Help Instruction File:

Decarbonation 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).
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Decarbonation Module Overview

Table of Contents

1.0 OBJECTIVE 5

2.0 OVERVIEW AND APPLICATION 5

2.1 Purpose and Understanding of Decarbonation 5

2.1.1 What is Inorganic Carbon and how is it distributed in water? 5

2.1.2 How does Inorganic Carbon affect the treatment of mine drainage? 7

2.1.3 What factors control how Inorganic Carbon is transferred from mine drainage to the atmosphere 

(decarbonation)? 9

2.2 Equipment and Treatment Configurations 12

2.2.1 Decarbonation Reactors 13

2.2.2 Decarbonation Approaches 13

2.2.2.1 Mixing Water into Air 13

2.2.2.1.1 Surface Aerators 13

2.2.2.2 Mixing Air into Water 14

3.0 DECARBONATION MODULE OVERVIEW 16

3.1 Layout and Workflow 16

3.2 Module Inputs 16

3.2.1 Water Quality and Flow Input 16

3.2.1.1. Raw pH, Temperature, Alkalinity, and TIC: 17

3.2.2 Decarbonation System Equipment and Installation: 17

3.2.3 Annual Cost Input 18

3.2.3.1 Aerator Operational Time: 18

3.2.3.2 Electrical Unit Cost: 18

3.2.3.3 Hydrated Lime Cost Savings Assessment: 19

3.2.4 Other Capital Items 19

3.2.5 Other Annual Items 19

3.3 Module Outputs 19

3.3.1 Sizing Summary 19

3.3.1.1 Decarbonation Design Output: 19

3.3.1.2 Water Quality Output: 19

3.3.1.3 Hydrated Lime Savings Assessment: 20

3.3.1.4 Non-Maelstrom Decarbonation Tank Sizing Summary / Maelstrom Decarbonation Tank Sizing 

Summary: 20

3.3.2 Capital Cost 20

3.3.3 Annual Cost 20

3.3.4 Net Present Value 21

3.3.4.1 Financial Variables 21

3.3.4.2 Cost Categories 21

3.3.4.3 Rationale for Recapitalization Recommendations 23
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3.4 Assumptions of Design Sizing and Costs 24

4.0 FIGURES 25

Figure 1: Relationship between gaseous and aqueous carbon dioxide species. 25

Figure 3: Deprotonation of TIC species when pH is raised during treatment. 26

Figure 4: Depiction of a shallow pond containing carbonated mine drainage. Red circles represent CO2

molecules. 26

Figure 5: Pictorial showing the mixing of compressed air into the water, which changes the distance 

between the CO2 molecules and the air/water interface (increases gradient), the air bubbles increase the 

overall surface area and enhance transfer. 27

Figure 6: Dye testing on a decarbonation unit to determine the CO2 transfer coefficient. 27

Figure 7: Dye testing on a Maelstrom Oxidizer decarbonation unit to determine the CO2 transfer 

coefficient. 28

Figure 8: Two surface aerators in a plug flow reactor providing decarbonation. 28

Figure 9: A surface aerator mounted in a continuous stirred reactor pulling water to the surface and 

creating droplets under the splash hood. Note the bubble froth promotes strong gas transfer, however, 

better results would be seen with froth in a plug flow reactor. 29

Figure 10: Fine diffuser aeration system with a piping grid at the bottom of a tank and disk diffusers. 30

Figure 11: Fine diffuser aeration system with a piping grid at the bottom of a tank and tube diffusers. 30

Figure 12: Ethylene Propylene diene monomer (EPDM) rubber disk diffusers. 31

Figure 13: Ethylene Propylene diene monomer (EPDM) rubber tube diffusers. 31

Figure 14: Fine bubble diffuser decarbonation system configured as a plug flow reactor. 32

Figure 15: Maelstrom Oxidizer aerator unit. The blower enclosed in acoustic silencing housing is shown 

in the background and the aeration unit is shown in the foreground. 32

Figure 16: Maelstrom Oxidizer aerator unit used to decarbonate anoxic and carbonated underground mine 

drainage. Compressed air blower not visible in photos. 33

Figure 17: Compressed air blower housed in acoustic-silencing housing. Maelstrom is located in a 

residential area and noise was a concern. AMDTreat does not include the cost of acoustic housing. 33

Figure 18: Maelstrom decarbonation tank showing vertical baffles separating the rows of air tubes. 34
4

Figure 19: Photo showing the air tube (steel tube in tank) with air tube housing removed. Compressed air 

from the plenum flows to the top of the air tube and down in between air tube and housing before exiting 

through the stainless-steel air nozzle band. 35

Figure 20: Air Tube removed for maintenance. The air orifices on the stainless-steel band at the bottom of 

air tube are pressure washed to remove scale and precipitate from clogging orifices. Note the orifices are 

directional and results in half of the air tube forcing air horizontally and countercurrent to the water flow. 

This causes strong mixing between water/air that is optimal for gas transfer. 36

Figure 21: Notice reduced water levels within the red box of the Maelstrom on the right side of the photo. 

The reduced airflow reduces the water elevation and is a sign the stainless steel air nozzle banks are 

clogged and require cleaning. 37

Figure 22: Sixty HP blowers enclosed in acoustic building to reduce noise. The Maelstrom Tanks sit 

above the building on top of the steel structure. 38

Figure 23: Spencer blower and electrical panel with variable frequency drive control panel inside of 

building shown in Figure 22. 39

Figure 24: Variable Frequency Drive control panel provides the ability to control the blower output. This 

provides the ability to alter the decarbonation/horsepower ratio to optimize treatment costs. 39
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1.0 Objective

In the AMDTreat program, the Decarbonation Module is considered an ancillary treatment component of 

a mine drainage treatment facility. Decarbonation is the process of transferring dissolved carbon dioxide 

(CO2(aq)) from mine water to the atmosphere to reduce acidity and lower alkali chemical consumption. 

This module provides guidance on sizing and estimating the performance of various Decarbonation 

systems configured as continuous stirred and plug flow reactors. 

The objectives of the overview are to (1) Provide an understanding of the application of Decarbonation in

mine drainage treatment and (2) Provide an overview of the Decarbonation Module to guide users in

developing a cost estimate to construct this type of treatment system component. This module, as well as 

most of the other AMDTreat modules, can also be applied to reverse cost model existing systems and 

system components to establish and evaluate future financial and investment decisions. The information

is presented in two sections, Overview and Application and Decarbonation Module Overview. 

2.0 Overview and Application

The Overview and Application section is organized into three parts: (1) Purpose and Understanding of 

Decarbonation, (2) Equipment and Typical Treatment Configurations, and (3) Application. 

2.1 Purpose and Understanding of Decarbonation 

To understand decarbonation, this section will be divided into three subsections. The subsections are: 

What is Inorganic Carbon and how is it distributed in water?, How does Inorganic Carbon affect the 

treatment of mine drainage?, and What factors control how Inorganic Carbon is transferred from mine 

drainage to the atmosphere (decarbonation)? .

