Help Instruction File:

Vertical Flow Pond (VFP)

Module Overview

Provided by the Office of Surface Mining Reclamation and Enforcement (OSMRE), the Pennsylvania Department 

of Protection (PADEP), the U.S. Geological Survey's (USGS) and the West Virginia Department of Environmental 

Protection (WVDEP).
1 

VFP Module Overview

Table of Contents

1.0 OBJECTIVE 5

2.0 TREATMENT COMPONENT OVERVIEW 5

2.1 Physical and Hydraulic Description 5

2.2 Application and Treatment Chemistry 6

2.3 VFP Conceptual Treatment Model 6

2.3.1 Freeboard Layer 6

2.3.2 Water Layer 6

2.3.3 Compost Mix Layer 7

2.3.4 Limestone Layer 7

2.3.5 Pipe Layer 7

2.3.6 Liner Layer 7

3.0 VFP MODULE OVERVIEW 7

3.1 Layout and Workflow 7

3.2 Module Inputs 8

3.2.1 Water Quality and Flow Input 8

3.2.2 Sizing Methods 8

3.2.2.1 Retention Time 8

3.2.2.2 Bureau of Mines 9

3.2.2.3 Alkalinity Generation Rate 10

3.2.2.5 User-Specified Limestone Quantity (tons) 11

3.2.2.6 User-Specified Dimensions 11

3.2.2.7 Volumetric Calculations 11

3.2.2.8 Calculations for Retention Time, Bureau of Mines, Alkalinity Generation Rate, and User-Specified 

Limestone Sizing Methods 12

3.2.2.9 Calculations for User-Specified Dimensions Sizing Method 14

3.2.3 System Properties 16

3.2.4 Layer Materials 16

3.2.4.1 Freeboard Layer 16

3.2.4.2 Water Layer 16

3.2.4.3 Compost Mix Layer 16

3.2.4.5 Limestone Layer 16

3.2.4.6 Pipe Layer 17

3.2.4.7 Liner Layer 17

3.2.5 Other Items 17

3.2.5.1 Valves 18

3.2.5.2 Intake Structures: 18

3.2.5.3 Flow Distribution 18

3.2.5.1 Water Level Control Structures 18
2 

3.2.5.1 Outlet Protection Structures 19

3.3 Module Outputs 19

3.3.1 Sizing Summary 19

3.3.2 Capital Cost 19

3.3.3 Annual Cost 19

3.3.4 Net Present Value 19

3.3.4.1 Financial Variables 19

3.3.4.2 Cost Categories 20

3.3.4.3 Rationale for Recapitalization Recommendations 22

3.4 Assumptions of Design Sizing and Error Messages 23

4.0 REFERENCES 24

5.0 FIGURES 25

Figure 1: Typical Inverted Trapezoidal Prism 25

Figure 2: Typical Cross Section View of a Vertical Flow Pond (VFP) Showing the Common Layers and 

Components 26

Figure 3: Plan View of the Vertical Flow Pond (VFP) to Illustrate Common Piping Layout and 

Components (Used in AMDTreat Piping Calculator) and the Layer Surface Areas 27

Figure 4: Installation of a clay liner during the construction of a VFP. 28

Figure 5: Excavator carefully covering the underdrain piping network (used for discharge and flushing) 

with limestone in preparation to construct a 3.0 ft thick limestone layer. A 1.5 ft thick compost layer will 

follow to finish the VFP. The excavator is covering a “spur” pipe. The spur pipes are connected to the 

larger “trunk” pipe oriented parallel to the bed. This is the type of piping configuration modeled in 

AMDTreat. 29

Figure 6: Constructed VFP after installation of compost layer. Vertical pipes are connected to 

underdrain system and used to maintenance underdrain pipes if clogging occurs. Clean out pipes are no 

longer typically used. 30

Figure 7: Newly constructed vertical flow pond. Inlet water control structure is shown. The reddish tint to 

the water in the background is due to low pH iron oxidation occurring on the top of the compost layer. 

Raw water pH is 2.8. 31

Figure 8. Two 2.5-acre vertical flow ponds flowing into a polishing wetland. Note the instream 

precipitation of metals caused by the alkaline effluent mixing with the acidic receiving stream. 32

Figure 9. Photo showing the path of a F3 Tornado that pasted directly over a VFP treatment system. No 

damage to the VFP was caused by the Tornado. 33
3

Figure 10. Photo showing the top of the Limestone Layer in a VFP after the compost layer was removed 

to perform an autopsy to identify the cause of failure. The 6 inch thick compost was insufficient to provide 

the retention time required to fully reduce the large ferric iron load to ferrous iron. Thus, the ferric iron 

oxidized and precipitated within the porosity of the limestone layer causing hydraulic failure. After this 

event, VFPs were designed with 12 to 18” of compost. 34

Figure 11. Close up photo of the 1.5 ft cross section of the Limestone Layer after removal of a thin and 

ineffective Compost Layer. Note the 1⁄2” layer of iron hydroxide on top of the Limestone Layer that caused 

hydraulic failure. In addition, iron hydroxide precipitate filled the void space within the 1 to 2” limestone 

fragments. 35

Figure 12. Rhodamine dye test to identify area where short circuiting is occurring. Dye was detected in 

the VFP effluent after 30 mins and the spread of the dye shown in photo remained constant for 8 hours 

indicating the location of the short circuit. This area was excavated and carefully rebuilt to eliminate the 

short circuit caused by poor construction practices. 36

Figure 13. Pourbaix diagram of the iron-sulfur system to help explain how the different layers in a VFP 

control the iron/sulfur geochemistry. The top left red circle represents the Eh/pH condition of the raw 

water that is dominated by ferric iron. As the water flows through the compost layer, the Eh decreases 

along the vertical black line and the Eh/pH condition may stabilize in the pyrite stability field denoted by 

the red circle at the bottom left. As the Eh lowers, ferric iron to ferrous iron and sulfate is reduced to 

sulfide and iron sulfide (pyrite) can form in the compost. If limestone fines are added to the compost the 

vertical line may be more diagonal to represent an increase in pH. The water then enters the limestone 

layer and the residual ferrous iron remains soluble as limestone dissolves and increases the pH. The 

horizontal black line represents the evolution of the Eh/pH condition as water travels though the 

limestone layer. Lastly, water is discharged from the VFP through a pipe and splashes on the ground 

before flowing to a settling pond. The increase in dissolved Oxygen increases the Eh condition and causes 

the remaining ferrous iron to oxidize and precipitate as Fe(OH)3. 37

Figure 14. Whiteish aluminum hydroxide sludge accumulating within the Limestone layer due to the 

increase in pH. Note the flush pipe had little effect on preserving the porosity near the pipe. 38

Figure 15. Gentleman on right turning a flush valve to flush a VFP. Note the flushing of the whiteish 

aluminum hydroxide precipitate. 39

Figure 16. Zonal Piping system. The smaller diameter pipes on the left are located at the top of the 

Limestone layer. Note the ferric hydroxide precipitate accumulated at the base of the pipe. Ferric iron 

not reduced in the compost layer precipitates at the top of the Limestone Layer as the pH is increased to 

> 4.0 and ferric iron becomes insoluble. The pipes on the right are at the base of the Limestone layer

where the pH is increased to 6 and aluminum becomes high insoluble. Note the accumulation of the 

whitish aluminum hydroxide precipitate. 40

Figure 17. Flow distribution structure. A discharge is piped to the structure and the flow is distributed 

between three pipes that flow to three VFPs arranged in parallel. 41

Figure 18. Inside view of Flow distribution structure. The untreated discharge enters through the bottom 

pipe and is distributed between the top three pipes that feed three VFPs in parallel. 42
4 

Figure 19 – Open Top Intake Structure with Weir. Water level is controlled by weir and baseflow is 

forced to flow through trashrack and into the intake structure. 43

Figure 20 – Flow Splitter Box Prior to Installation. Pipe Gaskets are cast in place in precast yard. 44

Figure 21 – Flow Splitter Box Installed with Orifice Plate to regulate Flow and Bypass Pipe to prevent 

Hydraulic Overloading of Treatment Cell 45

Figure 22 – Photo of Water Control Structure (Agri Drain brand) being installed into an embankment. 

