Help Instruction File:

Soda Ash 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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Soda Ash Module Overview

Table of Contents

1.0 OBJECTIVE 3

2.0 OVERVIEW AND APPLICATION 3

2.1 Physical and Chemical Properties 3

2.2 Benefits/Drawbacks, Equipment and Typical Treatment Configurations 5

2.2.1 Benefits 5

2.2.2 Drawbacks 5

2.2 Equipment and Typical Treatment Configurations 5

2.3 Application and Financial Comparison 6

3.0 SODA ASH MODULE OVERVIEW 7

3.1 Layout and Workflow 7

3.2 Module Inputs 7

3.2.1 Water Quality and Flow Input 7

3.2.2 Soda Ash Information 8

3.2.2.1 Soda Ash Dispenser 8

3.2.3 Chemical Consumption 8

3.2.3.1 Stoichiometric 8

3.2.3.2 Titration 9

3.2.3.3 User-Specified Quantity 9

3.2.4 Annual Cost Input 10

3.2.5 Other Capital Items 10

3.2.6 Other Annual Items 10

3.3 Module Outputs 10

3.3.1 Sizing Summary 10

3.3.2 Capital Cost 10

3.3.3 Annual Cost 10

3.3.4 Net Present Value 10

3.3.4.1 Financial Variables 10

3.3.4.2 Cost Categories 11

3.3.4.3 Rationale for Recapitalization Recommendations 13

3.4 Assumptions of Design Sizing and Costs 13

4.0 FIGURES 14
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Figure 1: 55-gallon barrel with hole cut into it containing soda ash briquettes, with mine drainage flowing 

through each end of the barrel to allow dissolution of soda ash. 14

Figure 2. Photo of a soda ash dispenser with soda ash briquettes placed within mine drainage flow path. 15
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1.0 Objective

The objectives of this overview are to: (1) Provide an understanding of the application of Soda Ash in 

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

developing an estimate of the cost to construct, operate, and maintain Soda Ash treatment systems. The 

module can also be applied to reverse cost model an existing system to establish and evaluate future 

financial and investment decisions. The information is presented in two sections, Overview and 

Application and Soda Ash Module Overview. 

2.0 Overview and Application

A basic understanding of the chemical and physical properties, the application, and equipment 

requirements of a soda ash treatment system are required to develop a treatment strategy using the 

AMDTreat software. These topics are discussed below before discussing the soda ashmodule interface 

and functionality to provide the necessary context. The Overview and Application section is organized 

into three parts: (1) Physical and Chemical Properties and (2) Benefits/Drawbacks, Equipment and 

Typical Treatment Configurations, and (3) Application and Financial Analysis. 

2.1 Physical and Chemical Properties

Soda ash is an alkali consisting of a white anhydrous, powdered or granular material with a chemical 

formula of Na2CO3, which is sodium carbonate. It is soluble in water and occurs worldwide naturally and 

specifically in the minerals trona and nahcolite. It is an essential raw material in the United States used in 

the manufacturing of glass, detergents, chemicals and other industrial products. In terms of health and 

safety, any exposure to soda ash may cause skin and eye irritation; however, soda ash does not have the 

serious concerns compared to other treatment chemicals, such as hydrogen peroxide or caustic soda. Soda 

ash is manufactured by an industrial process, the Solvay process, or mined from natural deposits. The 

Solvay process produces soda ash from brine, as a source of sodium chloride [NaCl] and from limestone, 

as a source of calcium carbonate [CaCO3]. The overall process chemical reaction is:

2NaCl + CaCO3 → Na2CO3 + CaCl2 

There are a number of chemical companies that manufacture soda ash. The delivery of soda ash typically 

comes in 50-lb bags when used for mine drainage treatment. Based on soda ash being administered via a 

dispenser, it is mostly utilized for low flow mine drainage discharges. Because soda ash is in a solid form 

and not pre-dissolved in a slurry mix (i.e. lime slurry) prior to adding to mine drainage, it is not ideal or 

cost effective for treating higher flow, high acidity discharges. 

While soda ash treatment systems are simple, dependable, and able to achieve effluent standards, the 

dosing of soda ash briquettes directly into mine drainage in order to dissolve without mechanical mixing 

results in poor dissolution ‘mixing’ efficiency of the solid briquettes. Soda ash reacts with water, 

dissociating into carbonate [CO3

2-

] and sodium ions [Na+] (equation 2). This reaction allows carbonate 

alkalinity to be available for acidity [H+] neutralization (equation 3). With the solid briquettes having a 

relatively small surface area available for chemical reactions and dissolution to occur; the mixing or 

dissolution efficiency (or lack thereof) is one of the biggest disadvantages of soda ash. In addition, soda 
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ash is considered a weak base compared to caustic soda [NaOH] or hydrated lime [Ca(OH)2]; therefore, 

all the alkali ions [CO3

2-

] will not disassociate in the mine drainage, requiring more chemical addition.