2.1.1 What is Inorganic Carbon and how is it distributed in water? 

A fundamental difference between organic and inorganic carbon is organic carbon is produced by living 

things. Organic carbon forms the basic organic compounds in living things and contains a carbonhydrogen bond (e.g. Methane - CH4). Inorganic Carbon is created by inorganic processes (e.g. volcanos, 

mineral dissolution, etc.) and lacks the carbon-hydrogen bond (e.g. graphite (C), carbon dioxide (CO2), 

carbon monoxide (CO), and bicarbonate (HCO3

-

)). It is important to note the hydrogen atom in 

bicarbonate is bonded to oxygen and not to carbon, thus it still meets the definition of inorganic carbon. 

In natural waters, the common inorganic carbon species are carbonic acid (H2CO3), bicarbonate (HCO3

-

), 

and carbonate (CO3

-2

). Total Inorganic Carbon (TIC) is the summation of these three inter-related 

species. They are inter-related because all three share carbonate as the “base” ion species. The difference 

between species is just the number of hydrogen atoms bonded to the oxygen contained in the carbonate 

ion. Carbonate is a deprotonated (lose a proton/H+) version of bicarbonate and bicarbonate is a 

deprotonated form of carbonic acid. So, what defines each species is the number of protons (H+

) bonded 

to the CO3

-2 ion. Not only do all three species share a common base species in the carbonate ion, they also 

contain a molecule of CO2. The fact that all three TIC species contain a molecule of CO2 shows that TIC 

is just a measurement of dissolved CO2 species in water.
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The link between atmospheric CO2(g) dissolving into water to form the aqueous TIC species or exsolving 

(outgassing) from the water to the atmosphere is described by the following equations and the pictorial in 

Figure 1: 

 

 

When disequilibrium exists between the atmosphere and a water body gaseous carbon dioxide will 

dissolve (undersaturated conditions) or exsolve (supersaturated conditions). When undersaturated, 

gaseous carbon dioxide dissolves to form aqueous carbon dioxide (Equation 1a) and a small portion of 

CO2(aq) will hydrate to form carbonic acid (Equation 1b). The combined concentrations of CO2(aq) and 

H2CO3 (sometimes indicated as H2CO3*) will be used interchangeably and referred to as carbonic acid. 

Carbonic acid can deprotonate twice, first to form bicarbonate (Equation 2) and then to form carbonate 

(Equation 3). Bicarbonate and carbonate are major components of alkalinity since they can accept protons 

(H+

). Since pH is a measurement of the concentration (activity) of H+ in solution, the incorporation of 

hydrogen ions by bicarbonate or carbonate will moderate decreases in pH resulting from the addition of 

CO2 or other acid to the water. 

By examining Equations 1 through 3, one may conclude that simply dissolving gaseous carbon dioxide 

into water will increase alkalinity since it appears to form bicarbonate and carbonate. In other words, one 

may suspect a treatment strategy to neutralize acidic mine drainage is to simply purchase a canister of

gaseous CO2 and inject it into the water to produce bicarbonate and carbonate alkalinity. However, 

dissolving CO2 will not inherently, by itself, produce alkalinity. To conceptualize this point, it is useful to 

recast the model of CO2(g) dissolving into water by using the following equations: 

 

Equations 5 & 6 show CO2(aq) reacting with hydroxyl ions (OH-

) to produce bicarbonate. Dissolving CO2 

into water simply converts existing hydroxyl alkalinity into bicarbonate alkalinity, thus dissolving CO2 

into water does not inherently change the overall concentration of alkalinity but changes the form of 
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alkalinity from hydroxyl to bicarbonate while also decreasing the pH. Dissolving CO2(g) into water will 

lower pH because as the concentration of hydroxyl is lowered, the concentration of protons (H+) will 

increase proportionally (to maintain the constant ionic product of water, Kw = [H+]˖[OH-

]). 

Equations 1 through 4 show dissolving gaseous CO2 into water will increase the concentration of the 

aqueous CO2 species and, hence, TIC, however, further explanation is needed to understand how the 

concentration of each carbonate species will change. Carbonate is a polyatomic ion and the number of 

protons (H+

) bonded to the CO3

-2 ion defines the TIC species. The dependance on proton reactions to 

form CO2 species illustrates a pH dependance. Protonated species, like H2CO3 will form in solutions with 

high concentrations of H+ (low pH) and deprotonated species, like CO3

-2

, will form in solutions with 

lower concentrations of H+ (high pH). HCO3

- will form within the range of the other two species. Figure 

2 relates pH to the distribution of inorganic carbon among the three species. The graph shows H2CO3

dominates at pH < 6.35, HCO3

- dominates within the range of 6.35 < pH < 10.32, and CO32- dominates at 

pH > 10.32. The graph also shows at a pH of 6.35 the concentrations of H2CO3 and HCO3

- are equal and 

at a pH of 10.32 the concentrations of HCO3

- and CO32- are equal. At all pH values, the sum of the three 

aqueous species equals TIC. 

2.1.2 How does Inorganic Carbon affect the treatment of mine drainage?

The previous discussion focused on explaining the interplay between CO2(g) and water and how TIC is 

distributed in water. Coal mine drainage in the eastern U.S. contains concentrations of CO2(aq) that are 

supersaturated compared to atmospheric conditions. The reaction between acidic mine water and 

carbonate minerals, or their byproducts, is the major source of CO2(aq) found in mine water (Equation 7). 

Since elevated concentrations of CO2(aq) are common in eastern coal mine drainage it is important to 

understand its effect on the treatment of mine drainage. 

The impact on understanding the interplay between CO2(aq) and pH is profound when treating mine 

drainage. When an alkali chemical, such as hydrated lime (Ca(OH)2), is added to mine drainage the 

resultant increase in pH (Equation 8) causes the TIC species to redistribute to maintain equilibrium 

conditions (Equations 9 & 10). This will cause deprotonation of TIC species as pH is increased and a 

release of acidity that will cause pH buffering and additional chemical consumption (Equation 11). 

Calcium Hydroxide dissolves in water to increase the hydroxyl concentration and, thus, pH increases: 

TIC species redistribute to establish equilibrium conditions as pH is increased. To illustrate this point, 

assume pH 6.3 mine drainage was dosed with Calcium Hydroxide to achieve pH 11. At a pH of 6.3, 

Figure 2 shows 50% of the TIC species is bicarbonate and 50% is carbonic acid. At pH 11, there is 
8

virtually no carbonic acid and there is less bicarbonate, but more carbonate than at pH 6.3. To achieve 

equilibrium at the higher pH, carbonic acid and bicarbonate must deprotonate to form carbonate, thereby 

increasing the carbonate concentration while decreasing the concentration of carbonic acid and 

bicarbonate. 

Carbonic acid fully deprotonates to produce to carbonate and releases two moles of acidity (H+): 

Bicarbonate deprotonates to increase the concentration of carbonate and releases one mole of acidity:

The deprotonation of carbonic acid and bicarbonate releases acidity that requires an additional dose of 

Ca(OH)2 to neutralize the acidity to prevent pH drop and maintain pH 11 conditions: 

 

The amount of acidity released depends on the initial pH and the final pH since pH determines how TIC 

is distributed among the species. Any change in the distribution of species between the initial and final pH 

must be achieved by protonation (pHfinal < pHinitial) or deprotonation (pHfinal > pHinitial) of the species. 