The water control structure is placed inside a corrugated pipe to protect against soil pressure. Without 

protection, the box could deform and prevent operators from being able to adjust stop logs to control 

water level. Stop logs are removed and shown to the right of the corrugated pipe. 46

Figure 23 – Looking down into an Agri Drain water control structure. Note the water flowing overtop of 

the stop logs, which set the water elevation in the VFP. Additional stop logs can be placed into the metal 

channels in the center of the box to increase the water elevation. 47

Figure 24 – Outlet protection structure designed to prevent channel erosion 48
5

1.0 Objective

A Vertical Flow Pond (VFP) is a passive treatment system (meaning that no machinery is required for its 

operation) that is typically used to treat acidic mine drainage of low to moderate flow with low to 

moderate acidity and dissolved metals, and low pH. 

The objectives of this overview are to: (1) Provide a basic understanding of the theory and application of 

VFP and (2) Provide an overview of the VFP Module to guide users in developing an estimate of the cost 

to construct, operate, and maintain VFPs for treating mine drainage. Therefore, this overview will be 

presented in two sections, Treatment Component Overview and VFP Module Overview. 

2.0 Treatment Component Overview

The following section is organized into three sections, (1) Physical and Hydraulic Description and (2) 

Application and Treatment Chemistry, and (3) VFP Conceptual Treatment Module. The first section

provides a physical description and hydraulic profile for a VFP to provide context for users of AMDTreat. 

The second section describes the common application of the VFP component, and the third section shows

a generalized conceptual model describing the treatment chemistry to help familiarize users with terms 

and concepts presented throughout the AMDTreat modules. 

2.1 Physical and Hydraulic Description

The geometric shape of a VFP is an inverted trapezoidal prism consisting of four distinct “layers” (Figure 

1). The bottom layer is filled with limestone ranging from 1.5 to 3.5 inches in size (AASHTO #1 or #3)

and is commonly 3.0 ft thick. Overlying the limestone layer is the compost mixture layer. The compost 

mixture layer is commonly between 0.5 and 1.5 ft thick and consists of mushroom compost or other 

composted organic matter. Some designers incorporate limestone fines, typically 10 to 25% by volume,

into the compost layer as additional treatment media. A 1.0 ft deep water layer is positioned above the 

compost mixture layer and serves to keep the compost saturated and anoxic. The topmost layer is the 

freeboard layer, which is used to accommodate changes in the water depth from either a large surge in 

flow or from a gradual decrease in the permeability of the compost and/or limestone layers due to the 

accumulation of metal precipitates on top of or within the layers. 

Mine water flows into the top of the VFP and then flows vertically downward through the water, compost 

and limestone layers. As shown in Figure 3, a piping network at the base of the limestone layer is 

designed to uniformly draw water from the limestone layer. The piping network converges to a single 

trunk pipe that extends through the embankment and either upwards via a pipe or through a water level 

control device that can regulate the water surface within the VFP and facilitate drainage of the VFP for 

maintenance. Treated water is then typically discharged to a rock-lined ditch that flows to a setting pond. 

Figures 2 and 3 are schematics showing typical material layers and piping network for VFPs. Figure 4

shows the installation of a clay liner during construction of a VFP, and Figure 5 shows a typical effluent 

and flushing piping network. Figure 6 shows the compost layer and Figures 7 and 8 are photos of

completed and operating VFPs. Figure 9 is an interesting aerial photograph taken in 1998 near Boynton, 

PA showing the aftermath of a F3 Tornado passing directly over two VFPs followed by a Wetland and 

Settling Pond. The VFP system was unscathed and continued to produce net alkaline water for more than 

15 years after without intervention. 
6

2.2 Application and Treatment Chemistry

A Vertical Flow Pond (VFP) is a type of “passive” treatment system used to treat net-acidic mine 

drainage. The term “passive” refers to the inability to exert operational control to adjust treatment to 

respond to changing influent or effluent conditions. Net-acidic mine drainage describes solutions that 

contain more acid-producing than acid-consuming species when the water is “treated”. Therefore, a netacidic solution is one where the acidity of the solution is greater than the alkalinity (net acidity = acidity –

alkalinity). VFPs are used to impart alkalinity to mine drainage and turn the net-acidic water to netalkaline (alkalinity > acidity) for the purpose of treatment. Once mine drainage is changed to a net 

alkaline condition, acid-producing reactions, such as the oxidation and precipitation of ferrous iron, can 

occur with the pH remaining above 6.0. 

Net-acidic coal mine drainage in the eastern U.S. is typically low pH (e.g., < 5) and may contain ferric 

and ferrous iron, aluminum, and manganese. Another, less common type of net-acidic mine drainage 

contains alkalinity, ferrous iron, and is characterized by pH ranging from 5.5 to 7.0. However, the acidity 

that will be released upon oxidation and precipitation of ferrous iron is greater than the alkalinity, making 

it net-acidic drainage. Thus, the pH will decrease to below 4.0 after precipitation. VFPs are best suited to 

treat low-pH net-acidic drainage, and anoxic limestone drains (ALD) are best suited for circumneutral pH 

net-acidic drainage. The solubility of aluminum is largely pH dependent, so the geochemical behavior of 

aluminum will be similar in Vertical Flow Ponds, Limestone Beds, and ALDs. As the mine water pH is 

increased to ~ 5.5, most of the aluminum will precipitate and cause hydraulic issues by clogging the 

limestone void spaces. However, Vertical Flow Ponds and Limestone Beds are much more suitable for 

treating aluminum-rich mine water since they have much larger cross-sectional area perpendicular to flow 

and are also capable of periodically flushing some of the precipitates as part of their normal operating 

conditions. Aluminum will still precipitate, but the larger surface area and flushing will allow the beds to 

function longer before hydraulic failure. In addition, Vertical Flow Ponds and Limestone Beds contain 

freeboard that allows the buildup of hydraulic head to continue to force the flow of water through the bed 

as aluminum precipitate accumulates in the limestone layer and impact the permeability of the bed. Since 

ALDs have no freeboard layer they lack the ability to increase hydraulic head without causing the 

rerouting of water around the ALD in surface mine discharges or increasing the mine pool elevation in 

underground mine discharges. Thus, ALDs are better suited for treating waters void of ferric iron, 

aluminum, and dissolved oxygen.

2.3 VFP Conceptual Treatment Model

The following discussion presents a conceptual treatment model for VFPs treating low pH mine drainage 

containing ferrous and ferric iron, aluminum, and manganese:

2.3.1 Freeboard Layer: The freeboard is designed to accommodate fluctuations in flow or 

increasing water elevation due to permeability or plugging of layer materials from metal hydroxides 

or sediment. It doesn’t perform a specific purpose pertaining to geochemical treatment of the mine 

drainage. 

2.3.2 Water Layer: The low pH drainage entering the water layer is prone to gas exchange reactions 

with the atmosphere (O2 in-gassing and CO2 degassing) and, under certain conditions, low-pH biotic 

iron oxidation may occur and increase the concentration of dissolved ferric iron. 
7

2.3.3 Compost Mix Layer: As the water flows vertically down into the compost mixture layer, the 

chemically reducing environment promotes the consumption of dissolved oxygen by organic matter 

to produce CO2. The decrease in redox potential promotes sulfate reduction and reduction of ferric to 

ferrous iron. The reduction of ferric to ferrous iron is important to keep iron soluble (dissolved) as 

pH is increased in the limestone layer. Otherwise, ferric iron is insoluble at pH > 4.0 and would 

cause the accumulation of ferric iron precipitate, and eventual clogging or short-circuiting of the 

limestone layer (Figures 10, 11, & 12). Sulfate reduction in the presence of ferrous iron will produce 

the formation of iron mono sulfides, thus some ferrous iron precipitation will occur in a highly 

functioning compost layer (Figure 13). Some designs opt to mix limestone fines into the compost to 

increase pH, which increases the kinetics of sulfate reduction. In addition, if the limestone fines 

increase the pH to ~ 5.5, aluminum precipitation will occur in the compost layer and help protect the

underlying limestone layer from becoming clogged. Periodic maintenance and replacement of the 

compost layer due to aluminum clogging is easier than removing the limestone layer. An additional 

benefit of the compost layer is the production of CO2 from the oxidation of organic matter. The 

increased carbon dioxide partial pressure (pCO2) production will drive limestone dissolution and 

increase alkalinity in the limestone layer. Typically, dissolved manganese remains unchanged in the 

water and compost mixture layers. 