Na2CO3 + H20  CO3

2- + 2Na+

CO3

2- + H+  HCOTypically, when determining to use soda ash as a treatment option, the only other option might be to use a 

caustic soda solution, in lieu of the soda ash. Therefore, treatment professionals must understand the 

relationship between neutralization capacity and unit cost between different reagents to make informed 

treatment decisions. Table 2 provides neutralization characteristics of commonly used caustic soda 

solutions relative to soda ash. The table shows that the caustic solutions have less neutralization capacity 

than an equivalent amount of soda ash. However, unlike caustic soda, soda ash is less soluble and 

requires placing briquettes in the mine drainage for dissolution of the solid soda ash particles. Thus, soda 

ash requires the continuous dissolution of the particles for CO3

2- ions to be available for mine drainage 

neutralization. Looking at the ‘lbs reagent/ton acidity’ ratio for the soda ash vs 50% NaOH solution, they 

are comparable when accounting for a 60% mixing efficiency for the soda ash. In this regard, caustic soda 

has big advantage over soda ash. Caustic soda is highly soluble, quick reacting and requires little mixing

in water. Simply dripping a caustic soda solution directly into turbulent flowing mine drainage provides 

sufficient mixing for treatment. A solution of caustic soda is denser than water so dripping into a pond or 

very slow-moving water may result in the accumulation of chemical at the bottom and result in decreased

chemical efficiency (actual dose > theoretical dose). 

Table 1: Neutralization characteristics of various caustic soda solutions compared to other common alkali 

treatment reagents. 

NA NaOH 

20%

NaOH 

25%

NaOH 

50% Soda Ash

mg of CaCO3 /mL reagent 306.0 399.8 959.0 2,396

lbs of CaCO3 /gal reagent 2.55 3.34 8.00 19.9

lbs of CaCO3 /lbs reagent 0.25 0.31 0.63 0.94

gallons reagent/ton acidity 

(CaCO3)*

783 599 249 100.5

lbs reagent/ton acidity (CaCO3)* 8,001 6,401 3,200 2,128 

(3,530)

* Represents the amount of reagent required to neutralize a ton of acidity (CaCO3)

Soda ash adjusted [60% mixing efficiency, (3,530)].

The purity of soda ash (% Na2CO3 by wt.) used in mine drainage treatment is approximately 99% pure. 

Soda ash is produced in various forms depending on the application and is mostly sized as briquettes (1” 

to 2 1/2”). The soda ash dispensers are typically 55-gallon barrels containing the soda ash product with 

holes cut into it to allow mine drainage to flow through and dissolve the briquettes (Figure 1). Slightly 

more sophisticated soda ash dispensers can be used, which consist of cone shaped metal bin with a flume 

feature that dispenses the soda ash into the mine drainage (Figure 2). 

Similar to other chemical treatment (i.e. caustic soda or pebble lime) the motivation to directly add soda 

ash into mine drainage is mostly due to a lack of electricity at a site and the nature (i.e. low flow) of the 
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mine drainage. Generally, sites with no electricity are forced to use caustic soda (NaOH) or soda ash due 

to their ability to be manually dispensed and passively mixed. However, given the very low capital cost 

for dispensing soda ash, minimal health and safety concerns and low operation and maintenance with

soda ash, an operator may choose to treat the mine drainage with soda ash compared to caustic soda. 

2.2 Benefits/Drawbacks, Equipment and Typical Treatment 

Configurations

This section summarizes some of the benefits, drawbacks, equipment, and treatment configurations for

using soda ash at mine drainage treatment sites. 

2.2.1 Benefits

The major reason why soda ash is popular is because of its simplicity, low operation and maintenance and 

low capital cost. The benefits of using soda ash as a mine drainage treatment chemical include: 

1. Can be placed in flow path of mine drainage (i.e. toe-of-spoil seeps, collection ditch) and allowed 

to dissolve; 

2. Dosing can be performed without electricity or the use of a metering pump;

3. Very small footprint of soda ash dispenser (i.e. 55-gallon drum);

4. Low operation and maintenance and low capital cost;

5. Relatively stable chemical allows for long-term storage in instances of ephemeral or periodic 

treatment;

6. No serious safety concerns with handling and storage;

7. Provides a good temporary treatment option until long-term treatment is evaluated. 

2.2.2 Drawbacks

The drawbacks of using soda ash as a mine drainage treatment chemical include: 

1. Poor ‘Mixing Efficiency’ due to using solid briquettes, making soda ash inefficient for higher 

flow mine drainage; 

2. Soda ash is a weak base, requiring additional chemical to be added in order to treat high acidity 

mine drainage. 

