Site preparation for construction of the P removal structure

As discussed in the post on site assessment, water must be channeled into a single location for a P removal structure inflow point.  That means that if you are working with a drainage ditch or a single subsurface drainage pipe, then it is pretty easy.  The flow in that case is already concentrated for you. 

For our site however, the flow was only somewhat concentrated along the gravel road in front of the poultry houses and on the East-West gravel road.  The flow from the field also combined with the gravel road into a natural drainage ravine.  We simply had to build some earthen berms to concentrate the flow into the P removal structure.  The figures below show the approximate location of the berms (in orange) and the P removal structure (in blue) for both an aerial view and a contour map.  The eastern berm was very short since it butted up against a steep hill, which acted as berm for channeling water.  

After the berm was constructed, we seeded with tall fescue and covered with an erosion-control mat:

Since then, the grass has been well established.

Also at this time, we cut the foundation for where the structure would be located.  As discussed on the previous post, we chose to use ¼” sized slag that was treated, which means that our structure required 40 tons of material, and would be approximately 33 ft long by 13 ft wide with a 20 inch depth of material.  This depth of material was chosen based on the slope of the site (see the contour map).  We simply cut into the ground 20 inches deep on the upslope side, and made a flat surface for 33 ft.  At 33 ft, the total change in elevation will be about 20 inches.  The cutaway view below shows the original ground surface relative to the “box shape” cut into the ground:

A word on drainage ditches

As discussed in the previous two posts, flow rate through the material is critical as it controls how much water will be able to be treated during a large runoff event.  Flow rate is controlled by the depth of the PSM, hydraulic conductivity of the PSM, and hydraulic head.  Hydraulic head is a function of the slope of the site.  Some sites have extremely low slope, and therefore, hydraulic head will also be naturally low.  This is the case on the Delmarva Peninsula where drainage ditches carry much of the surface water away from agricultural fields. 

Dr. McGrath has got around this problem by incorporating flow control structures to increase hydraulic head for the P removal structure, thereby increasing flow rate through the PSM and maintaining a more buffered and constant flow rate.  Our friends at Agri-Drain supply ideal flow control structures for achieving this task.  Essentially, the flow control structure acts as a miniature adjustable dam in the ditch.  The picture below show some of the P removal structures constructed by Dr. McGrath in drainage ditches and also some of the Agri-Drain flow control structures.

However, one must be cautious in the use of flow control structures to increase hydraulic head for the P removal structure because it can decrease the drainage upstream by backing up excessive water.  

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.     

Sizing the P removal structure: what is the required mass of P sorbing material (PSM) necessary for the job?

With the following information, we can now take the next step and estimate how much PSM is necessary for the job:

1. Annual P load at the site
2. Typical dissolved P concentration in runoff (or drainage) water to be treated
3. P removal goal i.e. the % of the annual P load that is desired to be removed
4. Characteristics of the locally available PSM

Annual P load and typical dissolved P concentrations

The annual P load was already determined as described in the previous post.  Turned out that at our site, we expect an annual P load of 22 kg.  This was calculated partly by knowing that our typical high-end dissolved P concentration is 2 mg/L.

P removal goal

Based on the large P load at this site, I am setting a range for removing the annual P load from 20 to 50%.
 

Characteristics of the locally available PSM

This is the part that requires a bit of work.  In order to properly design a P removal structure, one must develop a “design curve” for the PSM that they will be using in their structure.  A design curve is simply a quantitative description of the relationship between dissolved P loading of the PSM and the % discrete P removal.  This must be determined in a flow-through setting; a batch P sorption experiment will not suffice!  See Penn and McGrath (2011) and Stoner et al. (2012) for examples of this comparison.  We and other researchers have determined that in the context of a P removal structure, any assessment of P removal must be in a flow-through setting.  A batch sorption experiment in this context is only useful as an index to compare different PSMs, not to quantify how much P it will remove from a flowing solution.  There are several reasons why the batch sorption experiments fails in application to a flowing context such as a P removal structure:

1. Batch tests typically has a non-representative long contact time
2. Batch tests do not have constant removal of reaction products
3. Batch tests do not have constant addition of reactants (i.e. P) 

A design curve is specific with regard to the retention time (i.e. contact time) and the inflow P concentration that is moving through the PSM.  Below is an example of a design curve for an acid mine drainage residual (AMDR) that we collected from Eastern OK:

Note that this design curve is specific to an inflow P concentration of 1 mg P/L and a retention time of 30 seconds.  In other words, it takes 30 seconds for the inflowing solution to pass through the AMDR.  As you would expect, the P sorption is initially very high, but with further P loading the AMDR is not able to sorb as much P as it did previously.  The shape of this curve varies between PSMs, retention times, and inflow P concentrations.  This is discussed in detail in Stoner et al. (2012).

