By Dr. Chad Penn
USDA-ARS, National Soil Erosion Research Laboratory
![Chad Penn headshot](https://phosphorusalliance.org/wp-content/uploads/sites/42/2024/01/Chad-Penn-165x165-1.jpg)
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:
- Contains solid media with high affinity for P, commonly known as a “P sorption material”, or PSM.
- PSM is contained and placed in a hydrologically active area with high dissolved P concentrations.
- 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.
- The PSM is able to be removed and replaced after it is no longer effective.
![](https://phosphorusalliance.org/files/2019/09/P-rem-structure-diagram-1024x576.jpg)
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.
![](https://phosphorusalliance.org/files/2019/09/P-feoxide-sorption.jpg)
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.
![](https://phosphorusalliance.org/files/2019/09/PSMs-1024x256.jpg)
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.
![](https://phosphorusalliance.org/files/2019/09/modular-perf-boxes-300x186.jpg)
![](https://phosphorusalliance.org/files/2019/09/Dutch-filter-300x154.jpg)
![](https://phosphorusalliance.org/files/2019/09/golf-course-filter-300x189.jpg)
![](https://phosphorusalliance.org/files/2019/09/tile-drain-filter-top-down.jpg)
![](https://phosphorusalliance.org/files/2019/09/Blind-inlet-diagram-1024x368.jpg)
Choosing an Ideal Location
In order to qualify as a potential site for construction of a P removal structure, a site must possess:
- Flow convergence to a point where water can be directed into a structure, or the ability to manipulate the landscape
- At least 0.2 ppm dissolved P (DP) in water
- Hydraulic head required to “push” water through structure: function of elevation change or drainage ditch depth
- 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.
![](https://phosphorusalliance.org/files/2019/09/P-Trapp-1024x576.jpg)
Example P Removal Structures
![](https://phosphorusalliance.org/files/2019/09/Westville-structure-002-1024x576.jpg)
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.
![](https://phosphorusalliance.org/files/2019/09/Oedy-structure-1024x576.jpg)
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. For more information about how these types of structures are designed and work, please see our video here.
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.
![](https://phosphorusalliance.org/files/2019/09/P-removal-and-drawdown-fig-002-1024x576.jpg)
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 ([email protected])
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
USDA-ARS National Soil Erosion Laboratory – water filtration system and P-TRAP application video
Opinions expressed here do not necessarily reflect those of the Sustainable Phosphorus Alliance.