A modelling approach to the design of in situ agricultural drainage filters

Transcription

1 SoilUse and Management Soil Use and Management doi: /j x A modelling approach to the design of in situ agricultural drainage filters J. M. Mcgrath 1,C.J.Penn 2 &F.J.Coale 1 1 Department of Environmental Science and Technology, University of Maryland, College Park, MD, USA, 2 Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK, USA Abstract Agricultural drainage ditches provide a direct pathway for high nutrient loads such as phosphorus (P) from fields to surface waters. Most practices designed to reduce P loading from fields focus on overland flow and sediment-bound P; however, P transport in these landscapes can be dominated by subsurface transport of dissolved P. Filters have been designed and are currently being field-tested that utilize P sorbing materials to remove P directly from flow in ditches. Models have been developed to predict P removal and the time of filter effectiveness. While filter performance along with model development and validation is examined in separate papers, there is a need to translate these models into usable tools for watershed managers who have to decide on filter design. This study provides examples of how these models can be used to design P-removal structures containing electric arc furnace slag (EAFS) or flue gas desulphurization gypsum (FGDG). Through the use of realistic parameters for retention time, inflow P concentration and material characteristics, the presented examples demonstrate that EAFS would outperform FGDG across a range of inflow P concentrations (.5 1 mg L) and removing 28 65% more P. With an average inflow P concentration of 1.62 mg L (typical for ditches on the Delmarva Peninsula, United States), a 2-Mg EAFS filter would remove 42% more P than a 31-Mg FGDG filter. In addition, the EAFS filter would have a flow rate 1 times that of the FGDG filter. The predictive equations discussed in this paper can be used to determine the cost per unit of P removed of filters containing different materials. Keywords: Phosphorus removal, phosphorus sorbing materials, non-point source pollution, agricultural drainage ditches, industrial by-products, electric arc furnace slag, flue gas desulphurization, gypsum Introduction Phosphorus is a primary nutrient contributing to eutrophication of surface waters. The Delmarva Peninsula is in the Coastal Plain physiographic province of the Mid- Atlantic region of the United States (latitude: 37.7 to 39.8, longitude: )75.7 to )76.4 ) and separates the Chesapeake Bay from the Delaware Bay and the Atlantic Ocean. It includes all of Delaware and the eastern portions of Maryland and Virginia. Long-term nutrient surpluses on the Delmarva Peninsula have resulted in soils that are saturated in respect of their ability to sorb phosphorus (P) (Sims et al., 22, 28). High soil P saturation in combination with Correspondence: J. M. Mcgrath. mcgrathj@umd.edu Received May 211; accepted after revision November 211 coarse textured, artificially drained soils, and shallow groundwater tables have created a situation where large P losses to surface water may occur, primarily through subsurface pathways (Sims et al., 1998; Kleinman et al., 27; Vadas et al., 27). Even if P applications to these soils were to cease, it would likely take many years for P to be removed by crops before soil P saturation would be reduced through soil sorption processes (Eghball et al., 23; Schulte et al., 21). While particulate P losses can be controlled by reducing erosion, there are few viable options to reduce dissolved P losses, especially when they occur through subsurface pathways (Sims & Kleinman, 25). Numerous studies have shown that a range of materials rich in calcium (Ca), magnesium (Mg), iron (Fe) or aluminium (Al) are effective at reducing P solubility in amended manures or soil and thereby ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science 1

2 2 J. M. Mcgrath et al. at reducing the potential for dissolved P losses (Gallimore et al., 1999; Penn & Bryant, 26; Leader et al., 28; Brennan et al., 211; Penn et al., 211a). However, reductions in P solubility through soil or manure amendment may only be temporary by sequestering P through chemical sorption (specifically adsorption or precipitation), but not removing P from the system or addressing the underlying issue of soil P surpluses relative to crop demand (Penn & Bryant, 26). As a further development, P removal structures filled with P sorbing materials (PSM) or constructed treatment wetlands enhanced with PSM have been designed and evaluated to treat point and non-point pollution in agricultural runoff and effluent (Shilton et al., 26; Weber et al., 27; Lee et al., 21; Penn et al., 21). Specifically, P removal structures have been designed so that PSM can be removed