Assume pH 3 mine drainage is increased to pH 8.4. At pH 3 TIC is in the form of carbonic acid and at pH 

8.4 it has all deprotonated to produce bicarbonate. Inversely, of pH 8.4 mine drainage is decreased to pH 

3, bicarbonate must protonate to produce carbonic acid. If pH 8.4 mine drainage is treated to pH 12.3, all 

bicarbonate must deprotonate to produce carbonate. For more complicated conditions, say a pH 3 water is 

treated to pH of 9.5, a large portion of the carbonic acid deprotonates to produce bicarbonate and a 

smaller portion deprotonates to produce carbonate (Figure 2). Speciation modeling provides a method to 

predict the amount of acidity released as pH is changed. 

For example, assume a 1000 gallon/minute (gpm) acid mine discharge will be dosed with Ca(OH)2 to 

increase the pH from 4.0 to 8.4 for treatment. Assume the mine drainage contains a TIC of 30 mg/L and 

temperature of 12o C. The red line in Figure 3, noted as “Line #1”, represents the plot location of the pH 

4.0 untreated mine water on the TIC distribution graph. The purple line (line #2) represents the treatment 

pH end point of 8.4. The figure shows the percent distribution of the three inorganic carbon species for a 

given pH. Thus, the graph shows almost 100% of the TIC is in the form H2CO3 at the pH of the untreated 

water but ~ all of it deprotonates and converts to HCO3

- as the water is treated to a pH of 8.4. 

As Ca(OH)2 is added, H2CO3 deprotonates to form HCO3

- and release H+. The acidity released during the 

species transformation consumes additional Ca(OH)2 as shown in Equation 11. This release of acidity as 

pH is adjusted during mine drainage is a major cause of increased chemical consumption and cost. 
9

Thus far, the description of the TIC system and its effect on treatment has been described qualitatively; 

however, it can be quantitatively described as well. For example, consider the previously described 1000 

gpm discharge containing a pH of 4 and TIC of 30 mg/L (as C). This solution contains a H2CO3

concentration of 155 mg/L (as H2CO3). The deprotonation of H2CO3 , to produce HCO3

-

, as the pH is 

increased to 8.4 will release 125 mg/L of acidity (expressed as CaCO3) in producing HCO3

-. Given a 

flow of 1,000 gpm, an acidity of 125 mg/L (expressed as CaCO3), and a unit cost of $160/ton for Ca(OH)2 

, equates to an annual chemical cost of $34,910 to just neutralize CO2-based acidity. 

One strategy to minimize this nuisance reaction is to first decarbonate the water before adding chemical to 

adjust pH. The process of decarbonation is to remove CO2(aq) from mine drainage and exsolve it to 

Earth’s atmosphere as CO2(g). Lowering the initial TIC concentration will proportionally lower the acidity

and corresponding base required for neutralization. If the aqueous concentration of CO2 is greater than 

the equilibrium concentration with respect to the atmosphere in contact with the water, Equation 1 will 

proceed to the left (reactants side). Thus, H2CO3 will dissociate into CO2(g) that can escape into the 

atmosphere; all associated aqueous carbonate speciation reactions (Equations 1-3) would go to the left

and lower the TIC. 

2.1.3 What factors control how Inorganic Carbon is transferred from mine drainage to the 

atmosphere (decarbonation)? 

The pressure of Earth’s atmosphere is 1.0 atmosphere at sea level and is made up of a collection of gases, 

mostly nitrogen (N2) and oxygen (O2). It is useful to define the percent by volume, which corresponds to 

the partial pressure of each of the individual gases. The volumetric percentage of Carbon Dioxide in the 

atmosphere is 0.04% and, therefore, the partial pressure of CO2(g), is 0.0004 atmospheres. Henry’s Law 

relates the partial pressure of a gas in an atmosphere above a liquid (PCO2) to the equilibrium 

concentration of that gas dissolved into a liquid (CCO2) by using a gas-specific constant called Henry’s 

constant (kH) (Equation 12a). Henry’s Law accounts for differences in the solubilities of various gases as 

a function of its partial pressure:

Where: 

kH has the units of mmmmmmgggggg

LL∗aaaa

While we will be using the form shown in Equation (12a), note you may see the following form in some 

text books:

Where:

kH has the units of LL∗aaaa

mmmmmmgggggg
10 

The partial pressure of CO2(g) is 0.0004 atmospheres and a table of constants in a chemistry textbook 

indicates kH for the CO2/water system is .034 moles/L atm. Thus, the concentration of H2CO3 (CO2(aq)) in 

equilibrium with the atmosphere is ~ 0.84 mg/L, which is significantly less than the 155 mg/L calculated 

in the hypothetical mine drainage scenario previously presented. Therefore, there is a driving force to 

transfer CO2 from the water to the atmosphere to achieve equilibrium. This water is suitable for 

decarbonation since a concentration gradient exists that favors transferring gas from water to the 

atmosphere. However, the analysis thus far does not consider the amount of time it will take for 

decarbonation or whether the cost of decarbonation (capital, electrical, etc.) is more than the cost of 

adding chemical to neutralize the CO2 acidity as presented in Equation 10. 

The paragraphs above have shown the hypothetical mine drainage is susceptible to decarbonation but the 

mechanisms and factors that affect decarbonation have not been identified. To understand the factors, 

imagine that Figure 4 represents carbonated mine drainage emerging at the surface and being held in a 

shallow pond. Also, imagine the red circles represent CO2 molecules in the air and water and assume the 

concentration of CO2(aq) is greater than the concentration allowed by Henry’s Law when the water is 

equilibrated to atmospheric conditions. In this disequilibrium example, there is a concentration gradient, 

indicated by the upward arrow, so particles will “diffuse” in the direction of high to low concentration to 

achieve equilibrium. Diffusion can be thought of as the random movement of particles by natural means 

and not due to the movement of the fluid (advection). At equilibrium, the concentration gradient would 

be 0 and there would not be a driving force for the net transfer of gas molecules from or to the water. 

Try to think of the factors that would help increase the rate of transfer of CO2 molecules from the mine 

drainage pond to the atmosphere in a passive manner (without adding energy). Intuitively, one can 

imagine the rate of transfer will depend on the magnitude of the concentration (pressure) gradient (more 

particles = more likely to transfer). Furthermore, one can imagine the larger the surface area of the 

pond/air interface, the easier it would be to transfer CO2 molecules as the random motions have more of a 

chance to move across the interface. In addition, one could also image a shallow pond would be more 

efficient at transfer than a deep pond since a shallow pond would quicken transfer by minimizing the 

distance molecules can randomly move away from the interface surface. While the example is focused on 

CO2 molecules, one could imagine molecules with less mass, therefore atomic weight, (i.e. O2) would 

increase the rate of transfer as well since they move more quickly. 

