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A Tool for Trapping Dissolved Phosphorus

By Dr. Chad Penn

USDA-ARS, National Soil Erosion Research Laboratory



The P removal structure is a large, landscape scale filter for dissolved phosphorus (DP), intended to intercept and trap P from “hot spots” (i.e. legacy P) before reaching a surface water body. The P removal structure embodies four basic principles:


  1. Contains solid media with high affinity for P, commonly known as a “P sorption material”, or PSM.
  2. PSM is contained and placed in a hydrologically active area with high dissolved P concentrations.
  3. High DP water is able to flow through the contained PSM at a sufficient rate and contact time. Structures must be designed to handle peak flow rates since 90% of annual P losses are generally transported in less than 5% of the flow events, which are the largest flow events.
  4. The PSM is able to be removed and replaced after it is no longer effective.

Due to small amounts of PSM, poor contact between PSM and flowing water, and limited contact time, small in-line pipe filters and “socks” filled with filter media are not effective for removing appreciable amounts of P, nor do they remain effective for any significant amount of time. If a site contains appreciable P loads worth treating, then it will require several tons of filter media.

Phosphorus Sorption Materials (PSMs)

Many PSMs are by-products from different industries, and therefore can be obtained for low cost. Some PSMs are manufactured. However, all PSMs must first be screened for safety before use in a P removal structure. Some examples are shown below.

Types of P Removal Structures

P removal structures can appear in many different forms. They can be located on the surface or subsurface, in ditches, tile drains, drainage swales, drop inlets, blind/surface inlets, etc. Any unit that embodies the four basic principles is essentially a P removal structure. Several examples of P removal structures are shown.





Choosing an Ideal Location

In order to qualify as a potential site for construction of a P removal structure, a site must possess:

  1. Flow convergence to a point where water can be directed into a structure, or the ability to manipulate the landscape
  2. At least 0.2 ppm dissolved P (DP) in water
  3. Hydraulic head required to “push” water through structure: function of elevation change or drainage ditch depth
  4. Sufficient space to accommodate PSM

P removal structures are not a “silver bullet”, as they are intended only for sites that are legacy P sources, that is, excessive soil P concentrations that will remain elevated for decades, releasing dissolved P into drainage water even after ceasing P applications. This best management practice is not intended for trapping incidental losses of P from recently applied manure or fertilizer, as there are much less expensive BMPs for that type of P source, such as incorporation and proper timing of application.

Designing a P Removal Structure

Several inputs and target goals are required for designing a site-specific structure. The P-Trap software, which will soon become publicly available through the USDA-ARS, can be used to quickly design a structure with any effective and available PSM.

Example P Removal Structures

The confined bed structure shown above contained 40 tons of treated steel slag and intercepted runoff from a poultry farm with DP ranging from 2-15 ppm. This structure was able to remove 7.5 lbs DP in 2.5 yrs and handle ~1000 GPM flow.

The tile drain P filter shown above during construction, utilized a conventional septic tank. This structure contains 2.5 tons of a manufactured PSM (Fe-coated alumina) that can be regenerated after becoming saturated with P. This unit was designed to remove 40% of the 10 year cumulative DP load, and handle 300 GPM. After 10 years, the PSM will be regenerated in-situ.

Cost and Justification

The only other conservation practice that can truly remove P is soil P drawdown via plant uptake and harvest. Although necessary for solving the long term problem of legacy P, it requires many years (decades) for plant draw-down to reduce DP concentrations in drainage water. During that long draw-down period while dissolved P losses are still elevated, P removal structures can be used to trap P in drainage water. Cost of structures will vary dramatically with site and PSM utilized. Total costs are similar to waste water treatment: $50-500 dollars per pound of P removed.

Current and Future Efforts

Current research is focused on regeneration of PSMs and investigation of new by-products for reducing costs. The ability to regenerate PSMs in-situ, even once, will nearly cut the cost of P removal in half. The USDA-ARS National Soil Erosion Research Laboratory is also working closely with the NRCS, American Society of Agricultural and Biological Engineering, and American Society of Agronomy, in producing a series of training videos and certification program for design and construction of P removal structures. The P-Trap software will also aid in increasing the adoption of this new BMP. Many states include the P removal structure in their cost-share program, under the 782 Standard.

For more information, please contact Dr. Chad Penn at the USDA-ARS (chad.penn@ars.usda.gov)

Additional Sources:

Penn, C.J., and J.M. Bowen. 2017. Design and construction of P removal structures for improving water quality. Springer Publishing. ISBN 978-3-319-58657-1

Penn, Chagas, Klimeski, and Lyngsie. A Review of Phosphorus Removal Structures: How to Assess and Compare Their Performance. Water (20734441), August 2017. DOI 10.3390/w9080583

Opinions expressed here do not necessarily reflect those of the Sustainable Phosphorus Alliance.