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RECLAMATION and ENFORCEMENT

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Forward Cost Modeling      Estimating Annual Costs      Reverse Cost Modeling




Forward Cost Modeling Tutorial

Forward cost modeling is the process of trying to predict the future cost of building and maintaining a treatment system on a discharge.

The following treatment scenario is hypothetical; however, the scenario encompasses many of AMDTreat's modules and illustrates how the program can be used to estimate costs. Assume a reclaimed site has two discharges and the objective is to develop a cost estimate for treating both discharges.  The water quality and quantity data for both hypothetical discharges is shown in the Table below.

Parameter
Discharge #1
Discharge #2
Design Flow 10 gpm 17.7 gpm
Average Flow (median) 7.2 gpm 13.7 gpm
Hot Acidity to pH 8.2 157 mg/L 80 mg/L
Alkalinity to 4.5 < 5 mg/L 6 mg/L
pH 3.1 4.9
Ferric Iron 0 mg/L 0.35 mg/L
Ferrous Iron 46.8 mg/L 1.0 mg/L
Manganese 14.2 mg/L 34.4 mg/L
Aluminum 1.14 mg/L 2.70 mg/L
Sulfate 580 mg/L 2400 mg/L
Dissolved Oxygen 7.3 mg/L 5.63 mg/L

 

In order to save money, both discharges will be combined and treated with a single treatment system.  Two 500 ft-long  rock-lined ditches will carry both discharges to a point where the discharges will be combined and the single discharge will then flow 100 ft through a rock-lined ditch to a settling pond.  The purpose of the settling pond is to reduce the amount of treatment by precipitating many of the metals. The discharge will then flow from the settling pond to a Vertical Flow Pond and onto the final settling pond.  Since the VFP will not treat Manganese, a Caustic Soda treatment system will be constructed to raise the pH to precipitate Mn. The 3-acre site where the VFP and ponds will be built is covered with small diameter trees and brush, thus necessitating clearing and grubbing.  A road will not need to be built because an existing road is usable.  First we will perform a double check on the Lab's Hot Acidity value for Discharge #1 by using the Acidity Calculator, which is under the Tools menu.  The Acidity Calculator will calculate the amount of acidity from the hydrolysis of Fe, Mn and Al and the amount of acidity from the pH value.  Entering the data from Discharge #1 into the calculator results in an acidity value of  155.69 mg/L. This value is very close to the Lab's acidity value (157 mg/L) and provides confidence in the Lab's analysis. The minor differences in the values may be from out gassing of CO2 during sampling or analysis. Acidity values are very important as many of the AMDTreat's cost estimates are very sensitive to this value.  In theory, the VFP should not treat Ferrous iron, thus the VFP does not have to be designed to treat the acidity produced by the oxidation of this metal.  However, the organic matter commonly does not sufficiently reduce the water to keep Iron in the Ferrous state and Iron often oxidizes in the VFP. As a safety measure, the VFP will be designed to ensure enough limestone is present to treat any acidity produced from the oxidation of Iron. Net acidity for Discharge #1 equals Hot Acidity (see the Water Quality Help topic: Net Acidity, for a thorough discussion).  

Image of Acidity Calculations.

For Discharge #2, a large percentage of the Hot Acidity is probably a result of the oxidation of Manganese, since the pH is relatively high (i.e. low concentration of H+) and the concentrations of Al and Ferrous Iron are low.  It is uncommon for Mn to oxidize in a VFP, thus the Mn acidity contained in the Hot Acidity value should be removed to calculate a "design" acidity. The design acidity will be used in calculating the amount of limestone needed in the VFP to neutralize any non-Mn acidity encountered or produced in the VFP.

Design acidity = 16.76 mg/L (Hot Acidity (80 mg/L) - Mn Acidity (63.24)).

The Net Acidity for Discharge #2 can be calculated by subtracting Alkalinity from the Design Acidity. 

Net Acidity = 10.76 (Design Acidity (16.76 mg/L) - Alkalinity (6 mg/L)

 

 
Discharge # 1
Discharge #2
Hot Acidity
157 mg/L
80 mg/L
Calculated Acidity using Calculator
155.69 mg/L
80.98 mg/L
Design Acidity
155 mg/L
16.76 mg/L (less 63.24 mg/L Mn Acidity)
Alkalinity
<5 mg/L
6 mg/L
Net Acidity
155 mg/L
10.76 mg/L


The Mass Balance Calculator will be used to estimate the quantity and quality (loading) of the final discharge after Discharge #1 and #2 are combined.  The Mass Balance Calculator does not model any precipitation or dissolution that may occur after the waters are mixed. All mass is conserved and the calculator can be used to predict a "ball-park" bulk concentration (loading) after mixing occurs.  All of the lab data for Discharge #1 and #2 will be used in calculating the quality and quantity of the mixed water with the exception of the Hot Acidity value for Discharge #2.  A design acidity value of 10.76 mg/L will be used. 

Since the combined discharge has been characterized, we are now ready to use AMDTreat to estimate treatment costs!!

Three-Step Approach to estimating treatment costs

AMDTreat uses a 3-step approach to calculating a treatment cost: 1.Users enter water quality and quantity data, 2. Users “build” an active and/or passive treatment system by selecting the applicable treatment components from the software menu, and 3. Users customize each treatment system to site-specific conditions by controlling the size, quantity, and unit cost of treatment system components.

Step 1: Users enter water quality and quantity data

Before the treatment system (VFP, ponds, caustic, etc.) can be sized, the user must enter the combined discharge water data into the Water Quality Screen. The cost and sizing of the treatment system is very sensitive to the data entered.

AMDTreat offers two options to enter the Net Acidity of the combined flow.  The first method is to enter both a calculated acidity (tools menu) and a lab or field-determined alkalinity on the Water Quality screen.  AMDTreat will calculate a Net Acidity value by subtracting the Alkalinity value from the Calculated Acidity value. The second method is to manually enter a Net Acidity value on the water quality screen.  If the lab performs a Hot Acidity titration as outlined in Standard Methods (Method 2310), Hot Acidity should equal net acidity as the method requires the Lab to subtract alkalinity from the Hot Acidity value. For Further explanation, please refer to the Water Quality screen Help Topic:  Net Acidity (topic being developed for next version)

The design flow is used in sizing treatment systems (VFP, ponds, etc.) and average flow is used to estimate the annual amount of sludge and chemical reagent.  Therefore, design flow should approximate the largest flow the system will be able to treat and average flow should represent the flow the system is most likely to treat on a daily basis.  If a user enters too large a number for average flow, AMDTreat will overestimate the annual amount of chemical reagent the treatment system is likely to require.  Likewise, AMDTreat will overestimate sludge volumes. One important point is that Average Flow does not necessarily mean the value for this parameter should be the arithmetic mean.  Hydrology data is often non-normally distributed and data transformation cannot always cause it to approximate a normal distribution. The average flow may represent the median flow or some confidence interval. 

Parameters highlighted in black are not required by AMDTreat.  AMDTreat stores these numbers for reference purposes only; however, they can be used to help make treatment decisions. 

Step #2 Users "build” an active and/or passive treatment system by selecting the applicable treatment components from the software menu

Remember our hypothetical scenario was to build two 500 ft ditches to route two discharges to a single point.  Once the discharges are combined, a 100 ft rock-lined ditch will carry the combined discharge to a settling pond.  After the settling pond, a VFP, a caustic soda tank, and another settling pond will be used in a series to treatment discharge.  The site contains brush and small-diameter trees and will require clearing and grubbing. 

We are now ready to "build" our treatment system by selecting the appropriate treatment modules from the Costs screen.  First we will estimate the cost to construct 1100 ft of rock-lined trapezoidal-shaped ditch that will be needed to carry the water to the first settling pond.  Clicking the Ditching button on the Costs window opens the Ditching window.  The white text boxes on the Ditching window allow the user to enter information that will control the shape and cost of the ditch.  We will enter 1100 ft for the Ditch Length.  Variable #15 and #16, Cost to Place Rock and Excavation Unit Cost, default to $4.50/yd3.  Assume we have called several earth-moving contractors in the area and have found that the local cost to excavate dirt is $4.00/yd3.  The default cost can  be easily changed by double-clicking in the white box for variable #15 and #16 and entering a value of  $4.00/yd3.

