Phosphorus Removal Structures

Aerial photo of land
The area before the installation of the phosphorus removal bed is shown, adjacent to the two stage ditch.

What it is:

Phosphorus removal structures a landscape-scale filter that contains a reactive substrate, such as steel slag, water treatment residuals, or manufactured sorption materials that can effectively treat runoff water and reduce dissolved phosphorus loading into surface waters. The structures can be designed for many different scenarios including surface runoff filters, subsurface drainage tile filters, and ditch drainage filters.

Aerial photo of excavation in progress
The area is excavated, exposing the drainage tile that will be connected into the phosphorus removal bed.

How does it help:

Phosphorus removal beds are ideally placed in situations and sites with a known excess dissolved phosphorus issue. These sites can include fields with extremely high soil test concentrations. Fields with high concentrations of phosphorus have been shown to continually release dissolved phosphorus even after other measures such as nutrient management and phosphorus drawdown have been implemented.

According to Chad Penn, USDA-Agricultural Research Service, all phosphorus removal structures have the same four basic components:

  1. Contains a sufficient mass of an unconsolidated PSM (P sorption material). The material must have a strong capacity to adsorb P. PSMs are usually industrial by-products or manufactured. However, there are some PSMs that occur naturally.
  2. The PSM is contained and placed in a hydrologically active area that receives and/or exhibits high dissolved P concentrations.
  3. High dissolved P water is able to flow through the contained PSM at a suitable flow rate.
  4. The PSM is able to be removed and replaced after it is no longer effective at removing P or able to remove P at the minimum desired rate.
Aerial photo of multiple PVC pipes in soil
PVC pipe is installed at the bottom of the bed and acts as the drain for the treated water.

Planning ahead:

This conservation practice is relatively new. An effort is being made to train conservation staff on how to best utilize this practice. Phosphorus removal structures operate most efficiently under certain conditions and should be specifically designed for each location and PSM. Ideally, sites should be located where there are high dissolved phosphorus concentrations in water flow that can be directed into the structure. The size of the structure is highly variable based on parameters such as watershed, PSM used, target phosphorus removal and lifetime, etc.

Steel slag installed on top of PVC pipe
The treated steel slag in installed on top of the PVC pipe.

Management:

Management of the structure is dependent on which type of structure is installed. If there is high sediment concentrations entering the structure, additional practices should be utilized to reduce this. PSMs must be replaced after they have reached their effective lifetime. Efforts are being made to use materials that can be regenerated which will help reduce costs.

Aerial photo of PVC pipe in soil
An additional layer of PVC pipe in installed to evenly distribute the tile drainage water throughout the bed, over top of the treated steel slag. Filter fabric is installed on top of the pipe followed by the soil backfill to existing ground.

Additional Resources: