Designing Structures to Remove Phosphorus From Drainage Waters.

Excess phosphorus (P) in surface waters contributes to eutrophication.  An appreciable source of P to surface waters is P transported from agriculture, residential, and horticultural land.  Soils that have continuously received excess P beyond plant needs typically become “built up” to high levels of soil P.  These soils release dissolved P during rainfall/runoff events.  Current best management practices (BMPs) mostly address particulate P (i.e. P bound to soil particles) transport, not dissolved P.  Dissolved P is more damaging than particulate P because it is immediately 100% available to aquatic life.  Even if all P applications to high P soils are ceased and BMPs are implemented to reduce erosion (i.e. particulate P transport), dissolved P transport will continue to occur for at least 15 years, assuming that plants are harvested from the site.  In response to this problem, several groups have developed P removal structures, which are units filled with P sorbing materials and designed to channel runoff water through them while retaining the filter material and P.  The goal is to prevent P from entering a surface water body and allow filtered P to be removed from the watershed after the P-saturated material is removed.  The P sorbing materials utilized are typically by-products from various industries and include steel slag, FGD gypsum, drinking water treatment residuals, and acid mine drainage residuals.  A modeling tool has been developed for (1) sizing a structure based on filter media properties and watershed characteristics, (2) predicting the lifetime of a P removal structure, and (3) estimating total P removal.  In addition to the modeling tool, data from full scale filters will be presented.