and replaced after saturation with P. These structures have been evaluated in agricultural drainage ditches and stormwater retention ponds that serve as collection points for non-point runoff (Shilton et al., 26; Penn et al., 27, 21, 211b; Lee et al., 21). The PSM in these structures can be removed after they are spent, effectively relocating excess P from P saturated fields (Bird & Drizo, 29; Grubb et al., 211). Numerous materials including industrial by-products have been evaluated in the laboratory as potential PSMs for fieldscale P removal filters (Drizo et al., 1999; Bowden et al., 29; Penn & McGrath, 211; Penn et al., 211a; Stoner et al., 211). Penn et al. (211a) evaluated various industrial by-products to determine P sorption and desorption potential and mechanisms associated with these materials. They found that most P was sorbed through adsorption or precipitation with calcium (Ca), magnesium (Mg), iron (Fe) or aluminium (Al) oxides and hydroxides and that ph, buffer capacity, ionic strength and common ion effects played a large role in sorption efficiency and capacity of these materials. Gypsum, a by-product of flue gas desulphurization (FGD), has been evaluated in field-scale demonstrations on the Delmarva Peninsula as a potential PSM in P removing structures (Penn et al., 27). Steel slag is another material that has been evaluated as a potential PSM under various settings (Penn & McGrath, 211). Penn & McGrath (211) and Stoner et al. (211) developed models using flow-through experiments to predict the P sorption capacity and projected lifespan of various PSM. These models have subsequently been validated in the field for steel slag and produced reasonable estimates of P removal (Penn & McGrath, 211; Penn et al., 211b) and are currently being validated for FGD gypsum. There is a need to translate these laboratory-based findings into recommendations applicable at the field scale so that land managers can design and deploy P removal filters as a best management practice to control non-point dissolved P losses. Therefore, the objectives of this study were to evaluate the benefits and concerns associated with using models to design P removal structures that contain electric arc furnace steel slag (EAFS) and FGD gypsum (FGDG) as PSM. Methods Modelling approach A plot of P added to a PSM versus discrete P removal under specific conditions is known as a design curve. Design curves were developed by Penn & McGrath (211) and Stoner et al. (211) for coarse EAFS (>.64 cm) and FGDG, respectively. The design curves use inflow P concentration and retention time (RT) to predict P removal efficiency for a specific PSM. The efficiency of a filter material can be defined in either discrete or cumulative terms. Discrete efficiency refers to the amount of P removed per unit P added to the material and can be defined as the per cent discrete efficiency, which is the percentage of the P coming in contact with the material that is removed, or as mass discrete efficiency, which is the mass of P removed from the inflow per unit mass of material (mg kg). Material efficiency can also be described in cumulative terms as either the cumulative percentage of P removed or the mass of P removed per unit mass of material when at the point where the outflow concentration equals the inflow concentration. The design curve used to determine cumulative and discrete efficiencies of filter materials is defined below by equations 1 6. The design curve can be derived using slope and y-intercept parameters that are dependent on RT and inflow P concentration. RT, or the amount of time a molecule of water is in contact with the PSM as water flows through it, is a function of material physical characteristics (porosity and permeability), filter design (cross-sectional area and thickness) and head pressure. RT can be modified as a function of filter design, by restricting flow or modifying the filter size (thickness and cross-sectional area). In addition, storm size and hydrology control the amount of water entering a retention pond or drainage ditch and therefore head pressure on the filter, which in turn influences RT. Inflow P concentration is typically a fixed variable from the perspective of filter design as it is controlled by landscape hydrology and P source characteristics. However, P concentration is also related to flow rate and therefore varies with RT. Because RT and P concentration cannot be completely controlled, predictions of filter efficiency can be difficult. Nonetheless, it is useful to consider the design curve when planning a filter installation using expected P concentrations and possible RTs. In addition, if the end user is only concerned with a certain portion of a storm event, the design curve can be used to construct a filter specifically for that portion of the storm hydrograph. For example, the highest concentrations of P in runoff are typically associated with a larger flow rate. Therefore, it may be advantageous to design a filter that has a high flow-through rate, low RT and the ability to remove high P concentrations. The design curves provided by Penn & McGrath (211) and Stoner et al. (211) were arrived at through empirical modelling ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science, Soil Use and Management