3. Based on the unit cost of soda ash, it is not cost-effective to treat high-flow mine drainage

compared to other alkaline chemicals. 

2.2 Equipment and Typical Treatment Configurations

The soda ash treatment systems are the simplest setup of all the passive or active treatment systems. As 

mentioned, soda ash has only really been utilized in remote situations where operators have to treat an 

impacted seep or low flow mine drainage. . In these remote scenarios where there is no electric available 

that may be required for other active chemical treatment options and/or there is insufficient real estate

available to construct a passive treatment system is where soda ash is feasible. Soda ash may be utilized 

as temporary treatment while long-term treatment options are evaluated. Therefore, operators place a soda 

ash dispenser in the flow path of the mine drainage in order to increase the alkalinity and neutralize the 

acidity of the mine drainage. 
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Soda Ash Dispenser

It is recommended that soda ash dispensers, similar to Figure 2 be utilized for soda ash dispensing with 

mine drainage. These dispensers have a flume type base that allows for the mine drainage to make contact 

with the soda ash briquettes allowing for dissolution of the Na2CO3. This allows for fresh briquettes to 

continuously enter the mine drainage as briquettes dissolve, compared to placing the briquettes into a 

barrel all at once (Figure 1) and the briquettes lose their reactivity over time, requiring personnel to 

manually add additional chemical and possibly stir up the less reactive material. 

The module allows for the user to enter and/or adjust the default unit cost for the soda ash dispenser. The 

default cost for the dispenser is $1,500. No other equipment items are included with the treatment setup 

of soda ash; however, any additional items can be included in the ‘Other Capital Items’ section of the 

module. 

2.3 Application and Financial Comparison

The following is a quick cost assessment between soda ash and caustic soda for a low-flow acidic mine 

drainage source; whereby, no electric would be available, and an automated caustic soda system would 

not be an option. Table 2 includes the parameters that are used for the assessment, table 3 provides the 

estimated chemical consumption and annual chemical costs for both soda ash and a 20% caustic soda 

solution. 

Table 2. Summary of mine drainage and chemical parameters used in comparison. 

Mine 

Drainage 

Flow

Mine 

Drainage 

Acidity

Soda Ash 

Purity / Mixing 

Efficiency

Caustic Soda 

Purity / Mixing 

Efficiency

Soda 

Ash Unit 

Cost

Caustic 

Soda (20%) 

Unit Cost

10 gpm 100 mg/L 

as CaCO3

99% / 60% 99% / 100% $0.15/lb $0.70/gal

Table 3. Monthly chemical consumption and annual chemical costs. 

Soda Ash 

50-lb bags 

per Month

Soda Ash Annual 

Chemical Cost Caustic Soda 

gals/month

Caustic Soda (20%)

Annual Chemical Cost

13 $1,230 145 $1,321

There are some assumptions in selecting the default values in both the soda ash and caustic soda 

assessment; however, based on evaluating a scenario with a low flow source and no electric available, 

soda ash is comparable to the 20% caustic soda solution despite the lower mixing efficiency of soda ash. 

As the flow may increase and/or net acidity increase, it will likely become more cost effective to consider 

caustic soda and a stronger solution (25% or 50%) of caustic soda. Additional variable(s), other than the 

capital costs, that would need to be considered might be labor costs. For example, the soda ash would 

require approximately thirteen 50-lb bags per month; but if the flow is variable the caustic soda drip valve 

may need adjusted manually to ensure proper dosage. Again, soda ash is most likely utilized in low flow 
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conditions and/or on a temporary basis until a more permanent treatment system can be designed and 

constructed if mine drainage treatment is required on a long-term basis. 

3.0 Soda Ash 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 six sections: (1) Water Quality and Flow Input, (2) 

Soda Ash Information, (3) Annual Cost Input, (4) Other Capital Items, and (5) Other Annual Items. The 

workflow for the module is for users to start at the top left-hand side. Enter the Typical Flow and Net 

Acidity and AMDTreat calculates the annual acidity loading. Then, within the Soda Ash Information 

section the cost of the soda ash dispenser is specified. Also, the user selects the purity and mixing 

efficiency of the soda ash and selects the method to estimate the soda ash consumption (Stoichiometric, 

Titration, or User-specified). Please note, the ‘Mixing Efficiency’ is used in the calculations for the 

Stoichiometric and Titration options, but not for the User-Specified method (see Section 3.2.3.1 and 

3.2.3.2). AMDTreat uses this information along with the calculated acidity load to estimate the annual 

soda ash consumption. The Annual Cost Input section allows the user to specify the unit cost of soda ash. 

