INCLINED PLATE SETTLERS TO TREAT STORMWATER SOLIDS

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1 INCLINED PLATE SETTLERS TO TREAT STORMWATER SOLIDS James C. Elligson, J. Bradley Mikula, Shirley E. Clark, Ph.D., P.E., Christopher D. Roenning, Julia M. Hafera, and Kelly A. Franklin Environmental Engineering Program Pennsylvania State Harrisburg, Middletown, PA 17057, USA ABSTRACT It has been well-documented that, in stormwater runoff, many of the problem pollutants are associated with the particulate fraction. Past characterization of urban runoff and source contributions has shown the following: (a) soil disturbance increased the TSS and turbidity in the runoff; and (b) correlations were observed between TSS and particulate runoff concentrations of chromium, copper, and zinc, indicating that solids removal may reduce total metals concentrations. The first concern when investigating innovative treatment methods is determining the needed level of stormwater control. Specific treatment goals usually specify about 80% reductions in suspended solids concentrations. In most stormwater, this would require the removal of most, if not all, particulates greater than about 10 μm in diameter, which is about 1% of the 1-mm size that must be removed to prevent sewerage deposition problems. The ability of inclined cells (inclined plates/tube settlers) to provide excellent treatment of stormwater for a variety of pollutants was demonstrated by Pitt et al. (1999) in the report on the multi-chambered treatment train (MCTT) at the University of Alabama at Birmingham. This project is adding to that body of knowledge by investigating the potential of inclined plate settlers to treat stormwater runoff both in the field (at the City of Harrisburg Public Works Yard) and through a full-scale laboratory demonstration. Inclined plate settlers can be designed in one of two ways through the use of Stokes Law and through the use of the Hjulstrom diagram, which accounts for scour and re-suspension. The test device in this research was sized using the Hjulstrom diagram. The results showed that the Hjulstrom diagram may be a very effective tool to predict the performance of inclined plate sedimentation devices based on the particle size for which 100% control is desired. Once the runoff s particle size distribution is known, estimating the average percent removal for the system would be trivial, and could be done using Stokes Law. A sieve analysis of the influent and effluent, using a 250-μm sieve, demonstrated that the inclined plates were capable of removing particles in >250-μm size range, even when these particles were a substantial part of the mass load to the system. An analysis of the TSS and SSC 5609

2 data demonstrates that inclined plate settlers are able to achieve the high removals desired for particles with a density similar to that of sand. As with most stormwater sedimentation devices, the plate settlers operate as designed and removals for small and/or lighter particles are not as high as they are for large sand particles. Removal efficiency also was not dependent on continuous operation of the system. Interevent drying of the plates did not affect performance. KEYWORDS Urban runoff, sedimentation, inclined plates, particulate removal, particulate-associated pollutants INTRODUCTION The preferred method for treating solids in stormwater runoff has been through sedimentation in detention facilities. Using the terminology of the water and wastewater treatment industry, three types of sedimentation may occur in stormwater detention basins: Type I (discrete particle settling), Type II (floc settling) and Type III (zone/blanket settling). Type III settling is the settling that would occur near the bottom of the detention pond as the solids interact sufficiently to create a blanket which moves downward with the characteristics of a large, thin layer settling through water. In a wet detention basin or other wet sedimentation device (any device where the trapped water is not completely discharged), zone/blanket settling is likely to occur below the outlet structure and therefore, would have little impact on the solids removal efficiency of the device. Discrete particle and floc settling (Types I and II) are the settling methods of concern in design stormwater sedimentation facilities. Design has been based on determining the desired surface overflow rate (which is equal to the flow through the basin divided by the basin s surface area). This is the flow rate through the outlet structure when the basin is full, i.e., before activation of any freeboard area or overflow/bypass channel. If it is assumed that the particle settling in the sedimentation basin occurs by Type II settling (floc settling) where the interactions of particles affect the settling rate (usually by creating a larger, less dense particle), then the sedimentation characteristics of the floc must be determined in the laboratory, typically through the use of a settling column. The floc s settling characteristics, usually measured as total suspended solids (TSS) at specific depths in the water column over time, are graphed. Then the desired effluent TSS or removal efficiency and water depth are used to select the retention time in the basin. To size the basin (volume), the effluent flow rate at design flow is multiplied by the retention time required to achieve the desired removal efficiency. Most stormwater sedimentation facilities are designed assuming Type I, or discrete particle, settling. In Type I settling, it is assumed that the particle settles without interacting with other particles and that the settling occurs in laminar flow conditions (small Reynolds numbers). Therefore, the settling rate is determined by the balance of the gravitational forces on the particle and the buoyancy of the water. The primary equation used to determine settling rate is Stokes Law, which calculates the settling velocity of a particle, given its size, diameter and density. 5610

