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.  

Site data collection required for structure design

In addition to the estimates of dissolved P concentrations in runoff at the site location, further information was required:

1.   Peak runoff flow rate
2. Average annual flow volumes and dissolved P load
3. Hydraulic head

The average annual flow volume and peak flow rate was calculated by using site information required for estimating the NRCS curve number (ftp://ftp.wcc.nrcs.usda.gov/wntsc/H&H/training/runoff-curve-numbers1.pdf).  This required watershed information regarding soil type (used to determine hydrologic soil group), ground cover, greatest length of flow, and slope.  Each of these parameters, except for soil type and flow length (determined by soil survey and contour maps) were determined via site visit.  The wonderful NRCS folks at the local Stilwell office (Chad Kacir, Andy Inman, and others) were responsible for conducting the site survey for the curve number method and also calculations for peak flow rate and average annual flow volumes.  

Peak runoff flow rate estimate

The curve number is used in conjunction with the precipitation depth for the design storm.  In our case, we wanted to design our structure for a 2yr-24hr storm, which produces about 4 inches of rainfall as estimated by standard USDA rainfall tables (http://www.nws.noaa.gov/oh/hdsc/PF_documents/TechnicalPaper_No40.pdf).

For our site, the curve number (CN) was 78; the cover was mostly pasture.The curve number method resulted in a runoff depth of 1.97 inches for our 9 acre watershed, predicted for a 2yr-24hr storm.  Next, the runoff depth is used to calculate the peak flow rate through use of the Soil-Cover-Complex method and “time of concentration”.  The time of concentration is calculated using the curve number and the previously determined greatest length of flow via curve number method.  For our site, the greatest flow length to our proposed structure location was 1050 ft, which resulted in an estimated time of concentration of 24 minutes.  Based on this time of concentration, the estimated peak discharge was 0.9 cfs/acre-inch runoff.

For our scenario:

0.9 cfs/acre-inch runoff  * 9 acres * 1.97 inches runoff = about 16 cfs peak runoff rate.

Therefore, our goal was to design a structure that could handle a flow rate of at least 16 cfs so that it would be able to treat all of the runoff produced from a 2 yr-24hr storm.

Average annual runoff volume and P load estimate

Next, the annual flow volume is necessary in order to determine what the annual load of dissolved P is.  To achieve this, the runoff coefficient method was used.  An example is shown here: http://watershedmg.org/sites/default/files/docs/wmg_calculating_runoff_worksheet.pdf ) This calculation is simply based on cover for estimating the runoff coefficient, watershed area, and average annual rainfall depth.  For our site, the annual rainfall depth was 44 inches.  Based on our site, the average annual runoff volume is 12 inches/yr, or 9 acre-ft. 

Our grab samples indicated dissolved P concentrations between 1 and 2 mg/L.  Using the higher value of 2 mg P/L for over-design, this resulted in an estimated load of 22 kg of P/yr transported in runoff from the watershed. 

Therefore, our estimates for the required mass of PSM for this site will be partly based on the predicted value of 22 kg dissolved P lost in runoff per year. 

Hydraulic head

Hydraulic head is critical for achieving the desired flow rate through the P removal structure.  As discussed in the previous post, the runoff water must flow through the structure in order for P removal to occur.  Simply put, the hydraulic head is the force that “pushes” the water through the structure. The hydraulic head can be estimated by conducting an elevation survey of the proposed structure location. 

Based on the elevation survey, we will have an appreciable amount of hydraulic head to achieve the desired flow rates (i.e. 16 cfs for a 2yr-24hr storm).

New Site Location and Assessment

There are three requirements for site location of a P removal structure:

1. Elevated dissolved P concentrations in runoff.  I have found that it is generally not worthwhile unless the dissolved P concentrations are > 0.25 ppm.
2.  Hydraulic connectivity.  In other words, the runoff produced at the site must have the potential to reach a surface water body; otherwise there is not really a problem.
3.  Potential to channel the runoff water into a single point for treatment.  Sometimes this is already completed for you if there is a drainage ditch, culvert, subsurface drainage outlet, etc.  Otherwise, you have to physically alter the flow so that it will converge into a single point.

The P removal structure will be constructed on a poultry farm located in Eastern OK.  The producer volunteered the site; he is an excellent cooperator and a progressive steward.  At the location, there are several poultry houses in a small 9-acre sub-watershed.  Poultry litter spillage often occurs near the entrance to the house and as a result of the connective hydrology at the site, P can potentially be transported to a nearby creek, which is located within the Illinois River Watershed.  Based on a site survey and visual observations during rainfall-runoff events, the following potential location within the site was chosen for the P removal structure:

The proposed site is located on the side of hill.  Starting in September, 2012, “grab” samples of runoff were taken during runoff events for analysis of dissolved P.  This is especially critical for the obvious reason that construction of a P removal structure is not justified if there is not sufficient runoff dissolved P!  This is why we initially refer to this site as a “potential” location because we did not know if it was a good candidate until we started taking some samples. It turned out that the dissolved P tested in the runoff samples ranged from 1 to 2 ppm.  Due to the elevated P concentrations, the site hydraulic connectivity, and the potential to channel water to a single point for treatment, this site becomes a perfect location for constructing a P removal structure!