3 Designing in situ agricultural drainage filters 3 using flow-through cells. The general equation for discrete P removed as a percentage of P added takes the form of fðxþ ¼be mx ð1þ where f(x) is discrete P removal as a percentage of P added at point x, x is the cumulative P added (mg kg), m is the slope and b is the y-intercept. Penn & McGrath (211) used multiple linear regression models to predict the slope (equation 2) and y-intercept (equation 3) for coarse EAFS based on the RT and inflow P concentration (P i ) and arrived at the following predictive equations: logð mþ ¼:856RT :7416P 2:53493 ð2þ logðbþ ¼:65416RT :864P 2:4399 Stoner et al. (211) used the same method to determine the following equations for predicting slope and y-intercept for FGDG: logð mþ ¼ :7616RT :5743P 1:1482 ð4þ logðbþ ¼ :29576RT :28579P þ 2:13484: Inserting average or target concentrations and RTs in equations 2 5 and then solving equation 1 from the y-intercept to the x-intercept allows construction of a design curve representing the efficiency achieved over the lifespan of the PSM. The cumulative amount of P sorbed by a filter material can be predicted by integrating equation 1 from a to z (equation 6): Z z Cumulative P removed ðmg kg 1 Þ¼ be mx dx ¼ b ð m emz e ma Þ where a and z are the amount of P added (mg kg) at the beginning of the range of interest (typically ) and the point where f(x) = (equation 1) or P sorption ceases, respectively. Application of the model The above approach was applied to coarse EAFS and FGDG. Equations 1 6 were used to describe the behaviour of these two materials over a range of RTs and P concentrations, showing how the design equations behave and the materials differ. Finally, the equations are applied to several filter designs to estimate the performance of potential filter designs. Results and discussion Assessing material efficiency using the design curve When designing a filter, equations 1 6 can be used to modify the filter design to achieve a predetermined amount of P removal, to estimate filter lifespan or to evaluate several filter materials to determine which would best suit the specific site s a ð3þ ð5þ ð6þ needs (e.g. flow rate, P removal efficiency, size and lifespan). Figure 1a, b provides a design curve constructed for coarse EAFS and FGDG using a RT of 1 or 1 min and an inflow P concentration of 1 or 1 mg L. When the line crosses the x-axis, the PSM has ceased to remove P; in other words, the outflow P concentration equals the inflow P concentration. Overall material performance can be described as the cumulative amount of P removed per unit mass of PSM (Figure 2). This value can be derived by integrating the curve equation as described by equation 6. Material efficiency, as described by the percentage of P removed from the inflow for each increment of P added, is quite different for EAFS and FGDG. Overall efficiency is higher for EAFS at higher P concentrations and lower RTs, whereas FGDG has higher efficiency at low P concentrations and long RTs. Physical and chemical characteristics for the FGDG and EAFS are provided by Stoner et al. (211) and Penn et al. (211b), respectively. Stoner et al. (211) found that only FGDG had a statistically significant negative slope parameter and y-intercept parameter (equations 4 and 5) for RT, indicating that this material would significantly remove more P from solution as RT increased. Although the FGDG was able to provide sufficient amounts of soluble Ca to the solution for potential Ca phosphate precipitation, the ph was not buffered sufficiently to allow such a reaction to occur Discrete P removal (mg/kg) Discrete P removal (mg/kg) EAFS: RT = 1 min EAFS: RT = 1 min FGDG: RT = 1 min FGDG: RT = 1 min P added (mg/kg) Figure 1 (a) Design curves for coarse electric arc furnace slag (EAFS) and flue gas desulphurization gypsum (FGDG) with inflow phosphorus concentration of 1 mg L at retention times (RT) of 1 and 1 min and (b) design curves for coarse EAFS and FGDG with inflow phosphorus concentration of 1 mg L at RT of 1 and 1 min. (a) (b) ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science, Soil Use and Management

