Sizing the P removal structure part two. I know how much material I need: how should it be oriented to capture and treat the runoff?

The last post dealt with determining the required mass of your PSM for your site in order to remove a targeted amount of P in a given amount of time.  That mass of PSM could be oriented many different ways.  Two extremes are “wide and shallow” or “narrow and deep”!  

This is the part of the design that is really flexible and somewhat unique to the site.  Every site is different.  Different people will have different ideas for how to place the PSMs in the landscape filter.  However, the bottom line is that the water must flow through the material in an amount of time (i.e. retention time) that is sufficiently short enough to treat most of the water.  Any method that you want to do this is sufficient, as long as there is enough retention time for appreciable P sorption.  As discussed on the previous post, only Ca-dominated materials that are not buffered to a high pH seem to require a long retention time.  Gry Lyngsie at University of Copenhagen has some excellent data on that topic, using isothermal titration calorimetry. 

In other words, there is more than one way to skin a cat and I know that a lot of people are going to have some really great ideas for the actual physical layout on their particular sites.  Many of our European friends have demonstrated some really good ideas for moving the water through the PSMs.  Some folks like to design the water flow from the bottom of the sorption bed upward, laterally, or from the top downward.  There might be reasons at a particular site to choose one flow direction over another.  One reason to design flow movement from the bottom to the top is that you have the advantage of gravity in limiting the loss of the solid PSM material as they tend to stay settled in that situation.  A disadvantage to this type of design is that it often results in the PSM being saturated with water all the time, which could be a problem with Fe-dominated PSMs that can dissolve under reducing conditions.  An advantage to flow design from the top downward is that it is free-draining and avoids saturation with water during non-flow events. 

Regardless of the water flow direction through the material, flow rate is not faster for the top-down, bottom-up, or lateral direction.  For each of those situations, the flow rate is dependent on the hydraulic head, thickness of the PSM layer, and the hydraulic conductivity of the PSM.  In any of those situations, the standard Darcy Equation can be used to design the structure after you have determined the required mass of PSM, peak design flow rate, and site limitations such as area and slope (i.e. hydraulic head). 

Most of the structures that I build are designed to move water through the PSM from the top-down into buried subsurface drainage pipes, as shown in the diagram above.  This allows the structure to drain freely after the flow event has ceased.  Other folks do it differently.  Dr. McGrath also builds most of his structures to drain from the top down with subsurface drainage pipes.  One exception is his metal “boxes” that consist of a box made of expanded metal with a perforated drain in the center; the PSM is placed in box between the walls and the perforated pipe that drains it.  Such a condition would require some type of a radial flow equation to predict flow rate.    

How to design an appropriate layout that is suitable for the site limitations and peak flow rate

 Usually, the most limiting factor in structure design is the hydraulic conductivity of the PSM.  The dichotomy is that PSMs which have the best P sorption ability tend to have poor hydraulic conductivity, and PSMs with large hydraulic conductivity have low P sorption ability: 

Using a material with a low hydraulic conductivity translates to designing a structure that has a much larger area, since the thickness of the sorption bed must be lower in order to achieve a reasonable flow rate.  This is not a problem if your site is not limited in available area. 

Determining the layout of the structure for a particular PSM is a function of the following parameters:

1. Required mass of PSM
2. Hydraulic conductivity of the PSM
3. Porosity of PSM
4. Bulk density of PSM
5. Target peak flow rate for the structure
6. Maximum area for structure at site
7. Maximum hydraulic head at site

    Consider the site in which this blog is dedicated to (OK poultry farm): we have several PSMs available to us as described in the previous post in which a given mass will remove a certain amount of P.  However, some of the materials have a low hydraulic conductivity.  The table below shows the potential layout for several PSMs that we had to choose from.  Note that this table was developed specifically for our site characteristics, and the ability to handle the flow rate expected for a 2 yr-24hr storm event (16 cubic ft/s).

From the table above, it becomes apparent that the PSMs with lower conductivity tend to have a greater P sorption ability, as those materials (WTR, AMDR, and fly ash) all require relatively small amounts of PSM.  However, those materials also tend to require large areas.  On the other hand, Use of the sieved steel slag also requires a large amount of area, not because of limited hydraulic conductivity, but because of the physical constraint of housing a large mass of material.   For our site, we chose to use the treated slag since it was a “happy medium” between the low hydraulic conductivity-high P sorption materials such as WTR, AMDR, and fly ash, and the high hydraulic conductivity-low P sorption materials such as the sieved steel slag.

The suitable layout for the different PSMs shown in the above table was estimated using the software that we developed for designing P removal structures.  Please contact us if you are interested in obtaining a license.