After entering site-specific information into the Costs screen, AMDTreat calculates a total cost of $8,472 to build the 1100 ft trapezoidal-shaped ditch. The total cost is not only displayed in the Ditching screen, but also in the Costs window.  The Costs window shows the cost of the ditch is $8,472 and the cost for the construction of the ponds is $5,000.

Next we will estimate the cost to build the first settling pond. AMD Treat 4.x provides an enhanced Ponds module to size and cost treatment impoundments.   The Ponds module is designed to provide a cost estimate for any type of pond, sludge, sediment and erosion control, etc.  The Ponds module attempts to design a sludge pond that is large enough to hold the estimated sludge volume produced between sludge removal events while maintaining a desired water retention time. AMDTreat estimates the volume needed to hold sludge by estimating the volume of sludge that would be produced if all Iron, Manganese, and Aluminum precipitated. The user can control the volume of sludge by varying the numbers used to represent Percent Solids and Sludge Density. Another sizing criteria for Ponds is a desired retention time.  The user can enter a desired retention time and AMDTreat will calculate the theoretical volume of trapezoidal-shaped pond needed to hold the water for the desired retention time. The total pond volume for Ponds is the summation of the expected  volume of sludge and the volume needed to retain water for a desired retention time.  The Ponds module also provides the user with two options for sizing a trapezoidal-shaped pond.  The first option termed "Retention Time"  allows the user to enter a desired retention time. AMDTreat will calculate the pond volume needed to store water for a desired retention time.  The second option named "Pond Size" allows the user to enter a desired freeboard length, freeboard width, and pond depth and AMDTreat calculates the dimensions and volumes of a trapezoidal-shaped pond.  The specifics on calculations and equations are discussed in the Ponds help files.

Now we will use the Ponds module to size and cost the pond. The purpose of the first pond in the treatment system is to help reduce treatment costs by precipitating metals.  A large annual cost in water treatment is sludge removal.  To minimize sludge removal costs, we will design the pond large enough to hold approximately 5 years of sludge (assuming a sludge density of 8.4 lbs/yd3 and  a percent solids of 3.0%).  Furthermore, we will use a synthetic liner to line the entire pond.  Clicking on the Ponds button in the Costs window opens the Ponds module.  The following data will be entered to reflect our design and local costs: Sludge removal frequency = .2 (once every 5 years), Desired Retention Time = 100 hrs (over design to help eliminate the effects of short circuiting), Sludge Density = 8.4 lbs/yd3, Percent Solid = 3.0%, Freeboard Depth = 4.0 ft and Excavation Unit Cost =  $4.00/yd3.  Be sure to name the pond by typing Tutorial Pond #1 in the Ponds Name block. This will help you to keep track of which pond is which after the second pond is added.

AMDTreat estimates the cost to construct the pond is $10,077. The Costs window shows the total capital cost is $18,549 (summation of $8,472 (ditch) and $10,077(settling pond). 

Next we will estimate the cost to construct the VFP.   Users size a VFP treatment system by selecting one of the five radio buttons that represent sizing methodologies. The first sizing methodology, which appears on the VFP screen as "VFP Based on Acidity Neutralization," uses Design Flow and Net Acidity from the Water Quality window to calculate the tons of limestone needed to neutralize acidity for the design life of the VFP (variable # 12 on the VFP screen). The second sizing methodology, "VFP Based on Retention Time," calculates the tons of limestone needed to retain the water in the void spaces of the limestone bed. The third sizing methodology, "VFP Based on Alkalinity Generation Rate," calculates the tons of limestone needed for a user-defined Alkalinity Generation Rate (g/m2/day).  The fourth sizing methodology, "VFP Based on Tons Limestone Entered," allows users to enter the amount of limestone AMDTreat will use to size the treatment system. The final methodology, "VFP Based on Dimensions," allows the user to size the VFP by entering values defining the length and width of the VFP at the top of the freeboard.  This methodology is useful in back calculating costs (reverse cost modeling) for a VFP already constructed, and for sizing a VFP for locations with limited space for construction. Below is a table listing the variables that affect each of the sizing methodologies.
 