4 4 J. M. Mcgrath et al. P (mg/kg) RT = 1 min P = 1 mg/l RT = 1 min P = 1 mg/l RT = 1 min P = 1 mg/l EAFS FGDG RT = 1 min P = 1 mg/l EAFS suffers relative to FGDG owing to EAFS continuing to sorb P at a low discrete efficiency much longer than the FGDG. Overall, P removed by EAFS will be greater than by FGDG, but P passing through the filter will also be greater over the lifetime of the filter. However, this is primarily because the EAFS filter will last much longer before it ceases to sorb P (i.e. when inflow = outflow P concentration). Therefore, it might be advantageous if designing a filter using EAFS to plan on removing the EAFS well before it reaches the point where it ceases to sorb P. The design curve equation can be solved for cumulative P added at the point where the cumulative removal is equal to or greater than the target efficiency. Figure 2 Cumulative phosphorus removal by coarse electric arc furnace slag (EAFS) and flue gas desulphurization gypsum (FGDG) derived by integrating the area under the design curve for given retention times (RT) and inflow P concentrations (P). quickly. Precipitation of Ca phosphate is favoured if ph > 6.5 (Lindsay, 21). In addition, the EAFS was well buffered above ph 6.5 throughout sorption, which increased the kinetics of the Ca phosphate precipitation reaction (Stoner et al., 211). In addition, the EAFS slag contained some amorphous Fe oxyhydroxides, which will remove P from solution by fast ligand exchange reactions. Figure 2 shows the cumulative P removed by EAFS and FGDG for each of the four scenarios modelled in Figure 1. The cumulative P removed indicates the mass of P removed per unit mass of PSM and is determined by solving equation 6 using the four combinations of RT (1 and 1 min) and inflow P concentration (1 and 1 mg L) presented in Figure 2. Electric arc furnace slag (EAFS) outperforms FGDG in each situation. However, at the longer RT of 1 min, FGDG discrete efficiency approaches or exceeds that of EAFS for both the 1 and 1 min RT at 1 mg L inflow P concentrations. It is important to note that while EAFS clearly removes more P than FGDG before reaching equilibrium at a RT of 1 min, more P would bypass EAFS than the FGDG. This is because the slope of the design curve for EAFS is much shallower at the low RT; therefore, while the material is still removing P, it is doing so at a much lower discrete P removal efficiency than the FGDG. The cumulative efficiency is useful for comparing potential filter materials and is determined by integrating equation 6 from zero to the point z where f(x) (equation 1) equals zero. The cumulative efficiency at a 1-min RT for EAFS is 12 and 1% at 1 and 1 mg L inflow P compared to 26% for the FGDG at the same concentrations. The opposite is true when the RT is extended to 1 min. At this longer RT, EAFS has a cumulative efficiency of 34 and 3% and the FGDG has a cumulative efficiency of 16% for inflow concentrations of 1 and 1 mg L, respectively. Therefore, in designing filters, it is important to recognize that the cumulative efficiency of Material selection The design curve equation can be manipulated to determine at which RT or inflow P concentration the FGDG surpasses the EAFS in cumulative P removal. For example, in Figure 3, the design curve equation (equation 6) was solved at a realistic range of RTs for EAFS and FGDG at an inflow P concentration of 1 and 1 mg L. With a 1 mg L inflow P concentration, EAFS and FGDG achieve the same cumulative P removal of 89 mg kg at ca. 13-min RT, while if the inflow P concentration is 1 mg L, a RT of 14 min yields the same cumulative P removal of 35 mg kg. Note the decreasing P removal with increasing RT for slag. As discussed by Stoner et al. (211), this is likely to indicate fast P sorption kinetics. Apparently, the P sorption kinetics are so fast that the P removal is more limited by the rate at which P is added to the material rather than the rate of the reaction. Stoner et al. (211) showed that while the Ca found in FGDG (CaSO 4 ) was more soluble than the Ca hydroxide minerals found in EAFS, the Ca minerals in the EAFS were able to remove P more quickly than FGDG by Ca phosphate precipitation because the ph of these minerals was more highly buffered. A wellbuffered material allows for greater precipitation because Cumulative P removed (mg/kg) EAFS 1 mg/l FGDG 1 mg/l EAFS 1 mg/l FGDG 1 mg/l 1 15 Retention time (min) Figure 3 Effect of retention time on cumulative P removal at inflow phosphorus concentrations (P) of 1 and 1 mg L ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science, Soil Use and Management