3 Complete sedimentation is assumed for all particles whose settling velocity is greater than the surface overflow rate. For particles whose settling velocities are less than the surface overflow rate, partial sedimentation occurs if the influent water column is well mixed. As noted above, one of the primary controlling factors for sedimentation is the surface area available for settling. Various methods hydrodynamic separators, chemically-enhanced coagulation followed by detention have been used to enhance, and reduce the surface area required for, sedimentation. Another method, common in the water treatment industry for enhancing sedimentation efficiency, is the use of inclined plate settlers. Inclined tubes (or plate settlers) increase solids removal by reducing the distance particles travel to the chamber floor and by reducing scour potential (Davis, et al.1989). The main settling chamber operates much like a settling tank, but with the tube settlers increasing the effective surface area of the tank. The increase in performance is based on inclined cells that overlap each other. Each cell forms the ceiling of the next cell, etc. The projected area of each base forms the settling surface of each cell. However, the horizontal distance between each plate is a fraction of the horizontal projection of the cell base. Hence, the efficiency in settling surface is obtained by this cellpacking arrangement. If the plates are relatively flat and close together, the increase in performance is greater than if the plates are steeper and wider apart. The effective increase is usually about 3 to 5 fold, and in the drinking water industry where this technology has been studied and optimized, the increase has been about 10 fold. The drinking water industry uses these inclined plates or tubes to save on plant space, increase settling of the floc from the treated water, and ultimately to increase the filter run life. Previous Use of Inclined Plates and Tube Settlers to Treat Stormwater Runoff Inclined plate settlers (where water to be treated was fed from the bottom of the plates) have been used in combination with filtration to treat stormwater runoff. This configuration was tested in the Multi-Chambered Treatment Train (MCTT) (developed by Dr. Robert Pitt, University of Alabama). Pilot-scale testing was performed in Birmingham, Alabama (where samples were collected after every unit process in the treatment train, thus allowing an evaluation of the effectiveness of a specific process) at the university s maintenance yard/remote parking lot. The results showed that the sedimentation chamber (with the inclined plates) was very effective at removing both solids and solids-associated pollutants (solids removal typically > 80%, while removal of total zinc ranged from 60% to > 90%). Typical results for the plate settlers are shown in Figure 1 for a representative organic compound (bis 2-ethylhexyl phthalate) and a representative metallic compound (zinc), as well as for total suspended solids. As the results show, statistically significant removals occur in the settling chamber. In fact, the polishing filter had very little polishing to perform. Zinc is considered one of the more difficult metals to remove because it is typically not as well sorbed to the sediments as other metals. Therefore, to find a 45 50% removal of zinc indicated that removal of other metals would be significantly greater. For most of the observed storm events, the majority of the pollutant removal occurred in the inclined plate chamber, while the filtration chamber provided almost no additional benefit. 5611

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5 Figure 1. Removal efficiencies for the MCTT (Pitt et al. 1999). The inclined cell settling chamber mimicked the completely mixed settling column bench-scale tests previously conducted by Pitt, et al. (1995) and used a hydraulic loading rate (depth to time ratio) for removal estimates. This loading rate was equivalent to the conventional surface overflow rate (SOR), or upflow velocity, for continuous-flow systems, or the ratio of water depth to detention time for batch systems. Compared to conventional detention devices, the retention of the settled material was enhanced through the use of inclined tube settlers which prevent scouring velocities from re-suspending previously settled particles. These results showed promise for the removal of solids suspended in stormwater runoff using inclined plates to enhance sedimentation. The limitation to the MCTT tests was very low flow rates used in the sedimentation chamber. Low flow rates require large sedimentation areas which require larger footprints for the device. If inclined-plate settlers are to be effective as catch basin replacements, the footprint needs to be small and the treatment flow rate needs to be higher than the flow rate through the MCTT. A second limitation was that grab samples were not collected and flows were not measured throughout the testing. Therefore, flow rates could not be correlated to instantaneous pollutant removals. The question that remained was how do inclined plates behave in terms of solids removal at higher flow rates? Description of the Current Inclined Plate Treatment System for Stormwater Runoff The inclined plate technology was tested in the Terre Kleen device (patent #US 6,676,832 B2), which combines a baffle, screen, internal by-pass duct and inclined sedimentation cells to create a primary chamber, sedimentation chamber, and oil, litter and debris storage into a self-contained 5613