2.3.4 Limestone Layer: The drainage exiting the compost mixture layer and entering the limestone 

layer should be anoxic and elevated in CO2. If the drainage entering the limestone layer contains 

aluminum, precipitation will occur as the pH is increased to above 5.5. Over time, the aluminum 

precipitation will decrease the permeability and cause hydraulic issues within the limestone layer

(Figure 14). If the drainage contains ferrous iron, some oxidation of ferrous iron may occur, but the 

amount should be limited due to the lack of electron acceptors (O2(aq)) and the slow kinetics of abiotic 

iron oxidation at pH ~ 6.5. The amount of alkalinity the limestone can produce depends on the 

amount of metal acidity released in the limestone layer and the pCO2 of the solution. Considering 

these limitations, there are instances of VFPs incapable of making a solution net alkaline. A properly 

functioning limestone layer will discharge a solution pH between 6.2 and 7.0. Ferrous iron will 

remain soluble and dissolved manganese will be unaffected. VFPs are not a suitable treatment 

solution for manganese. In theory, the net alkaline drainage containing ferrous iron and manganese is 

collected in the piping network at the bottom of the limestone layer and discharged to a ditch or 

settling pond.

2.3.5 Pipe Layer: The Pipe Layer is discussed in more detail in the Layer Materials section, as it 

doesn’t perform a specific purpose pertaining to geochemistry treatment of the mine drainage.

2.3.6 Liner Layer: The Liner Layer is discussed in more detail in the Layer Materials section, as it

doesn’t perform a specific purpose pertaining to geochemistry treatment of the mine drainage.

3.0 VFP Module Overview

3.1 Layout and Workflow

In general, inputs are on the left-hand side of the module and calculated outputs are on the right. The 

module inputs on the left-hand side are arranged into five sections: (1) Water Quality and Flow Input, (2) 

Sizing Methods, (3) System Properties, (4) Layer Materials, and (5) Other Items. The workflow for the 

module is for users to start at the top left-hand side. Enter the Design Flow and Net Acidity, then select a 
8 

method to determine the amount of limestone contained in the limestone layer of the VFP. Next, users 

specify design parameters such as the depth of the compost mixture layer, and unit costs for items 

affecting the capital cost to construct the VFP. Finally, users can enter any capital cost not captured by the 

module into the Other Items section. 

Module output is provided on the right-hand side of the module. Module outputs on the right-hand side 

are arranged into four sections: (1) Sizing Summary, (2) Capital Cost, (3) Annual Cost, and (4) Net 

Present Value. The sizing summary provides dimensional details for the system. The estimated cost to 

construct and operate the VFP is provided under the Capital Cost and Annual Cost headings. Lastly, users 

can opt to conduct a Net Present Value (NPV) to obtain the total cost to operate and maintain a treatment 

system for a defined time period. 

A general overview of the module input and output sections is presented below, however, users are 

directed to the numerous tool tips located in the module that provide additional detailed information, such 

as definitions of terminology. In most cases, the tool tips are accessed by clicking on the information icon 

( ) in each of the subheadings in the module. 

3.2 Module Inputs 

3.2.1 Water Quality and Flow Input: The Water Quality and Flow Input section is where users 

specify the design flow and water quality to size the passive treatment system component. The 

definitions for the required input can be found in the tool tip for this section. Click on the information 

icon ( ) on the right side of the Water Quality and Flow subheading. 

3.2.2 Sizing Methods: Users can select from five different methods to size the VFP: (i) Retention 

Time, (ii) Bureau of Mines, (iii) Alkalinity Generation Rate, (iv) User-specified Limestone Quantity, 

and (v) User-specified Dimensions. The module offers two different approaches to sizing VFPs. The 

first approach calculates the mass of limestone (tons) in the limestone layer before determining other 

dimensions. The Retention Time, Bureau of Mines, Alkalinity Generation Rate and User-Specified 

Limestone Quantity methods utilize this approach. The first approach is the most common as it relates 

the sizing of the treatment system based on flow and water quality characteristics. The second 

approach requires users to specify the length and width of the freeboard layer (user-specified 

Dimensions method) before calculating the remaining dimensions of the VFP. The second approach 

is often used when the land area available for treatment is restricted, when reverse engineering, or 

when developing a cost estimate for an existing treatment system. 

Calculate Mass of Limestone in Limestone Layer

3.2.2.1 Retention Time – The user enters the desired time to retain the water in the void 

spaces between the individual pieces of limestone. The mass of limestone required to achieve 

the user-specified retention time is calculated by multiplying the Design Flow rate by the 

Retention Time to determine the volume required to retain the water (Equation 1). The 

calculated volume is then divided by the porosity of the limestone layer to calculate the total 

volume of the limestone layer (Equation 2). 
9

Volume Required for Water (yd3

) = 

Limestone Layer Volume (yd3

) =

The porosity represents the percent (%) of void space in the limestone layer (volume of the 

voids in between individual pieces of limestone/total volume of the voids and the limestone).

Users can specify a value for porosity to match the gradation of limestone they plan to use in 

the limestone layer. Users can specify the porosity value under the Limestone section, which 

is located under the Layer Materials subheading. Based on the user-specified porosity value,

AMDTreat calculates the Bulk Density of the limestone. The mass of the limestone required in 

the limestone layer is calculated by multiplying the limestone layer volume by the Bulk Density

(Equation 3).

Limestone Mass (tons) =

Results from bench-scale testing indicate that a retention time of 16 hours is a good balance

between maximizing alkalinity production and the size of the treatment system (mass of 

limestone). Some professionals conduct “cubitainer” testing to determine the amount of time 

required to achieve net alkaline conditions. A cubitainer is filled with limestone and mine 

drainage, with care taken to prevent off-gassing of carbon dioxide (CO2) from the mine 

drainage, and alkalinity is measured over time. Some professionals find it appropriate to add 

a safety factor to the retention time since the retention time and contact between limestone 

and mine drainage will change as soon as limestone starts to dissolve. 

See Section 3.2.2.7 to 3.2.2.9 for volume calculations for the other layers.

3.2.2.2 Bureau of Mines - The Bureau of Mines was an agency in the U.S. Department of 

Interior that is now retired but was instrumental in conducting some of the early research in 

how to design passive treatment systems (Hedin and Watzlaf, 1994). A method they 

developed to determine the mass of limestone needed in passive treatment systems was to 

perform two calculations. The first calculation is to determine the mass of limestone required 

to retain the water for a given retention time. This calculation is the same as Equation 3,

described above for the retention time sizing method. 
10

A second calculation (Equation 4) is used to determine the mass of limestone required to 

neutralize the acid loading of the mine drainage for a specified time frame (Neutralization 

Period). The mass of limestone required to neutralize the acid loading is determined by 

multiplying the user-specified Design Flow by the Net Acidity and the Neutralization Period. 

The resultant value is divided by both the Purity and Dissolution Efficiency of the limestone 

to correct for the portion of limestone that will not dissolve to produce alkalinity and the fact 

that larger pieces of limestone are not likely to efficiently dissolve due to a loss of surface 

area from weathering and mineral precipitate coatings (i.e. iron hydroxide) that form on the 

surface and disrupt dissolution. A Dissolution Efficiency of less than 100% will increase the 

mass of limestone required in the VFP to compensate for the mass of limestone that is 

unlikely to dissolve. Users can specify the Purity and Dissolution Efficiency of limestone 

under the Limestone section, which is located under the Layer Materials subheading. 

