Groundwater Recharge of Urban Runoff after Biological Treatment in Geotextile Filters Cevat Yaman 1, Eyup Korkut 2 and Roger Marino 3

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1 Groundwater Recharge of Urban Runoff after Biological Treatment in Geotextile Filters Cevat Yaman 1, Eyup Korkut 2 and Roger Marino 3 1 Civil, Architectural and Environmental Engineering, Drexel University, Philadelphia, PA, 1914; PH (267) ; cevat@drexel.edu 2 Civil, Architectural and Environmental Engineering, Drexel University, Philadelphia, PA, 1914; PH (215) ; korkut@drexel.edu 3 Civil, Architectural and Environmental Engineering, Drexel University, Philadelphia, PA, 1914; (732) ; roger.marino@drexel.edu Summary This paper describes a Best Management Practice (BMP) to remove pollutants from urban runoff prior to infiltration, adapting a system developed for onsite wastewater treatment (Yaman, C., 23; Korkut, E., 23). Alternating layers of non-woven geotextile filters and granular soil host a biomass to extract and degrade substrate. Pilot plant development using primary effluent removed total suspended solids (TSS) and biochemical oxygen demand (BOD) to well within secondary treatment standards, as well as reducing both ammonia (NH 3 ) and nitrate (NO 3 ). There was minimal decrease in permeability while the system was dosed cyclically at a hydraulic loading rate (HLR) of 1 gpd/ft 2. Urban runoff will typically be more dilute, and is generated intermittently providing opportunity for re-aeration and decomposition between events. Sustainable recharge depends on preventing clogging of the infiltration surface by either inert or degradable materials. Removing fine soil particles can be done by physical means, so the contribution of the new system is in removing and decomposing organic material upstream of the infiltration surface. Scanning electron microscopy (SEM) confirmed that a discontinuous floc formed within the complex, porous fabric structure, providing both ample permeability and high surface area for substrate and oxygen transfer. However, in the event of clogging, the upper geotextile can be replaced without disturbing the underlying infiltration surface. The system can be installed as adjuncts to catch basins or inlets at sites of poor quality runoff generation, e.g., parking lots. It would not only enhance local stream habitat by reducing offsite runoff frequency and the conveyed pollutants, but would also sustain dry weather base flow in by year-round recharge. Application and Layout Directing runoff to infiltration recharges the groundwater that eventually becomes dry weather streamflow. The logical focus is on impervious surfaces. They most disrupt natural hydrology by preventing infiltration and generating runoff in almost any storm. Moreover, paved surfaces tend to produce the most polluted runoff, often thought as being mobilized in the first flush of the initial ¼ to ½ of runoff per event. Figure 1 shows an arrangement to divert runoff from an inlet to a recharge chamber, treating the flow before infiltration, which is needed anyway to minimize clogging. If the goal is to infiltrate up to 1/2 depth per event, about 4 ft 3 would be generated from a 1, ft 2 parking lot. With 2-4 events annually in Eastern cities, a half-foot of infiltration will be 1

2 restored annually. It would be distributed through the year, including summer storms when light rainfall on vegetated surfaces would be lost to evapotranspiration. Parking lot Storm inlet House Diversion Sedimentation Infiltration Sanitary sewer to wastewater treatment plant Storm sewer to river Treatment Plant River Figure 1 Layout of a Local Runoff Treatment and Recharge System Materials entrained in urban runoff can be classified as one of seven types: 1. Screenable floating litters 5. Colloidal organic solids 2. Settleable organic and inorganic solids 6. Dissolved organics 3. Colloidal inorganic solids (silt and clay) 7. Dissolved inorganic solids 4. Hydrocarbon fuel and lubricant globules and solutes Only the last three can be allowed to reach the infiltration surface. Larger materials must be removed upstream. The longevity of the infiltration surface is enhanced if degradable material (types 4, 5 & 6) is also removed upstream. Figure 2 shows a schematic profile. To storm sewer Weir Curb Filter/Screen Skimmer for oil/grease Manhole Manhole Sediment Stormwater inlet Sediment Sedimentation Geotextile Gravel Geotextile Sand Geotextile Infiltration Ground water Figure 2 Profiles of a Local Runoff Treatment and Recharge System 2