Finally, users can specify additional capital and annual costs not considered by the module under the 

Other Capital Items and Other Annual Items sections. 

Module output is provided on the right-hand side. 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 estimates of chemical consumption in lbs/year and the number of 50-bags/year for the 

dispenser. The estimated cost to construct and operate a soda ash treatment system is provided under the 

Capital Cost and Annual Cost sections. Lastly, users can opt to conduct a Net Present Value (NPV)

analysis to obtain the total cost to operate and maintain a soda ash 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 Typical Flow and Net Acidity values for the source of mine drainage. These values are 

used to (1) estimate the annual chemical consumption and (2) potentially estimate the size or cost of a 

soda ash dispenser. 

The definitions for Typical Flow and Net Acidity 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 Input subheading. In 

short, Typical Flow is the flow rate “typically” experienced at the site (e.g., median flow). This flow 

rate is used to calculate the annual chemical consumption so one must take careful consideration to 

calculate this value. 
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Net Acidity represents the acidity released and neutralized when the base is added to achieve the 

desired treatment pH. For eastern coal mine drainage, the acidity producing species will be the 

hydrolysis of Al3+, Fe2+, Fe3+, and Mn2+, the precipitation of CaCO3, and the deprotonation of H2CO3

and HCO3

-

. For treatment pH > 10, the hydrolysis of Mg2+ and hydroxylation of cations must also be 

considered.

3.2.2 Soda Ash Information: The user can enter the cost of soda ash dispenser and adjust this 

capital cost as needed. The purity and mixing efficiency can also be adjusted. Typically, the purity of 

soda ash is very high (~99%); however, the mixing efficiency is poor compared to other chemicals 

(i.e. caustic soda). Based on setups that utilize soda ash for treatment, there is essentially not any 

‘mixing’ of the soda ash briquettes, but rather, the soda ash briquettes are placed in the flow path of 

the mine drainage allowing them to dissolve as the AMD flows through the dispenser. Therefore, the 

mixing efficiency default is set relatively low at 60%, and the user can adjust this value based on the 

site-specific conditions. Next the user can select a method, either stoichiometric, titration, or userspecified, that calculates the soda ash consumption and is discussed further in Section 3.2.3.

3.2.2.1 Soda Ash Dispenser: The default cost of a soda ash dispenser is $1,500. The user can 

adjust this value based on the type of dispenser that is chosen or already existing at a site. A metal 

cone-shaped hopper with a retrofitted flume channel to allow mine drainage to flow through the 

soda ash is assumed with this cost (Figure 2). This hopper-style dispenser will allow the soda ash 

to be self-feeding as the soda ash dissolves in the flowing mine drainage. However, it should be 

noted this will still require personnel to check periodically that no plugging or dysfunction of the 

dispenser has occurred. For example, the unreacted or coated soda ash briquettes may need 

removed or broken up on a consistent basis in order for fresh briquettes to enter the mine drainage

flow. 

Since soda ash is not highly reactive, all the soda ash may not react (i.e. disassociate) in the mine 

drainage; therefore, mixing efficiency is a parameter that can be adjusted and is used in the 

calculations for the stoichiometric and titration methods. 

3.2.3 Chemical Consumption: The Chemical Consumption section offers the user three methods to 

estimate the annual chemical consumption, Stoichiometric, Titration, and User-Specified Quantity. 

3.2.3.1 Stoichiometric – This method estimates the annual chemical consumption by using the 

user-specified information under the Water Quality & Flow Input and Soda Ash Information

sections. This method uses the values for Typical Flow and Net Acidity to calculate the annual 

acidity loading in Calcium Carbonate Equivalents in the following equation:

Where: 

A.L. = Annual Acidity Load in g CaCO3/yr

Flow = Typical Flow in gal/min

Acidity = Net Acidity in mg/L as CaCO3
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After the Annual Acidity Load is determined, the program uses the stoichiometric 

relationship between CaCO3 and Na2CO3 to re-express the acidity load in terms of Na2CO3.

Where: 

Na2CO3 L = Annual Acidity Loading in g/yr expressed as Na2CO3

Finally, the program uses the designated purity and mixing efficiency to determine the annual 

amount of soda ash required to neutralize the acidity load. 

