Help Instruction File:

Clarifier 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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Clarifier Module Overview

Table of Contents

1.0 OBJECTIVE 5

2.0 OVERVIEW 5

2.1 Settling Tests 5

2.1 Clarifier Options: Conventional versus Solids Contact 7

2.1.1 Overview of Conventional Clarifiers 7

2.1.2 Overview of Solids Contact Clarifiers 8

2.2 Treatment Options 10

2.2.1 High Density Sludge 10

2.2.2 Enhanced Flocculation Sludge Recirculation System 11

3.0 CLARIFIER MODULE OVERVIEW 13

3.1 Layout and Workflow 13

3.2 Module Inputs 14

3.2.1 Water Quality and Flow Input 15

3.2.2 Clarifier Design 15

3.2.3.1 Conventional 15

3.2.3.2 Solids Contact 15

3.2.3.3 Clarifier Construction Material 16

3.2.3.4 Clarifier Water Height, Freeboard, and Sludge Blanket Depth 16

3.2.4 Equipment 16

3.2.4.1 Tank Coatings, Motors, Pumps etc. 16

Overflow Weir Unit Cost 16

Catwalk with Handrail Unit Cost 17

Density Current Baffles 17

Tank Protective Coating 17

Impeller Motor 17

Rake Drive Motor 17

3.2.4.2 Solids Recirculation Options 17

High Density Sludge Recirculation Pump 17

Enhanced Flocculation Sludge Recirculation System 18

3.2.4.3 Sludge Disposal Pumps 18

Safety Factor 20

Pump Efficiency and Pump Safety Factor 20

3.2.4.4 Foundation 20

3.2.4 Sludge Disposal Pipeline (HDPE) 20

3.2.4.1 Pipe Selection and Sizing 21

3.2.4.2 Pipe Sizing Evaluation and Error Routine 21

3.2.4.2.1 Total Static Head 21

3.2.4.2.2 Pipeline Length 21

3.2.4.2.3 Incidental Head Losses 21
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3.2.5 Other Capital Items 22

3.2.6 Annual Cost Input 23

3.2.6.1 Electric Unit Cost 23

3.2.6.2 Clarifier Operational Time 23

3.2.6.2 Sludge Generation 23

3.2.6.3 Sludge Disposal 23

3.2.7 Other Annual Items 25

3.2.8 Other Annual Items 25

3.3 Module Outputs 25

3.3.1 Sizing Summary 25

3.3.2 Capital Cost 26

3.3.3 Annual Cost 26

3.3.4 Net Present Value 26

3.3.4.1 Financial Variables 26

3.3.4.2 Cost Categories 26

3.3.4.3 Rationale for Recapitalization Recommendations 28

5.0 FIGURES 30

Figure 1: Settling test determining the settling rate for a specific concentration of flocculated aluminum 

hydroxide produced solids. 30

Figure 2: Results of ten settling tests to relate the rise rate to the concentration of produced solids. 31

Figure 3: The settling flux rate characterizes the optimal solids loading. 31

Figure 4: Typical concrete conventional clarifier used to in mine drainage treatment in the eastern U.S. 32

Figure 6: Concrete-walled conventional clarifier. Note the open trough located under the walkway 

(catwalk) to the center well. Diluted polymer flocculant is added at hydraulic jump in the trough where 

the water falls by six inches to provide the mixing necessary to disperse the diluted polymer throughout 

the waste stream. Particle-to-particle interactions occur within the center well to promote agglomeration 

of particles. 33

Figure 8: Photo of a 110 ft diameter conventional clarifier. Note red-colored sludge blanket is within five 

feet of the surface indicating the need to pump sludge. Reducing the volume of the sludge bed allows the 

sludge blanket to reposition lower in the clarifier. 35

Figure 9: Photo a center well and rake drive system that is over 55 years old. The center well is the large 

circular steel structure that extends approximately 6 ft below the water and the smaller diameter cylinder 

that extends through the center well to the bottom is the rake drive system. The steel armatures that 

extend outward from the rake drive to the outer wall is the rake. The angled plows attached to the bottom 

of the rake are used to transport settled sludge to the center of the clarifier for evacuation using a sludge 

pump. 36

Figure 11: A newly constructed concrete clarifier showing the sloped floor and the sludge sump and 

sludge pump line extending under the floor. 38
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Figure 14: Bright green dye introduced into conveyance trough that connects the reaction tank to the 

center well of the clarifier. The open conveyance trough is located under the catwalk to the center well. 

Also, note the corrosion of the steel catwalk and handrailing extending to the center well. 41

Figure 16: Alternative view of dye test shown in Figures 14 and 15. Before the installation of the flow 

baffle in the center well, dye preferentially flowed on one side of the clarifier. 43

Figure 17: Dye test on a large 220 ft diameter steel clarifier. Note rotation caused by rake movement. 44

Figure 19: Cloud of suspended iron hydroxide that followed the rake rotation after conversion from 

hydrated lime to hydrogen peroxide. 46

Figure 20: Schematic of a Solids Contact Clarifier (courtesy of Westech Engineering). Blue arrows 

signify the flow of water and the red arrow indicates how the sludge is pulled back to the surface to mix 

with raw water. 47

Figure 22: Arial view of newly constructed concrete-walled Solids Contact Clarifier treating ferruginous 

mine drainage. Sludge pumps are housed in the white building and the effluent is discharged from silver 

piping into a wetland where total Fe is reduce4d from 0.5 to 0.03 mg/L. 48

Figure 24: A picture showing the reaction well of a newly constructed solids contact clarifier. The twelveinch inlet pipe transports the raw mine drainage into the draft tube where it is reacted with hydrated lime 

and recycled sludge. 50

Figure 25: A steel mix tank to condition the recycled sludge with hydrated lime. The white pipe is the 

sludge recycle line and the smaller diameter black pipe entering the top right of the tank is the lime slurry 

line. A mechanical mixer is used to flash mix the sludge and lime slurry. The lime/sludge mixture is then 

discharged to the reaction tank where it reacts with the untreated influent water. 51

Figure 26: Outfall of sludge conditioning tank showing the sludge/lime slurry mixture being discharged 

into the reaction tank to react with the untreated influent. 51

Figure 27: The HDS conditioning tank was retrofitted to separate the lime slurry and sludge due to 

constant scaling and maintenance issues with the tank. The recirculated sludge is shown discharging from 

the blue pipe at the left of the photograph and the lime slurry is discharging from the conditioning tank. 52

Figure 32: The construction of a 220 ft diameter welded steel clarifier with a concrete floor. Note the 

missing steel panels that allow for equipment access into the clarifier during construction. 56

Figure 35: Photo showing a buildup of iron in the valleys of the sawtooth overflow weir. 59

Figure 36: Worker lowering pump to clean inside of clarifier. In addition, the iron-coated overflow weir 

and transport trough (working standing in trough) will be cleaned to restore an even distributed flow 

across the weir. 59

Figure 37: Works replacing the steel bolts that attach the overflow weir to the concrete wall with stainless 

steel bolts. The corrosion caused some of the bolts to fail causing the weir to fall into the clarifier (Figures 

38 and 39). Stainless steel fasteners must be used when treating mine drainage. Note the top of the ladder 
4

is resting against the saw-toothed overflow weir and the weir is resting on support beams that secure the 

weir to the concrete wall. 60

Figure 43: Overland sludge pipeline to injection borehole. Most sludge lines are buried but this one is 

supported by wooden cribbing. 64

Figure 44: Raw water entering a reaction tank, showing clear, low total suspended solids water. The 

sludge injection well was located a few hundred feet from the raw water pumping well in a semi-isolated 

section of the mine. Sludge was successfully pumped for fifteen years without recirculating sludge back 

to the raw water pump. However, eventually the sludge disposal migrated to where the raw water pump 

recirculated sludge back to the reaction tank (Figure 45). A new sludge injection borehole was drilled a 

few thousand feet away in a more isolated and down-dip section of the mine. 65

Figure 45: Photo showing sludge injected in the underground mine is being captured by the raw water 

pump and recirculated with the raw water back to the reaction tank. See caption in Figure 44. 66

Figure 47: Additional photo of geotubes show in in Figure 46. The geotubes will be buried in place as 

part of a reclamation plan to reclaim the highwall in shown in the photo. Soil will be placed over the final 

elevation of the geotubes and vegetated as part of the plan. 67

Figure 48: Sludge is pumped to the polymer mixer in the brown hose on the hillside. Polymer is added in 

the grey tubing near the sludge inlet and the polymer and sludge are passively mixed using a series of 

pipe bends. Two outlet hoses provide the flexibility to switch between two geotubes. When a geotube is 

filled, it is allowed to dewater and sludge is then pumped to the second geotube. The spigot is used to 

check the flocculation of the sludge to help calibrate the polymer dose. The PVC mixer can be 

constructed for a few hundred dollars. 68

Figure 49: Sludge is pumped through the red hose and into the geotube where the polymer flocculated 

particles are larger than pores in the geotube, which causes the solids to be retained in the bag while the 

water seeps through the fabric, dewatering the solids. 69

Figure 50: Geotubes being used in parallel. The geotube in the foreground was totally filled with sludge 

the previous day and is currently being allowed to dewater. Sludge will be pumped again in the next few 

days and the empty geotube will be used. The weekly sludge pumping will be alternated between 

geotubes to provide dewatering time. A gravel pad was constructed for the geotubes and has a 1% slope 

towards the perimeter ditching to capture the dewatering water. The geotubes are staked to the ground to 

prevent rolling as the tube is filled. In this photo, the sludge is being pumped from a pond and the 

variable sludge concentration during pumping makes it difficult to achieve a correct polymer dose. Thus, 

there are times where unaggregated particles leak through the pore space and into the perimeter ditch 

explaining the red coloration. 70
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1.0 Objective

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

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

an estimate of the cost to construct, operate, and maintain clarifiers at mine drainage treatment sites. The

information is presented in two sections, Overview and Clarifier Module Overview. It needs to be 

stressed the Clarifier module is a tool for reverse cost modeling or developing rough costs for the 

construction and operation of clarifiers and not for final design. 

2.0 Overview

In the treatment of coal mine drainage in the eastern U.S., ponds and conventional clarifiers are the two 

common technologies used to separate suspended particles from water; a process known as clarification. 

Coal companies are more apt to use ponds for clarification, whereas government entities are more 

inclined to use clarifiers. However, in large pump and treat scenarios both entities commonly use 

clarifiers. Clarifiers are settling tanks used for solid-liquid separation and they also include a sludge 

management system . 

A basic understanding of the data requirements and application of clarifiers is required to develop a 

treatment strategy using the AMDTreat software. These topics are discussed below to provide the 

necessary context before discussing the clarifier module interface and functionality. The Overview 

section first provides a description of how clarifiers are typically utilized to treat coal mine drainage in the 

eastern US which is then followed by additional sections: (1) Settling Tests and (2) Clarifier Options, and 

(3) Treatment Options. 