3 Floating litter and coarse soil particles are removed in inlets or catch basins. The filter shown can be cleaned or replaced, and in addition, an oil skimmer (e.g., disposable socks or metal trap) can intercept oil and grease. The second unit removes fine soil. To prevent standing water supporting insect breeding, this chamber has a pervious base for drainage. Microorganisms attached to organic particles would colonize the replaceable geotextile filter to treat this seepage. The layered filter in the last chamber supports an aerobic biomass. While it grows at each dose, re-aeration after drainage and the lack of fresh substrate will cause decomposition to re-open pore spaces, for an overall endogenous (near starved) condition that limits filter clogging. Porous Filters and Biological Clogging Sand filters (Figure 3) are often used to treat runoff, and are also common in onsite (septic system) wastewater disposal. In the latter use, sand filters can produce high quality effluent low in BOD, TSS and ammonia. Important variables include filter depth, organic loading rate, and dosing frequency, as indicated in Figure 3. 1 dose/day Liquid fills the pores of filter medium. Some particles pass through the filter untreated. 4 doses/day Liquid partially fills the pores of the filter. 24 doses/day Liquid dose flows around filter medium in a thin film. Most pores are open. Figure 3 Effect of Hydraulic Loading Rate on Filters (Redrawn from Tchobanoglous, 1998) The concerns with using sand filters for runoff infiltration are the allowable hydraulic loading rate (HLR) and frequent maintenance. Surfaces must be periodically scraped to mitigate clogging from fines and biological growth, because only 4% of the volume is pore spaces connected by narrow channels. Filtration separates solids from a conveying fluid as it flows through media with minimal head loss. Embedded geotextile soil filters must function untended for an indefinite service life. They are designed to keep particles from being mobilized and not physically contact the filter at all. However, runoff filters must handle a continuous supply of organics and inert particles. When backwashing or frequent replacement is not possible, as in passive drainage systems, soil particles must be removed upstream, and the degradable material must decompose to restore pore space. 3

4 Geotextiles as Biomass Attachment George Koerner (1993) investigated biological clogging of landfill leachate collection filters. Stable permeability was found in only certain types, implying that the biomass growth stabilized with continuous channels through the filter. Leachate quality was improved by substrate decomposition. This problem was seen as an opportunity to treat dilute runoff. Issues included finding types of geotextiles that support biomass, the types of treatment that occur, available hydraulic capacity, and the useful life. Geotextiles are pervious textiles, a class of geosynthetics, polymeric materials used in infrastructure projects due to features such as controlled properties, rapid installation, and volumetric compactness (Koerner and Soong, 1995). Synthetic fibers are classified by composition (polypropylene, polyethylene or polyester), thickness (denier) and length (filaments or short staples). The main feature controlling engineering behavior is the arrangement of fibers from the manufacturing process. Figure 4 is an SEM picture of a woven product, while Figure 5 shows particle clogging of a woven geotextile. Figure 6 is an SEM picture of a non-woven, needle-punched product. High porosity and intersecting channels indicate favorable conditions for hosting a biomass without clogging. Figure 4 SEM Photo of a Woven Geotextile Figures 7 and 8 show the morphology of the non-woven geotextiles after extended permeation with clarified wastewater. It is evident that the high porosity and complex structure do support and host biomass while still maintaining high permeability. Bench Scale Testing The process had four stages: - Filtration of suspended organic material with attached microorganisms. - Growth of an active biomass within the fabric. - Absorption of dissolved substrate by the biofilm. - Biodegradation of organic material. 4

5 Geotextile Particle Figure 5 Clogging Mechanism of Woven Geotextile (Redrawn from Mlynarek, 199) Figure 6 SEM Photo of Clean Nonwoven Geotextile. The test liquid was primary effluent from a wastewater treatment plant whose tributary area is served by combined sewers. Dry weather flow is sanitary, while wet weather flow also includes urban runoff. Primary treatment removes settleable solids, but finer and dissolved organic materials are removed in secondary (biological) treatment. The test method was one pass, one-dimensional permeation of primary effluent through columns with multiple filter layers as shown on Figures 9 and 1. Questions addressed included: - Which geotextile types attract microorganisms and support their growth? 5

6 - What types and degree of treatment can occur? - What hydraulic capacity can be maintained? Figure 7 SEM Picture of Nonwoven Geotextile with Biomass Figure 8 SEM Picture of Nonwoven Geotextile with Biomass The experiments were conducted in four phases: - Phase I: Baseline water permeability of composite filter columns - Phase II: Geotextile selection based on wastewater permeability and treatability - Phase III: Extended parametric study of biological treatment 6