The factors that affect the passive transfer of molecules between air and water are represented in Fick’s 

Law. Fick’s Law describes the rate of passive diffusion of molecules in response to a concentration 

gradient. Fick’s law can be expressed in several ways as shown in Equations 13 through 15. Equation 10 

describes the “net rate” of particles moving through an area in response to a change of pressure (particles 

in a volume) over a distance. The rate of diffusion represents the amount of gas transported over time and 

is governed by several variables including: (1) the concentration/pressure gradient (P1 - P2), (2) the 

surface area of the interface where the transfer takes place (air/water interface)(Sarea), and (3) the 

Diffusion Coefficient which is specific to the gas/liquid system (DCO2). 

 
11

 

 

The Diffusion Coefficient describes the quantity of a gas that is moving through a cross section per unit 

time when the concentration gradient is unity (units of L2

/Time). The Diffusion Coefficient is not only a 

proportionality constant but also incorporates characteristics like molecular weight and solubility. 

Graham’s Law (16) provides the link between molecular weight (MW) and Diffusion Coefficient 

showing D is inversely proportional to the square root of the molecular weight of the gas. 

Thus, heavier molecules, like CO2, will diffuse slower than lighter molecules, like O2. The Diffusion 

Coefficients for CO2 and O2 in water are 1.91 and 2.42 (cm2

/sec @ 25C), respectively, which shows O2 

diffuses about 1.26 times faster than CO2. This makes sense since the O2/ CO2 ratio of 1/√MW of the 

gases is 1.3, which is very similar to the ratio of diffusion coefficients.

Notice Equations 12 through 14 describe steady state diffusion since the diffusion rate or flux of gas 

being transferred is not changing as a function of time, thus the rate of diffusion is constant and is at 

steady state. One reason why the rate of diffusion is constant is because the concentration gradient (P1 -

P2) is constant with time. When decarbonating mine drainage, the concentration gradient will change with 

time since decarbonation will continuously lower the concentration of CO2(aq) as gas is outgassed to 

atmosphere. Therefore, a more robust model that considers a variable rate is required to model 

decarbonation and size decarbonation reactors. 

In addition to needing a non-steady state diffusion equation (rate changes with time) to model 

decarbonation, we need to consider non-passive mechanisms to transfer CO2. Instead of relying on the 

random motion of molecules, why not add an advective component (transfer mechanism due to bulk fluid 

motion) to a decarbonation system to drive molecules in a direction to promote transfer. Moreover, 

instead of designing a decarbonation system with a fixed distance (concentration gradient) between the 

bottom of the fluid and the transfer interface like shown in Figure 4 (e.g. pond), why not create a short 

transfer distance to increase the gradient by either injecting compressed atmospheric air to create bubbles 

in mine drainage (Figure 5) or creating water droplets and splashing them into the air. Creating either 

small air bubbles in water or small water droplets in air will increase the transfer surface area and enhance 

the rate of transfer. In either case, the cumulative surface area of the bubbles or water droplets will be 

orders of magnitudes greater than the transfer surface area of a pond or ditch, the rate of transfer will be 

greater by orders of magnitude. 
12

Both the enhanced transfer due to convection/mixing processes and the fact that the rate of transfer will 

change with time requires a modified version of Fick’s Law: 

Where: 

d[CCO2(aq)]/dt = the rate of change in the concentration of CO2(aq) (moles L-1 sec-1

)

-kLa = “bulk” volumetric (gas) transfer coefficient (sec-1

)

Ci = Initial Concentration of CO2(aq) (moles L-1

)

Cs = Saturation Concentration of CO2(aq) under atmospheric conditions (moles L-1

)

This non-steady state equation shows the rate of CO2 transfer changes with time and depends on both the 

concentration gradient and the gas transfer coefficient. The gas transfer coefficient is considered a “bulk” 

coefficient since it contains both “kL” which represents the mass transfer coefficient that describes the 

transport of gas to the atmosphere and “a” which represents the gas-liquid exchange area per unit volume 

of liquid (e.g. bubble surface area). Furthermore, kL represents a host of mechanisms that affect transfer, 

such as diffusion, advection, mixing effects, and the type of reactor used (batch, plug flow, stirred). It is 

impracticable to be able to separate and quantify all of the individual mechanisms that affect kL and a, 

thus gas transfer coefficients are empirically derived by measuring the performance of operating 

decarbonation devices. Transfer coefficients are process and equipment specific and it may be difficult to 

scale the performance of a decarbonator at one site into the design of a decarbonator for another site. 

The AMDTreat team used Equation 16 and empirically measured gas transfer coefficients for a variety of 

combinations of different mechanical devices (surface aerators, bubble diffusers) paired with different 

chemical reactor designs (continuous stirred, plug flow). Water samples were collected to characterize 

the change in CO2 concentration in the decarbonation unit and dye testing was used to determine retention 

time (Figures 6 and 7). The information was used to determine the gas transfer coefficient for different 

types of decarbonation equipment. This will be further discussed in Section 2.2, Equipment and 

Treatment Configurations. 

2.2 Equipment and Treatment Configurations

Decarbonation is the process of transferring CO2 from the water to the air, the reverse of carbonation. The 

decarbonation module offers the ability to model and predict decarbonation performance using different 

decarbonation devices paired with different chemical reactor designs. The two primary methods for 

mechanical decarbonation involve either pumping compressed air into water or pumping water into the 

air. Both processes enhance the transfer mechanism by creating small particles of air or water with large 

surface areas. AMDTreat offers the ability to model both approaches using several different 

decarbonation units. 
13 

2.2.1 Decarbonation Reactors 

AMDTreat can model several decarbonation devices configured as continuous stirred or plug flow 

reactors (Table 1). Users may also use their own Custom option to enter a transfer coefficient and model 

the performance as either a plug flow (PFR) or continuous stirred reactor (CSTR). Recall, the default 

transfer coefficients provided represent the performance of both the decarbonation device and reactor as a 

system. A key difference between PFR and CSTR is that the concentration of CO2(aq) changes with 

retention time (or distance allow the flow path) in a PFR but is assumed to be a constant average 

concentration in a CSTR because the CSTR is assumed to be instantaneously and completely mixed. 

Therefore, every drop of high CO2(aq) influent water entering the CSTR is instantly diluted to the average 

concentration of the entire reactor. Furthermore, the CO2(aq) concentration of the CSTR is assumed to be 

the same as the effluent concentration since the reactor is assumed to be completely mixed. In other 

words, the concentration gradient between the water and atmosphere is relatively low and constant 

throughout the entire volume in a CSTR. In a PFR, a large concentration gradient may result from the 

initially elevated CO2(aq) concentration in influent to the reactor, which results in greater potential for gas 

transfer upon aeration. As the CO2 is removed, the concentration gradient decreases as a function of 

distance or volume (retention time) through the PFR. Because the initial gradient between CO2(aq) in the 

solution versus CO2(aq) in equilibrium with the solution is relatively high, the same aeration rate in a PFR

results in a gas transfer rate that will be greater than that for a CSTR of similar volume (using the same 

aeration technology). For this reason, PFR is the preferred reactor choice for decarbonation. 

Decarbonation Device Default k La (sec-1) Plug Flow Reactor Continuous Stirred 

Reactor

Surface Aerator 0.00067 NA x

Fine Bubble Diffuser 

Aerator

0.00116 x NA

Maelstrom Oxidizer 0.03195 x NA

Custom User Specified x x

Table 1: Combination of decarbonation devices, reactor types, and default gas transfer coefficients used in 

AMDTreat. The Custom option allows users to enter a customized kLa for either a plug flow or continuous 

stirred reactor decarbonation system. 