6 concrete structure. The inclined cells are stacked in the sedimentation chamber and operate in parallel. Similar to the operation of the inclined tubes in the MCTT, the flow enters the sedimentation chamber at the bottom of the inclined plates. The finer particles settle out in the inclined plate cells and fall into a separate storage hopper away from the flow path below the plates. The design of the grit chamber prevents re-suspension during successive storms. Sedimentation devices have typically been designed using Stokes Law (laminar flow settling) to predict the removal efficiency of particles of a known size and specific gravity. However, Stokes Law does not account for the potential of resuspension and transport of previously deposited particles in the sedimentation device, yet these processes are important in the operation of a sedimentation device. The Terre Kleen device was designed using the Hjulstrom diagram (Figure 2), which graphically represents the interaction of the three processes sedimentation, solids transport, and erosion. The Hjulstrom diagram relates flow rate/velocity to particle size for the three processes. The curve of interest for the design of a treatment device is the sedimentation curve. The transport and erosion sections are also of interest because they represent the resuspension/scour of previously-settled particles thereby allowing a designer to predict what would happen if the flow rate through the device was higher than anticipated. Figure 2. Traditional Hjulstrom diagram (used to predict bed movement in a natural water system). 5614

7 The Hjulstrom diagram (Figure 2) was used to develop the anticipated-performance table (Table 1 Sizing Chart) for the inclined plates based on the size of the smallest particle for which 100% control was required. The table is based on the sedimentation curve from Figure 2. Table 1. Sizing Chart for the Terre-Kleen. Anticipated Performance Capacity (cfs) Loading rate in gpm/ft 2 Capacity (gpm) Minimum Particle Size Removal (µm) Grit Chamber Settling Area (ft 2 ) Use of Particle Size to Determine Removal Efficiency (Versus Percent Removal) The sources of the sediments contained in urban runoff will determine the particle size distribution. Table 2 shows the particle size distribution from three different sources in urban landscapes residential runoff, highway (interstate) runoff and street dirt. The residential runoff was collected at the end of the pipe as the runoff water entered a detention facility; the highway runoff was collected from a section of U.S. Interstate as it entered the storm drainage system; and the street dirt sample was directly collected from the roadway and before potentially being entrained in road runoff. Table 2. Particle Size Distributions. Sieve Size Analysis (%) Particle Size (μm) Smaller than Sieve Size Analysis Percent Residential a Particle Size (μm) Smaller than Sieve Size Analysis Percent Highway Runoff b Particle Size (μm) Smaller than Sieve Size Analysis Percent Street Dirt c 10 < d 30 = d 60 = 1800 ~ na ~2000 a House et al (summarized in Pitt and Clark 2006). b Cristina et al c Pitt

8 While increasing loads of sediments in waterways causes problems to aquatic habitats typically through smothering eggs and larvae and abrading fish gills, sediments also are known to be repositories for many urban runoff pollutants. In particular, phosphorus and many metals have been documented to be associated with stormwater particulates to a large degree. Table 3 highlights several of the known pollutant associations with specific particle sizes in urban runoff (Morquecho 2005). Samples were collected at five storm drain inlets and the table values are the concentrations in the solution where particles greater than the size noted have been sieved out of the solution. As noted in the table, to see substantial reductions in many pollutants through sedimentation, the particle size required for control may be in the range of 5 20 μm. Table 4 demonstrates that for the dissolved fraction, a significant portion of the material (with the exception of copper) is already bound to colloidal material, likely ensuring that it will not associate with settleable solids. Table 3. Particle Size Associations for Various Pollutants at Storm Drain Inlets (Summarized from Morquecho 2005). Sieve Size for Copper (μg/l) Total Phosphorus Turbidity (NTU) Treatment a Chemical Oxygen Demand (mg/l) (mg/l as PO 4 ) Unsieved a Analyzed sample contains particle sizes less than that shown in the table. Table 4. Ionic vs. Colloidal/Bound Fractions for Pollutants in the <0.45-μm (Filtered) Sample. Average % Ionic Average % Bound Zinc Copper Cadmium Lead INCLINED PLATE EVALUATION METHODOLOGY Penn State Harrisburg (PSH) has been contracted to independently evaluate an inclined plate settler that was designed using the Hjulstrom diagram. This evaluation is being accomplished in two phases a laboratory testing on a full-scale device using simulated stormwater and a field evaluation of the full-scale device during actual rainfall-runoff events at a public works yard in the Harrisburg, Pennsylvania area. Individual grab samples have been collected throughout the storm events in both phases and analyzed for total suspended solids (TSS), suspended sediment concentration (SSC) and particle size distribution (PSD). The analytical methods and detection limits are given in Table 5. PSD, the analyte of interest for this paper, is analyzed using a Coulter Multisizer 3 after the samples have been sieved with a 250-μm ASTM certified stainless steel sieve. The Multisizer 3 results 5616