2.1 Settling Tests

Understanding the relationship between the concentration and settling rate of precipitated particles and 

effluent clarity is important. Generally, low concentrations of precipitated particles (produced solids)

result in a fast particle settling rate but will result in poor effluent clarity. When particle concentrations 

are low, flocculated particles tend to settle uninterrupted and without influence from other particles that 

result in a fast-settling rate. However, the low solids concentrations will result in infrequent collisions 

between particles and will result in both poor flocculation and effluent clarity. Particle collisions and 

interactions are needed to agglomerate smaller particles into large particles to produce low turbidity 

effluent. Alternatively, if the concentration of solids becomes too high, constant particle-to-particle

interactions will hinder particle settling and result in a slow settling rate. On the other hand, the numerous 

particle interactions promote flocculation and produce a clear effluent. Therefore, the goal of settling 

tests is to identify the optimum balance between the concentration of solids required to produce clarity 

and settling rate to minimize the size of the clarifier. 

It is important to note the term “solids concentration” refers to the total concentration of suspended solids 

entering the clarifier or settling basin. The suspended particles include the particles “produced” or 

precipitated during treatment reactions, particles of unreacted hydrated lime or other reagents, and the 

solids that are or planned to be recirculated from the clarifier underflow back to the reaction tank 

(external recirculation) or to the center well (internal recirculation). 
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In general, if the total suspended solids (precipitated + recycle + lime) predicted to enter the clarifier is:

• < 150 mg/L, there is a high likelihood of poor effluent clarity and solids from the clarifier 

underflow (sludge) will need to be recycled to the reaction tank (external recirculation) or 

internally recirculated within the clarifier to increase the total suspended solids concentration to 

achieve effluent clarity;

• > 150 < 800 mg/L, there may be enough produced solids to produce effluent clarity without the 

need of recirculating solids;

• >800 < 3000 mg/L, the optimal concentration of suspended solids to produce effluent clarity 

while producing a fast-settling rate; and

• > 3000 mg/L, will produce clear effluent but will have slow, hindered, settling rates and require 

high dose of polymer flocculant. May require dilution to lower solids concentration. 

Bench-scale settling tests are used to determine the relationship between solids concentration, settling 

rate, and effluent clarity. The tests are used to determine the settling rate and thickening potential of the 

metal hydroxide particles precipitated during the treatment process. For settling to occur, the water

velocity in the clarifier (rate of rise) must be less than the velocity (settling rate) required to settle the 

produced solids (solids produced during precipitation reactions) to achieve effluent standards. 

Settling tests are performed by adding enough treatment chemical to a 2 L container of mine drainage to 

achieve the treatment pH required to meet effluent standards in a full-scale treatment system. Next, an 

anionic polymer flocculant is added to agglomerate particles into larger more settable solids. Lastly, the

treated water is added to a 2000 mL cylinder to measure the bulk settling rate (Figure 1). The settling 

rate is measured and the clarity of the supernate is recorded. The solids are resuspended and sampled to 

determine the Total Suspended Solids (TSS) concentration. If the supernate is clear and the settling rate 

acceptable, the results can be used to size the clarifier. If clarity is poor, then the produced solids 

concentration will have to be increased by recirculating solids from the clarifier underflow (settled 

sludge) or increasing the treatment pH. 

If the concentration of produced solids is low and/or the settling tests produced poor clarity, a series of 

settling tests are performed to identify the optimal concentration of solids. In settling test situations 

where raw water quality results in low produced solids, additional solids must be produced by treating 

tens or hundreds of gallons of additional mine drainage. The supernate is decanted and the additional

settled solids are collected to perform the tests. A portion of settled solids is added to a 2000 mL beaker 

and the decanted supernate is added until 2000 mL is achieved. A polymer flocculant is added to 

agglomerate the particles and the flocculated solution is added to a 2000 mL graduated cylinder. The 

solution is gently mixed and a setting rate is measured. This test is repeated using differing concentrations

of sludge to produce a graph of settling rate versus concentration of suspended solids (Figure 2). One

goal is to determine the lowest concentration of suspended solids that will produce a clear supernate and 

fast settling rate. This will help determine the solids loading that will have to be pumped from the 

underflow of the clarifier back to the reaction tank or center well to achieve effluent clarity. Next a graph 

of settling flux is produced by multiplying the suspended solids concentration by the settling rate (Figure 

3). This provides information on the optimal concentration range for flocculation that achieves the highest 

settling flux, or movement of solids through the clarifier. Figure 3 shows that the optimal settling flux was 
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achieved with a solids concentration between 1,200-2,200 gm/L, which produces a rise rate of between 4 

and 2 gpm/ft2 or settling rates of 0.53 and 0.27 ft/min. This indicates the water velocity must be much less 

than 0.27 ft/min to provide a factor of safety. 

Numerous settling tests have shown varying concentrations of ferruginous mine drainage treated with 

hydrated lime resulted in hydraulic loading ranging 2 to 7 gal/ft2

/day. Likewise, aluminum dominated 

mine drainage produced rise rates ranging from 1 to 7 gal/ft2

/day. After reviewing both bench scale 

testing and data from in-service clarifiers, the optimal suspended solids concentration for producing 

effluent clarity (Total Fe < 0.5 mg/L) appears to range from 1,500 to 2,500 mg/L for most coal mine 

drainage producing aluminum, iron, and manganese hydroxide sludges. 

Settling tests are used as a proxy to simulate the behavior and characteristics of solids that will occur in a 

full-scale treatment system. Any bench-scale treatment used to create the produced solids must mimic the 

full-scale treatment strategy. Even with perfect bench scale testing, design and construction flaws in full 

scale treatment systems, like issues with flocculation, floc shearing, or hydraulic issues, can cause 

disparity between settling tests results and the actual performance. Thus, it is important to use safety 

factors to adjust bench scale results. It is common practice to multiply the settling rates from bench tests 

by, at least, 0.5 to compute the maximum design settling rate.

Many industrial settings have a design advantage in that designers know and can control the water quality 

characteristics of the produced wastewater stream before designing a clarifier. In mining scenarios, 

treatment systems may have to be constructed before mining commences and water quality is known. In 

addition, the pumped water quality is often unknown for Abandoned Mine Land (AML) projects. In these

instances, the water quality of the surface discharge may be different than the quality deeper in a 

chemically stratified mine pool; thus collecting water from the discharges may not be appropriate to

conduct settling tests. Piezometers can be installed near the proposed pumping site to try to better 

characterize the water for settling tests, but, even then, the final chemical composition of the water that 

will be pumped is unknown since the capture zone will be an amalgamation of different water qualities 

throughout a stratified mine pool. There are many nuances that can affect the concentration of produced 

solids, such as the concentration of carbon dioxide in the water, that is hard to characterize when 

proposing to treat a chemically stratified mine pool. The preceding discussion provides the reasoning to 

justify the use of robust safety factors for design criteria in many mine drainage treatment scenarios. 

2.1 Clarifier Options: Conventional versus Solids Contact

While there are many types of clarifiers, over 99% of the ones used to treat coal mine drainage in the 

eastern U.S. are conventional clarifiers. There are at least two solids contact clarifiers in existence to treat 

mine drainage and there have been attempts to use inclined plate clarifiers, however, problems with iron 

hydroxide and calcite scale on the plates eventually caused the decommissioning of at least one of the

incline plate clarifiers. AMDTreat offers users the ability to cost model two types of clarifiers, 

conventional and solids contact. 

2.1.1 Overview of Conventional Clarifiers

Within the context of AMDTreat, conventional clarifiers describe circular settling tanks with the 

mechanical means to continuously manage and remove sludge (Figures 4 & 5). Typically, conventional 
8

clarifiers receive pH or Eh adjusted mine drainage that has been dosed with a polymer flocculant prior to 

entering the clarifier. There are a few situations where a tank is used to mix polymer and mine drainage to 

promote flocculation before discharging to the clarifier. However, it is more common to add polymer at a 

“hydraulic jump” or turbulent point in a conveyance trough or pipe that transports the water from the 

reaction tank to the center well of the clarifier (Figure 6). The center well acts as an inlet feed system to 

feed the water to the clarifier at an optimal velocity and flow distribution. Since many mine drainage 

treatment systems do not use external flocculation tanks, “flocculating” center wells are often 

incorporated into the clarifier design (Figure 7). Flocculating center wells are often larger than traditional 

center wells are sized to promote the agglomeration of flocculated particles. Water should flow out of the 

center well in an even radial fashion. If the velocity of the water is less than the settling velocity of the 

suspended particles, then settling is initiated. If not, solids are discharged from the clarifier. The zone of 

active settling and high suspended solids concentration is called the sludge blanket (Figure 8). Suspended 

particles that rest on the bottom of the clarifier make up the sludge bed. A raking and sludge pump

(sludge removal) system is used to manage solids inventory. The raking system consists of plows 

attached to steel armatures that extend radially from the center of the clarifier (Figure 9). A geared drive 

system provides the rotational force needed to move the rake assembly through the sludge bed. The rake 

plows are used to transport the sludge toward the sludge sump, also sometimes referred to as a sludge 

cone, located at the bottom center of the clarifier (Figures 10 & 11). Sludge sumps are typically 100 to 

500 gallons and will completely fill within a few minutes when treating in the thousands of gallons per 

minute range and producing low density sludge (< 1.0% solids by wt.). If the sludge pumping schedule is 

less frequent than the time it takes to fill the sludge sump, a sludge bed will form that extends laterally out 

to the side walls of the clarifier. Infrequent sludge pumping will cause the buildup of the sludge bed and

the raking system will no longer effectively transport sludge to the sump until pumping occurs. Instead, 

the rakes will continue to rotate through the sludge bed and agitate and stir the sludge particles. The 

agitation facilitates sludge thickening, a process that involves the rearrangement of particles into a more 

compact arrangement by displacing water contained in the sludge. The thickening process increases 

sludge density and will reduce the volume of the sludge bed and lessen the frequency of sludge pumping. 

Deciding how to manage solids inventory to control thickening and the sludge blanket is a major 

objective of plant operators. Some operators chose to periodically dewater and clean the clarifier. Figure 

12 shows an operator using a hose connected to a pump to pressure wash the clarifier walls, floor, and 

rakes. Figure 10 was taken after the clarifier shown in Figure 12 was cleaned. 

When starting up a newly constructed treatment system or after changing treatment chemicals or strategy, 

a clarifier may become disrupted (Figure 13) and require troubleshooting. Troubleshooting usually starts 

with evaluating whether polymer is being properly dispensed and distributed in the wastewater stream. 

The sludge pumping plan is also evaluated to ensure the solids inventory is being properly managed. Dye 

tests are used to identify flow distribution, velocity, and retention time issues (Figures 14, 15, 16, 17). 

Simple devices that show the position of the rakes are useful in troubleshooting the formation of clouds of 

suspended solids (Figures 17 & 19). For both newly constructed and older clarifiers, laundering weirs 

should be inspected for levelness to prevent preferential flow paths. Scrapping or rubbing sounds may 

indicate the rakes are misaligned. Frequent visual checks of the torque indicator meter helps to monitor 

whether excess torque is being applied to the drive system. 