2.2.2 Decarbonation Approaches 

2.2.2.1 Mixing Water into Air

2.2.2.1.1 Surface Aerators 

Surface aerators include a large collection of aeration devises, which include floating aerators, fountain, 

and spray aerators. All these devices pump or push water to the surface to create small droplets of water 

that mix with the air to cause transfer. Some of the devices have specially designed splash guards or 

nozzles to promote the formation of small droplets. Floating surface aerators are common decarbonation 

devices at mine drainage treatment facilities. Floating aerators are relatively inexpensive, and ponds and 

reactors can easily be retrofitted to install a surface aerator. They can be installed in PFRs (Figure 8) and 

in CSTRs (Figure 9), however, the decarbonation module in AMDTreat only offers the ability to model 

surface aerators installed in a CSTR tank. Users input the desired retention time and AMDTreat selects a 
14

tank depth based on the flow rate. AMDTreat estimates the cost to purchase and install a tank based on 

user input and selections and does not attempt to evaluate whether the design can achieve the predicted 

performance. 

The AMDTreat team collected field data and calculated KLa for several sites using surface aerators for 

decarbonation. Empirical data were collected on the rates of CO2 outgassing during an aeration 

experiment at one AMD site described by Cravotta (2015) and at several active or passive treatment 

AMD sites in Pennsylvania that employed various aeration or other treatment technologies Values for 

kL,co2 were estimated from the linear slope of Ln(C0-CS)/(Ct-CS)] versus t, where t is elapsed time during 

the aeration experiment or travel time between measurement points. Either dye tests were used to 

determine retention time or residence time was computed by dividing the estimated water volume by the 

measured flow rate. No attempt was made to explicitly consider the effects of water depth, wind, and 

other hydrodynamic parameters on the gas exchange rates or solute transport (e.g. Rathbun, 1998; Zappa 

et al., 2003). The empirical values corrected to 20 °C for kL,CO2 which resulted in a default KLa for surface 

aeratorsof 0.00067 sec-1 Users can use the “custom” option in AMDTreat to input a customized KLa and 

model decarbonation in either a PFR or CSTR configuration. When choosing a customized KLa it is 

recommended that the user validates the selection based upon field testing of similar equipment designs 

or available verifiable performance data. 

2.2.2.2 Mixing Air into Water

2.2.2.2.1 Fine Bubble Diffuser Aerator

Fine bubble diffuser aerators are available in many different configurations and styles. In mine drainage 

treatment, compressed air blowers push air through a grided piping network installed in the bottom of a 

tank or pond reactor (Figures 10 & 11). Diffuser disks or tubes installed throughout the piping network 

transform the compressed air into bubbles less than a millimeter in diameter (Figures 12 & 13). Typically,

a check valve separates the diffuser head from the piping network. When applied to mine drainage 

treatment scenarios, the diffuser heads are susceptible to fouling with scale or precipitate. Flexible head 

aids in cleaning. The head is typically constructed of a flexible material such as EPDM rubber to aid in 

removing scale. Some designers opt for larger orifices (larger bubbles) to help prolong and manage 

clogging issues with iron hydroxide. 

AMDTreat assumes the bubble diffuser system consists of a grided piping network installed in the bottom 

of a rectangular tank constructed of either concrete or bolted or welded steel and configured as a CSTR. 

The AMDTreat team collected field data and calculated KLa for several sites using bubble diffusers for 

decarbonation (Figure 14). The field-collected data was used to develop a default KLa for bubble diffusers 

of 0.00116 sec-1

. Users can use the “custom” option in AMDTreat to input a customized KLa and model a 

bubble diffuser decarbonator in either a PFR or CSTR configuration.

AMDTreat estimates the cost to purchase and install a tank based on user input and selections and does 

not attempt to evaluate whether the predicted performance can be achieved with the selected design. 

Users may want to use the decarbonation option in the PHREEQ-N-AMDTreat model to help predict 

performance. 
15

2.2.2.2.2 Maelstrom Aerator (Oxidizer) 

The Maelstrom’s official trade name is the Maelstrom Oxidizer and is manufactured and marketed by 

Somerset Environmental Solutions (SES) (Figures 15 & 16). The name “Maelstrom Oxidizer” implies its 

primary function is to oxidize ferrous iron. While it can accelerate iron oxidization, the primary 

geochemical function is gas exchange which has a secondary effect of accelerating iron oxidation as 

exsolving CO2(aq) will increase pH. The Maelstrom is typically sized for one minute of retention time 

configured as a PFR where water enters one side of the tank and flows to the other side. It is a popular 

decarbonation device in the Appalachian coal fields because of its small footprint, ease of installation, and 

effectiveness. Most other decarbonation devices require engineering to properly size and design the 

decarbonation device, tank, and foundation. Maelstrom is a turn-key system that is offered in four 

standard sizes and can be combined in various sizes to achieve the needs of a site. The cost typically 

covers all the componentry and the installation of the system on a foundation. 

The Maelstrom consists of three parts, the compressed air blower, the plenum box, and the decarbonation 

tank. The blower compresses atmospheric air (Figure 17) and forces the compressed air into a 0.5 ft 

height plenum box. The plenum is physically the bottom part of the decarbonation tank system and 

serves to distribute air to the numerous air tubes in the overlying decarbonation tank (Figure 18). The air 

flows to the top of the air tube and down between the tube and tube housing before exiting to the water 

through the stainless-steel air nozzle band (Figure 19). The relatively large orifices on the band are shaped 

to cause airflow countercurrent to water flow which creates violent mixing between the air bubbles and 

water. While the air orifices are relatively large, fouling due to calcite, aluminum, and iron scale does 

occur and requires periodic maintenance (Figure 20). Maintenance usually involves using a cordless 

driver to remove a bolt that attaches the air tube housing to the air tube. The housing is removed, and the 

orifices are pressured washed to remove the scale. One benefit is the maintenance can occur while the 

system continues to operate. The need for maintenance is visually monitored by visually inspecting 

changes in water level in each row of air tubes in the tank. Figure 21 shows a drop in water level due to 

the lack of air volume caused by a clogged section of air tubes. The water flow in Figure 21 is from the 

top of the photo to the bottom. 

Some opt to house the blower in an acoustic-silencing housing to lower noise levels in situations where 

noise is a concern (Figures 17 & 22). Adding a variable frequency drive (VFD) to the electrical control 

panel provides the ability to control the blower motor speed which controls horsepower and airflow 

(Figures 23 & 24). This allows operators to calibrate the system to achieve the optimal cost-savings 

(decarbonation/HP). Neither of these options are included in the AMDTreat cost estimates for the 

Maelstrom. 

The default KLa used in AMDTreat for the Maelstrom is 0.03195 sec-1

. The typical height of Maelstrom 

units is 5.0 ft (plenum-0.5, water depth-2.5, air allowance-0.5, &freeboard-1.5 ft)
16 

2.2.1.2.3 Custom Decarbonation

AMDTreat provides the option for users to enter a custom KLa to model decarbonation configured as a 

continuous stirred or plug flow reactor. When choosing this option it is recommended that the user 

validates the selection based upon field testing of similar equipment designs or available verifiable 

performance data. 

