A Phosphorus Removal Structure (PRS) is an edge of field practice that removes dissolved phosphorus (DP) from drainage water leaving the field. The practice is best suited to sites where a history of DP concentrations in water leaving the site are measured at 0.2 mg/L DP or greater. These sites are best candidates for the P Removal Structure practice due to efficiency of filtration and the capital cost involved with the installation.
The practice requires the ability to divert concentrated flows of water into a structure containing P absorbent media. The water flows through the media where the DP attaches to a media lowering P levels of the treated water leaving the site.
The structures can take on many styles and forms (See Figure 1), but each possesses the following core components:
- Enough of an unconsolidated Phosphorus Sorption Material (PSMs). PSMs are usually industrial by-products or manufactured materials composed of Fe, Al, or Ca with different P adsorptive characteristics.
- Water with high DP concentration flow through the PSM in the structure at a suitable flow rate while allowing enough contact time based on the PSM characteristics.
- Plan for the ability to remove and replace PSM after it is no longer effective at removing P at the minimum desired rate. Some materials are available that allow renewing the adsorptive capacity without removal of the PSM.
Structure placement is site dependent. Structures can generally be placed in non-production spaces such as buffer areas near the field tile outlet or surface concentrated flow. In some cases, field edges may need to be taken out of production to accommodate the structure. There are several media types available with different absorbance efficiencies which allow some design flexibility. This provides options to adjust filter size to conform to the space available. This is an engineered practice, and it is recommended to consult with a Professional Engineer for design and installation recommendations. An online tool P-Trap is available from USDA-ARS to use as a planning tool. Natural Resources Conservation Service has an interim standard 782 Phosphorus Removal System that can be consulted.
Figure 1. Example Phosphorus Removal Structure Types for treating surface and subsurface water.
The Phosphorus Removal Structure can be utilized for many situations where DP is a resource concern in receiving waters: urban, agricultural, golf course, horticultural, and wastewater. Much of the early work using P removal structures were done with municipal, domestic, and agricultural wastewater where the structures were often used in conjunction with treatment wetlands. Phosphorus Removal Structures have been placed in line with surface drainage water, in conjunction with drainage tile, or in ditch areas; it is possible to “stack” this practice with nitrogen bioreactors. Placement is only limited by the practicality of directing enough high concentration DP water through the filter material while allowing for a functional drainage system.
In a field setting, it is best to measure water leaving a field site at several different flow conditions to quantify the actual DP concentrations leaving a site. Taking a grab samples under a high, moderate and low flow conditions is a reasonable method to use. The best sites for a PRS have water with DP concentrations greater than 0.2 mg/L which is four times the desired DP concentration target (0.05 mg/L) for many receiving water bodies such a streams, rivers and lakes.
These field sites are often associated with high Soil Test Phosphorus (STP) levels, generally 2-3 times the agronomic STP need of many crops grown. Fields with a STP value of 100 mg/kg Mehlich 3 or greater would be a candidate for water testing to confirm DP concentrations. Fields with high STP will remain high for many years due to the P buffering capacity of soils. In these high STP field situations there are few conservation practice alternatives to reduce DP losses. The long term needs at these sites make the PRS a cost-effective option.
Sites with high concentrations of DP in drainage water are often associated with high STP levels from past field management. Phosphorus removal structures can be installed to trap the lost P while in-field practices to lower STP are implemented for reducing the source of DP. More information on the relationship of soil test level to DRP losses can be found on this site.
The PRS immediately removes DP from drainage water and surface runoff at the edge of a field. The ideal field site is where high levels of DP (>0.2 mg/L) are measured in water and other conservation practices cannot be effectively deployed to reduce DP in sensitive watersheds. The practice will be most cost effective when implemented at sites where long-term DP reduction are needed while infield practices are used to reduce DP sources in the field. The primary in-field practice for reducing the source of DP is reduced or no application of additional P while drawing down STP levels to environmentally acceptable levels.
Properly designed P Removal Structures can result in reductions of 16-71% in DP concentration and loading. These results were from a summary of 40 different field scale trials using various designs and PSM materials. (Penn, et al, 2017) Water 2017, 9, 583; doi:10.3390/w9080583
Characteristics of the ideal site for construction of a P removal Structure include:
- Flow convergence to a point where water can be directed into a structure, or the ability to manipulate the landscape to concentrate water flows.
- Dissolved P (DP) in water of at least 0.2 mg/L.
- Hydraulic head required to “push” water through structure which is a function of elevation change or drainage ditch depth.
- Sufficient space to accommodate PSM chosen.
P removal structures require careful design to reach peak absorption efficiencies and maintain subsurface tile drainage. Design inputs needed to determine size and function for a PRS fall into three categories.
- Site hydrology and water quality characteristics
- Target removal and Lifetime needs
- PSM characteristics.