                                                          

For this example, we are going to use a combination of methodologies #1 and #2 to size the VFP.  A common methodology for sizing a VFP is to calculate enough limestone to neutralize the acidity for the life of the system while maintaining a desired retention time (16 hrs). In theory, at the end of the design life and after all of the limestone dedicated to acidity neutralization is dissolved, there will still  be enough limestone to provide the design retention time.  This sizing methodology entails summing the tons of limestone calculated by AMDTreat's sizing methodologies "VFP Based on Acidity Neutralization" and "VFP Based on Retention Time" and entering the tonnage into the fourth sizing methodology, "VFP Based on Tons of LS Needed."

Immediately after opening the VFP window, AMDTreat uses default values for quantities and pricing of treatment components, along with data from the water quality screen, to calculate the tons of limestone required for each of the 4 sizing methodologies. AMDTreat calculated 84.64 tons of limestone are required to neutralize the acidity for 20 years (assuming the limestone is 90% pure and 100% efficient). AMDTreat also calculated that 507.83 tons of limestone are needed to retain the water in the void spaces for 16 hours (assuming the LS bed is 35% void and the Density of Loose Limestone is 100 lbs/ft3).   The summation of 84.64 and 507.83, will represent the tonnage of limestone needed for the sizing methodology previously discussed.  After selecting the 4th sizing method, enter the summation (643tons) into the Limestone Needed variable box (#8).  AMDTreat offers two different methodologies for estimating the cost of the piping system for the VFP, "AMDTreat Piping Costs" and "Custom Piping Costs."  Users that know the amount of pipe to be used in the VFP should choose the "Custom Piping Costs" methodology and enter the specific lengths and costs of pipe.  Users that would like to calculate a "ball-park" figure for piping costs should choose "AMDTreat Piping Costs" for AMDTreat uses a piping routine to estimate the cost to purchase and place a single layer of pipe at the bottom of the limestone bed.  AMDTreat offers detailed explanations for all of the routines and variables for each of the different treatment methodologies by simply clicking on the help button located in the bottom right-hand corner of all of AMDTreat's windows. Clicking on the help button provides the user with a better understanding of the piping system design AMDTreat uses to calculate a piping cost.

The user can print all of the values used in calculating the capital cost of the VFP by using AMDTreat's report function.  AMDTreat offers two options to print a report.  A user can print individual screens (see below) by clicking on the Report button located in the bottom right-hand corner of every screen or a user can print every screen the user has activated by choosing Print Report from the File menu.

AMDTreat estimates the cost to build the VFP is $23,221. This value will be summed with the other capital costs on the Costs screen to arrive at a total capital cost of $69,327.  

Next we will estimate the cost to construct a Caustic Soda Treatment system.  Recall that our hypothetical scenario was to place a caustic soda treatment system directly downstream of the VFP.  The purpose of the Caustic Soda system is twofold.  First, it will act as a backup treatment system in case the VFP does not perform as expected (past experience has shown the treatment success of VFPs can be highly variable).  Secondly, caustic soda will be used to raise the pH to approximately 9.5 to precipitate the Mn in the discharge, since the VFP will not treat the Mn. 

After the Caustic Soda window is activated, AMDTreat uses information from the Water Quality screen to provide the user with chemical consumption rates that can be used in determining  the size of caustic tank needed. The "default" consumption rates will estimate the amount of caustic soda needed to raise the pH to 8.3. (Titration end point for Hot acidity calculation.)  Since our goal in this hypothetical scenario is to raise the pH to 9.5, a hypothetical consultant was hired to perform a chemical titration on the discharge to calculate the exact amount of caustic needed to raise the pH to 9.5.  Basically, the consultant added Caustic Soda to a known amount of discharge water until the pH stabilized at 9.5.  The amount of caustic soda added divided by the volume of discharge used in the titration represent the "Caustic Titration Volume" (variable #15 on the Caustic window). The results showed 0.0001 gallons of caustic is needed per gallon of discharge to raise the pH to 9.5.  After entering 0.0001 into variable #15, AMDTreat updates the consumption rates to reflect the use of the titration values.  Approximately 3 gallons of caustic soda will be needed per day to treat an average flow rate of 20.90 gpm.  A 2,500 gallon caustic tank will be used to help lower the annual cost of delivering caustic to the site because a 2,500 gallon tank should only have to be refilled once every 2 years.