3.0 Decarbonation 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) 

Equipment and System Installation, (3) Annual Cost Input (4) Other Capital Items, and (5) Other Annual 

Items. The workflow is for users to begin at the top left-hand side and continue down on the left-hand side 

entering all the appropriate input parameters. 

Module output is provided on the right-hand side of the module. Module outputs are arranged into four 

sections: (1) Sizing Summary, (2) Capital Cost, (3) Annual Cost, and (4) Net Present Value. The Sizing 

Summary section provides the calculated Decarbonation Design Output, Water-Quality Output, Hydrated 

Lime Savings Assessment (if selected as part of the input), and Decarbonation Tank Sizing Summary

information. The estimated cost to install and operate the decarbonation tank, along with the user 

specified components, is provided under the Capital Cost and Annual Cost headings. The final output 

section includes the Net Present Value (NPV) analysis. This section provides an estimate of the total cost 

to operate and maintain this treatment system component 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 selecting the information icon (

) in each of the subheadings in the module. 

3.2 Module Inputs

3.2.1 Water Quality and Flow Input: The user specifies the design flow in this section. Normally 

the design flow is used to design the components of the treatment facility and represents the 

maximum flow of the mine drainage discharge or pumping well. This flow rate is used to calculate 

the tank retention time for the tank size that has been selected in the next module input section. If the 

treatment system only experiences the maximum flow for a small percentage of the time, it may be 

appropriate to use the 50 or 75 percentile flow as the design flow to avoid excess capital and annual 

costs. 
17 

3.2.1.1. Raw pH, Temperature, Alkalinity, and TIC: 

AMDTreat offers users two methods to calculate the concentration of CO2(aq) for the raw water. 

Users can use the radio button to select either the Alkalinity or TIC method. Both methods 

require users to specify the raw pH and temperature, and either define the alkalinity or TIC 

concentration. AMDTreat uses this information to speciate the water and calculate the 

concentration of CO2(aq). Please note, users must use the TIC method to compute CO2(aq) if the raw 

pH is < 4.5 (~ no alkalinity). 

Users must be aware that the accuracy of this model is highly sensitive to the accuracy of the 

initial input parameters used to calculate CO2(aq). Field-measured pH, alkalinity, and temperature 

is required, and it is important to try to obtain an in-situ measurement of pH to minimize 

accidental exsolving CO2 during measurement, causing pH change. The use of laboratory 

alkalinity values may result in artificially low results due to consumption by iron oxidation or 

calcite precipitation during sample transport. TIC samples must be carefully collected. The 125

ml amber glass bottles (w/septum) must be filled and capped underwater, as far below the surface 

as possible, in a manner to avoid exsolution of CO2. 

3.2.2 Decarbonation System Equipment and Installation: 

A decarbonation system consists of the decarbonation unit and a tank. Users make selections under 

the three subsections to size the decarbonation system. In the Decarbonation Systems Options section, 

users specify the desired decarbonation retention time and specify the type of decarbonator and 

reactor design. It is important to note the Maelstrom is a premanufactured turn-key system that does 

not require user-specified design options. Instead, AMDTreat uses the design flow and determines the 

optimal number and sizes of Maelstrom units (Table 2). The design retention time for a Maelstrom 

unit is ~ 1 minute. The cost of the purchase and installation for the Maelstrom estimated by 

AMDTreat includes a concrete foundation. 

Tank Size (LxWxH)(ft) Tank Volume (gal) Blower HP Blower CFM

5 x 5 x 5 300 5 400

5 x 10 x 5 600 10 1,000

10 x 10 x 5 1,200 20 2,000

5 x 15 x 5 1,850 30 3,000

*Maelstrom is sized for a 1 min. retention time; thus a 300-gal tank is sized for flows up to 300 gpm 

Table 2: Characteristics of Maelstrom decarbonation units. 

For non-Maelstrom decarbonation systems, the Non-Maelstrom Decarbonation Tank & Equipment 

Input section is used to specify the tank construction material, protective coating and depth and 

freeboard dimensions. AMDTreat sizes the tank in a square configuration. The protective coatings 

help to prevent corrosion and increase the longevity of the tank. It is standard to use coatings to 

protect equipment materials in the treatment of mine drainage and the type of coating affects the 

default equipment lifecycle used in the Net Present Value section of this module. Additionally, this 

section allows users to select and include the cost of stairs, hand railing, and a catwalk for the 

decarbonation tank. The stairs are assumed to be positioned at 37 degrees from horizontal and each 

section reaches 12 ft vertically. The catwalk is assumed to span across the entire tank. Users can opt 
18 

to include a variable frequency drive (VFD) to control the speed of the decarbonation motor. This 

provides the ability to optimize decarbonation while controlling electricity costs. Furthermore, a VFD 

can offer the treatment system additional performance flexibility with respect to varying the amount 

of CO2 removal in order to achieve a desired effluent alkalinity concentration. For example in certain 

scenarios or during seasonal conditions, allowing a certain amount of aqueous CO2 to remain in the 

raw water will allow for higher alkalinities in the effluent which in turn may be more desirable in 

achieving water quality restoration goals in the watershed. 

The water depth within the tank is assigned by AMDTreat based on the design flow (Table 3) and 

users specify the tank freeboard in this section. The total tank height is the summation of the water 

depth and the freeboard depth. 

Flow (gpm) <500 >500 < 1500 >1500<3000 >3000<4500 >5000

Tank Depth (ft) 3 5 10 15 20

Table 3: Default water depth based on design flow used by AMDTreat to size decarbonation tank. 

For additional information on catwalk, hand railing, stairs, and protective coatings, please refer to the 

Reaction Tank help file. The Reaction Tank module uses the same equipment as Decarbonation, 

except for the tank geometry and lack of a Fiberglass option in Decarbonation. 

The Non-Maelstrom Decarbonation Tank Foundation section is used to cost the tank foundation. 

Users specify the load-bearing capacity of the soils for the site along with the unit cost to purchase, 

deliver, and form a rebar-reinforced foundation. The load-bearing capacity of the solids changes the 

foundation area. Please see the Reaction Tank help file to learn more about how soil quality affects 

foundation sizing. The foundation area is assumed to be the footprint of the tank plus a one-ft apron 

and AMDTreat estimates the capital cost. 

3.2.3 Annual Cost Input: This section contains the unit costs that effects estimate of annual costs. 

3.2.3.1 Aerator Operational Time: Some mine drainage treatment systems do not continuously 

operate. For example, many underground mines have pump and treat systems that are designed to 

maintain a certain mine pool elevation to prevent a surface discharge. During dry climatic 

conditions, a mine pool may not have to be pumped for weeks until it fills to an elevation that 

triggers pumping. The Operational Time annual cost input provides users with the ability to 

specify the period of time the decarbonation system operators. The operational time will affect 

the annual electricity cost. 