Limestone Mass to Neutralize Acid (tons) =

The mass of limestone required to retain the water for a specified time (Retention Time) is 

added to the mass of limestone required to neutralize the acidity loading for the 

Neutralization Period to determine the total mass of limestone required for the VFP 

limestone layer (Equation 5). This method ensures the user-specified Retention Time is 

preserved, to generate maximum alkalinity, at the end of the user-specified Neutralization 

Period when the limestone mass dedicated to neutralizing the acidity load is nearly depleted.

Total mass of limestone (Bureau of Mines method) =

See Section 3.2.2.7 to 3.2.2.9 for volume calculations for the other layers.

3.2.2.3 Alkalinity Generation Rate - This method calculates the mass of limestone required 

based on an expected rate of alkalinity production, or acidity neutralization, per area of 

limestone. Since the limestone depth in VFPs is commonly 3 feet, the alkalinity generation 

rate could be viewed as the alkalinity production per unit volume (ft3

). Penn State University 

professor emeritus Dr. Art Rose developed this method (Rose, 2004) by evaluating 29 

operating VFPs and identifying a relationship between effluent net alkalinity and influent 

acidity (raw mine drainage) loading per unit area. Dr. Rose suggested a design guideline for 

the alkalinity generation rate (or acidity neutralization rate) of 25 grams of influent acidity/m2

of limestone surface area/day. 
11

AMDTreat calculates the mass of limestone by dividing the daily influent acidity load 

(gal/day) by the user-specified Alkalinity Generation Rate (g/m2

/day) to calculate the 

limestone surface area requirement (Equation 6). The surface area is multiplied by the userspecified limestone layer depth to determine the volume required in the limestone layer

(Equation 7). AMDTreat uses the user-specified porosity of the limestone material to 

determine the bulk density of loosely placed limestone. The limestone layer volume is 

multiplied by the bulk density to calculate the mass of limestone required, as shown in 

Equation 3.

Limestone Surface Area (ft2

) =

Limestone Volume (ft3) = Limestone Surface Area (ft2) x Limestone Layer Depth (ft) (7)

See Section 3.2.2.7 to 3.2.2.9 for volume calculations for the other layers.

3.2.2.5 User-Specified Limestone Quantity (tons) - If users have their own method for 

determining the mass of limestone, the User-Specified Limestone method can be used to 

calculate the dimensions of the limestone layer and remaining layers of the VFP. The mass 

of limestone is entered, and the user-specified porosity value for the limestone layer is used to 

calculate the volume and retention time of the limestone layer. See Section 3.2.2.7 for volume 

calculations for the other layers.

3.2.2.6 User-Specified Dimensions - Mine drainage discharges often emerge at the base of 

hillsides. The available land area, between the point of emergence and the receiving stream, 

for constructing a treatment system is normally limited. The use of other sizing methods may 

produce a VFP size that is larger than the available land area. A smaller VFP can be designed 

for the site, but it will most likely require more maintenance and prematurely plug due to 

higher metal loading per unit area/volume. To use the freeboard sizing method, enter the top 

length and width of the freeboard. AMDTreat then uses the user-specified side slopes and 

depths to calculate the dimensions and volumes of the VFP layers.

Volumetric Calculations 

3.2.2.7 Volumetric Calculations – AMDTreat models all passive treatment systems, including 

VFPs, as an “inverted” trapezoidal prism (Figure 1). AMDTreat calculates the overall 

dimensions of a passive treatment system by first breaking up the system into individual 

layers, such as the freeboard layer, water layer, compost mix layer, limestone layer, etc. Each 

individual layer and the geometric shape of the overall system is modeled as an inverted 

trapezoidal prism. AMDTreat sizing methods fall into one of two categories: (1) Users input 

information that is used to calculate a layer volume or surface area is calculated or users 

specify the mass of limestone, (2) Users specify the top width and length of the freeboard. 
12 

In either case, AMDTreat uses the information generated by the sizing method, along with 

user-specified depths, side slopes, and length to width ratios to determine the dimensions of a 

layer shaped as an inverted trapezoidal prism. What differs is the approached used to 

calculate the geometry. 

3.2.2.8 Calculations for Retention Time, Bureau of Mines, Alkalinity Generation Rate, and 

User-Specified Limestone Sizing Methods – The equation for an inverted trapezoidal prism is: 

Where: 

V = Volume of trapezoidal prism

LTWT =Length & Width of top of prism

LBWB =Length & Width of bottom of prism 

D = Depth

These sizing methods determine the volume of the limestone layer. Therefore, AMDTreat

calculates the volume of the limestone layer and users are required to specify the depth of 

the limestone layer, inside slope, and length-to-width ratio for the bottom of the 

limestone layer. The unknowns are the length and width of the top and bottom 

dimensions of the trapezoidal prism for the limestone layer. Since the depth and inside 

side slope of the VFP is known, the top width and length of the limestone layer can be 

written in terms of the bottom length and width to reduce unknowns in the equation. 

Furthermore, since the bottom length to width ratio is known, the bottom width can be 

written in terms of the bottom length. Now Equation (8) is rearranged to solve for the 

bottom length of the limestone layer. The resultant equation is a quadratic: 

Where: 

V = Volume of Limestone Layer 

LBLL = Bottom Length of Limestone Layer

D = Depth of Limestone Layer

S = Inside Side Slope of Vertical Flow Pond 

R = Length to Width Ratio of Bottom of Limestone Layer

Once the bottom length is known, the bottom width of the limestone layer (WBWL) is 

calculated by dividing the bottom length by the user-specified value for length-to-width 

ratio for bottom of the Vertical Flow Pond: 
13 

NOTE: AMDTreat will produce an error message if the calculated Bottom Width is less 

than 10 ft. See Section 3.4 for additional information. 

The two remaining unknown dimensions of the limestone layer are the top length and 

width. 

The top length of the limestone layer is calculated by: 

Where: 

LTLL =Top Length of Limestone Layer

LBLL = Bottom Length of Limestone Layer 

DLL = Depth of Limestone Layer

S =Inside Side Slope of Vertical Flow Pond (rise/run)

The top width of the limestone layer is determined by the same approach. Now that all 

the dimensions of the limestone layer are known, the dimensions of the compost mix, 

water, and freeboard layers can be determined using a similar approach: 

Compost Mix Layer

The bottom length of the compost mix layer (LBCML) is equal to the top length of the 

limestone layer (LTLL). 

The top length of the compost mix layer (LTCML) is calculated by: 

Where: 

S =Inside Side Slope of Vertical Flow Pond (rise/run)

DCML = Depth of Compost Mix Layer

Equation (8) is used to calculate the volume of the compost mix layer. 

Water Layer

The bottom length of the water layer (LBWL) is equal to the top length of the compost mix 

layer (LTCML). 
14 

The top length of the water layer (LTWL) is calculated by:

Where: 

S =Inside Side Slope of Vertical Flow Pond (rise/run)

DWL = Depth of Water Layer

The same approach used to calculate the bottom and top width of the water layer. Then 

Equation (8) is used to calculate volume of the water layer.

Freeboard Layer

The bottom length of the freeboard layer (LBFL) is equal to the top length of the water 

layer (LTWL). 

The top length of the freeboard layer (LTFL) is calculated by: 

Where: 

S =Inside Side Slope of VFP (rise/run) 

DFL = Depth of Freeboard Layer 

Equation (8) is used to calculate the volume of the freeboard layer. 

In addition to assuming all passive treatment systems are constructed as an inverted 

trapezoidal prism, an additional assumption is that the excavation volume is equal to the 

summation of all the individual layer volumes, except for the freeboard layer. It is 

assumed the Freeboard Layer is constructed with a portion of the excavated earth. Thus: 

3.2.2.9 Calculations for User-Specified Dimensions Sizing Method – When users specify the 

top width and length of the freeboard for a system, AMDTreat uses the user-specified values 

for inside side slope and the freeboard, water, compost mix, and limestone layer depths to 

calculate the dimensions and volumes of the individual layers of the VFP. 