2.1.2 Overview of Solids Contact Clarifiers

Solids Contact Clarifiers (SCC) are also known as a Sludge Contact and Draft Tube Clarifier (Figures 20, 

21, & 22). SCC provide two main benefits; enhanced flocculation and combination of several treatment 

processes into a single component. SCC increases effluent clarity by using a centrifugal impeller pump to 
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lift sludge from the bottom of the clarifier, through a draft tube where recycled sludge is mixed with the 

raw/influent water and chemical reagent before being discharged at the top of the center well (Figure 23). 

At this point, polymer flocculant is added and the water flows from the top of the center well down 

through the surrounding solids contact reaction well where flocculation occurs before entering the settling 

zone (Figure 24). The process of increasing the suspended solids concentration to enhance flocculation is 

further explained in Section 2.2.2. 

Another key feature is a SCC provides the flexibility to be operated as a chemical reactor, flocculation 

tank, and settling tank. Ph or Eh adjusting chemicals can be injected into the draft tube or influent line and 

the center well/reaction skirt will serve as a reaction tank to mix the influent water, chemical, and 

recycled sludge. Polymer flocculant can be injected at the top of the center well and the solids contact 

skirt acts as a slow mix flocculation tank. While all these processes can be completed within a SCC, 

some operators may prefer to use external chemical reactors and polymer flocculant mix system to 

separate the processes to aid in operational control and troubleshooting. 

Lastly, another important distinction between SCC and conventional clarifiers is the sludge thickening 

process. Sludge is not thickened, or is minimally thickened, in a SCC due to the pump continuously 

disturbing the sludge at the bottom of the clarifier and lifting it through the draft tube. It is common 

practice in the coal fields of the eastern U.S. to inject sludge back into the underground mine being 

pumped and treated. Thus, a drawback of not being able to thicken sludge is having to frequently pump a 

large volume of low-density sludge/water back into the mine that increases inflow. A SCC in 

Pennsylvania produces a 0.2 % solids sludge (by wt.) and must pump sludge to the underground mine at 

200 gpm every few minutes to manage solids inventory. 

It needs to be noted that the settling and clarity performance offered by a SCC can be achieved by 

combining a conventional clarifier with external sludge recirculation to a reaction tank (pumping sludge 

from clarifier to reaction tank to increase suspended solids). See Section 2.2.2 on Enhanced Flocculation. 

Advantages of a SCC compared to a conventional clarifier: 

• Improved settling rates which result in a smaller and less expensive clarifier; 

• Improved effluent clarity due to increased TSS loading in center well; 

• Provides the ability to be a turn-key treatment system that acts as a chemical reactor, flocculator, 

and sedimentation tank; and

• A low-density unthickened sludge will increase the longevity of injection wells into underground 

mines or surface mine backfill. 

Disadvantages of a SCC compared to a conventional clarifier: 

• Does not appreciably thicken sludge, which can affect the operation and final density if 

dewatering devices are required (e.g. presses, centrifuges); 

• Produces a low-density unthickened sludge. The constant recirculation of sludge from the bottom 

up through the draft tube for solids contact prevents thickening. This is a drawback for treatment 

systems injecting sludge into the same mine they are pumping from since it adds significant 
10 

amounts of water back into the mine. 

• May produce poor water quality during initial startup until enough sludge is produced to achieve

appreciable solids contact and enhanced flocculation. 

• While the overall cost is less due to size, the cost of the clarifier internals is more expensive; and 

• Due to their decreased size, the sludge blanket may rapidly expand upwards and be hard to 

control if large fluctuations in influent flow is experienced. 

2.2 Treatment Options

2.2.1 High Density Sludge 

The High-Density Sludge (HDS) process was developed in the late 1960s by Bethlehem Steel researchers 

P.D. Kostenbader and G.F. Haines. The first described the process in a 1970 Coal Age article titled, “ 

High-Density Sludge Treats Acid.” The paper describes the laboratory testing conditions and the 

demonstration project at the Mine 32 treatment plant, located near Johnstown, Pa, that was used to 

optimize and refine the process. The process involves recirculating a portion of the clarifier underflow 

(settled sludge) back to a small reactor where hydrated lime is added to the sludge to generate a pH 11 to 

12.4 sludge/hydrate slurry mixture. Presumably, the high pH condition creates an overall negative charge 

on the surface of the metal hydroxide sludge. The sludge/slurry mixture is then mixed with the raw water

and cations, ferrous and ferric iron, are adsorbed and then precipitated onto the surface of recirculated 

metal hydroxide sludge. Dissolved pollutants adsorbing and precipitating onto the surface of recycled 

sludge creates a denser sludge then the precipitation of individual particles of sludge in a water column 

that may or may not aggregate into larger denser particles. The Kostenbader paper provides some useful 

design criteria including a recycled sludge and lime mixing time recommendation of one minute in a 

reactor and a solids recycle ratio of between 20 to 30 (lbs recycled sludge/lbs precipitated sludge). 

Conventional mine drainage treatment produces sludge that ranges from 0.2 to 6% solids (by wt.) whereas

HDS typically produces 20 to 40 % solids. 

Advantages to producing high density sludge include: 

• Improved settling rates, thus smaller clarifiers;

• Improved effluent clarity due to increased TSS loading in center well; 

• Decrease in sludge pumping time due to increase in sludge density; 

• Less water being returned to the mine if injecting into underground mine workings; 

• Provides opportunity to redissolve and use unreacted lime or calcite in sludge during recirculation 

process;
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• Effective in removing trace elements; 

• Decrease in diameter of sludge pumping lines; and 

• May not require (or minimize) a polymer flocculant to achieve TSS standards. 

Disadvantages to producing high density sludge include: 

• Sludge injection wells may be more prone to clogging or have a shortened life span due to 

decreased flowability of the sludge; 

• A possible decrease in the service life clarifier rake drive mechanisms due to increased torque 

imparted by the higher density solids 

The equipment for a HDS system typically consists of a sludge line that pumps sludge from the sludge 

sump in the clarifier, a sludge pump with a variable frequency drive to control recirculation rate, and a 

tank or conveyance trough capable of mixing hydrated lime with the sludge. HDS equipment can be 

included in a treatment design and only used if it is required to achieve effluent standards. For a relatively 

low cost, it provides operators with flexibility to achieve compliance in variable conditions or if treatment 

standards change in the future. 

Many designs specify using a small mix tank to mix the recirculated sludge and lime for 1 to 5 minutes 

(Figures 25 & 26). However, some operators have complained about the maintenance caused by calcite 

scaling and other issues with the mixer and tank. There are instances where operators separated the 

recirculated sludge from the lime slurry to minimize the maintenance (Figure 27), which probably 

stopped the formation of HDS. While the Kostenbader paper discusses the Bethlehem Steel HDS design 

as using a conditioning tank to mix the sludge and lime, the majority, if not all, of Bethlehem’s HDS 

plants eliminated the conditioning tank and used a trough to dose the recirculated sludge with hydrated 

lime (Figure 28). The sludge recirculation line transitions to a trough directly under the lime dispenser 

and lime is dispensed directly into the trough. The sludge/lime slurry mixture flows about 30 feet before 

discharging into the reaction tank. The trough uses passive mixing and virtually eliminates maintenance 

issues encountered with the tank and mechanical mixer. While retention time in the trough is only 

seconds, it appears to be sufficient to condition the sludge for adsorption processes. The trough HDS 

systems that mimic the Bethlehem design have produced sludges that range from 35 to 40% solids by wt. 

Lastly, a common myth is the iron sludge needs to be oxidized as Fe(OH)3 (red sludge) for the HDS 

process to work. Many HDS plants in Pennsylvania, including the Bethlehem Mine 31 plant, recycle 

unoxidized Fe(OH)2 (green sludge) and generate sludge up to 40% solids by wt. 

2.2.2 Enhanced Flocculation Sludge Recirculation System 

Arguably, the most difficult challenge in operating a treatment system with a clarifier is to manage the 

suspended solids concentration of the effluent, especially for sites that generate produced solids 

concentrations of less than 200 mg/L. The term “produced solids” refers to the concentration of solids that 

is generated during the treatment process and flows to the clarifier for solid/liquid separation. The primary 

source of those produced solids results mostly from precipitation reactions and unreacted chemical. For 

example, if 60 mg/L of ferric iron is expected to precipitate as Fe(OH)3, then the produced solids 

concentration for iron would be 114.8 mg/L. If it is predicted that 50 mg/L of Hydrated Lime will 
12

recarbonate to calcite during the treatment process and remain suspended, then the produced solids from 

this reaction will be 67 mg/L. If these are the only two reactions that produced solids, then the total 

produced solids concentration is 182 mg/L (114 mg/L of Fe(OH)3 and 67 mg/L of CaCO3). 

Before entering a discussion on how to enhance sedimentation of a low produced solids waste stream, it is 

useful to define how the terms flocculation and agglomeration are defined and used in this help file. For 

the purposes of this help file, the process of flocculation includes using both a mixing strategy to promote

collisions between particles and the addition of a polymer flocculant to aggregate individual particles into 

large particles to increase settling rates. While flocculation is used to describe both the particle mixing 

and bonding strategies, agglomeration is used to just describe the process of aggregating individual 

particles to form a single larger particle. 

For mine drainage sites that use clarifiers, a general rule of thumb is a produced solids concentration of 

less than 150 mg/L may result in poor effluent clarity. Low concentrations of produced solids inhibit the 

agglomeration of particles by lowing the probability of particle collisions, even in well mixed situations. 

Upon initial precipitation, particles are less than 1μm in size and, if conditions are favorable, some of the 

individual particles will combined to form large aggregates due to the attractive nature of Van der Waals 

forces. Under this scenario, the low concentration of large aggregates will settle discretely and freely, 

without influence from adjacent particles. The aggregates have fast settled rates since their settling is not 

influenced or hindered by other particles in these solutions of low particle concentrations. On the other 

hand, the low concentration of particles means the remaining unaggregated micron-sized particles left in 

suspension will cause poor effluent clarity. In situations of low concentrations of produced solids, the 

Enhanced Flocculation (EF) process can be used to increase the concentration of produced solids to 

improve particle aggregation and effluent clarity. The EF process augments the produced solids 

concentration by recirculating solids from the clarifier sump back to the reaction tank to increase the 

probability of particle collisions (Figure 29). EF is like the HDS process in that sludge is recirculated 

back to the reaction tank but differs in that no chemical is added to condition the sludge to a high pH like 

in the HDS process. Sludge is simply recirculated to increase the produced solids concentration to 

improve the flocculation process. 