3.2.3.2 Electrical Unit Cost: Users must specify the unit cost of electricity for the site they are 

interested in evaluating. The true cost of electricity includes taxes, transmission, and distribution 

fees, therefore, one should take the total cost of a monthly electrical bill and divide by the kWh 

used to determine the electrical unit cost. This cost is used to calculate the annual electrical cost 

of all electrical components in the treatment system. In the case of decarbonation, it is used to 

estimate the cost to operate the decarbonation blower.
19 

3.2.3.3 Hydrated Lime Cost Savings Assessment: Users can opt to have AMDTreat perform a 

cost evaluation for decarbonating mine drainage. The cost evaluation consists of estimating the 

annual hydrated lime requirement to treat to a user-specified treatment pH and comparing that 

cost to the cost of first decarbonating mine drainage and then adding hydrated lime to the same 

treatment pH. The non-decarbonation cost, which consists of the annual hydrated lime cost, is 

compared to the decarbonated treatment cost, which includes both the electrical cost to operate 

the decarbonation blower and the annual hydrated lime cost, to calculate the net annual savings. 

This provides the user with an overall estimate of cost savings. See the help file sections for Post 

Decarbonation Solution and Hydrated Lime Savings Assessment (part of the Sizing Summary 

section) for additional information. 

3.2.4 Other Capital Items: The Other Capital Items section allows users to capture the capital cost of 

equipment and other items that are not included in this module. For example, in some cases users may 

want to include the cost of adding steel gridding across the entire top of the decarbonation tank to allow 

complete access instead of a catwalk. This can be accounted for here in the Other Capital Items section to 

capture the capital cost. 

3.2.5 Other Annual Items: The Other Annual Items section allows users to capture the cost of other 

annual items that are not included in this module. For example, users could include the annual 

subscription cost to conduct electronic surveillance on the treatment system in the Other Annual Items 

section. 

3.3 Module Outputs

3.3.1 Sizing Summary: The Sizing Summary section displays important calculated module outputs, 

such as estimates of CO2(aq) concentrations, prediction of post-decarbonation pH, decarbonation tank 

dimensions, and estimated blower airflow and horsepower. 

3.3.1.1 Decarbonation Design Output: This section provides an estimate of the calculated 

retention time for the decarbonation tank, an estimate of the % of CO2(aq) removed by the 

simulated decarbonation system, and provides the gas transfer coefficient (Kla) used to model the 

decarbonation system. The % of CO2(aq) removed is calculated by first speciating the water based 

on the user-specified pH, Temp and either the specified alkalinity or TIC concentration. Next 

AMDTreat assigns a Kla based on the decarbonation unit selected, an integrated form of Equation 

12 is used to predict the final CO2(aq) concentration after decarbonation. The % removal is 

determined from the initial and final CO2(aq) concentrations. 

3.3.1.2 Water Quality Output: This section provides the results of the speciation calculations for 

the pre and post-decarbonated water based on user-specified values for pH, Temperature, and 

either alkalinity or TIC. monitored to understand how sensitive speciation calculations are to user 

input and selections. This section is subdivided into two sections, Initial Solution and Post 

Decarbonation Solution. 

Initial Solution (raw water): This section provides the results of using the user specified 

values to speciate the raw water. The calculated value for the CO2(aq) concentration 

represents the amount of CO2 available for decarbonation. If the “Alkalinity” method 

(under the Water Quality & Flow Input section) was selected to estimate CO2(aq)
20 

concentrations, then the water is speciated and the TIC concentration is calculated and 

shown in this subsection. If the “TIC” method (under the Water Quality & Flow Input 

section) was selected to estimate CO2(aq) concentrations, then the water is speciated and 

the Alkalinity concentration is calculated (if present in the water) and shown in this 

subsection. 

Post Decarbonation Solution: This section provides estimates of the pH, TIC, CO2(aq), 

and Alkalinity after decarbonation has been simulated using the user-selected 

decarbonation device. It is important to note this section provides estimates assuming the 

solution solely consists of water and TIC species and does not consider the effect other 

reactions, such as iron oxidation or aluminum hydrolysis, will have on the post 

decarbonation pH, alkalinity, or TIC. Users are encouraged to use the PHREEQ-NAMDTreat tool to model the behavior of complex solutions in a decarbonation scenario. 

The decarbonation model is a simple tool that provides a rough estimate of the effect 

decarbonation will have on a carbonated solution. 

3.3.1.3 Hydrated Lime Savings Assessment: This section provides an estimate of the annual net 

savings of decarbonating mine drainage. This section is only active if the Hydrated Lime Cost 

Saving Assessment is checked under the Annual Cost Input section. The concentration of CO2(aq)

outgassed is provided under the Water Quality Output section by comparing the pre and postdecarbonation concentrations. This section recasts the concentration outgassed in terms of the 

acidity that would be released and consumed by an alkali chemical to achieve the treatment pH. 

The Ca(OH)2 Savings is the value for CO2 Acidity Outgassed recast in terms of Hydrated Lime 

and the Ca(OH)2 Cost Savings represents the chemical savings due to decarbonating the CO2

instead of neutralizing it during alkali addition. Cost to Operate Decarbonation Blower is 

calculated by using the blower HP and the unit cost of electricity to estimate the cost of 

Decarbonation. The Net Cost Savings is the Ca(OH)2 Cost Savings minus the Cost to Operate the 

Decarbonation Blower. 

3.3.1.4 Non-Maelstrom Decarbonation Tank Sizing Summary / Maelstrom Decarbonation Tank 

Sizing Summary: The last section in the Sizing Summary provides specifications and dimensions 

for the Decarbonation System. If a Maelstrom is selected, this section is titled Maelstrom 

Decarbonation Tank Sizing Summary and provides the number and size of tanks estimated by 

AMDTreat for the design flow, along with the total blower HP and airflow. If a decarbonation 

system other than a Maelstrom is selected, this section will be titled Non-Maelstrom 

Decarbonation Tank Sizing Summary and will display the tank dimensions along with blower 

estimates. 

3.3.2 Capital Cost: This section provides the estimated costs for the selected decarbonation components. 

If Maelstrom was selected, a single cost will be provided since Maelstrom is a turn-key system and the 

purchase cost includes the decarbonation system, foundation, and installation. For all other decarbonation 

units, users can either specify the installation costs or estimate the installation cost as a percentage of the 

capital cost. 

3.3.3 Annual Cost: The annual cost section provides an estimate of the annual cost to operate and 

maintain the Decarbonation system. The annual operation and maintenance cost can be either specified or 

estimated as a percentage of the capital cost. Similarly, the annual Electric cost can be specified or 
21 

estimated based on kW requirements of the electrical componentry and the specified unit cost of 

electricity. 

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 

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, single values for Term of Analysis, Rate of Return, and Inflation Rate are 

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. 
22 

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 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 17). 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 

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 = 

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

O&M cost from the Annual Cost section to the NPV 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 
23

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. 

3.3.4.3 Rationale for Recapitalization Recommendations:

Recapitalization recommendations are based on either manufacture recommendations 

(per discussions with manufactures) or professional experience of the AMDTreat Team. 