9 provide a particle size distribution for a sample for all particles between 1 and 240 μm. Particles greater than the upper limit of the Multisizer 3 (240 μm) are weighed and categorized together as >240 μm and are included in the mass distribution of the solids in the device s influent and effluent. Table 5. Testing Method and Detection Limits. Constituent Method No. Method Detection Limit Lab Reporting Limits SSC ASTM D mg/l 5 mg/l TSS EPA mg/l 5 mg/l PSD SM 2560 N/A 1,000 particles/ml The field test site (City of Harrisburg, Pennsylvania, Public Works Yard), is approximately 0.5 ha and is approximately 85% paved/roofing. The rest of the site is also impervious, but it consists of dirt roads and storage. Evaluation testing at the site prior to the field installation showed that the anticipated influent TSS was between 100 and 300 mg/l, with a median particle size (d 50 ) of approximately 60 μm. That characterization of the site held for the first six months of testing. During the summer of 2005, construction was completed on the Harrisburg municipal waste incinerator that is located on the same property. Runoff from the incinerator does not drain to the field test inlets; however, the dirt roads are used regularly by trucks traveling from the incinerator to the ash pile on the other side of this drainage area. The addition of regular traffic on the unpaved areas of the site has resulted in an increase in TSS to mg/l (flowweighted mean concentration). It also has resulted in a decrease in the d 50 entering the inclined plate settlers. The laboratory testing also was performed on a full-scale device using simulated stormwater. The simulant was a mixture of Sil-Co-Sil 250 (purchased from U.S. Silica) and concrete plant sand that had been sieved to a size less than 1000 μm. The sieve mixture is show in Figure 2. Tests were performed at multiple flow rates and influent concentration combinations (flow rate adjustments equivalent to changing the surface loading rates). Table 6 shows the combinations of flow rates and influent concentrations used in the full-scale laboratory testing. 5617

10 Mix % Passing 100.0% 90.0% 80.0% Percent passing 70.0% 60.0% 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 2 Micron 20 Micron 50 Micron 100 Micron 200 Micron 500 Micron 1000 Micron 2000 Micron 3000 Micron 4000 Micron 5000 Micron Particle size Figure 2. Sieve size analysis for the influent solids for the laboratory testing. Table 6. Full-Scale Laboratory Testing Conditions Target Flow Rate (gpm) 100 mg/l Influent 200 mg/l Influent 300 mg/l Influent 71 x x x 142 x x x 213 x x x 283 x x x 354 x x x 144* x 223* x 290* x 359* x 443* x 493* x *Device re-configured to block off plates to test higher surface loading rates than traditional design. 5618

11 RESULTS AND DISCUSSION The research question to be answered under the auspices of this paper is whether or not the Hjulstrom diagram can be used as a design tool for sedimentation devices, and secondly, whether there is an improvement in sedimentation above that predicted from the diagram if inclined plates are used to enhance sedimentation. The hypothesis was that the sedimentation curve could be used in combination with the desired flow rate to predict the minimum particle size for which 100% control would be accomplished (i.e., the particle would settle out and would not be resuspended). Influent and effluent samples at various flow rates were collected and analyzed for particle size distribution. The largest particle size remaining in solution was plotted on the diagram versus the measured flow rate into the device at the time the sample was collected. The results are shown in Figure 3. The data is denoted by different shades for the squares for the field and the full-scale laboratory tests. This distinction was made because of the potential differences in specific gravity of the particles. The field specific gravities are currently being analyzed, whereas it was assumed that the specific gravity of the particles in the laboratory testing was As can be seen in Figure 3, the Hjulstrom sedimentation line appears to provide a good representation of the particle size for which 100% control is possible at the higher surface loading rates. It is unknown why the samples collected at the lower surface loading rates did not follow the trend. This diagram was developed assuming a particle specific gravity of If the particles in the two samples in question had a specific gravity substantially less than 2.65 (such as if they were composed mostly of organic matter), then their sedimentation line would be shifted to the right. That shift to the right means that lower flow rates would be required for 100% control of a specific particle size whose specific gravity was less than 2.65, compared to a particle that did have the density of sand. 5619

12 Figure 3. Hjulstrom diagram in the Terre Kleen inclined plate settler (diagram assumes a particle specific gravity of 2.65). 5620

