BACKGROUND
Transport of phosphorus (P) to surface waters is one of the main causes of eutrophication. A significant source of this P is non-point transport from agriculture, horticulture, urban and suburban land through surface runoff. This occurs in the form of particulate P and dissolved P. Particulate P (i.e. P adsorbed onto soil particles) losses occur as soil is lost through erosion processes. This form of P loss is controlled with several best management practices (BMPs) that prevent erosion such as buffer strips or riparian zones. However, these BMPs have little or no effect on preventing dissolved P losses. Consider that dissolved P is more "potent" in regard to surface water eutrophication compared to particulate P losses because dissolved P is 100% bio-available upon deposition to the water body. Dissolved P losses occur from soils that have high levels of soil test P due to many years of excessive fertilization. Even when all P fertilizer applications to such soils cease, they will continue to release dissolved P to runoff for many years. Materials or chemicals with a strong affinity for P can potentially be applied directly to soils with high soil test P levels; this will immediately reduce concentrations of dissolved P in runoff from such soils. However, such reductions are only temporary because P is not truly removed from the system; the P is only temporarily tied up in an insoluble form. With time, the P will begin to dissolve again.
SUMMARY OF THE TECHNOLOGY
The P removal structures removes dissolved P in passing runoff water by utilizing various P sorption materials (PSMs), allowing for the trapped P to be removed from the watershed. An example of potential PSMs includes steel slag, drinking water treatment residuals, and flu-gas desulfurization gypsum. Many PSMs are inexpensive by-products of various industries. The model developed allows one to design a P removal structure for a targeted P load based on a user's chosen PSM. The user simply provides some basic parameters of their chosen PSM, typical runoff dissolved P concentrations and flow rates for the site location, and the desired "lifetime" of the structure. The lifetime of the structure is the time until the materials are "spent" and no longer effective at removing P. At that point the materials must be removed and replaced. On the other hand, the model can also be used to predict how long a structure will last for a given "cookie cutter" structure with specified PSMs.
POTENTIAL AREAS OF APPLICATION
- Runoff from barnyards with excessive P concentrations. These concentrations are very common in concentrated animal agriculture such as poultry, dairy, and swine industries.
- Runoff from agricultural fields with excessive P concentrations
- Golf course runoff
- Urban and suburban storm water drainage
- Agricultural drainage ditches from areas with excessive soil P concentrations
MAIN ADVANTAGES OF TECHNOLOGY
- Instantly removes dissolved P from runoff. Other best management practices only remove particulate P, not dissolved P.
- Allows for P saturated spent materials to be removed, thereby allowing for a true "removal" of P from a watershed. Simple broadcast application of PSMs to soils only temporarily ties up P (i.e. temporarily reduces P solubility).
- Allows one to easily predict how much P will be removed by a given structure and also measure how much is actually removed by a structure. This is very valuable in regard to total maximum daily load (TMDL) regulations for non-point source runoff since no other best management practice can be quantified as easily.
- Predicts how long a given structure will last.
STATE OF DEVELOPMENT
The model for designing P removal structures will be developed into user-friendly software. The model is being developed from several years of laboratory characterization of many different PSMs collected from around the country. After individual models for each material is developed, all data will be combined to produce one large, overall model for any material. Over half of the materials characterization has been completed, and the remainder will be completed by late spring 2011. A P removal structure has been constructed on Stillwater Country Club golf course located in Stillwater, OK. This structure utilizes steel slag as the PSM. The structure receives drainage from a 152 acre watershed which includes residential areas, golf course, and undeveloped land. During the summer, the structure receives runoff from daily golf course irrigation. Thus far, the structure has removed 37% of all runoff flowing into it and has handled several large storms (> 2 inches) and flow rates (> 100 gpm) even though the structure is relatively small for the watershed size (only 3 tons of slag). The model for the steel slag used for design/prediction of this structure is complete and has been accurately predicting the performance of the structure. The model predicts that this structure will last for 15 months and remove an overall of 30% of the dissolved P load.