Tutorial
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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 ftlong rocklined 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 rocklined 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 3acre 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 CO_{2} 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).
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 nonMn 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 "ballpark" 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!!
ThreeStep Approach to estimating treatment costs
AMDTreat uses a 3step 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 sitespecific
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 fielddetermined 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
nonnormally 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 rocklined 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 smalldiameter 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 rocklined trapezoidalshaped 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/yd^{3}. Assume we have called several earthmoving contractors in
the area and have found that the local cost to excavate dirt is $4.00/yd^{3}.
The default cost can be easily changed by doubleclicking in the white box for
variable #15 and #16 and entering a value of $4.00/yd^{3}.
After entering sitespecific information into the Costs screen, AMDTreat
calculates a total cost of $8,472 to build the 1100 ft trapezoidalshaped 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 trapezoidalshaped 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 trapezoidalshaped 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 trapezoidalshaped 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/yd^{3} 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/yd^{3},
Percent Solid = 3.0%, Freeboard Depth = 4.0 ft and Excavation Unit Cost =
$4.00/yd^{3}. 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 userdefined
Alkalinity Generation Rate (g/m^{2}/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/ft^{3}). 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
"ballpark" 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 righthand 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 righthand 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 userdefined 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 3acre site had to be cleared and grubbed of smalldiameter 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
earthmoving 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 FlexiModule 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.
Back to Top
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 userdefined 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 (worsecase 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 yd^{3} 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.
Back to Top
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 WhatIf Cost Modeling Costs and Water Quality Summary Screens
As with the forward cost modeling, reverse cost modeling will use the same
threestep 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 sitespecific conditions inputting the size, quantity, and
unit cost of treatment components. While it will be simple to obtain some
onsite parameters, others will not be so evident. For example, without design
plans and specifications (or asbuilt), 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 3acre area, 500 linear feet of stone access road, 120 linear feet of
rocklined upflow and insystem ditches, Vertical Flow Pond (VFP), aerobic
wetland, and 200 linear feet grasslined 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
Figure 6 Wetland Module
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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