public Utilities (SPU). She has over 30 years of experience in stormwater management and site remediation. Beth currently leads the City s source control programs for the Lower Duwamish and East Waterway Superfund sites and is also the lead engineer on preliminary engineering for a regional stormwater treatment system that will use an active treatment system to treat runoff from an urban basin in Seattle. Page 11