The capital cost for the caustic system is added to the total capital cost on the Costs screen. Because of site and safety conditions,  a concrete slab will be need to be constructed underneath the caustic tank.  The cost of building the footer and pouring the slab is approximately $5,000.  The caustic soda module does not have the ability to incorporate such costs, so the Other Costs module will be used to capture costs not included in the primary treatment modules.

As seen above, the cost of the slab is entered on the Other Cost screen as a capital cost. Note that the Costs screen has two areas where Other Costs are displayed and subtotaled, one on the capital costs area and one on the annual costs area. Since the cost for the concrete slab is marked as a capital cost on the Other Costs screen (see above) it is added to the total capital cost on the Costs screen. 

Lastly, we will estimate the cost to construct the final settling pond.  Because of land limitations, the maximum size of the settling pond is 275 x 150 ft. We will add a second pond from the Ponds module to size and estimate the cost of the 275 x 150 ft settling pond. To add the settling pond press the Add button. This will create a new pond screen using the AMDTreat default values. After creating the new pond screen give it the name "Tutorial Second Settling Pond". Ponds offers 2 different sizing methodologies, "Retention Time" and "Pond Size."   The sizing option, "Pond Size" will be used since the maximum size of the settling pond is already known. AMDTreat uses the length and width values along with using user-defined values for freeboard and water depth to back calculate the pond volume. In the case below, the user input site specific information about the pond including the length and cost per foot of pipe, and added a clay liner and a baffle. The baffle cost was estimated by taking the width of the pond and multiplying that times the approximate cost per foot ($9.00) of building and installing a baffle.

Recall that, at the beginning of the tutorial, we stated that the 3-acre site had to be cleared and grubbed of small-diameter trees and brush.  Most modules offer two options for estimating the cost to clear and grub a site.  The first option, termed Land Multiplier, calculates the area of the treatment system and multiplies the area by a Land Multiplier to estimate the total amount of area that needs to be cleared and grubbed to ensure that earth-moving equipment can operate freely. This option should be used if the user does not know the number of acres that require clearing.  Thus, if the user does not know the number of clear/grub acres, the user should estimate clearing and grubbing costs on all treatment modules needed for the whole treatment system. For example, if a user is estimating the cost of constructing a VFP, Mn Removal Bed, and a settling pond for a particular site, the user should use the Land Multiplier method on each of the treatment systems to calculate the total cost of clearing and grubbing for the site. If the user only calculated the cost to clear and grub for one of the treatment systems, for example VFP, AMDTreat will calculate clear and grub cost by multiplying the surface area of the VFP and by the Land Multiplier. Therefore, in this example, the cost for clear and grubbing the whole treatment system would not have been estimated, but rather, just the cost of clearing the grubbing the area required to build the VFP.  The second option, termed Clear/Grub Acres, allows the user to enter the total acreage that needs to be cleared and grubbed.  Since we know 3 acres needs clearing and grubbing, we will use Clear/Grub Acres to estimate the cost.  Even though AMDTreat offers the option to estimate the cost to clear and grub on most of the treatment modules, users that opt to use the Clear/Grub Acres option should only estimate the cost to clear and grub on ONE of the treatment modules to avoid duplication of clearing and grubbing costs. We have chosen to add the cost of clearing and grubbing the whole site, for all treatment systems (VFP and 2 ponds), into the cost of constructing the second pond. 

Note the Flexi-Module feature of the Ponds screens. The user can select for viewing and editing either pond in this two pond treatment system by using the dropdown box in the Current Ponds section.

