Scrubbing Phosphorous from Runoff Water February 27, 2015

Steel slag is a filter material for removing dissolved phosphorus from runoff and drainage water. Researchers at Oklahoma State University tested the hydraulic conductivity of sieved steel slag samples. Size fractions included >6.35, >3.36, >2.38, >0.5 mm, and non-sieved. The smaller sized slag (>0.5 mm) removed more phosphorus and had a longer lifetime than larger slag (>6.35 mm).


Phosphorus is a plant nutrient that can encourage eutrophication in surface waters. Many best management practices (BMPs) prevent the movement of phosphorus bound to sediment. Yet, these practices do little to reduce dissolved phosphorus levels in runoff or drainage water. High phosphorus levels in soil will need time for plant removal before they are no longer a source of dissolved phosphorus.

The USGA funded research at Oklahoma State University to design landscape filters. The filters contain material to prevent dissolved phosphorus from entering surface waters. The design of these filters allow for the replacement of the material after it is no longer effective.

Steel slag is a by-product left over when separating the desired metal from its raw ore. Steel slag has the ability to remove and bind phosphorus during runoff events. The use of slag in “interception ditches” removed dissolved phosphorus from runoff water. A few highlights from the research include:

  • The smaller sized slag (>0.5 mm) removed more phosphorus and had a longer lifetime than larger slag (>6.35 mm).
  • The smaller slag removed 33% of the phosphorus over 18 months. The larger slag size removed 25% of the phosphorus over 6 to 8 months.
  •   Used slag from a phosphorus removal structure was able to keep phosphorus when exposed to clean water.
  •   Trenches filled with slag removed dissolved phosphorus in runoff like expensive structures.
  •   Dissolved phosphorus removal by slag decreased with retention time. Consider the retention time when constructing a phosphorus removal trench or structure.

Road construction is currently the most common use for fresh steel slag. Steel slag from landscape filters also could be used for roads. The phosphorus will remain bound to slag from filters if not exposed to acidic conditions.

Additional Information:

Improving the Ability of Steel Slag to Filter Phosphorus from Runoff

Phosphorus Remediation

Scrubbing Phosphate from Runoff

Designing Structures to Remove Phosphorus from Drainage Waters     

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