5 Designing in situ agricultural drainage filters 5 precipitation of Ca phosphates generates acidity in solution. As previously described, the EAFS material also contains some amorphous Fe oxyhydroxides that can remove P through fast ligand exchange reactions. Often the PSM to be used will be determined by availability and cost of materials. If FGDG were to be used and the average inflow P concentration is known to be 1mg L, the filter can be designed to exceed a RT of 13 min to achieve greater efficiency, whereas if EAFS is the material of choice, the filter should be designed to have a much lower RT. However, often a filter must be designed to handle a range of P concentrations, in which case equation 6 can be solved for a range of concentrations at several RTs to aid in filter design. Figure 4 shows RTs of.5, 13 and 2 min over a range of P concentrations from 1 to 1 mg L. Flue gas desulphurization gypsum exhibits the highest cumulative removal at the longest RT but is not much different than EAFS at the lowest RT, especially at higher concentrations. Cumulative P removal increases with inflow P concentration for both materials, but the slope of the curve is steeper for the EAFS, indicating that it is more responsive to variable P concentration. This increase in P removal with increasing inflow P concentrations is typical of materials that sorb P by Ca phosphate precipitation as the precipitation reaction is driven by the amount of phosphate available in solution (Stoner et al., 211). In addition, the FGDG has a higher cumulative removal at longest RT, while the opposite is true Cumulative P removed (mg/kg) EAFS: RT =.5 min EAFS: RT = 13 min EAFS: RT = 2 min FGDG: RT =.5 min FGDG: RT = 13 min FGDG: RT = 2 min Inflow P concentration (mg/l) Figure 4 Effect of inflow phosphorus (P) concentration and retention time (RT) on cumulative P removal. for the EAFS. This is attributable to the Ca minerals in the FGDG being highly soluble, but the Ca minerals in the EAFS having a highly buffered ph. Filter design and sizing Figures 1 4 demonstrate that coarse EAFS will continue to remove P for much longer than FGDG in terms of the amount of P added. Also, per unit mass, the discrete efficiency of EAFS typically exceeds that of FGDG at realistic P concentrations and RTs. Because EAFS is more efficient, per unit mass in situ treatment structures utilizing EAFS typically contain much less material than FGDG filters. In addition, owing to the material characteristics, FGDG can easily be dumped directly into a ditch or pond bed and then be removed and spread (after P saturation) on a field, whereas it is more convenient to contain EAFS in a box-type structure. Currently, box-type structures and tile-drained beds are being evaluated in several locations (Penn et al., 27, 21, 211b; Penn & McGrath, 211). In Table 1, the characteristics of a potential filter design are presented based on measured material parameters, and the flow rate and RT are calculated based on Darcy s law (Middleton & Wilcock, 1994). Equation 7 presents a simplified version of Darcy s law that can be used to solve for flow rate as shown in Table 1, Q ¼ ka h þ T ð7þ T where flow rate (Q) equals the product of the coefficient of permeability (k), cross-sectional area (A) and the hydraulic gradient across the filter. The hydraulic gradient is defined as the sum of head above the filter (h) and the filter thickness (T) divided by filter thickness. The flow rate can then be used in equation 8 to determine RT in seconds. RT ¼ kv Q : If a constant head of.61 m is assumed with 2 Mg of EAFS and 31 Mg of FGDG (as shown in Table 1), equations 7 and 8 can be used to calculate RTs of.28 and 11.5 min, respectively. Using the flow rates calculated with equations 7 and 8 and the filter dimensions presented in Table 1, equations 1 6 can be used to plot the cumulative P removed ð8þ Table 1 Example of filter flow characteristics and retention times for electric arc furnace slag (EAFS) and flue-gas desulphurization gypsum (FGDG) Material Density (kg L) Hydraulic conductivity (cm s) Porosity (L L) Head Filter thickness Filter width Filter length Volume (L) Mass (kg) Flow rate (L s) Retention time (min) EAFS FGDG ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science, Soil Use and Management

6 6 J. M. Mcgrath et al. across a realistic range of inflow P concentrations (Figure 5) for both materials. Under these conditions, the EAFS is able to remove more cumulative P across a range of realistic P concentrations. A design curve for each example filter is presented in Figure 6 using long-term mean P concentrations found in agricultural drainage ditches on the Delmarva Peninsula (1.62 mg L; J.M. McGrath, personal communication). With an inflow P concentration of 1.62 mg L, the design model predicts that the 2-Mg EAFS filter would cease to sorb P after L of flow, and the FGDG would cease to sorb P after L of flow. This represents 31.7 kg of P added to the EAFS filter and 16.7 kg of P added to the FGDG filter over the lifetime of the filters. At equilibrium, or filter failure, the cumulative efficiency of the Cumulative P removed (g) EAFS: RT =.28 min, size = 2 Mg FGDG: RT = 11.5 min, size = 31 Mg Inflow P concentration (mg/l) Figure 5 Cumulative phosphorus (P) removed by coarse electric arc furnace