 
15 

The bottom length of the freeboard layer (LBFL) is calculated by: 

Where: 

LBFL = Bottom Length of Freeboard Layer 

LTFL =User-Specified Top Length of Freeboard Layer

DFL = Depth of Freeboard Layer

S =Inside Slope of VFP (Run/Rise) 

The bottom width of the freeboard layer is calculated using the same approach. 

NOTE: AMDTreat will produce an error message if the calculated Bottom Width is less 

than 10 ft. See Section 3.4 for additional information. 

The volume of the freeboard layer is determined by: 

Where: 

VFL = Volume of trapezoidal prism-shaped Freeboard Layer

WTFL = Top Width Freeboard Layer

LTFL = Top Length Freeboard Layer 

WBFL = Bottom Width Freeboard Layer 

LBFL = Bottom Length Freeboard Layer 

D = Depth of Freeboard Layer

Water Layer

The top length of the water layer (LTWL) is equal to the bottom length of the freeboard 

layer (LBFL). 

Now that the top width and length of the water layer is known, the user-specified values 

for inside slope and depth of water layer are used in Equation (19) to calculate the bottom 

width and length of the water layer. Equation (20) is used to calculate the volume of the 

water layer. 

Compost Mix and Limestone Layers 

The same approached used to calculate the dimensions of the water layer are used to 

calculate these layers. 

Lastly, the excavation volume is determined by adding the volumes for the water, 

compost mix, and limestone layers (Equation 18). The model assumes the excavated 

earth is used to construct the freeboard layer and any excess material is disposed of onsite. 
16

3.2.3 System Properties: The Systems Properties section allows the user to input the inside slope of 

the pond, the limestone layer bottom length-to-width ratio, and the excavation unit cost. AMDTreat 

suggests default quantities and unit costs for construction of a single VFP.

3.2.4 Layer Materials: Unlike active treatment systems which require the continuous addition of 

chemicals to maintain treatment of mine drainage, passive treatment systems typically consist of

impoundments filled with enough reagent (e.g., limestone, compost) to provide treatment for many 

years. For some passive systems, such as VFPs, different reagents are used at different elevations 

within the impoundment to manipulate the geochemistry and control oxidation/reduction and 

solubility reactions as the mine drainage flows through the system. AMDTreat considers each reagent 

or material as a “layer” in the impoundment. Both “reagent” and “material” are used to describe 

layers because not all layers are comprised of reagents or materials meant to manipulate water 

chemistry. For example, the freeboard layer is designed to accommodate fluctuations in flow or 

increasing water elevation due to permeability or plugging of layer materials from metal hydroxides 

or sediment. The piping layer is located at the very bottom of the impoundment and is designed to 

transport water out of the VFP or to flush the system in order to remove metal hydroxide precipitates 

that accumulate in the limestone layer. The Layer Materials section is where users can control the 

depth and other characteristics of the materials used in the various VFP layers. 

3.2.4.1 Freeboard Layer: Users specify the depth of the freeboard layer designed to produce 

enough storage to accommodate increases in the water layer due to increased flow events or 

clogging of the treatment media. In most passive systems, the influent to the VFP is within or at 

the top of the freeboard layer. An emergency spillway is incorporated into the freeboard layer 

just below the influent elevation to prevent the restriction of water flowing into the VFP and to 

prevent water from overtopping the embankment. The bottom of the freeboard layer is the top of 

the water layer for the VFP. A typical freeboard layer depth is between 1.0 and 3.0 feet. 

3.2.4.2 Water Layer: The water layer maintains the saturation of the treatment media and helps to 

disperse the water flow across the surface area of the VFP. The depth of the water layer is often 

manipulated to maintain equilibrium as the treatment media clogs with sediment or metal 

hydroxide precipitates and to control gas exchange of the lower layers with the atmosphere. At 

times, the water elevation is manipulated to control the growth of aquatic plants that may occur in 

VFPs. A typical water layer depth is between 1.0 and 2.0 feet. 

3.2.4.3 Compost Mix Layer: The compost mix layer is used to control the redox potential as 

described in the Application and Treatment Chemistry section located above. Users can specify 

the thickness, unit cost, and placement cost of compost in the Compost Mix Layer section. Users 

can also allow limestone fines to be mixed with the compost to increase pH and promote 

aluminum precipitation. Limestone fines are commonly specified to comprise roughly 10 to 25% 

by volume of the compost mixt layer. A typical compost mix layer depth is between 1.0 and 2.0

feet. Thickness less than 1.0 ft may not create a chemically reducing environment and thickness 

greater than 2.0 feet may compact and cause permeability issues. 

3.2.4.5 Limestone Layer: The limestone layer is the main source of alkalinity generation in a

VFP. Users can specify the depth, purity, dissolution efficiency, porosity and unit cost for both 

the material and placement of limestone in this section. A typical limestone depth is 3.0 feet and 

limestone used in passive treatment systems such as VFPs normally have purity greater than 85% 

calcium carbonate (CaCO3) by weight. The dissolution efficiency is used to compensate for 
17

limestone losing surface area and becoming less reactive as it dissolves/weathers and as a safety 

factor for additional limestone to compensate for potential plugging issues. 

3.2.4.6 Pipe Layer: The pipe layer considers the piping required between the limestone and liner 

layers necessary to force the water to flow vertically through the treatment media, collect the 

water and discharge it out of the VFP. The VFP module offers users two methods to estimate the 

cost to purchase and install an effluent piping network, the AMDTreat Piping Calculator or 

User-Specified Piping Layout. 

The default design of the AMDTreat Piping Calculator assumes a large-diameter 

“trunk” of solid pipe that extends along the middle of the limestone layer bottom length

(Figures 3 & 5). “Spur” pipes are typically smaller diameter pipes that extend 

perpendicular from the trunk pipe across the entire bottom width of the limestone layer. 

The spur pipes are perforated and are intended to evenly draw water downwards through 

the treatment media into the pipes and into the trunk pipe for discharge from the VFP. 

Users can specify the spacing between the spur pipes to promote an even flow 

distribution or to maximize flow velocity during flushing events to remove accumulated 

metal precipitates from the limestone layer.

Historically, designers have developed a wide variety of piping systems to accommodate 

the hydraulic functionality of VFPs. In many instances piping systems designed to flush 

the limestone layer are physically separated from the primary discharge flow pipe 

network (Figures 14 & 15). Additionally, some designers have incorporated zonal flush 

pipe systems which allow for flushing metal precipitates in areas of the limestone bed 

that are most susceptible to plugging, such as the area of the bed nearest the influent and 

at the top of the Limestone Layer (Figure 16). Users that desire to customize their piping 

design can select User-Specified Piping Layout and specify the size and length of trunk 

and spur pipes.

3.2.4.7 Liner Layer: Many mine sites lack the soil characteristics required to prevent a leaky 

passive treatment system. AMDTreat offers three types of liner systems that prevent water from 

leaking from the impoundment, including Clay, Synthetic (i.e. PVC), and Geosynthetic Clay 

liners. The volume of clay required is calculated from the entire inside sloped area of the pond, 

from the top of freeboard to the base of the limestone (bottom of pond), plus the bottom area of 

the VFP and multiplying it by the user-specified thickness (typically 0.5 to 1.0 foot) and unit cost 

to purchase and install the clay liner. The area required for Synthetic or Geosynthetic Clay liners

is determined by calculating the inside sloped area and the bottom area of the VFP similar to the 

clay liner and adding an additional 2.0 ft of length on all sides to accommodate the incorporation 

of the liner into the embankment near the top of the freeboard layer to secure the liner system. 

AMDTreat provides the option of adding Non-woven Geotextile as a separate material for all 

liner systems. Non-woven geotextile separation material is commonly used to protect the 

Synthetic or Geo-Synthetic Clay liner from the excavated material inside the surface of the pond 

and/or the limestone layer.