The Costs screen above shows the final capital cost for the treatment system is $83,535.  On the Costs screen the user can see a cost summary itemized by treatment structure, including the cost of the five thousand dollar concrete pad listed in the Other Costs module. The user can also see that there is only one instance of each of the structures with the exception of Ponds where there are two. (See the green blocks under the columns labeled A.) The user can also tell that none of the instances of the structures are (S)uspended from the calculation. (Note the zero in the pink blocks under the columns labeled S.)

It is now time to estimate the annual costs.


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Estimating Annual Costs

AMDTreat offers the users the ability to estimate 7 different annual costs, Sampling, Labor, Maintenance, Pumping, Chemical Cost, Oxidant, and Sludge Removal. For this example we will use all of AMDTreat's annual costs, except Pumping and Oxidant, since there is no pumping or oxidant use at this site.  

Sampling

Periodically, we will want to sample the system to monitor its performance. We will estimate the cost to sample the site on a quarterly basis (4 samples/12 months = .33 samples/month) for two sample points, upstream and downstream of the treatment system. This site is approximately 1/2 hour, one way,  from our water sampling consultant. AMDTreat estimates the annual cost of sampling will be $428.  Notice that this annual cost is added to the Costs Screen under Annual Cost. 

Labor

Labor costs can represent any type of labor cost; however, in our case we will have it represent the labor cost associated with paying someone to periodically check to ensure the caustic soda system is functioning properly. For this scenario, we will estimate the cost of overseeing the functioning of the caustic system on a weekly basis.

Maintenance 

AMDTreat's Maintenance routine estimates the annual amount of money that should to be set aside to pay for routine maintenance. Routine maintenance can include the cost of fixing embankments, fixing roads, replacing fixtures, etc. AMDTreat uses a method common in the construction industry to estimate annual maintenance costs.  AMDTreat estimates the annual maintenance cost by multiplying the capital costs by a user-defined percentage. It is logical to think that a caustic system costing $20,000 dollars requires more yearly maintenance than a caustic system costing $2,000. AMDTreat allows the user to set different percentages for four categories. Those four categories are active, passive, ancillary and other capital costs.   In our example, the total amount of money required for annual maintenance is $1191. This money may not be entirely used every year to maintain the system. It will accumulate over the years to provide a fund that can be drawn from when necessary. (Note: The percentages used in the example below are to demonstrate the flexibility of the software and should not be used as default values in the program.)

Chemical Cost

Often the major cost in active treatment is the annual purchase and delivery cost of the chemical reagent.  In our scenario, the caustic soda treatment system is not the primary treatment system, thus our annual chemical cost will be relatively low.

Sludge Removal

Removal of sludge from settling ponds can be quite expensive. Estimating sludge volumes is difficult. AMDTreat offers two options for estimating the annual amount of sludge.  By default AMDTreat uses information from the Water Quality screen (average flow, concentrations of Mn, Al, and Fe) along with values for Percent solids and sludge density to estimate the annual amount of sludge if all metals are precipitated (worse-case scenario). The estimated sludge volume is highly sensitive to the values used for average flow and percent solids. The value used for percent solids will differ depending on how one plans to remove the sludge.  If water is diverted from a settling pond full of sludge, and the water is allowed to evaporate from the sludge, a higher value for percent solids can be used. On the other hand, if a Mud Cat will be used to continuously clean a sludge pond, a lower value for percent solids will be used. It is best to use high and low values representing percent solids to provide a range of estimates for sludge volume. For a more accurate estimation, AMDTreat offers the ability to estimate sludge volumes from sludge titration data.  During a sludge titration, the treatment chemical is added until the desired pH endpoint is obtained. Metals will precipitate and the water will be allowed to rest for a given period of time to allow for sludge compaction.  For more information, refer to the sludge removal help file. 