While increasing the concentration of solids improves effluent clarity by promoting particle interactions 

and agglomeration, the increased particle interactions prevent the free and discrete settling of individual 

particles that produce fast settling rates. Instead, as particle concentrations increase, the interactions 

between neighboring particles hinders discrete particle settling and causes the particles to settle as a 

group. Group settling has a lower settling rate than free settling due to particle collisions redirecting 

particles from the settling direction, upward velocities caused from settling particles displacing water, and 

from changes in viscosity when particle concentrations transform the solution to a slurry. The 

agglomeration that occurs during group settling produces good effluent clarity but slows the overall 

settling rate. There is a point when the particle concentration increases, and the slurry is effectively 

“hindered” from settling in any reasonable time. The rate of hindered settling may become slower than 

the rate of rise in the clarifier and particles will be discharged from the clarifier. Therefore, managing an 

EF system is a balancing act of finding a center well concentration that achieves low turbidity but still 

produces acceptable settling rates.

In mine drainage, hindered settling typically occurs when center well concentrations approach 5,000 to 

10,000 mg/L. A rule of thumb is to adjust the sludge recirculation pump to achieve a produced solids 
13

concentration of 2000 to 2500 mg/L in the center well. Typically, this level of EF produces an optimal

balance between effluent clarity and settling rates. Operating clarifiers with EF within this range typically

produce effluent suspended iron concentrations of 0.2 to 0.6 mg/L. 

**The Enhanced Flocculation option is only available to conventional clarifiers since Solids Contact 

Clarifiers inherently contain enhanced flocculation through internal recirculation of sludge particles. 

3.0 Clarifier Module Overview

The module provides a capital, annual, and recapitalization (net present value) costs for a clarifier and 

sludge handling and disposal activities. AMDTreat estimates the diameter of the clarifier after users 

specify the Design Flow, suspended metal concentrations, a Hydraulic Loading value and selects a type 

of clarifier (Conventional or Solids Contact). Under the Clarifier Design section, users can specify the

clarifier tank be constructed from concrete, welded steel, or bolted steel that sits atop of a concrete floor. 

Under the Equipment heading, users can specify whether to include a protective coating for the tank and 

either accept default values or specify unit costs for items like the overflow weir, catwalk, and density 

current baffle. In addition, users must specify the power requirement for motors to facilitate the 

calculation of the annual electrical cost to operate the clarifier. For Conventional Clarifiers, users can opt 

to include a High-Density Recirculation Pump or an Enhanced Flocculation system. The power 

requirement and number of sludge disposal pumps must also be determined along with the cost estimate 

for the clarifier and sludge pump foundations under the Equipment heading.

Next, the Sludge Disposal Pipeline (HDPE) section is used to specify the characteristics of the buried 

HDPE pipeline that will transport sludge from the clarifier to be disposed of in injection boreholes or

dewatered in geotubes (filter bags) and disposed of in a landfill.

The Other Capital Items section is used to account for items and costs that are not included in the module. 

The Annual Cost Input section contains a list of items that affect the annual cost estimate. Users can 

specify the Operational Time of the clarifier, which is important in instances, like mine pool management, 

where the clarifier is not constantly operating. The Operational Time will affect the estimates of sludge 

production, pumping, disposal, and electrical costs to operate the clarifier. The Sludge Generation section

is where users characterize the sludge that will affect annual sludge handling and disposal costs. Users 

must estimate the percent solids (by wt.) of the thickened sludge and specify whether minerals other than 

aluminum, iron, and manganese (specified in the Water Quality and Flow Input section) will add mass to 

the sludge. AMDTreat allows users to specify the amount of calcite that will precipitate and the mass of 

other nuisance minerals, such as, Mg(OH)2 that will increase sludge mass and volumes. Finally, Annual 

Cost Input contains a Sludge Disposal section that allows users to determine whether sludge from the 

clarifier (underflow) will be disposed of in an injection well into an underground coal mine or pumped 

into a geotube, allowed to dewater, and excavated, transported, and disposed of in a landfill. 

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) 
14 

Clarifier Design, (3) Equipment, (4) Sludge Disposal Pipeline (HDPE), (5) Other Capital Items, (6) 

Annual Cost Input, and (7) Other Annual Items. The workflow for the module is for users to start at the 

top left-hand side. Enter the Design and Typical Flow and Suspended metal concentration of the 

produced solids. In other words, these are the concentration of solids precipitated from the raw water after 

chemical addition (pH/Eh adjustment). These concentrations should not include concentrations from 

recirculating clarifier underflow. The suspended metal concentrations can be obtained from treatability 

tests after adjusting the raw water to a target treatment pH or Eh. AMDTreat will assume the metals 

precipitate as metal hydroxides (e.g. Fe2+ as Fe(OH)2, Fe3+ as Fe(OH)3, Al3+ as Al(OH)3, Mn2+ as 

Mn(OH)2). 

Next, select either a conventional or solids contact clarifier and specify a hydraulic loading and 

construction material (e.g. steel or concrete clarifier) under the Clarifier Design section. AMDTreat uses 

the information to determine the size of the clarifier. Then specify equipment options and costs for items 

like Tank protective coatings and catwalks and determine whether High Density Sludge (HDS) or 

Enhanced Flocculation (EF) is necessary. The Sludge Disposal Pipeline (HDPE) section is used if users 

plan to pump the clarifier underflow through a buried HDPE pipeline. Users select a Nominal Pipe Size 

and SDR ratio and specify a total static head and pipeline length and AMDTreat estimates the fluid 

pressures and velocities provides an error message if the design criteria are outside acceptable limits. 

Errors can be fixed by changing the pipe size, SDR ratio or water quality (affects fluid viscosity, which 

affects pressure). Users then use the Annual Cost Input section to specify unit costs and make selections 

that affect annual costs. Users can specify the operational time of the clarifier for sites that do not 

continuously operate. The operational time is used to refine estimates of annual costs and sludge volumes. 

The Annual Cost Input section also includes sections for Sludge Generation and Sludge Disposal. Users 

must specify the predicted % solids (by wt.) of the underflow (sludge) and specify the amount of calcite 

or other miscellaneous precipitate that will add to the mass of sludge beyond aluminum, iron, and 

manganese. The Sludge Disposal section offers the ability to estimate the cost to use sludge injection 

boreholes to dispose directly into an underground mine or pump to a geotube for dewatering to haul for 

offsite disposal. Finally, users can specify additional capital and annual costs not considered by the 

module under the Other Capital Items and Other Annual Items headings. 

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

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

Present Value. The Sizing Summary section provides estimates of clarifier dimensions, center well 

suspended solids concentrations, and sludge production estimates. The estimated cost to construct and 

operate the Clarifier is provided under the Capital Cost and Annual Cost headings. Lastly, users can opt 

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

for a defined time period. 

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

directed to Help files located on AMDTreat’s website and 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
15 

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

specify the Design and Typical Flow and Metal Concentrations. These values are used to estimate (1) 

the size of the clarifier and (2) annual sludge volumes. 

The definitions for Design and Typical Flow and Metal Concentrations can be found in the tool tip 

for this section. Click on the information icon ( ) on the right side of the Water Quality and Flow 

subheading. In short, Design Flow is used to size the clarifier and Typical

Flow is used to calculate annual sludge estimates and costs. Design Flow is the maximum flow the 

clarifier can accept and Typical Flow is the flow rate “typically” experienced at the site. Ferrous 

Iron, Ferric Iron, Aluminum, and Manganese represent the concentrations of suspended metals 

predicted to be removed by the clarifier. While users input the concentration of the suspended metal, 

AMDTreat assumes they are precipitated as metal hydroxides (e.g. the value entered for suspended 

Fe2+ is recalculated as Fe(OH)2) for calculating the mass and volume of sludge. 

3.2.2 Clarifier Design:

3.2.3.1 Conventional – This option is used to size a Conventional clarifier (see section 2.1.1 

for description). A default value of 0.50 gal/min/ft2 is used to represent the Hydraulic 

Loading required to size a conventional clarifier without recirculating underflow solids back 

to the reaction tank to increase solids loading. The default value has been used to successfully 

size several large, hydrated lime treatment plants with > 600 mg/L of suspended (metal 

hydroxide) solids entering the center well, however, users are encouraged to conduct setting 

tests to characterize site specific conditions. The Hydraulic Loading value could be equal to 

the value used for a Solids Contact clarifier if the Enhanced Flocculation Sludge 

Recirculation System is selected under the Equipment section. Adding external recirculation 

of underflow solids to a Conventional clarifier approximates the performance of a Solids 

Contact clarifier. 

The Design Flow rate is divided by the Hydraulic Loading value to calculate the surface area 

of the clarifier.

When a Conventional clarifier is selected, the Impeller Motor options under the Equipment 

section is deactivated since impeller motors are unique to solids contact clarifiers. 

 

3.2.3.2 Solids Contact – This option is used to size a Solids Contact clarifier (see section 

2.1.2 for description). A default value of 0.70 gal/min/ft2 is used to represent the Hydraulic 

Loading (rise rate) required to size a Solids Contact clarifier, however, users are encouraged 

to conduct setting tests to characterize site specific conditions. 

The Design Flow rate is divided by the Hydraulic Loading value to calculate the surface area 

of the clarifier.

When a Solids Contact clarifier is selected, the High-Density Sludge Recirculation Pump and 

Enhanced Flocculation Sludge Recirculation System options under the Equipment section are 

deactivated. 
16

3.2.3.3 Clarifier Construction Material – Users can opt to develop cost estimates for 

clarifiers tanks constructed of concrete, welded steel, or bolted steel. Regardless of tank 

construction material, AMDTreat assumes all clarifiers have a concrete floor (Figure 30). 

Figure 11 shows a newly constructed concrete walled clarifier with a concrete floor and 

Figure 31 shows a welded steel concrete clarifier with a concrete floor. 

In general, a concrete-walled clarifier is the most expensive to construct but the most durable

(Figures 4 & 6), while bolted steel is the cheapest to construct but may require increased 

maintenance due to the potential for corrosion to occur at each bolt location. Welded Steel 

clarifiers are more maintenance than concrete clarifiers, due to periodic sandblasting and

painting, but are more durable than bolted steel clarifiers. Figures 32 through 34 show the 

construction of a welded steel clarifier. 

Users can select to include cost estimates to include protective tank coatings under the

Equipment section that may extend the lifecycle and lessen the maintenance of a clarifier. 

3.2.3.4 Clarifier Water Height, Freeboard, and Sludge Blanket Depth – Users must specify a 

water and freeboard depth to allow AMDTreat to calculate the dimensions of the clarifier. 

The Sludge Blanket Depth is an optional parameter that does not affect cost calculations. 

3.2.4 Equipment: This section has two subsections: Tank Coatings, Motors, Pumps, etc. and 

Foundation. 

3.2.4.1 Tank Coatings, Motors, Pumps etc.: This section is used to specify the unit costs for various 

parts of the clarifier and to specify or estimate power requirements. 

Overflow Weir Unit Cost – Users can specify the unit cost of a peripheral overflow (laundering) weir 

located around top of the clarifier wall. The overflow weir transports water from inside to outside of 

the clarifier. Figure 6 shows a blue saw toothed overflow weir directly inside of the concrete wall. 