3.2.5 Other Items: The Other Items section allows users to consider the capital cost of inlet and outlet 

structures, valves, and flow distribution structures. AMDTreat suggests default quantities and unit 

costs for construction of a single VFP. 
18

3.2.5.1 Valves: - Valves can be used at various locations in the piping network of passive 

treatment systems. They can be placed at the intake of a collection system, downflow of a flow 

splitting device to shut off flow to a cell during maintenance, or at the downstream end of a 

system to allow for flushing of the limestone layer. Other valve configurations are possible, 

depending on system design requirements, which vary due to designer’s preference or water 

quality constraints. Valves can range from heavy duty stainless steel industrial models to lighter 

grade simple Polyvinyl Chloride (PVC) valves, again depending on system requirements, 

intended lifecycle, and designer’s preference. The cost of the valves can vary greatly depending 

on the materials used for construction and intended duty cycle. 

3.2.5.2 Intake Structures: Various methods of intake systems are used in passive treatment 

system design. Deep mine discharges with intact wet seals can be collected with simple pipe 

connections or open top structures to allow for inspection and cleanout. In cases where there is no 

pipe discharging from a deep mine, custom intakes need to be designed based on site constraints, 

water quality and flow to collect the deep mine discharge and connect it to a conveyance pipe or 

open channel leading to a flow splitting device or treatment system. Surface discharges or seeps 

can be collected with subsurface drainage infrastructure (French drains) or open top devices such 

as catch basins with appropriately designed openings. Stream intakes are typically designed to 

capture base design flows while allowing stormwater flows to bypass the intake, assuming the 

surface discharge is diluted with non-AMD stormwater runoff. These types of intakes typically 

consist of concrete boxes or weirs with pipe openings to convey the base flow to a flow splitter or 

treatment system. There are many options available to capture water for treatment, and designers 

should select an alternative that fits the discharge and does not create an undue maintenance 

burden. Depending on water chemistry, any concrete used in intake structures must be coated 

inside and out with a heavy-duty epoxy paint or other suitable coating to prevent dissolution of 

the concrete from acidic conditions. An example of an open-top concrete intake structure is 

shown in Figure 19 at the end of this document. 

3.2.5.3 Flow Distribution: Many passive treatment systems include multiple treatment cells or

bypasses that require that the flow is split into multiple parts. Flow can be split with concrete 

structures or by large diameter pipes with multiple outlets. As mentioned in the Intake structure 

section, depending on water chemistry, any concrete used in intake structures must be coated 

inside and out with a heavy-duty epoxy paint or other suitable coating to prevent dissolution of 

the concrete from acidic conditions. Sample Flow Distribution Structures are shown in Figures 

17, 18, 20 & 21. 

3.2.5.1 Water Level Control Structures: Water level control structures are exactly as the name 

implies, a structure or series of structures used to control the level of water within the treatment 

component. Typically, the structure allows for the adjustment or manipulation of the water level 

for various reasons such as the need for greater head pressure to force the water through the 

treatment materials and outlet piping or lowering of the water level for maintenance. Common 

water level outlet structures may include a PVC box-like structure with removable stop logs 

(Figures 22 & 23) or vertical standpipes with Fernco fittings (Figure 16) both of which are 

installed in the downstream embankment and hydraulically connected to the piping layer within 

the treatment component and the effluent pipe for discharge to the subsequent treatment 

component.
19

3.2.5.1 Outlet Protection Structures: These are typically rip-rap structures constructed in a 

trapezoidal shape, which are at the outlet of the effluent pipe from the water level control 

structure and the inflow to the next downstream treatment component. The purpose of these 

structures is to dissipate the energy generated by the gravity flow of water downslope in the 

effluent pipe from a water level control structure and prevent scouring/erosion at the point of 

discharge. A secondary benefit of the outlet protection structures is the potential to add dissolved 

oxygen into the water as it is agitated from striking the rip-rap material. It is important that an 

outlet protection structure be properly designed to provide the most appropriate rip-rap size (stone 

size) and the actual dimensions of the structure. Refer to Figure 24 for a photo of an outlet 

protection structure situated at the pipe discharge from a water level control structure. 

3.3 Module Outputs

3.3.1 Sizing Summary: The Sizing Summary section contains all the calculated module outputs, such as 

the dimensions, volumes, and plug flow retention times for the various layers. The mass of limestone 

required for the limestone layer is provided on the right side of the Sizing Summary heading. 

3.3.2 Capital Cost: This section provides the estimated costs for the various user-specified components 

and the total estimated cost to construct the entire VFP. 

3.3.3 Annual Cost: Since a VFP is a passive treatment system component, the only annual cost 

considered is the cost for maintenance. Annual costs, such as sampling, labor, and snow removal are 

“project-wide” costs since they apply to an entire treatment site, not each individual treatment component. 

Sampling, Labor, and Site Development (i.e. Clearing/Grubbing/Revegetation) modules are located under 

the “Project Modules” section of the main user interface and capture these types of annual costs. 

Annual maintenance costs for VFPs include periodic “flushing” of the piping network to evacuate metal 

hydroxide precipitates from the limestone layer, maintaining clear influent and effluent piping, and 

manipulating the water level control structure (e.g. Agri Drain) to change the water level in the VFP. It is 

assumed that some years will not require the full amount of funds set aside for maintenance, but funds 

will eventually be used over the system life. Users can estimate the annual maintenance costs by 

assuming a percentage of the capital cost (the more expensive the system, the more expensive the 

maintenance) or specify an annual maintenance cost amount. 

3.3.4 Net Present Value: The Net Present Value (NPV) section determines the cost to operate a 

treatment system component over a specified time period. The NPV calculates the present-day financial 

investment required to generate the income to pay for future operation and equipment/materials

replacement costs. Both Financial Variables and Cost Categories are required to calculate the NPV. 

3.3.4.1 Financial Variables - The Term of Analysis, Inflation Rate, and Rate of Return are 

three variables used in the NPV calculations. The default values for these terms are 

shown under the Net Present Value section of each module. Users must access the Net 

Present Value menu at the top of the main user interface to change the default values as 

they would apply to all modules used for an entire treatment system. While NPV is 

determined for each AMDTreat module activated by the user, the goal is to determine a 
20 

total NPV for an entire mine drainage treatment system project (a collection of cost 

estimates for individual modules creates a treatment system project in AMDTreat). 

Therefore, a single value for Term of Analysis, Rate of Return, and Inflation Rate is 

applied to all modules and cannot vary between modules. 

• Term of Analysis: The time period used by the NPV calculation to determine the 

financial investment required to pay for all future costs of the treatment system. 

• Inflation Rate: Represents the average price increase of goods and services over 

time. AMDTreat uses the inflation rate to calculate the future cost of the annual 

operation and maintenance (O&M) and recapitalization items. 

• Rate of Return: Describes the expected profit on an investment. 

3.3.4.2 Cost Categories - For each treatment module, AMDTreat provides a list of 

recommended equipment and materials that require recapitalization. In addition, 

AMDTreat provides recommendations (default values) for life cycle and replacement 

percentage. Users can click on the default values for Life Cycle or Replacement 

Percentage and use the +/- buttons to change the default values. In addition, users can 

select Custom Cost and enter a new cost to represent the current cost of the equipment. 

Users can add new recapitalization items or deactivate/delete existing items for 

calculating the NPV. 

An example of how the recapitalization variables are used to determine NPV is to 

consider the following hypothetical scenario. Assume a vertical turbine pump has a life

cycle of 50 years but requires the pump motor to be rebuilt every 20 years. Assume the 

present-day cost to purchase the motor is $500,000, and the cost to remove, rebuild, and 

reinstall the pump motor is $20,000. Now assume we want to determine the amount of 

investment required today (NPV) to generate the income to pay for the future cost of 

rebuilding the pump motor over a 50-year Term of Analysis, which is also equal to the 

life cycle of the pump. Assume an Inflation Rate of 5.0% and Rate of Return of 8.1%. 