For our treatment scenario, we will use AMDTreat's default method for estimating sludge volumes. AMDTreat estimates 51 yd3 of sludge will be produced on an annual basis. AMDTreat offers 5 different methods for calculating the cost to remove the sludge.   We will use the first option, termed "Sludge Removal by $ per Gallon."  Tiff Hilton, a leading expert in the field of designing treatment systems for mine drainage has found that, over a course of 20 years, the average cost to remove sludge, regardless of the method used, is approximately 5 cents for every gallon of sludge.

The total cost for removing sludge will be approximately $610.  

For our treatment scenario, the total capital cost of the system is $83,535 and the total annual cost is $4,049.

 

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Reverse Cost Modeling Tutorial

While forward cost modeling is the process of predicting the cost of building and maintaining a treatment system, reverse cost modeling is the process of back calculating the cost to build a currently constructed treatment system. 

Reverse Cost Modeling provides costing information for an existing system, i.e. obtains the cost to replace/rebuild that system.  The procedure of reverse cost modeling is to utilize the modules within AMDTreat  to calculate the cost to replace/rebuild an existing system or just part of the system.  This could be used for grant application purposes to develop costs to construct a similar treatment system.  Site specific information (size of ponds, lengths and size of ditches, depth of materials used, etc.) can be gathered about an existing treatment system, and input into the modules. Of course, sufficient, appropriate influent water quality data must also be input. (See Figure 1) Please see the water quality help menu  for further discussion on this matter. Further caution is warranted when applying an existing successful system to another discharge location, as success for one discharge does not automatically guarantee success for a different water discharge with different water quality and quantity parameters.


Figure 1 What-If Cost Modeling Costs and Water Quality Summary Screens

As with the forward cost modeling, reverse cost modeling will use the same three-step approach to estimate treatment costs: 1. Users enter water quality and quantity data, 2. Users select the existing active and/or passive treatment system components from the software menu, and 3. Users customize each treatment system to existing site-specific conditions inputting the size, quantity, and unit cost of treatment components. While it will be simple to obtain some on-site parameters, others will not be so evident.  For example, without design plans and specifications (or as-built), depth of the organic layer in wetlands may be hard to determine.  Historical information gained from people knowledgeable of the system's construction techniques should be used or reasonable assumptions must be made to arrive at existing parameters.

The components of the existing passive treatment system are: site encompasses a 3-acre area, 500 linear feet of stone access road, 120 linear feet of rock-lined up-flow and in-system ditches, Vertical Flow Pond (VFP), aerobic wetland, and 200 linear feet grass-lined ditch for the discharge flow to the receiving stream.  The respective modules are chosen and include Roads, Ditching, a 200' by 50' Aerobic Wetland (with 0.5 feet of standing water, 1 foot layer of compost and a 0.5 foot of clay liner), and a 244' by 142' Vertical Flow Pond (with 1.5 feet of free standing water, 1 foot of compost and a 0.5 foot thick clay liner).  The fields for each module are filled in as shown in Figures 2 through 7.  

Note that a different approach to clearing and grubbing was used for this example.  The clear and grub radio button was checked for each module, giving an area for each component. Another method that could have been used was to measure on site the overall acreage that was cleared and grubbed to build the treatment system. Then apply that number of acres to only one module. The total cleared and grubbed area for all components of the existing system can be put in the aerobic wetland module as 3 acres. 


Figure 2 Access Road


Figure 3 Access Road Module


Figure 4 Ditch Module


Figure 5 Wetland

Image of Wetland Module.
Figure 6 Wetland Module

Image of Vertical Flow Pond.
Figure 7 Vertical Flow Pond

Reverse cost modeling is the act of determining approximate cost to construct an existing facility by taking field measurements and entering them into AMDTreat. Therefore, when reverse cost modeling, the sizing based on dimensions method should always be selected. Note that the VFP and aerobic wetland dimensions were obtained from onsite measurements.

The total cost to replace the example treatment system is $165,971 and includes the construction and engineering costs. (See Figure 1 for a cost breakout.) Sampling and maintenance costs were not included in this example, but could be added by simply choosing the respective modules and adding the costs. The methodology to add these costs would be identical to the forward modeling methodology covered under Estimating Annual Costs section of this tutorial.


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Page Last Modified/Reviewed: 7/2/15

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