Site characteristics will determine the amount of water that flows to potential installation locations. Measurements of the DP concentration over several different flow conditions combined with estimates of annual flow volume can be used to estimate the load (mass) of DP that is delivered. Sizing of a P removal structure is a function of the annual P load, the chosen P removal amount and lifetime, and the characteristics of the PSM to be used. The most important PSM characteristic is its ability to remove P, as quantified by a “P removal curve”. The P removal curve is simply a mathematical description of P removal under flowing conditions for a given P inflow concentration and retention time (RT), expressed as a function of P loading (i.e., P added per unit mass of PSM). Physical characteristics of the PSM, especially porosity and saturated hydraulic conductivity (i.e. its ability to conduct water) are especially important when it comes to designing PRS to achieve the desired P removal at the chosen flow rate and RT.
An important factor in design is PSM selection. Generally, any product with a high affinity for P, suitable physical characteristics, and is safe for use in waterways can work as a PSM. Types of PSMs available include drinking water treatment residuals, fly ash, mine drainage residuals (Fe oxides), steel slag, metal filings, and manufactured PSM. The most cost effective PSM known at this point, is metal filings/turnings mixed (5-8%) with clean pea-gravel. A graphic of PSM’s is found in Figure 2.
Manufactured PSMs tend to absorb P more efficiently but are higher cost products. While manufactured PSMs are higher cost, less material mass is needed to perform the desired P capture, lowering the amount of space needed for the PRS. For example, a subsurface tile drain filter designed to remove 35% of a 5-year Dissolved P load using treated steel slag would require 40 tons while a manufactured Fe-rich PSM would only require 2-5 tons.
When treating subsurface tile drainage water do not use untreated electric arc furnace slag or blast furnace slag. The bicarbonate contained in the tile water will cause premature failure with these PSM options. Aluminum-treated slag can be used for tile drains if properly sieved to removed fines. Regular non-treated sieved slag works well for treating surface water, especially when used as the gravel in blind inlets.
A variety of materials have been evaluated and are included in the P-Trap software database. New materials are always being evaluated at USDA Agricultural Research Service, National Soil Erosion Research Laboratory in West Lafayette, IN. Contact the USDA Erosion Laboratory if your PSM of interest is not found in the P-Trap database.
Figure 2. Type of PSM Materials. (Source: Chad Penn)
The PSMs become saturated over time, meaning its ability to absorb P decreases with P loading. Systems can be designed to last 2 to 15 years depending on desired cost, site location constraints as well as the amount of P reduction needed. Although P-saturated PSM loses absorption efficiency, it remains a net P sink because the P is usually tied up tightly. Although the structure may not be able to absorb more P flowing through it, it will not release the P it has already absorbed. The PSM, depending on what type it is, can be treated and reused, recycled, or disposed of properly. Slag, for example, can be re-used again as road construction material.
Phosphorus removal structures should always be constructed with an emergency over-flow in cases of flow rates exceeding the capacity of the structure. While this is simple to do for a surface PRS, and subsurface tile drain PRS should be utilized in conjunction with a water table control structure that diverts tile water into the PRS but allows overflow to drain in situations of high flow rates or clogged PSM.
Another consideration is access to the filter for maintenance. If there is a long-term site need to reduce P beyond the initial structure’s design life, considerations to location and design of the structure to make renewing or recharge the PSM should be made.
The cost of PRS range from $3 to 20K, depending on the size, site, PSM, and P removal goals. In general, the cost of P removal on a per unit mass basis is in the same price range as a PRS used wastewater treatment setting though DP removal is much more difficult in an agricultural non-point setting.
A P removal structure is essentially a landscape-scale filter for trapping dissolved P in drainage water through a media that contains phosphorus sorption materials (PSMs). The reaction can occur with iron, aluminum, calcium or other metal ions in the media resulting in adsorption of the P to the surface, or precipitation. The resulting water outflow has a reduction in the amount of dissolved P contained. Figure 3 show the chemical process of P retention in a PRS. Figure 4 shows basic process of directing water through the PSM, then providing a path for treated water to leave the PRS.
Figure 3 .Process of P retention is PSM in a Phosphorus Reduction Structure.(Source: Chad Penn)
Figure 4. Example of P Removal Structure with surface water. (Source: Chad Penn)
Training modules for certification in PRS design are currently underway by the NRCS. See “House of Phos” on twitter (https://twitter.com/ChadPenn12) for updates and announcements of webinars for P-Trap training as well as opportunities to watch a PRS be constructed.
Questions, concerns or suggestions for website content on this practice. https://agbmps.osu.edu/submit/email-general-questions-comments-or-concerns
Extensive design details are provided in Natural Resources Conservation Service has an interim standard 782 Phosphorus Removal System
Online planning tool P-Trap is available from USDA-ARS.
Other Design and Performance References
Special Issue "Advances and Challenges in Improving Water Quality with Phosphorus Removal Structures: Scaling Up to the Field" Open Access. https://www.mdpi.com/journal/water/special_issues/phosphorus_removal [Accessed 2021, Feb 5]
Utilization of Steel Slag in Blind Inlets for Dissolved Phosphorus Removal. Open Access. https://www.mdpi.com/2073-4441/12/6/1593 [Accessed 2021, Feb 5]
Development of a Regeneration Technique for Aluminum-Rich and Iron-Rich Phosphorus Sorption Materials. Open Access. https://www.mdpi.com/2073-4441/12/6/1784 [Accessed 2021, Feb 5]