Figure 35 is an up-close photo of an iron coated overflow weir. Coatings of iron, calcite, algae, 

leaves, or other debris will lead to short circuiting to unclogged sections of the weir and will cause 

settling problems. Figure 36 shows an operator ready to both clean the inside of the clarifier and 

brush the overflow weir to remove the iron coating. 

It is highly recommended to use stainless steel bolts to fasten the center well structure and overflow 

weir when treating mine drainage. Figure 37 shows workers replacing the steel bolts that fasten the 

yellow saw-toothed overflow weir to the concrete wall at the Hollywood AMD treatment plant in 

Pennsylvania. Note the ladder is resting on the saw-toothed weir and the yellow weir is resting on 

and bolted to the steel beam extending from the concrete wall. Figure 38 shows the corrosion of the 

bolts that attach the steel support beam to the weir and to the concrete wall. Some of the steel bolts 

corroded to the point of failure and half of the weir disconnected and fell into the water causing a 

short circuit and elevated suspended solids in the effluent (Figure 39). The clarifier was in its 9th year 

of operation when the failure occurred. 
17

Catwalk with Handrail Unit Cost – The Catwalk with Handrail is used to walk across the top of the 

clarifier to access the center well to work on the drive system and possibly sludge recirculation lines 

and or pumps. AMDTreat offers the ability to calculate the cost of a Catwalk extending from the 

outside clarifier wall to the center well (Half Length) or across the entire top of the clarifier (Full 

Length). The unit cost can be set to $0.0 to eliminate the cost. Figures 10 and 14 show examples of a 

steel catwalk with handrailing extending half the diameter of the clarifier. 

Density Current Baffles – Density Current Baffles are used to disrupt density currents caused by 

differences in suspended solids concentrations and temperatures within the clarifier (Figures 31 & 

40). Temperature differentials along the outside wall, bottom, and top of the clarifier will change 

viscosity and create areas of different velocities, which will affect particle settling. In winter

conditions, surface cooling may cause vertical convection currents that will hinder particle setting. 

Decreases in water temperature will increase viscosity and may lead to an increase in suspended 

particles in the effluent. Likewise, the momentum and suspended solids concentration of the water 

exiting the bottom of the center well may cause a density current near the top of the sludge bed which 

may concentrate lighter solids. Density current baffles have a proven track record of successfully 

reducing effluent suspended solids concentrations in a number of Pennsylvania mine drainage 

treatment systems. 

Tank Protective Coating – Users can opt to include tank coatings to prevent corrosion and extend the 

life cycle of the clarifier. The tank coating affects the life cycle assigned to the clarifier under the Net 

Present Value section. Manufactures recommend periodically reapplying coatings for full protection. 

Impeller Motor – This option is only available when Solids Contact is selected. The impeller motor 

draws solids from the bottom of the clarifier up the draft tube to be mixed with the influent water and 

increase the solids loading to improve flocculation. The horsepower of the Impeller Motor is required 

to estimate the annual electricity cost. AMDTreat can estimate the power requirement of the Impeller 

Motor based on the size of the clarifier or users can specify the motor power. 

Rake Drive Motor – User must specify the horsepower of the rake drive motor for AMDTreat to 

calculate the annual electrical cost to transport sludge to the sludge sump. 

3.2.4.2 Solids Recirculation Options – Users can opt to include the High Density Sludge process or 

Enhanced Flocculation in a conventional clarifier. Both methods require users to specify the 

difference in water elevation between the water elevation in the clarifier and the reaction tank to 

estimate the power requirement to pump underflow from the bottom of clarifier back to the reaction 

tank. Users must specify the elevation difference at the top of the Solids Recirculation Options

section. 

High Density Sludge Recirculation Pump: Selecting this option adds the cost of incorporating a 

slurry pump to pump the underflow back to the reaction tank. The High-Density Sludge (HDS) 

process is explained in Section 2.2.1. Users must specify the difference in elevation between the 

water level in the clarifier and the reaction tank to activate the HDS option. In addition, users can 

have AMDTreat estimate the size of the HDS slurry pump or opt to specify the capacity of the 

pump. For AMDTreat to estimate the pump capacity users must specify the Sludge Recycle 

Ratio. The value represents the ratio of the iron loading being pumped from the underflow back 

to the reaction tank to the iron loading in the untreated water. P.D. Kostenbader and G.F. Haines 

(1970) recommended an iron recycle ratio of between 20 and 30 to optimize density. Both the 

AMDTreat estimate, and the User Specified method uses the difference in water elevation and the 
18

HDS pump capacity (gpm) to estimate the capital cost of the pump and the annual electrical cost 

associated with the HDS system. 

Enhanced Flocculation Sludge Recirculation System: Users can opt to include an Enhanced 

Flocculation Sludge Recirculation System into a conventional clarifier to approximate the 

flocculation and clarity of a Solids Contact clarifier. While a Solids Contact clarifier recycles 

solids within the clarifier, the enhanced flocculation pumps underflow from a conventional 

clarifier back to the reaction tank to achieve a desired suspended solids concentration in the 

center well. Users specify the desired suspended solids concentration in the center well and 

AMDTreat uses this value along with the values for Typical Flow and Ferrous and Ferric Iron 

concentrations to calculate the flow rate of the underflow required to achieve to achieve the 

center well concentration. AMDTreat estimates the horsepower required to calculate the 

estimated annual electrical cost. 

3.2.4.3 Sludge Disposal Pumps – Users specify the number of horizontal centrifugal sludge pumps, which 

should be at least two for redundancy. Users can opt to have AMDTreat estimate the size and capacity of 

the pumps or users can specify a capacity. If users choose to have AMDTreat estimate the pump capacity, 

users must specify the Desired Time to Pump Sludge each Day and the Safety Factor under the Sludge 

Disposal Pump Sizing heading and specify the Total Static Head, sludge Pipeline Length and Nominal 

Pipe Size under the Sludge Disposal Pipeline section. 

AMDTreat uses four steps to estimate the power requirement of the sludge disposal pump.

Step #1: Determine Density and Specific Gravity of Solids within the Sludge

Mine drainage sludge is a composition of water and precipitated minerals, such as oxides, hydroxides, 

and carbonates (Figure 41). The first step is to determine a weighted density and specific gravity of the 

composite precipitated solids. AMDTreat assumes the dissolved metal concentrations entered into the 

Water Quality & Flow Input section are precipitated as the hydroxide solids shown in Figure 1. 

AMDTreat converts 100% of each metal concentration to the equivalent concentration of a precipitated 

solid. AMDTreat calculates the % mass of each of the precipitated solids relative to the total mass of 

precipitated solids. The percent mass of each solid is multiplied by the corresponding specific gravity to 

determine the weighted density of each precipitate. The weighted density of the precipitates is summed to 

calculate an overall weighted density and specific gravity for the composite precipitated solids (Figure 

42). 

The composite sludge density is used to calculate the composite specific gravity of the solids in the 

sludge. After the composite sludge density is determined, AMDTreat uses the Typical Flow and the 

concentration of sludge to calculate the daily solids load that makes up the sludge. Recall, sludge consists 

of mineral solids and water. At this point, the daily solids load is characterizing the daily mass of the solid 

precipitate and does not include the water portion of the sludge. The daily solids load is divided by the 

composite density to determine the daily volume of sludge produced. 

Users can also include additional mass due to the precipitation of calcite or other miscellaneous 

precipitated solids. If Calcite is expected to precipitate, users can specify the precipitation concentration 

under the Sludge Generation heading of the Annual Cost Input Section and it will be included in the

estimate of daily sludge production. If other minerals are expected to precipitate, users enter the

concentration of the precipitated form of the mineral in the Miscellaneous Solids input field along with a
19

density under the Sludge Generation heading of the Annual Cost Input Section. For example, if bench 

scale testing showed 100 mg/L of Mg2+ will precipitate as a hydroxide, users can specify that 242 mg/L, 

in this case Mn(OH)2, of Miscellaneous Solids will precipitate and be added to the mass of sludge. 

AMDTreat will use the Miscellaneous Solids value along with the Miscellaneous Solids Density to

calculate mass of the additional sludge. 

Step #2. Calculate the Specific Gravity and Density of the Sludge

Sludge is a combination of mineral precipitate and water. Sludge is often characterized by specifying the 

% Solids of the sludge by weight. 100 grams of sludge that is 10% solids by wt. contains 10 grams of 

precipitated mineral and 90 grams of water. Users can specify the expected % Solids by wt. of the sludge 

under the Sludge Generation heading of the Annual Cost Input section. The % Solids should represent 

the solids content of the thickened sludge being pumped from the clarifier. The values for % Solids and 

the specific gravity of the composite precipitated solids (solid content of the sludge) is used to calculate 

the specific gravity of the Sludge: 

The specific gravity of the sludge is multiplied by 8.43 lbs/gal to calculate the sludge density. 

Step #3: Calculate Daily Sludge Production

The cumulative concentration of precipitated solids (Step #1) is multiplied by the Typical Flow rate to 

calculate the daily loading of precipitated solids. The daily loading is divided by the Sludge Density (Step 

#2) to calculate the daily volume of sludge produced. 

Step 4: Calculate the Power Requirement of the Sludge Pump

The flow capacity of the sludge pump is determined by dividing the daily sludge volume by both the

Sludge Pumping Time and Safety Factor, which are specified by the user under the Equipment heading. 

The Horsepower of the pump is calculated by:

HP = Horsepower of sludge pump

Flow = Required Flow Rate of sludge pump

Total Dynamic Head = (Dynamic Pipe Losses + Static Head)*(1+(% Incidental Head Losses/100)

Efficiency = The Pump Efficiency is used to describe the how efficient the pump is at converting all the 

electrical power to pumping power. It is expressed as a decimal.

Safety Factor = Expressed as a decimal
20

AMDTreat uses the information to estimate the pumping power (HP) required transport the sludge 

through the pipeline. The variables describing the pipeline are described in the Sludge Disposal Pipeline

section and the variables presented in the Sludge Disposal Pump Sizing heading are described here. 

Safety Factor: The Safety Factor is convenient way to increase the overall capacity of the pump. 

If AMDTreat estimates the need of a 20 HP pump, a Safety Factor of 2.0 will increase the pump 

estimate to 40 HP. 

Pump Efficiency and Pump Safety Factor: The Pump Efficiency is obtained from the manufacture 

is expressed as a decimal. The Pump Efficiency is used to describe the how efficient the pump is 

at converting all the electrical power to pumping power. The Pump Safety Factor is a way to 

increase the capacity of the pump to overcome any uncertainty with the assumed percent solids 

(by wt.), viscosity of the sludge or estimates in head loss or pressure drops over time. The 

estimated pump capacity is divided by both the Pump Efficiency and Safety Factor. For example, 

assume a user specifies a Pump Efficiency of 0.75 and a Pump Safety Factor of 0.85 and 

AMDTreat estimates an 82 HP pump is required to achieve the flow rate and head pressure in the 

sludge line. The final pump size would be 129 HP or 56% larger since 82/0.75/0.85 = 129 HP. 