Other P removal structures:

Located on the Stillwater Country Club Golf Course in Stillwater, OK

  

 ------------------------------------------------------------------------------------------------------------

This structure is located on a poultry farm in Maryland.  A retention pond captures runoff from around poultry houses.  The pond drains through the P removal structure before flowing out into drainage ditches.

------------------------------------------------------------------------------------------------------------

  

 Below is a picture of a structure while under construction in Maryland.  The structure will remove P from a drainage ditch.

Contributors

The construction of the P removal structure featured in this blog is brought to you through funding by the 

NRCS-CIG program: http://www.nrcs.usda.gov/wps/portal/nrcs/main/national/programs/financial/cig/

This project would not be possible without them or the support from the Illinois River Watershed 

Partnership: http://www.irwp.org/

Previous funding that has resulted in development of this technology includes:

United States Golf Association: http://www.usga.org/default.aspx

Maryland Department of Agriculture: http://www.mda.state.md.us/

Chesapeake Bay Trust: http://www.cbtrust.org

Delaware Department of Natural Resources: http://www.dnrec.delaware.gov/Pages/Portal.aspx

The principal investigators on this project include:

Dr. Chad Penn, Oklahoma State University

Dr. Josh McGrath, University of Maryland

Dr. Josh Payne, Oklahoma State University

Dr. Jeff Vitale, Oklahoma State University

Dr. Garey Fox, Oklahoma State University

  

Introduction

The purpose of this blog is to provide a case-study example of designing and constructing a phosphorus (P) removal structure, step by step.  A P removal structure is intended to filter dissolved P (DP) from runoff using industrial by-products, before the runoff reaches a surface water body.  Through this best management practice (BMP), the trapped P is retained in the structure, thus allowing P to be removed from the watershed at clean-out.  The justification for construction of a P removal structure is 3-fold:

General information about P removal structures can be found at the following websites: 

Slide show and video: 

http://www.extension.org/pages/67669/designing-structures-to-remove-phosphorus-from-drainage-waters

Extension publication: http://usgatero.msu.edu/v11/n02.pdf

SUNUP Video: http://www.youtube.com/watch?v=KAIXmFEQY1k

OK Gardening Video: http://www.oklahomagardening.okstate.edu/category/es/2013/112313-scrubbing

More detailed information can be found in the following publications:

Penn, C.J., J.M. McGrath, E. Rounds, G. Fox, and D. Heeren.  2012.  Trapping phosphorus in runoff with a phosphorus removal structure.  J. Environ. Qual. 41:672-679. 

Penn, C.J., R.B. Bryant, M.A. Callahan, and J.M. McGrath.  2011. Use of industrial byproducts to sorb and retain phosphorus.  Commun. Soil. Sci. Plant Anal.  42:633-644.

Penn, C.J., R.B. Bryant, P.A. Kleinman, and A. Allen.  2007.  Removing dissolved phosphorus from drainage ditch water with phosphorus sorbing materials.  J. Soil Water Cons.  62:269-276.

Penn, C.J., J.M. McGrath, and R.B. Bryant.  2010.  Ditch drainage management for water quality improvement. In “Agricultural drainage ditches: mitigation wetlands for the 21rst century”.  Ed. M.T. Moore.  151-173. 

Similar work by other researchers:

Active wetlands - the use of chemical amendments to intercept phosphate runoffs in agricultural catchments. http://wwf.fi/mediabank/4368.pdf

Klimeski, A., Chardon, W.J., Turtola, E. and Uusitalo, R. 2012. Potential and limitations of phosphate 

retention media in water protection: A process-based review of laboratory and field-scale tests. 

Agricultural and Food Science 21: 206–223.

Vohla, C.; Koiv, M.; Bavor, H. J.; Chazarenc, F.; Mander, Ü. Filter materials for phosphorus removal from wastewater in treatment wetlands-A review Ecol. Eng. 2010, 10.1016/j.ecoleng.2009.08.003

SupremeTech. http://www.supremetech.dk/SUPREMETECH.htm

Mining waste byproduct capable of helping clean water: http://www.usgs.gov/newsroom/article.asp?ID=3482&from=rss

Wastewater treatment with by-products: http://publications.polymtl.ca/860/

Removal of nutrients from tile drainage in The Netherlands: Dr. Stefan Jansen