Phosphorus Removal Structure (NRCS 782)

Effectiveness

Properly designed P Removal Structures can result in reductions of 16-71% in DP concentration and loading. These results were from a summary of 40 different field scale trials using various designs and PSM materials. (Penn, et al, 2017) Water 2017, 9, 583; doi:10.3390/w9080583

Considerations

Characteristics of the ideal site for construction of a P removal Structure include:

• Flow convergence to a point where water can be directed into a structure, or the ability to manipulate the landscape to concentrate water flows.
• Dissolved P (DP) in water of at least 0.2 mg/L.
• Hydraulic head required to “push” water through structure which is a function of elevation change or drainage ditch depth.
• Sufficient space to accommodate PSM chosen.

P removal structures require careful design to reach peak absorption efficiencies and maintain subsurface tile drainage. Design inputs needed to determine size and function for a PRS fall into three categories.

1. Site hydrology and water quality characteristics
2. Target removal and Lifetime needs
3. PSM characteristics.

Site characteristics will determine the amount of water that flows to potential installation locations. Measurements of the DP concentration over several different flow conditions combined with estimates of annual flow volume can be used to estimate the load (mass) of DP that is delivered. Sizing of a P removal structure is a function of the annual P load, the chosen P removal amount and lifetime, and the characteristics of the PSM to be used.  The most important PSM characteristic is its ability to remove P, as quantified by a “P removal curve”. The P removal curve is simply a mathematical description of P removal under flowing conditions for a given P inflow concentration and retention time (RT), expressed as a function of P loading (i.e., P added per unit mass of PSM).  Physical characteristics of the PSM, especially porosity and saturated hydraulic conductivity (i.e. its ability to conduct water) are especially important when it comes to designing PRS to achieve the desired P removal at  the chosen flow rate and RT.

An important factor in design is PSM selection. Generally, any product with a high affinity for P, suitable physical characteristics, and is safe for use in waterways can work as a PSM. Types of PSMs available include drinking water treatment residuals, fly ash, mine drainage residuals (Fe oxides), steel slag, metal filings, and manufactured PSM. The most cost effective PSM known at this point, is metal filings/turnings mixed (5-8%) with clean pea-gravel.  A graphic of PSM’s is found in Figure 2.

Manufactured PSMs tend to absorb P more efficiently but are higher cost products. While manufactured PSMs are higher cost, less material mass is needed to perform the desired P capture, lowering the amount of space needed for the PRS. For example, a subsurface tile drain filter designed to remove 35% of a 5-year Dissolved P load using treated steel slag would require 40 tons while a manufactured Fe-rich PSM would only require 2-5 tons.

When treating subsurface tile drainage water do not use untreated electric arc furnace slag or blast furnace slag. The bicarbonate contained in the tile water will cause premature failure with these PSM options.  Aluminum-treated slag can be used for tile drains if properly sieved to removed fines.  Regular non-treated sieved slag works well for treating surface water, especially when used as the gravel in blind inlets.

A variety of materials have been evaluated and are included in the P-Trap software database. New materials are always being evaluated at USDA Agricultural Research Service, National Soil Erosion Research Laboratory in West Lafayette, IN.  Contact the USDA Erosion Laboratory if your PSM of interest is not found in the P-Trap database.

Figure 2. Type of PSM Materials. (Source: Chad Penn)

The PSMs become saturated over time, meaning its ability to absorb P decreases with P loading. Systems can be designed to last 2 to 15 years depending on desired cost, site location constraints as well as the amount of P reduction needed. Although P-saturated PSM loses absorption efficiency, it remains a net P sink because the P is usually tied up tightly. Although the structure may not be able to absorb more P flowing through it, it will not release the P it has already absorbed. The PSM, depending on what type it is, can be treated and reused, recycled, or disposed of properly. Slag, for example, can be re-used again as road construction material.

Phosphorus removal structures should always be constructed with an emergency over-flow in cases of flow rates exceeding the capacity of the structure.  While this is simple to do for a surface PRS, and subsurface tile drain PRS should be utilized in conjunction with a water table control structure that diverts tile water into the PRS but allows overflow to drain in situations of high flow rates or clogged PSM.

Another consideration is access to the filter for maintenance. If there is a long-term site need to reduce P beyond the initial structure’s design life, considerations to location and design of the structure to make renewing or recharge the PSM should be made.

Cost

The cost of PRS range from $3 to 20K, depending on the size, site, PSM, and P removal goals.  In general, the cost of P removal on a per unit mass basis is in the same price range as a PRS used wastewater treatment setting though DP removal is much more difficult in an agricultural non-point setting.