13 Because of the interest by regulators in the control of TSS and SSC, those parameters were also measured. Table 7 shows the TSS and SSC removals for the inclined plates during the laboratory testing. Table 7. Loading Rate vs. Percent Removal in the Inclined Plates. Loading Rate (gpm/ft 2 ) Reconfigured for Increased Overflow Rate Influent Solids (mg/l) Effluent Solids (mg/l) Difference % Removal N N N N N N N N N N N N N N N Y Y Y Y Y Y Y Most sedimentation devices have focused on removal of large particles. The Coulter Counter was used in this evaluation to determine the particle sizes in the range of μm. The sieve analysis was used to determine the fraction of the particles in the sample greater than 250 μm. Table 8 shows the results for the 250-μm sieve. As can be seen, most of the 250-μm particles (if not all) are removed by the inclined plate settlers. Out of the 28 events samples, only three sampling events had substantial effluent concentrations of these large particles (> 10 mg/l). It is unknown why these three events had the elevated effluent concentrations. They were not related directly either to flow rate through the device (it was predicted that higher loading rates would worsen removals) or to influent concentration in the >250-μm range. In fact, the performance of the inclined plates for solids removal was improved at the higher flow rates, which is opposite of what would be predicted. 5621

14 Table 8. Concentration of Influent and Effluent Solids Greater than 250 μm. Influent Concentration > 250 μm (mg/l) %Influent > 250 μm Effluent Concentration > 250 μm (mg/l) %Effluent > 250 μm * * * * * 5.26 *Sample results within the method detection limit. CONCLUSIONS Based on this data, it appears that the Hjulstrom diagram may be an effective tool for predicting the performance of devices using inclined plates once the particle size for which 100% control is desired has been selected. Once the runoff s particle size distribution is known and the level at which 100% control is desired, estimating the average percent removal for the system is fairly trivial, using Stokes Law. In addition, the sieve analysis using the 250-μm sieve demonstrated that the inclined plates were capable of removing particles in that size range, even when they were a substantial part of the mass load to the system. An analysis of the TSS and SSC data demonstrates that inclined plate settlers are able to achieve the high removals desired for particles that have a density approximately the same as sand. Removal efficiencies will not be as high for particles that are smaller in diameter and/or lighter. Short-circuiting of lightweight particles is often a limitation of sedimentation devices unless a skimming baffle is installed. Another concern in sedimentation device efficiency is the impact of resuspension/scour. This was not noticed during the laboratory tests, although it has been a concern in the field testing because of the lighter flocs formed by the oils attached to the fine sediment. Enhanced sedimentation is gaining acceptability as the stormwater management industry moves toward the control of specific particle sizes in order to better treat particulate-associated 5622

15 pollutants. Other research has focused on determining the association of pollutants with specific particle sizes in runoff. The combination of these two data sets will allow for better design of sedimentation devices to control particle-associated pollutants. In addition, as new data is added to this evaluation and as that data is correlated with particle density measurements, it is anticipated that a more complete comparison of inclined plate settlers with traditional open sedimentation will be possible. It is anticipated that similar, or likely better, removals will be seen with inclined plate settlers compared to traditional sedimentation, and with the added benefit of a substantially smaller device footprint. The caveat to this must be that, to control specific pollutants in general urban runoff, the particle size required for control is substantially smaller than what is the current design criterion for sedimentation devices. Only a few land uses, such as highways, have a substantial fraction of their pollutants associated with large particles (greater than μm). ACKNOWLEDGMENTS The authors would like to thank Terre Hill Concrete Products, Inc., for the opportunity to use a Terre Kleen device to evaluate the effectiveness of inclined plate settlers. In addition, the authors would like to thank the Harrisburg Public Works department for the field testing site. REFERENCES Cristina, C.; Tramonte, J.; Sansalone, J.J. (2002). A granulometry-based selection methodology for separation of traffic-generated particles in urban highway snowmelt runoff. Water, Air, and Soil Pollution. 136: House, L.B.; Waschbusch, R.J.; Hughes, P.E. (1993). Water Quality of an Urban Wet Detention Pond in Madison, Wisconsin, U. S. Geological Survey, in cooperation with the Wisconsin Department of Natural Resources. USGS Open File Report , Morquecho, R. (2005). Pollutant associations with particulates in stormwater. Ph.D. Dissertation. University of Alabama, Tuscaloosa, Alabama. 200 pages. Pitt, R.; Clark, S. (2006). Interactions between catchbasin and street cleaning in urban drainages and sediment transport in storm drainage systems. Best Management Practices (BMP) Technology Symposium: Current and Future Directions, American Society of Civil Engineers Pitt, R. (1979). Demonstration of Nonpoint Pollution Abatement Through Improved Street Cleaning Practices, EPA-600/ , U.S. Environmental Protection Agency, Cincinnati, Ohio. 270 pgs. 5623