The Safety Factor and the Pump Safety Factor may be redundant, and users can simply use a 

Pump Safety Factor of 1.0 to eliminate redundancy. 

3.2.4.4 Foundation – This section affects both the foundation of the clarifier and the concrete pad that 

houses the sludge disposal pumps. The first step in estimating the capital cost of the foundation is to 

specify, or accept the default, Concrete and Material Placement unit cost used to calculate the cost to 

purchase, deliver, and form (including rebar) the concrete foundation. Next, users must specify a Site 

Solids Condition of Poor, Average, or Excellent (Poor = 1,500 lbs/ft3

, Average = 3,500 lbs/ft3, Excellent = 

4,500 lbs/ft3

). The specified soil condition is used to determine the foundation area. The user should be 

aware that mine subsidence concerns are not addressed in the AMDTreat foundation design and that these 

concerns if applicable need to be addressed separately by the system designers with the appropriate input 

from qualified Geotechnical Engineers. 

For either sizing method, the foundation volume is multiplied by the Concrete and Material Placement 

Unit Cost to estimate the capital cost of the foundation. If no foundation is required, users can make the 

Concrete and Material Placement $0.0 per yd3

.

3.2.4 Sludge Disposal Pipeline (HDPE): 

The sludge disposal pipeline is used to transport sludge from the bottom of the clarifier to either an 

injection borehole or to a geotube. AMDTreat assumes the construction of the pipeline consist of 

excavating a trench and installing a bed of gravel on the floor of the trench to cushion the pipe. HDPE 

pipe is installed on top of the gravel bed and the trench is backfilled and compacted. Users can opt to 

include the costs of air/vacuum release assembles to prevent air locks and pig launchers to unclog the 

pipeline. 

The width of the trench is calculated by adding 24” to the outside diameter of the pipe selected by the 

user. Trench depth is determined by adding 42” to the diameter of the pipe. The trench wide and depth are 

multiplied to determine the cross-sectional area of the trench. This area is multiplied by the user-specified 

length of the pipeline to estimate the excavation volume and cost. The volume and mass of gravel bed is 

determined by first multiplying the length of the pipeline by two since the width of the trench is ~ two 

feet. This determines the area of the bottom/top of the gravel bed. This value is multiplied by the user-
21 

specified pipe bedding thickness to estimate the volume of the gravel bed. two the user-specified pipe 

bedding thickness. A bulk density of 95 lbs/ft2 is used to convert the bed volume to a mass of gravel. 

3.2.4.1 Pipe Selection and Sizing- The HDPE pipeline can be sized with a 1.5-, 2.0-, 3.0-, 4.0-, or 6.0-

inch nominal pipe size. Users must also select a Standard Dimension Ratio (SDR) for the pipe. SDR 

describes the correlation between the outside diameter of the pipe and the pipe wall thickness (SDR = 

outside diameter / wall thickness). SDR is used for pressure rating pipe with lower SDR numbers having 

higher pressure ratings. Table 1 provides the relationships between pipe dimensions and SDR Rating. 

Table 1: Relationship between Nominal Pipe Diameter, SDR, Pressure Rating, and Inside Pipe Diameter 

(Courtesy of Performance Pipe Website). 

3.2.4.2 Pipe Sizing Evaluation and Error Routine - In addition to specifying the pipe size and SDR, the 

total static head, pipeline length and incidental head losses must be specified. 

3.2.4.2.1 Total Static Head - The total static head is the difference in elevation between the outlet 

(discharge head) and the inlet (suction head). 

3.2.4.2.2 Pipeline Length - The pipeline length is the distance between the clarifier and the 

injection borehole or geotube. 

3.2.4.2.3 Incidental Head Losses – The loss in head (pressure) as the fluid flows through the 

pipeline due to friction caused by pipes, valves, fittings, bends, and tees. 

AMDTreat uses the input values to calculate the pressure within the sludge pipeline during pumping. To 

calculate the pressure, AMDTreat first calculates the required flow rate of the sludge pump (see section 

3.2.4.3, Step 4). Next, it uses the flow rate, along with the inside diameter and length of the sludge 

pipeline to estimate the Dynamic / Frictional Head Pipe Losses. These losses are added to the Total Static 

Head and Incidental Head Losses to determine the Total Dynamic Head. The Total Dynamic Head is 

used to calculate the Total Dynamic Head (TDH) Pressure within the pipeline. 

AMDTreat performs four error checks and one warning check to ensure the sludge pipeline is sized 

properly. Error messages require users to change input values to eliminate the errors before AMDTreat 

will proceed with cost estimations. Warning messages are designed to notify users of a potential issues 

but AMDTreat will continue to conduct cost estimation without changes to input values. 

The four error checks are: 
22 

1. Compare the calculated THD pressure to the pressure rating of the pipe show in Figure 3. If the 

calculated pressure exceeds the allowable pressure, the following error message is displayed, and 

users will have to change input variables to prevent the exceedance: 

“The pressure for the SDR selected has been exceeded. Increase in pipe diameter to decrease 

pressure or a decrease in SDR to increase the allowable pressure.”

2. If the calculate THD pressure is greater than 335 psi, which is the maximum pressure for the SDR 

piping in AMDTreat, the following error message appears: 

“Maximum pressure for HDPE pipe has been exceeded. Increase pipe diameter or decrease flow 

rate to decrease pressure, or design project using steel or some other high-pressure pipe.” 

3. If the calculated velocity of the pumped sludge is greater than 15.0 ft/sec, the following error

message appears and users will have to change input variables to reduce the flow velocity: 

“Pipe Fluid Velocity is greater than 15 ft/s, which results in excessive wear and head loss. 

Increase pipe diameter or decrease flow rate.”

4. If the calculated velocity of the pumped sludge is less than 2.0 ft/sec, the following error message 

appears, and users will have to change input variables to prevent the possibility of sludge settling 

within the pipeline: 

“Pipe Fluid Velocity is less than 2 ft/s which creates the possibility of sludge settling in line. 

Decrease pipe diameter or increase flow rate.” 

The one warning message is:

1. If the user selects a pipe diameter and SDR value that has a pressure rating that is much higher 

than the calculated pipe pressure, AMDTreat will provide a warning message to the user notifying 

them a pipe with a higher SDR number, which is less expensive due to a thinner wall thickness, 

could be selected without errors. Users do not have to change any input values and can disregard 

the warning message. The warning message is: 

“Selected SDR is heavier than required. Consider increasing SDR to use a lower pressure rated 

pipe which results in decreased costs.”

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, a small number of Lime

treatment systems have electronic surveillance to notify authorities if unauthorized persons attempt to 

interfere with the equipment. 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. 
23

3.2.6 Annual Cost Input: This section contains the variables and activities that affect the annual 

treatment costs.

3.2.6.1 Electric Unit Cost - Users must specify the unit cost of electricity for the site they are 

interested in evaluating in order to estimate the annual cost to operate the motors and pumps 

associated with the clarifier. 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 for a clarifier, including the rake motor drive, HDS 

pump, enhance flocculation pump, sludge pump impeller motor, etc. 

3.2.6.2 Clarifier 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 time period the clarifier operates. Changes to the operational time will affect the 

annual electricity cost and the estimated sludge production. 

3.2.6.2 Sludge Generation- The clarifier module calculates the annual sludge production to

estimate the annual costs associated with pumping and disposing of the sludge. Under this 

subheading, users can specify values for variables that affect annual sludge production. 

% Solids of the sludge underflow – This percentage represents the mass of solids relative to the 

mass of sludge being pumped from the clarifier for disposal. The annual sludge production 

estimate is very sensitive to this value, so it is important to properly characterize the % solids to 

produce accurate cost estimates. 

Calcite as CaCO3 – AMDTreat allows users to specify the concentration of aluminum, iron, and 

manganese that will contribute to sludge. In addition, calcite is a common precipitate especially in 

instances where lime or hydrated lime is being used for pH adjustment or in waters with elevated 

Total Inorganic Carbon containing a treatment pH > 7. Kinetically, calcite does not precipitate to 

equilibrium as quick as the metal hydroxides so it can be difficult to estimate the amount of 

calcite that will precipitate before the water is discharged from the treatment systems. Users can 

enter the estimated concentration of calcite that will precipitate during treatment and AMDTreat 

will include the mass in the annual sludge production estimate. 

Miscellaneous Solids – There are instances where precipitates other than aluminum, iron, 

manganese, and calcite contribute to sludge generation. For example, magnesium will precipitate 

as Mg(OH)2 if the treatment pH is > 10. Users can enter the concentration of miscellaneous solids 

that are predicated to precipitated and AMDTreat will use the concentration and the specified 

Miscellaneous Solids Density to add it to the sludge volume. If 100 mg/L of Mg is predicted to 

precipitate as Mg(OH)2, users would enter a value of 239 mg/L for Miscellaneous Solids. 

Miscellaneous Solids Density – Users must specify a density representative of the Miscellaneous 

Solids predicted to precipitate. 

3.2.6.3 Sludge Disposal – The capital and annual costs associated with sludge handling, 

dewatering, and disposal of sludge is organized under the Annual Cost Input section. AMDTreat 
24

offers two methods for sludge handling and disposal for a clarifier, Injection Borehole and 

Geotube. 

Injection Borehole - The most common method for sludge handling and disposal when operating 

a mine drainage treatment facility is to pump the sludge from the clarifier sump to a borehole that 

injects the sludge into an abandoned underground coal mine (Figure 43). Ideally, the borehole is 

installed into a different mine complex than the one being pumped and treated; however, this is 

not always possible, and it is common to pump and inject within the same mine complex. In any 

case, the injection borehole must be located into a portion of a mine that minimizes 

communication with the pumping well (e.g., downdip and into a non-subsidence area) (Figures 

44, & 45). In addition, the mine pool must be circumneutral pH to prevent the redissolving

sludge back into the mine pool. 

A best practice is to always have two sludge injection boreholes installed and connected to the 

treatment system and a third borehole sited with all necessary access easements. The two installed 

boreholes provide redundancy, and the pre-determined location of the third borehole allows for 

easy implementation when one of the primary boreholes eventually fail. 

The Underground Mine Disposal Borehole section provides users with the ability to model the 

capital cost of installing up to three sludge injection boreholes. Users select the desired borehole 

diameter and the borehole multiplier. The multiplier increases the size of the borehole to 

accommodate casing and grout. Next, the depth to the mine is entered and users can either accept 

the default drilling and installation cost or specify a cost. AMDTreat multiplies the borehole 

depth by the installation cost to determine the capital cost. The annual cost of electric to pump to 

the borehole is determined by the electrical unit cost and the sludge pumping frequency. 