Knowledge of the equation of a design curve is necessary for choosing the necessary amount (mass) of PSM for your structure.  This curve can serve several purposes, depending on what you desire:

1. Estimate the lifetime of the structure if a given mass of a specific PSM is to be placed in the structure.  In this case, “lifetime” is defined as the amount of time until the P removal structure is no longer able to sorb P that flows into it.
2. Upon integration of the design curve, estimate how much P will be removed by the structure during that lifetime. 
3. Upon integration of the design curve, estimate how much of the PSM (i.e. mass) will be necessary to remove a desired amount of P under the condition of the design curve.     

For our purposes at this particular site, our use of the design curve will be for purpose number 3.  For example, at our site we have several PSMs locally available to us: steel slag from Fort Smith, drinking water treatment residuals (WTRs) from Tulsa, fly-ash from Muskogee, and an AMDR.  All of these are potential options for us.  Other PSMs may be available in different regions.  Some areas may have a greater abundance of steel slag or WTRs.  Perhaps you live in Western PA (God’s country) where AMDRs are highly available.  There may be some PSM that is unique to your region.

Considering our conditions previously determined: annual P load of 22 kg, typical inflow P concentration of 2 mg/L, and appropriate design curves for each material, it would require:

• 5.5 tons of the Tulsa WTR to remove 37% of the annual P load
• 3.1 tons of our AMDR to remove 49% of the annual P load
• 131 tons of sieved steel slag (1/4” and greater diameter) to remove 21% of the annual P load
• 41 tons of a treated sieved steel slag (1/4” and greater diameter) to remove 45% of the annual P load

Model for predicting the design curve instead of directly measuring it

We developed a model for predicting the equation of a design curve for a specific PSM, as a function of the retention time, inflow P concentration, and selected PSM characteristics (chemical and physical).  This model was developed for the following reasons:

1. Conducting a flow-through experiment for every single individual PSM sample and every possible flow condition (i.e. inflow P concentration and retention time) is extremely time consuming and expensive.
2. There is great variation in P sorption behavior between different PSMs.
3. There is great variation in P sorption behavior the same type of PSMs that come from different sources.
4. There can be variation in P sorption behavior among the same type of PSMs that come from the same source, but are produced at different times.   
5. It is much easier and less expensive to measure certain chemical and physical characteristics of PSMs to use in predicting a design curve, than it is to conduct many flow-through P sorption experiments. 

Through the tireless work of a former graduate student, Dustin Stoner (a living legend as far as I am concerned), we have developed a model to predict the equation for a design curve for a specific material under specific flow conditions.  One thing that we learned from conducting > 1000 flow through P sorption experiments, is that under the relatively short retention times that we are working with in the world of runoff and subsurface drainage P removal structures (seconds up to 20 minutes), retention time usually has little impact on P removal (see Stoner et al., 2012).  This is true for materials that dominantly remove P via fast kinetics by Al and Fe sorption (ligand exchange) and for Ca-rich materials that have a relatively high pH AND are buffered to a high pH.  For example, flue gas gypsum is an example of a Ca-rich material that is NOT highly buffered with regard to pH, and therefore the retention time does have a dramatic impact on P removal in a flow-through setting.  Gypsum is one of the few materials that I have observed this for.  For most PSMs, retention time does not have much impact at our scale of retention time that we are aiming for.  However, for materials such as gypsum, increased retention time will greatly increase P removal (see McGrath et al., 2012).  

While the details of this model for predicting the design curve will not be discussed here, the model is at the heart of the current software that we are developing, which essentially helps one to design a site-specific P removal structure in the same manner in which this blog describes.  If you are interested in a license to that software, please contact me.