slag (EAFS) filter or flue gas desulphurization filter at specific filter size and retention time (RT) across a range of inflow P concentrations. Cumulative P removed (kg) EAFS: RT =.28 min, size = 2 Mg FGDG: RT = 11.5 min, size = 31 Mg P added to the filter (kg) Figure 6 Cumulative phosphorus (P) removed by coarse electric arc furnace slag (EAFS) or flue gas desulphurization gypsum (FGDG) filters designed at the given retention times (RT) and filter sizes with an inflow P concentration of 1.62 mg L over their lifespan in terms of P added up to the point where the filters cease to sorb P as predicted by the design curve equation. EAFS and FGDG filters would be 1.7 and 14.8%, respectively. In addition, at failure, this translates into a cumulative P removal per unit mass of material for the EAFS and FGDG of 172 and 8 mg kg, respectively. One important consideration is total P bypassing the filters. If we assume the filters are designed to handle most flow rates they would encounter, then the design curve can be used to predict total P bypassing the filter over its lifetime. Because the EAFS continues to operate longer than the FGDG in terms of the amount of flow passing through the filter, and the discrete efficiency of both materials decreases with the amount P removed, the cumulative P that has passed through the EAFS is much higher at the end of its lifespan than the FGDG. The EAFS and FGDG filters described above would remove 3.4 and 2.5 kg of P over their predicted lifespan, respectively. However, 28.3 kg of P would bypass the EAFS filter, while only 14.2 kg would bypass the FGDG filter during its shorter lifespan. Therefore, if cumulative P load moving down stream was the primary design consideration, the equations could be used to estimate when filter materials should be replaced or rejuvenated. Similarly, a specific outflow P concentration might be the objective of a filter designer. In this case, the design curve could be solved for that value, and timing of the filter material replacement could be determined. In the example presented in Table 1 and Figures 5 and 6, the EAFS filter was able to treat almost twice the flow volume and remove 38% more P over its lifespan with 36% less material (by mass). Even more significant from a designer s perspective is that the EAFS is able to do so at a much higher flow rate ( L s) than the FGDG (17.39 L s). One of the limitations of the FGDG filter is that it has a very slow flow rate and therefore often much of the water passes over the filter untreated because these coastal plain systems tend to be flashy with very low base flow and very high storm flow rates. Nonetheless, FGDG might be more attractive from a cost perspective in some regions, and in large ditches, it might be advantageous to design a large bed filter. Such a filter could have a much greater cross-sectional area, allowing somewhat higher flow rates and still effectively removing P from inflow, particularly during base flow events. Conclusion Assuming equal cost of material transport and filter construction per unit mass of material, the highest costeffectiveness in terms of unit P removed per dollar spent would be achieved using the coarse EAFS. The EAFS filter with 2 Mg of material would remove 25 6% more P over its lifespan than the FGDG filter with 31 Mg of material across a concentration range of.5 1 mg L. With an expected inflow concentration of 1.62 mg L, it can be estimated from the design curve that the 2 Mg slag filter would remove 38% more P than the 31 Mg FGDG filter. In addition, the EAFS offers the opportunity to rejuvenate the ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science, Soil Use and Management

7 Designing in situ agricultural drainage filters 7 PSM in situ using the method of Penn & McGrath (211), thus permitting a longer lifespan before complete PSM replacement if required. Finally, the coarse EAFS has the advantage of a higher flow rate than the FGDG, so the potential for the filter to be overtopped during larger storm events is reduced. Overall, the in situ treatment of agricultural drainage, particularly in flat, low-lying areas such as the Delmarva Peninsula, shows much promise. The ability to predict filter effectiveness for multiple materials during filter design greatly aids decisions on the use of filters. Acknowledgements This project was funded in part by a USDA-NRCS Conservation Innovation Grant (Award number NRCS 69-3A ) and a Pioneer Grant provided by the Chesapeake Bay Trust. References Bird, S.C. & Drizo, A. 29. Investigations on phosphorus recovery and reuse as soil amendment from electric arc furnace slag filters. 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Water Science & Technology, 56, 135. ª 212 The Authors. Journal compilation ª 212 British Society of Soil Science, Soil Use and Management