Where: 

Na2CO3 lbs = lbs/yr of Na2CO3 to Neutralize the Annual Acidity Loading

Mix E = Mixing Efficiency %

Purity = Purity %

3.2.3.2 Titration – The titration method allows users to input the results of field or bench 

acidity titrations that empirically determine the required dose of a chemical to achieve the 

desired end point (e.g., treatment pH). Users must enter the results of the titration data in 

terms of pounds (lbs) of soda ash required to treat a gallon of mine drainage. Since the soda 

ash purity is inherently contained in the titration input value, AMDTreat does not use a value 

for Purity when the Titration method is selected. However, users can adjust the Mixing 

Efficiency value if they feel the actual mixing efficiency will differ from the efficiency 

experienced during the titration. 

Where: 

Mix E. = Mixing Efficiency

Adj. Na2CO3 lbs = Annual Soda Ash Consumption adjusted for Mixing Efficiency 

3.2.3.3 User-Specified Quantity - This method allows users to specify the annual soda ash

consumption. This method is typically selected when AMDTreat is being used to evaluate 
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long-term water treatment liability using the Net Present Value calculations or if the quantity 

of soda ash is known for an existing system.

3.2.4 Annual Cost Input: The user can adjust the unit cost, or price per pound, of soda ash. The default 

value for soda ash is $0.15 per pound and is assumed that a bulk order consisting of a pallet(s) of soda ash

is purchased. In addition, this unit cost does not include delivery, so the user should consider the delivery 

cost based on the site location. The default unit cost is estimated based on pricing in 2019-2020; but 

similar to other chemicals, the unit cost can fluctuate and the user should adjust this value accordingly by 

checking prices with local suppliers. 

3.2.5 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, if someone wanted to 

invest in an automated dispenser, this additional feature could be included in Other Capital Items. Since 

this is uncommon it was not included the cost module, however, users who want to include this capability 

can input the cost into the Other Capital Items section to capture the capital cost. 

3.2.6 Other Annual Items: The Other Annual Items section allows users to capture the annual cost of 

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

annual electric cost of an automated soda ash dispenser 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 the estimated soda ash consumption in pounds per year and total number of 50-lb soda ash bags 

per year. 

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

and the total estimated cost to install the soda ash system. Users can opt to estimate the installation cost 

by specifying it as a percentage (cost-multiplier) of the capital cost or by entering a user-specified

installation cost. 

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

maintain the soda ash treatment system. Users can select to have AMDTreat estimate the annual chemical 

cost or specify a known annual chemical cost. Specifying an annual chemical cost is often used when 

AMDTreat’s Net Present Value calculations are being used to estimate water treatment liability. The 

annual operation and maintenance (O&M) for the soda ash treatment system can either be user-specified 

or estimated as a percentage of the capital cost. The latter method assumes the more expensive a system 

the more costly it is to maintain. 

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 
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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, 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 c0hange 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 
12

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 5). 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. 

• 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. 
13

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 and active 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 two recapitalization items. However, based on 

the items the user selects to include in the module will determine which ones are included 

in the NPV section. 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 section. For example, 

the default value for the lifecycle of a poly storage tank is 12 years due to degradation 

from ultraviolet light. However, users may opt to increase the life cycle if the poly tank 

will be housed in a building. Users are free to fully customize the replacement items, 

including adding new items or deleting default items. 

Soda Ash Dispenser: The life cycle of a soda ash dispenser depends on the type of 

material (i.e. steel, poly) the dispenser is constructed. In almost all instances, these 

dispensers will be placed outdoors and exposed to weather. As a default, the AMDTreat 

team feels 20 years is an estimate as long as the dispenser is maintained. However, the 

user should consider adjusting this value based on the site conditions and dispenser 

quality. A longer or shorter life cycle may be appropriate if the dispensing unit is 

cleaned and painted when corrosion becomes visible or if the dispenser is constructed of

a poly material. 

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 soda ash module. 

1. The Stoichiometric method used to estimate the annual chemical consumption relies on a 

properly determined value for Typical Flow and Net Acidity. Many people use the Standard 

Method 2310 (Hot Acidity) procedure to determine Net Acidity. In most instances, a Hot Acidity 

titration result will not accurately describe the base requirement to achieve effluent standards. A 

cold acidity titration is the best method to determine the Net Acidity encountered during 

treatment and for estimating chemical consumption. 
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4.0 Figures 

Figure 1: 55-gallon barrel with hole cut into it containing soda ash 

briquettes, with mine drainage flowing through each end of the barrel to 

allow dissolution of soda ash. 
15 

Figure 2. Photo of a soda ash dispenser with soda ash briquettes placed 

within mine drainage flow path.