The goal is to place the money in a relatively secure investment vehicle to generate 8.1% 

annually. The NPV will calculate the size of investment required to generate income for 

future costs. 

There are several ways to model the replacement cost. One way is to replace 4% of the 

present-day cost of the pump (4% of $500,000 = $20,000) with a life cycle of every 20 

years. If the Term of Analysis is 50 years, then the entire pump would not require 

recapitalization since the life cycle of the pump is 50 years. However, the motor would 

require two replacements (50 years / 20 years = 2.5 rounded down to 2). 

To determine the NPV to recapitalize rebuilding of the motor, AMDTreat calculates the 

future cost to rebuild the motor at each life cycle, 20 and 40 years. The program uses the 

Inflation Rate to inflate the present-day default cost to rebuild the motor in 20 and 40 

years from now. While the present-day cost to rebuild the pump motor is $20,000, the 

future cost to rebuild the motor in 20 years at a 5.0% Inflation Rate is $53,065 and 

$140,799 in 40 years (Equation 22). Assuming an 8.1% Rate of Return, the 50-year NPV

for the pump is $17,422. In other words, an initial investment of $17,422 is needed at an 
21 

annual Rate of Return of 8.1% to generate the investment income required for the two 

motor rebuilds over the 50-year life cycle of the pump. 

Cost to rebuild pump motor in 20 years = 

PPPPPPPPPPPPPP DDDDDD CCCC × (100% + II IIII II RR )20 = $20,000 × (100% + 5%) 20 = $53,065 (22) 

Cost to rebuild pump motor in twenty years is equal to present day cost multiplied by parentheses one 

hundred percent plus inflation rate close parentheses to the power of twenty.

• Annual Operation and Maintenance Cost: By default, AMDTreat transcribes the annual 

O&M cost from the Annual Cost section to the Net Present Value section. The program 

assumes the module is being used to first estimate the annual cost for a treatment system 

component, so it automatically transcribes the annual cost to the NPV section. If this is 

not the case or the user wants to use some other annual cost, the “Use Custom” box can 

be selected to allow the user input of a different annual cost to utilize in the NPV 

calculation. 

• Recapitalization Cost: Certain treatment system components, especially mechanical and 

water conveyance equipment, require periodic replacement. The recapitalization cost of 

an item is an estimate of the amount of money required to pay for future replacement 

costs for the item. In addition to the Financial Variables described above, three additional 

values are required to calculate the NPV of recapitalization costs, the Present-Day 

Equipment Cost, the Life Cycle, and the Replacement Percentage. 

• Default Cost: This represents the current cost to purchase the equipment or material.

• Life Cycle: The time frame between equipment or material replacement is termed as its 

Life Cycle. Some equipment manufacturers provide recommended life cycles for their 

equipment to provide consumers with an estimate of how long the equipment is expected 

to be operational. Some life cycles, such as those used for treatment media (limestone), 

are based on best professional judgement. Some operators prefer to periodically purchase 

and replace equipment before failure to preserve the continuity of operations, while 

others wait until failure to replace an item.

• Replacement Percentage: The Replacement Percentage is an adjustment factor to the 

Default Cost to accommodate situations where the entire piece of equipment or all of the 

material does not require recapitalization. For example, a passive treatment component 

may be designed to contain enough limestone to neutralize the acidity load for 20 years, 

however, the accumulation of metal hydroxide precipitates within the void space of the 

limestone layer may require that 25% of the limestone be replaced every 7 years to 

prevent hydraulic failure such as plugging or short-circuiting. For this scenario, the 

initial cost of the limestone making up the limestone layer is discounted by 75% and 

assigned a life cycle of 7 years to determine the amount of money required to cover the 

cost of replacing 25% of the limestone layer every 7 years over the Term of Analysis. 
22

3.3.4.3 Rationale for Recapitalization Recommendations:

Recapitalization recommendations are based on professional experience of the 

AMDTreat Team and may not apply to all situations. Users are encouraged to customize 

the recapitalization assumptions to their treatment scenario. AMDTeat Team members 

are located in Pennsylvania and West Virginia and have collective experience in design, 

funding, and/or operation/maintenance for over 100 passive treatment systems. Most 

passive treatment systems constructed in Pennsylvania and West Virginia are less than 15 

years old, thus the industry is still gaining experience in lifecycles for various 

components. However, the AMDTreat Team held discussions on personal experience to 

develop a list of recapitalization recommendations. Users may have different experience 

and opinions. 

By default, AMDTreat includes a list of six recapitalization items for VFP. The items are 

listed even if they are not selected for the treatment system. The purpose of this is to 

inform the user of items they should think about recapitalizing or including in their 

treatment design. For example, Liner is listed as a recapitalization item even if no liner is 

selected in the cost estimation. In this case the recapitalization cost is set to $0.00 but the 

item is still shown as a reminder to the user that a Liner is a common item that should be 

considered. If a liner is included in the cost estimation, the estimated liner cost is used to 

calculate a recapitalization cost for liner. Users can delete or modify any of the default 

Recapitalization items by either deselecting the item or by setting the Replacement % to 

zero. If the item is deselected the Total Cost for the item will still be shown but the cost 

will be subtracted from the Net Present Value Cost, shown in the Net Present Value 

Heading.

Limestone: The limestone can require maintenance and periodic partial replacement. 

Limestone becomes clogged with metal precipitate as the compost mix effectiveness 

decreases. Therefore, the default recommendation in AMDTreat is to replace 50% of the 

limestone every 10 years. The limestone replacement would likely be completed at the 

same time the Compost Mix is replaced. 

Compost Mix: A portion of the Compost Mix may need to be replaced as it becomes 

less effective producing anaerobic conditions. Based on the historical experience of 

maintaining VFPs, AMDTreat recommends planning to replace 50% of the Compost 

every 10 years.

Liner: Often liners are destroyed, especially synthetic, while conducting maintenance on 

passive treatment systems, if the equipment operator is inexperienced with passive 

systems. Thus, AMDTreat recommends planning to replace a portion of the liner while 

the Limestone and/or Compost Mix is being replaced. The default assumption is to 

replace 50% of the Compost every 10 years, so the same assumptions are recommended 

for Liner recapitalization.

Pipe: Piping is often exposed at the surface and can become damaged by vandalism, 

falling trees, and other factors. AMDTreat assumes 50% of the piping will require

replacement after 25 years of service.

Other Items: The Other Items section in AMDTreat includes Flow Distribution 

Structures, Water Level Control Structures, and Outlet Protection Structures. These 
23 

structures are often durable, and many treatment systems have never experienced 

replacement of these items, however, AMDTreat Team members have had at least one 

experience of each of these items requiring replacement, thus the Team wanted to include 

these items for replacement consideration. 

3.4 Assumptions of Design Sizing and Error Messages

AMDTreat is a cost estimation model that uses assumptions to provide treatment sizing and both capital 

and annual cost estimates. While there are many assumptions in the program, the assumptions that follow 

are important for the VFP module. 

1. The VFP is assumed to be constructed on a flat surface, where the water, compost, and limestone 

layers are excavated at or below the existing or original ground. A portion of that material is used 

to construct the freeboard embankments which are considered fill material above the existing or 

original ground (see Figure 2). The program assumes an on-site balance of cut and fill material. 

However, if this is not the case, the user can modify excavation cost default values to reflect the 

anticipated conditions. 

2. The geometric shape of a VFP is an inverted trapezoidal prism. The volumetric equation for a 

trapezoidal prism is used to calculate the VFP layer volumes. 

3. AMDTreat requires the bottom length of the VFP must be equal to or lager than the Bottom 

Width. In addition, AMDTreat will produce an error message if the input values create a Bottom 

Width less than 10 ft. The error message notifies users they must change the Design Flow rate, 

values associated with the selected sizing methodology (e.g. retention time), the inside slope of 

the VFP or the depths associated with any of the layers. In most cases, users will focus on 

decreasing the depth of one of the layers to eliminate the error. AMDTreat assumes a VFP with a 

Bottom Width of less than 10 ft is not practical to construct. 