In either case, the recommendations may not apply to all situations. Users are encouraged 

to customize the recapitalization assumptions to their treatment scenario. AMDTeat 

Team members located in Pennsylvania and West Virginia have a collective experience 

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

The AMDTreat Team held discussions on personal experience to develop a list of 

recapitalization recommendations. Users may have different experience and opinions 

than those listed. 

By default, AMDTreat includes a list of six recapitalization items for a non-Maelstrom 

decarbonation system and one for a Maelstrom system. The only recapitalization item for 

a Maelstrom is for the Blower. 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 NPV Cost, shown in the NPV Heading. For example, the 

default value for the lifecycle of the aerator/blower is 10 years due to experience with 

decarbonation systems. However, users may opt to periodically rebuild the blower motor 

instead of purchasing new. In this case, users could opt to add a recapitalization cost to 

rebuild the motor every 10 years and replace the entire blower unit every 30 years. Users 

are free to fully customize the replacement items, including adding new items or deleting 

default items. 

Decarbonation Tank: The Life Cycle of a steel reaction tank depends on how well the 

bottom of the tank is protected from ground moisture, how often it is sandblasted and 

painted, and whether the tank is vandalized. After considering the condition and life of 

many steel tanks, the AMDTreat team feels 35 years is a good estimate as long as the 

tank is on a concrete or gravel foundation or on wooden cribbing to prevent contact with 

ground moisture. A much longer Life Cycle may be appropriate if the tank is cleaned, 

primed, and painted when corrosion becomes visible. Concrete reaction tank Life Cycle 

is defaulted to 75 years based upon observations of existing systems that AMDTreat 

members have observed. This assumption is only valid if protective coatings are 

periodically reapplied. 
24

The default Life Cycle for a fiberglass tank is 25 years. This value was determined after 

discussing tank longevity with chemical distribution companies and tank manufactures 

assuming the tank would be placed outside and open to the atmosphere. 

Catwalk & Stairs with Handrails: The default Life Cycle of both the Catwalk and 

Stairs with handrails is set at 20 years at 100% Replacement. These are safety and access 

items so the user should consider if the life cycle value needs adjusted. 

Tank Protective Coating: This item would only apply to the concrete and steel tanks. 

The fiberglass tank would not have a protective coating on the tank. Depending on the 

specific coating (Alkyd, Epoxy, or Zinc/Urethane) selected the default Life Cycle will be 

different. 

Decarbonation Aerator/Blower: The default Life Cycle is 10 years and at 100% 

Replacement after surveying several treatment operators. 

Variable Frequency Drive: The default Life Cycle is 20 years and at 100% 

Replacement.

3.4 Assumptions of Design Sizing and Costs

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 Decarbonation module. 

1. AMDTreat assumes the gas transfer coefficients assigned to each of the decarbonation devices 

are representative of that type of device and scalable. This is not the case since the specifics on 

the decarbonation devices and tanks are not known and are not represented in AMDTreat. The 

transfer coefficients were measured by the Team at numerous locations and a representative 

coefficient was selected for a type of decarbonation system. These coefficients may not apply to 

your situation. A prudent approach would be to visit a site with a desired decarbonation system, 

measure the transfer coefficient, and use the coefficient and AMDTreat to model different sizing 

scenarios. 

2. The effect of Decarbonation on species concentrations assumes an ideal solution containing only 

water and TIC species. Mine drainage is a complex solution and various reactions can occur 

during Decarbonation that will affect results. This module should be used for approximation only. 

Fully scale geochemical models, like PHREEQ-N-AMDTreat or other codes should be used to 

fully predict solution changes. 

When choosing a Custom Decarbonation option AMDTreat team recommends that the user 

validates the selection based upon field testing of similar equipment designs or available 

verifiable performance data. 5.0 Figures 
25 

4.0 Figures 

Figure 1: Relationship between gaseous and aqueous carbon dioxide 

species. 

Figure 2: Distribution of TIC based on pH. 
26

Figure 3: Deprotonation of TIC species when pH is raised during 

treatment. 

Figure 4: Depiction of a shallow pond containing carbonated mine 

drainage. Red circles represent CO2 molecules.
27

Figure 5: Pictorial showing the mixing of compressed air into the water, 

which changes the distance between the CO2 molecules and the air/water 

interface (increases gradient), the air bubbles increase the overall 

surface area and enhance transfer.

Figure 6: Dye testing on a decarbonation unit to determine the CO2

transfer coefficient. 
28

Figure 7: Dye testing on a Maelstrom Oxidizer decarbonation unit to 

determine the CO2 transfer coefficient. 

Figure 8: Two surface aerators in a plug flow reactor providing 

decarbonation. 
29

Figure 9: A surface aerator mounted in a continuous stirred reactor 

pulling water to the surface and creating droplets under the splash hood. 

Note the bubble froth promotes strong gas transfer, however, better 

results would be seen with froth in a plug flow reactor. 
30

Figure 10: Fine diffuser aeration system with a piping grid at the bottom 

of a tank and disk diffusers.

Figure 11: Fine diffuser aeration system with a piping grid at the bottom 

of a tank and tube diffusers.
31

Figure 12: Ethylene Propylene diene monomer (EPDM) rubber disk 

diffusers.

Figure 13: Ethylene Propylene diene monomer (EPDM) rubber tube 

diffusers.
32

Figure 14: Fine bubble diffuser decarbonation system configured as a 

plug flow reactor. 

Figure 15: Maelstrom Oxidizer aerator unit. The blower enclosed in 

acoustic silencing housing is shown in the background and the aeration 

unit is shown in the foreground. 
33

Figure 16: Maelstrom Oxidizer aerator unit used to decarbonate anoxic 

and carbonated underground mine drainage. Compressed air blower not 

visible in photos.

Figure 17: Compressed air blower housed in acoustic-silencing housing. 

Maelstrom is located in a residential area and noise was a concern. 

AMDTreat does not include the cost of acoustic housing.
34

Figure 18: Maelstrom decarbonation tank showing vertical baffles 

separating the rows of air tubes. 
35

Figure 19: Photo showing the air tube (steel tube in tank) with air tube 

housing removed. Compressed air from the plenum flows to the top of 

the air tube and down in between air tube and housing before exiting 

through the stainless-steel air nozzle band. 
36

Figure 20: Air Tube removed for maintenance. The air orifices on the 

stainless-steel band at the bottom of air tube are pressure washed to 

remove scale and precipitate from clogging orifices. Note the orifices are 

directional and results in half of the air tube forcing air horizontally and 

countercurrent to the water flow. This causes strong mixing between 

water/air that is optimal for gas transfer.
37

Figure 21: Notice reduced water levels within the red box of the 

Maelstrom on the right side of the photo. The reduced airflow reduces 

the water elevation and is a sign the stainless steel air nozzle banks are 

clogged and require cleaning. 
38

Figure 22: Sixty HP blowers enclosed in acoustic building to reduce 

noise. The Maelstrom Tanks sit above the building on top of the steel 

structure. 
39

Figure 23: Spencer blower and electrical panel with variable frequency 

drive control panel inside of building shown in Figure 22. 

Figure 24: Variable 

Frequency Drive control 

panel provides the ability to 

control the blower output. 

This provides the ability to 

alter the 

decarbonation/horsepower 

ratio to optimize treatment 

costs.