7 - Phase IV: Confirmation tests at the best indicated hydraulic loading rate Air Pump Peristaltic Pump 125 cm Peristaltic Pump PWD Primary Effluent Tank 5 cm 15 cm 25 cm Gravel GT-1 Gravel GT-2 Sand Treated Effluent GT-2 Sand 4 in 4 in Figure 9 Schematic of Geotextile Filter Columns Figure 1 Geotextile Filter Columns The first two phases indicated that only non-woven needle punched geotextiles hosted biofilm in the interior porosity. In Phase III, the hydraulic loading rate (HLR) was reduced using eight plant samplings in stages from16 gpd/ft 2 to 9gpd/ft 2. Application varied from continuous flow to intermittent with a dose and drain cycling. The intent was to balance the organic loading rate, oxygen supply, and metabolic rate to produce an endogenous (starved) condition with a stable biomass that mineralized the substrate at a stable permeability. At the early high loading rates, biological clogging required upper filter replacement after sample runs of a week each. Figures 11 and 12 show that the primary effluent that was the influent to the columns had TSS varying from 3 to 7 mg/l, and BOD 5 from 3 to 7 mg/l. Reduction of each to 12 mg/l or less was obtained rapidly, even at the high (16 gpd/ft 2 ) HLR. Figure13 shows that the initial ammonia concentrations of up to 25 mg/l were reduced to below 1 mg/l after the fourth round, due to a better oxygen availability and biomass acclimation. 7

8 TSS Removal Rates for Column-1 TSS concentration (mg/l) Initial TSS Final TSS Rounds Figure 11 TSS Removal Rates of Geotextile Filters BOD 5 Removals for Column initial BOD Final BOD Rounds Figure 12 BOD 5 Removal Rates of Geotextile Filters Permeability behavior of the geotextile filter is shown in Figures 14 and 15. The first shows the incremental decrease with each sampling round. Decreasing the HLR first to 18 gpd/ft 2 and then to 9 gpd/ft 2 significantly slowed down the rate of?k loss. Figure 16 shows that the final permeability appeared to approach an asymptotic value of about 1/5 of the initial value at an HLR of 9 gpd/sf 2. It was found that adding more layers of geotextile did not improve the treatment, and the lowest one slowly clogged, as shown on Figure 16. The filter shown in the center is a clean sample, and the top sample is in the lower left corner, showing the imprint of the overlying gravel. The middle, and bottom filters are counterclockwise from there. NH 4 Removal vs Rounds for Column-1 NH 4 Concentration (mg/l) Initial NH4 Final NH4 Rounds Figure 13 NH 3 Removal Rates of Geotextile Filters 8

9 K Residual vs HLR for Column 1 HLR (gpd/sf) a45 Rounds 3a 5a 67 5a K Residual (cm/sec) (3a and 5a indicate fresh GT replacement) Figure 14 Permeability Change vs Hydraulic Loading Rate HLR vs?k loss for Column-1?K loss (cm/sec) HLR (gal/ft 2 /day) Rounds Figure 15 Hydraulic Loading Rate vs?k loss of Geotextile Filter Figure 16 Geotextile Samples with Biomass 9

10 Thus, only two geotextile filter layers were used in the final Phase IV. Table 1 shows that after two rounds (14 days permeation), the permeability loss in the two filter column was negligible, while the layered sand/gravel control column lost 13%. The treatment was similar to that in Phase III, with more rapid ammonia reduction, probably due to the better air circulation. The control column also provided good TSS and BOD 5 removal, but minimal ammonia reduction. Table 1 Permeability Results of the Two-filter Final Phase K initial cm/sec K final cm/sec AK loss cm/sec K loss % Aerobic Geotextile Column 1 Granular Column References 1. Yaman, C., Geotextiles as Biofilters in Wastewater Treatment, Ph.D. Thesis, Drexel University, Korkut, E., Geotextiles as Baffles in Wastewater Treatment, Ph.D. Thesis, Drexel University, Koerner, G. R., Performance Evaluation of Geotextile Filters Used in Leachate Collection Systems of Solid waste Landfills, Ph.D. Thesis, Drexel University, Koerner, R. M. and Soong, T., Use of Geosynthetics in Infrastructure Remediation, Journal of Infrastructure Systems, Vol.1, No.1, Mlynarek, J., Rollin, A. L., Lafleur, J. and Bolduc, G., Microstructural Analysis of a Soil/Geotextile System, Geosynthetics: Microstructure and Performance, ASTM STP 176, I. D. Peggs, Ed., ASTM, Philadelphia, PA, Tchobanoglous, G., Small and Decentralized Wastewater Management Systems, Mc-Graw-Hill Series in Water Resources and Environmental Engineering,