4. None of the sizing methods provide a “design life” that properly accounts for both: a) the amount 

of limestone required to neutralize the acidity loading for a defined time frame, and b) hydraulic 

and treatment issues caused by the accumulation of metal hydroxide precipitates within the void 

spaces of the treatment media. Some sizing methods, such as the Bureau of Mines, account for 

reagent requirements. Others, like the Alkalinity Generation Rate, indirectly try to address 

plugging issues by controlling the metal loading applied to a unit surface area. 

5. In general, the goal of the VFP sizing methods is to provide an estimate of the mass of limestone 

required to generate net alkaline conditions. None of the sizing methods are based on achieving 

effluent water quality criteria or requirements. That said, a properly sized VFP treatment system 

should generate pH > 6.0 and be net alkaline, which will decrease aluminum and ferric iron 

solubility to less than 1.0 mg/L. Some VFPs discharge ferrous iron as they are designed to 

promote ferrous iron solubility within the compost mix layer and virtually all VFPs will not affect 

dissolved manganese concentrations. 
24

4.0 References

Hedin, R.S. and Watzlaf, G.R., 1994. The effects of anoxic limestone drains on mine water chemistry. 

Proceedings of the 3rd International Conference of the Abatement of Acidic Drainage, Volume 1, 

Pittsburgh, PA. U.S. Bureau of Mines SP 06B-94, p. 185–194.

Rose, A. W., 2004. Vertical flow systems – Effects of time and acidity relations. Proceedings, American 

Society of Mining and Reclamation, Morgantown, WV, April 18-24, 2004, p. 1595-1615. 

https://www.asrs.us/Portals/0/Documents/Conference-Proceedings/2004/1595-Rose.pdf
25

5.0 Figures 

Figure 1: Typical Inverted Trapezoidal Prism
26

Figure 2: Typical Cross Section View of a Vertical Flow Pond (VFP) Showing the 

Common Layers and Components
27

Figure 3: Plan View of the Vertical Flow Pond (VFP) to Illustrate Common Piping 

Layout and Components (Used in AMDTreat Piping Calculator) and the Layer Surface 

Areas
28

Figure 4: Installation of a clay liner during the construction of a VFP.
29

Figure 5: Excavator carefully covering the underdrain piping network (used for discharge 

and flushing) with limestone in preparation to construct a 3.0 ft thick limestone layer. A 

1.5 ft thick compost layer will follow to finish the VFP. The excavator is covering a 

“spur” pipe. The spur pipes are connected to the larger “trunk” pipe oriented parallel to 

the bed. This is the type of piping configuration modeled in AMDTreat.
30

Figure 6: Constructed VFP after installation of compost layer. Vertical pipes are 

connected to underdrain system and used to maintenance underdrain pipes if clogging 

occurs. Clean out pipes are no longer typically used.
31

Figure 7: Newly constructed vertical flow pond. Inlet water control structure is shown. 

The reddish tint to the water in the background is due to low pH iron oxidation occurring 

on the top of the compost layer. Raw water pH is 2.8.
32 

 

Figure 8. Two 2.5-acre vertical flow ponds flowing into a polishing wetland. Note the 

instream precipitation of metals caused by the alkaline effluent mixing with the acidic 

receiving stream. 
33

Figure 9. Photo showing the path of a F3 Tornado that pasted directly over a VFP 

treatment system. No damage to the VFP was caused by the Tornado.
34

Figure 10. Photo showing the top of the Limestone Layer in a VFP after the compost 

layer was removed to perform an autopsy to identify the cause of failure. The 6 inch thick 

compost was insufficient to provide the retention time required to fully reduce the large 

ferric iron load to ferrous iron. Thus, the ferric iron oxidized and precipitated within the 

porosity of the limestone layer causing hydraulic failure. After this event, VFPs were 

designed with 12 to 18” of compost. 
35

Figure 11. Close up photo of the 1.5 ft cross section of the Limestone Layer after removal 

of a thin and ineffective Compost Layer. Note the 1⁄2” layer of iron hydroxide on top of 

the Limestone Layer that caused hydraulic failure. In addition, iron hydroxide precipitate 

filled the void space within the 1 to 2” limestone fragments. 
36

Figure 12. Rhodamine dye test to identify area where short circuiting is occurring. Dye 

was detected in the VFP effluent after 30 mins and the spread of the dye shown in photo 

remained constant for 8 hours indicating the location of the short circuit. This area was 

excavated and carefully rebuilt to eliminate the short circuit caused by poor construction 

practices.

 
37

Figure 13. Pourbaix diagram of the iron-sulfur system to help explain how the different 

layers in a VFP control the iron/sulfur geochemistry. The top left red circle represents the 

Eh/pH condition of the raw water that is dominated by ferric iron. As the water flows 

through the compost layer, the Eh decreases along the vertical black line and the Eh/pH 

condition may stabilize in the pyrite stability field denoted by the red circle at the bottom 

left. As the Eh lowers, ferric iron to ferrous iron and sulfate is reduced to sulfide and iron 

sulfide (pyrite) can form in the compost. If limestone fines are added to the compost the 

vertical line may be more diagonal to represent an increase in pH. The water then enters 

the limestone layer and the residual ferrous iron remains soluble as limestone dissolves 

and increases the pH. The horizontal black line represents the evolution of the Eh/pH 

condition as water travels though the limestone layer. Lastly, water is discharged from 

the VFP through a pipe and splashes on the ground before flowing to a settling pond. The 

increase in dissolved Oxygen increases the Eh condition and causes the remaining ferrous 

iron to oxidize and precipitate as Fe(OH)3. 
38

Figure 14. Whiteish aluminum hydroxide sludge accumulating within the Limestone 

layer due to the increase in pH. Note the flush pipe had little effect on preserving the 

porosity near the pipe. 
39

Figure 15. Gentleman on right turning a flush valve to flush a VFP. Note the flushing of 

the whiteish aluminum hydroxide precipitate. 
40

Figure 16. Zonal Piping system. The smaller diameter pipes on the left are located at the 

top of the Limestone layer. Note the ferric hydroxide precipitate accumulated at the base 

of the pipe. Ferric iron not reduced in the compost layer precipitates at the top of the 

Limestone Layer as the pH is increased to > 4.0 and ferric iron becomes insoluble. The 

pipes on the right are at the base of the Limestone layer where the pH is increased to 6 

and aluminum becomes high insoluble. Note the accumulation of the whitish aluminum 

hydroxide precipitate. 
41

Figure 17. Flow distribution structure. A discharge is piped to the structure and the flow 

is distributed between three pipes that flow to three VFPs arranged in parallel. 
42

Figure 18. Inside view of Flow distribution structure. The untreated discharge enters 

through the bottom pipe and is distributed between the top three pipes that feed three 

VFPs in parallel.
43

Figure 19 – Open Top Intake Structure with Weir. Water level is controlled by weir and 

baseflow is forced to flow through trashrack and into the intake structure.
44

Figure 20 – Flow Splitter Box Prior to Installation. Pipe Gaskets are cast in place in 

precast yard.
45

Figure 21 – Flow Splitter Box Installed with Orifice Plate to regulate Flow and Bypass 

Pipe to prevent Hydraulic Overloading of Treatment Cell
46

Figure 22 – Photo of Water Control Structure (Agri Drain brand) being installed into an 

embankment. The water control structure is placed inside a corrugated pipe to protect 

against soil pressure. Without protection, the box could deform and prevent operators 

from being able to adjust stop logs to control water level. Stop logs are removed and 

shown to the right of the corrugated pipe. 
47

Figure 23 – Looking down into an Agri Drain water control structure. Note the water 

flowing overtop of the stop logs, which set the water elevation in the VFP. Additional 

stop logs can be placed into the metal channels in the center of the box to increase the 

water elevation. 
48

Figure 24 – Outlet protection structure designed to prevent channel erosion