Geotube Disposal - In instances where injection to an unground mine is not possible, geotubes 

can be used to dewater and temporary store sludge (Figures 46 & 47). Sludge is periodically

pumped from the clarifier sludge sump and dosed with additional polymer flocculant as a 

thickening agent before being pumped into a geotube (Figure 48). The flocculant aggregates the 

particles to a size larger than the pores of the geotube fabric so the sludge is retained within the 

geotube while the water can discharge through the fabric (Figure 49). Operationally, geotubes are 

often used in parallel where one bag is filled with sludge and allowed to dewater while the other 

bag is filled (Figure 50). If sludge pumping is daily, several bags operating in parallel may be 

needed as it often takes 24 to 48 hours to dewater and generate the space to accept additional 

sludge. After completely filling a geotube, it may take several weeks or a month to dewater and 

produce a sludge capable of being excavated from the geotube and transported in a dump truck. A

rule of thumb is that sludge must be at a minimum of 20% solids by wt. before it can be excavated 

from the geotube. There are geotubes that are made to fit inside a roll of container and, once 

filled, a roll off container truck can simply transport the geotube to the landfill for disposal. 

However, it is more common to use an excavator to remove the dewatered sludge from the 

geotube and use a dump truck to transport the sludge to a disposal location. 

The annual cost to use geotubes to handle sludge is determined by having users estimate the 

dewatered solids content of the sludge before it is excavated for disposal. The default assumption is the 

sludge will be 40% solids by wt. Users can also specify the size of the geotube and AMDTreat uses the 

information to estimate the annual number of geotubes. This module assumes the polymer equipment 

sized and specified in the Polymer module will be primarily used to flocculate particles flowing from the 

reaction tank to the clarifier. It is assumed the same equipment will be used to dose the sludge when it is 
25 

pumped form the clarifier to the geotube. The polymer model accounts for the cost to dose the produced 

solids flowing into the clarifier and the Clarifier module accounts for the annual polymer used to dose the 

sludge flowing to the geotube. Therefore, users are required to specify the type of polymer (emulsion or 

dry), the % active ingredient, the dose, and the unit cost and the clarifier module will predict the annual 

amount and cost of polymer. Finally, users must specify the roundtrip distance, excavator and 

transportation unit costs, and Landfill tipping fee. AMDTreat uses the information to estimate the annual 

excavator time required to remove the dewatered sludge from the geotube and load the dump truck. Then 

AMDTreat estimates the annual time and cost to transport the sludge to the disposal site. Users can 

specify any disposal cost in the Landfill Tipping Fee section. 

3.2.7 Other Annual Items: The Other Annual Items section allows users to capture the capital cost of 

equipment and other 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. 

3.2.8 Other Annual Items: The Other Annual Items section allows users to capture the capital cost of 

equipment and other 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 sizing characteristics of the clarifier and estimated sludge production. Most of the sizing summary 

outputs are self-explanatory, however, there are a few that warrant further explanation. 

Sludge Production @ Typical Flow Rate & Operational Time – Sludge loading rates shown under this 

heading are calculated using the “Typical Flow Rate”. The reported values represent the sludge 

production for the specified Operational Time located in the Annual Costs Input section of the module. 

For example, if a mine pool management plan only requires a pool to be pumped and treated for six 

months annually, users would enter 24 hrs/day and 182.5 days/year into the input fields for Operational 

Time. AMDTreat would use the information to compute the sludge production at the Typical Flow rate

for six months of treatment. 

Est. Sludge Pumping Time @ Operational Time – Values reported under this subheading represent the 

required sludge pumping based on the Operational Time users specified under the Annual Cost Input 

section. The reported value for hours/day represents the amount of daily sludge pumping required while 

the treatment plant is operating. For example, if a user specifies the Operational Time is 6 hours/day for 

365 days/year, the reported value for hours/day represents the daily sludge pumping time during the 6 hrs 

of daily operation. The hours/year represents the cumulative hours of pumping on an annual basis. 

Calculated Pressure Class – AMDTreat uses the user-specified input and calculates the highest SDR 

rating (lowest pressure class) for the set of conditions. AMDTreat compares the Calculated Pressure Class 

to the SDR selected by the user as part of error checking. If the user selects a pressure class of SDR pipe 

higher than the calculated, AMDTreat produces a Warning Message informing the user they can use a 

thinner, and less expensive, pipe. 
26 

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

and the total estimated cost to construct the Clarifier system. Users can opt to estimate the installation cost 

by specifying it as a percentage of the capital cost or by entering a cost. 

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

maintenance the clarifier component of the treatment system. The annual Maintenance can either be 

specified by the user or estimated by assuming it is a percentage of the capital cost. The latter method 

assumes the more expensive system are more costly to maintain. Finally, users can specify the annual 

electric cost or AMDTreat will estimate the annual electric cost for all the electrical equipment contained 

in the clarifier. 

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

clarifier component of the 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, a single value for Term of Analysis, Rate of Return, and Inflation Rate is 

applied to all modules and cannot vary between modules. 

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

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

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

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

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

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

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

recommended equipment and materials that require recapitalization. In addition, 

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

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

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

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

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

calculating the NPV. 

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

consider the following hypothetical scenario. Assume a vertical turbine pump has a life
27 

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

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

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

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

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

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

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

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

future costs. 

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

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

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

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

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

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

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

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

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

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

$140,799 in 40 years (Equation 8). 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 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.
28 

• 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. 

• 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, the clarifier assumes the Sludge 

Recirculation Pump will be rebuilt every five years at a cost of 17% of the purchase 

price. 

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 in 

Pennsylvania and West Virginia and have collective experience in design, funding, 

and/or operation/maintenance of large treatment systems using clarifiers. The AMDTreat 

Team held discussions on personal experience and spoke to manufactures to develop a 

list of recapitalization recommendations. Users may have different experience and 

opinions than those listed. 

By default, AMDTreat includes a list of eight recapitalization items for clarifiers. Users 

can delete or modify any of the default Recapitalization items by either deselecting the 

item or by setting the Replacement % to zero. If the item is deselected the Total Cost for 

the item will still be shown but the cost will be subtracted from the Net Present Value 

Cost, shown in the Net Present Value Heading. For example, the default value for the 

lifecycle of a Catwalk is 20 years assuming it is not sandblasted and repainted at first 

sight of corrosion. However, if a catwalk is periodically repainted, users may opt to 

increase the Lifecyle. Users are free to fully customize the replacement items, including 

adding new items or deleting default items. 

Clarifier Tank and Internals: A Clarifier Tank consists of the outer wall and concrete 

floor. The Internals consists of the drive mechanism, rakes, and center well. The Life 

Cycle of a Clarifier Tank and Internals depends on the type of construction material. A 

concrete clarifier is likely to last much longer than a bolted or welded steel tank. There 

are concrete clarifier tanks that are over 65 years old still in operation. Likewise, there are 

welded steel clarifiers that are 50 years old still in operation, however, they are 

sandblasted and painted every 7 to 10 years at a significant cost. After a discussion with 

a clarifier manufacture, the AMDTreat team recommended a lifecycle of 75 years for 

concrete clarifiers and 35 years for steel clarifiers. The 35-year recommendation assumes 

the steel will not be maintained through sandblasting and painting. The lifecycle 

recommendations can be adjusted to reflect a planned maintenance strategy. 

Catwalk: AMDTreat assumes the installation of painted steel catwalk and handrailing. 

A life cycle of 20 years is recommended for situations where the catwalk will be allowed 

to corrode without maintenance. If the catwalk is periodically sandblasted and painted, 
29

the life cycle can be increased. There are some well maintained catwalks that are 50 years 

old. The replacement life cycle can be increased if an aluminum catwalk is installed, like 

in many municipal situations. 

Tank Protective Coating: Protective coatings are applied to clarifier outer walls during 

installation to slow corrosion and increase longevity. Protective coatings can be 

periodically reapplied to manufacture recommendations. AMDTreat includes the 

manufacture’s recommended life cycle for each type of protective coating. 

Sludge Disposal Pump Rebuild and Replacement: The AMDTreat team has personal 

experience with operating mine drainage plants and held discussions with plant operators 

to try to estimate the life cycles for sludge disposal pumps. In general, sludge pumps are 

commonly rebuilt or repaired numerous times before being replaced. The Team decided a 

five year rebuild/repair schedule and 20 year replacement life cycle. 

Sludge Disposal Borehole: Sludge boreholes are used to dispose of sludge into 

underground coal mine voids or into the backfill of reclaimed surface mines. There are 

several way to try to increase the longevity of disposal boreholes including trying to 

prevent sludge thickening before pumping and pumping clean water for several minutes 

after injection to clear the borehole of any sludge. However, eventually sludge boreholes 

lose capacity to accept sludge and new holes must be drilled. While it is impossible to 

accurately predict the longevity of a borehole, we recommend a 15-year replacement 

assumption unless there is other information available. 

Clarifier Cleanout: Some operators prefer to periodically dewater and clean the clarifier 

to prevent scale from accumulating on the rakes and potentially increasing torque and 

damaging the drive unit. Many operators clean the clarifier by using pressurized water to 

abrade the solids and scale from surfaces and a trash pump is placed in the sludge sump 

to remove the abraded solids from the clarifier. Cleaning the surfaces provide an 

opportunity to inspect the rakes to look for signs of corrosion.
30

5.0 Figures 

Figure 1: Settling test determining the settling rate for a specific concentration of flocculated 

aluminum hydroxide produced solids.
31

Figure 2: Results of ten settling tests to relate the rise rate to the concentration of produced 

solids.

Figure 3: The settling flux rate characterizes the optimal solids loading. 
32

Figure 4: Typical concrete conventional clarifier used to in mine drainage treatment in the 

eastern U.S.

Figure 5: Hydrated Lime mine drainage treatment plant using two conventional clarifiers 

operating in parallel.
33

Figure 6: Concrete-walled conventional clarifier. Note the open trough located under the 

walkway (catwalk) to the center well. Diluted polymer flocculant is added at hydraulic jump in 

the trough where the water falls by six inches to provide the mixing necessary to disperse the 

diluted polymer throughout the waste stream. Particle-to-particle interactions occur within the 

center well to promote agglomeration of particles. 
34 

Figure 7: Example of a flocculating center well. Polymer flocculant is added to pH-adjusted 

mine drainage at a hydraulic jump in the conveyance trough. The center well is designed to 

dissipate flow velocity and promote agglomeration of particles. Note transport rakes, located 20 

ft under the water surface can be seen (see red arrow). 
35

Figure 8: Photo of a 110 ft diameter conventional clarifier. Note red-colored sludge blanket is 

within five feet of the surface indicating the need to pump sludge. Reducing the volume of the 

sludge bed allows the sludge blanket to reposition lower in the clarifier. 
36

Figure 9: Photo a center well and rake drive system that is over 55 years old. The center well is 

the large circular steel structure that extends approximately 6 ft below the water and the smaller 

diameter cylinder that extends through the center well to the bottom is the rake drive system. The 

steel armatures that extend outward from the rake drive to the outer wall is the rake. The angled 

plows attached to the bottom of the rake are used to transport settled sludge to the center of the 

clarifier for evacuation using a sludge pump. 
37 

Figure 10: A different angle of the same clarifier shown in Figure 5. Note the rectangular sludge 

sump located in the bottom center of the clarifier along with the steel sludge line embedded into 

the concrete flow of the clarifier. A sludge pump housed in the operations building pumps sludge 

from the sludge sump through the sludge line to an injection borehole that terminates in an 

underground mine. Also note the corrosion on the steel catwalk and handrailing extending to the 

center well. 
38

Figure 11: A newly constructed concrete clarifier showing the sloped floor and the sludge sump 

and sludge pump line extending under the floor.
39 

Figure 12: Photo of an operator using a pressurized hose to clean iron hydroxide scale from the 

walls, floor, and rakes of the clarifier. Notice the lack of scale to the left of the operator 

compared to the right. Figure 9 shows the empty clarifier after it was cleaned. 
40 

Figure 13: Photo of a disrupted clarifier discharging the sludge blanket due to an incorrect solids 

inventory handling plan. A change to both the elevation to maintain the sludge blanket and the 

sludge pumping schedule fixed the issue.
41

Figure 14: Bright green dye introduced into conveyance trough that connects the reaction tank to 

the center well of the clarifier. The open conveyance trough is located under the catwalk to the 

center well. Also, note the corrosion of the steel catwalk and handrailing extending to the center 

well. 
42 

Figure 15: Dye showed a preferential flow path of water on one side of the clarifier that 

increased water velocity causing poor settling and turbid effluent. Metal baffles were welded into 

the center well to improve flow distribution. This created a radial flow path emanating from the 

center well and resulted in improved effluent clarity. into the center well to redistribute the 

watering flowing down through the clarifier. 
43

Figure 16: Alternative view of dye test shown in Figures 14 and 15. Before the installation of 

the flow baffle in the center well, dye preferentially flowed on one side of the clarifier. 
44

Figure 17: Dye test on a large 220 ft diameter steel clarifier. Note rotation caused by rake 

movement. 
45 

Figure 18: A 5-gallon jug tied to a rake arm informs the operator the position of the rakes. The 

treatment strategy was changed from hydrated lime to hydrogen peroxide, which produced a less 

dense sludge. Soon after the conversion, a cloud of suspended iron solids appeared (Figure 19) 

and followed the floating jug indicating the rake speed was resuspending the less density 

peroxide sludge. A variable frequency drives connected to the drive motor was used to slow the 

speed of the rakes to fix the issue. 
46

Figure 19: Cloud of suspended iron hydroxide that followed the rake rotation after conversion 

from hydrated lime to hydrogen peroxide.
47

Figure 20: Schematic of a Solids Contact Clarifier (courtesy of Westech Engineering). Blue 

arrows signify the flow of water and the red arrow indicates how the sludge is pulled back to the 

surface to mix with raw water. 

Figure 21: Concrete walled solids contact clarifier. Draft tube and reaction skirt/well are 

underwater and not visible.
48

Figure 22: Arial view of newly constructed concrete-walled Solids Contact Clarifier treating 

ferruginous mine drainage. Sludge pumps are housed in the white building and the effluent is 

discharged from silver piping into a wetland where total Fe is reduce4d from 0.5 to 0.03 mg/L.
49

Figure 23: Top view looking down at Center Well of a newly constructed Solids Contact 

Clarifier. The blue cylindrical tube in the middle of the photo that extends down to the concrete 

floor is the draft tube. The larger diameter blue cylinder on top of the daft tube is the impeller. 

The outer blue steel wall is the reaction well/skirt. The blue impeller spins and pulls settled 

sludge from the bottom up the draft tube where it is mixed and discharges it into the reaction 

well where the solids flocculated with the treated mine drainage. The mixture flows down the 

reaction well and into the settling zone of the clarifier.
50

Figure 24: A picture showing the reaction well of a newly constructed solids contact clarifier. 

The twelve-inch inlet pipe transports the raw mine drainage into the draft tube where it is reacted 

with hydrated lime and recycled sludge. 
51

Figure 25: A steel mix tank to condition the recycled sludge with hydrated lime. The white pipe 

is the sludge recycle line and the smaller diameter black pipe entering the top right of the tank is 

the lime slurry line. A mechanical mixer is used to flash mix the sludge and lime slurry. The 

lime/sludge mixture is then discharged to the reaction tank where it reacts with the untreated 

influent water. 

Figure 26: Outfall of sludge conditioning tank showing the sludge/lime slurry mixture being 

discharged into the reaction tank to react with the untreated influent. 
52

Figure 27: The HDS conditioning tank was retrofitted to separate the lime slurry and sludge due 

to constant scaling and maintenance issues with the tank. The recirculated sludge is shown 

discharging from the blue pipe at the left of the photograph and the lime slurry is discharging 

from the conditioning tank. 
53

Figure 28: HDS trough system developed by Bethlehem Steel. Lime slurry is dosed into the 

trough directly inside the building. The sludge/lime slurry mixture travels 30 feet before being 

discharged into the reaction tank to mix with the untreated influent water. The conditioned 

sludge in this photo has a higher percentage of lime since the photo was taken during a sludge 

pumping event. When sludge is pumped from the clarifier to an injection well, the amount of 

sludge recirculated decreases.
54 

Figure 29: Example of an Enhanced Flocculation treatment system. Sludge from the clarifier is 

pumped back into the reaction tank where it is mixed with mine water treated with hydrogen 

peroxide. The pipe is recirculating approximately 250 gpm of 4% solids by weight back to the 

reaction tank.
55 

Figure 30: Workers constructing a rebar form to pour a sloped concrete floor of a clarifier. 

The vertical form is to pour the concrete side walls.
56 

Figure 31: A newly constructed welded steel conventional clarifier with a concrete floor. 

Note the greyish density curtain baffle extending the perimeter of the inside wall under the 

laundering weir. 

Figure 32: The construction of a 220 ft diameter welded steel clarifier with a concrete floor. 

Note the missing steel panels that allow for equipment access into the clarifier during 

construction. 
57 

Figure 33: A viewpoint of the inside of the welded steel clarifier shown in Figure 31. 
58 

Figure 34: A viewpoint of the inside of the welded steel clarifier shown in Figures 31 & 32. 

Note the inside wall is in the process of being painted beige. A protective coating was also 

applied to the steel to retard corrosion. The steel panel weld marks are clearly visible on the 

unpainted section. 
59

Figure 35: Photo showing a buildup of iron in the valleys of the sawtooth overflow weir. 

Figure 36: Worker lowering pump to clean inside of clarifier. In addition, the iron-coated 

overflow weir and transport trough (working standing in trough) will be cleaned to restore an 

even distributed flow across the weir. 
60

Figure 37: Works replacing the steel bolts that attach the overflow weir to the concrete wall with 

stainless steel bolts. The corrosion caused some of the bolts to fail causing the weir to fall into 

the clarifier (Figures 38 and 39). Stainless steel fasteners must be used when treating mine 

drainage. Note the top of the ladder is resting against the saw-toothed overflow weir and the weir 

is resting on support beams that secure the weir to the concrete wall. 
61 

Figure 38. Photo showing the corrosion of the fasteners that connect weir (top of photo) to the 

steel support beam and that connect the beam to the concrete wall. The steel fasteners were 

replaced with stainless steel. 
62 

Figure 39: Photo showing a portion of the yellow saw-toothed weir fell 

into the clarifier because corrosion from the mine drainage caused the 

fasteners to fail. 
63 

Figure 40: The blue fiberglass Density Current Baffle extends (courtesy of NEFCO) extends 

along the periphery of the outer wall just below the overflow weir. 

Figure 41. Assumed solid phases and specific gravities for ions 

Figure 42. Example of how a composite sludge density is calculated 

Ion Solid S.G.

Al3+ Al(OH)3 2.42

Fe2+ Fe(OH)2 3.4

Fe3+ Fe(OH)3 4.25

Mn2+ Mn(OH)2 3.25

CaCO3 2.71

Ion mg/L % Mass Solid S.G.

Weighted Density of 

Solids in Sludge 

(lbs/gal)

Al3+ 30 40% Al(OH)3 2.42 8.08

Fe2+ 15 20% Fe(OH)2 3.4 5.67

Fe3+ 20 27% Fe(OH)3 4.25 9.46

Mn2+ 10 13% Mn(OH)2 3.25 3.62

CaCO3 0 CaCO3 2.71

Misc. Solid 0

Total 75 Composite Solids in Sludge = 3.22 26.83
64

Figure 43: Overland sludge pipeline to injection borehole. Most sludge lines are buried but this 

one is supported by wooden cribbing. 
65

Figure 44: Raw water entering a reaction tank, showing clear, low total suspended solids water. 

The sludge injection well was located a few hundred feet from the raw water pumping well in a 

semi-isolated section of the mine. Sludge was successfully pumped for fifteen years without 

recirculating sludge back to the raw water pump. However, eventually the sludge disposal 

migrated to where the raw water pump recirculated sludge back to the reaction tank (Figure 45). 

A new sludge injection borehole was drilled a few thousand feet away in a more isolated and 

down-dip section of the mine. 
66

Figure 45: Photo showing sludge injected in the underground mine is being captured by the raw 

water pump and recirculated with the raw water back to the reaction tank. See caption in Figure 

44. 
67 

Figure 46: Large geotubes receiving sludge pumped from a clarifier. Eventually, the geotubes 

will be buried in place for final disposal. 

Figure 47: Additional photo of geotubes show in in Figure 46. The geotubes will be buried in 

place as part of a reclamation plan to reclaim the highwall in shown in the photo. Soil will be 

placed over the final elevation of the geotubes and vegetated as part of the plan. 
68

Figure 48: Sludge is pumped to the polymer mixer in the brown hose on the hillside. Polymer is 

added in the grey tubing near the sludge inlet and the polymer and sludge are passively mixed 

using a series of pipe bends. Two outlet hoses provide the flexibility to switch between two 

geotubes. When a geotube is filled, it is allowed to dewater and sludge is then pumped to the 

second geotube. The spigot is used to check the flocculation of the sludge to help calibrate the 

polymer dose. The PVC mixer can be constructed for a few hundred dollars. 
69

Figure 49: Sludge is pumped through the red hose and into the geotube where the polymer 

flocculated particles are larger than pores in the geotube, which causes the solids to be retained 

in the bag while the water seeps through the fabric, dewatering the solids. 
70

Figure 50: Geotubes being used in parallel. The geotube in the foreground was totally filled with 

sludge the previous day and is currently being allowed to dewater. Sludge will be pumped again 

in the next few days and the empty geotube will be used. The weekly sludge pumping will be 

alternated between geotubes to provide dewatering time. A gravel pad was constructed for the 

geotubes and has a 1% slope towards the perimeter ditching to capture the dewatering water. The 

geotubes are staked to the ground to prevent rolling as the tube is filled. In this photo, the sludge 

is being pumped from a pond and the variable sludge concentration during pumping makes it 

difficult to achieve a correct polymer dose. Thus, there are times where unaggregated particles 

leak through the pore space and into the perimeter ditch explaining the red coloration.