(2) in which OM = annual operating and maintenance costs, in dollars per year; CC = annual capital cost, in dollars per year based on 8% interest rate and 20-yr ...
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OPTIMIZING GAC
SYSTEMS 1
By Robert M. Clark, M. ASCE ABSTRACT: Granular Activated Carbon (GAC) is an effective technique for removing synthetic organics from both ground and surface waters. Questions have been raised, however, over the cost of using GAC in this manner. In order to provide insight into these cost issues, the Drinking Water Research Division has developed a series of cost curves for various unit processes including those related to GAC. This paper presents a modification of these cost curves into continuous analytic equations. These equations can be used to estimated unit cost for various treatment conBgurations. Sensitivity analysis can be performed and design configurations listed. Regional options are explored using these equations as the basis for analysis. INTRODUCTION
Granular activated carbon (GAC) is recognized as an effective technique for removing synthetic organics from both ground and surface waters (4). Questions have been raised, however, over the cost of using GAC in this manner. This paper is intended to provide information which may assist in answering some of these cost related questions and to suggest ways in which minimum cost designs for GAC may be achieved. The cost data presented is based on a research effort conducted by the Drinking Water Research Division of the U.S. Environmental Protection Agency. The original data was developed by Culp/ Wesner/Culp (C/W/C), Consulting Engineers. The purpose of the project was to develop operating and costs for water treatment processes that are sufficiently accurate for preliminary planning and flexible enough for cost effectiveness studies. Capital and operations and maintenance (O&M) costs were developed for a series of unit processes and presented as a function of a given process design parameter. ZurheideHerman, a cost-estimating specialist reviewed the developed costs (2). DATA SOURCE
The construction cost for each unit process is presented as a function of the specific process design parameters which was determined to be the most useful and flexible under varying conditions, such as loading rate, detention time, or other conditions that can vary because of de'Engrg. Systems Analyst, Drinking Water Research Div., Municipal Environmental Research Lab., Cincinnati, Ohio 45268. Note.—Discussion open until July 1, 1983. To extend the closing date one month, a written request must be filed with the ASCE Manager of Technical and Professional Publications. The manuscript for this paper was submitted for review and possible publication on August 25, 1981. This paper is part of the Journal of Environmental Engineering, Vol. 109, No. 1, February, 1983. ©ASCE, ISSN 0733-9372/83/0001-0139/$01.00. Proc. No. 17676. 139
J. Environ. Eng. 1983.109:139-156.
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signer's preference or regulatory agency requirements. These curves are not the final capital cost for the unit process because they do not include costs for general contractors' overhead and profit, administration, engineering and legal fees, fiscal determinations and interest during construction, which are added separately. Operation and maintenance requirements were developed for building-related energy, process energy, maintenance material, and labor. Chemical costs must be added separately. The following unit process cost curves are those most closely related to GAC (5). Gravity Carbon Contactors—Concrete Construction.—Concrete gravity carbon contactors are essentially the same as concrete gravity filtration structures. The contactors were assumed to be completely housed and the costs are for the contactor only, excluding surface wash and backwash pumping facilities, the initial carbon charge, and carbon handling equipment outside of the pipe gallery. Gravity Carbon Contactors—Steel Construction.—Steel gravity carbon contactors are large diameter, field-erected structures. The contactors were sized for downflow operation and an application rate of 5 gpm/ sq ft (0.0034 m3 • m~2 • s' 1 ). Spent carbon is removed from the contactors through multiple carbon drawoff pipes in the underdrain support plate. Regenerated carbon is returned to the top of the contactors, which are completely housed. The costs exclude surface and backwash pumping facilities, the initial carbon charge, and carbon handling equipment outside of the building. Pressure Carbon Contactors.—Carbon contactors are shop-fabricated, cylindrical pressure vessels. The contactors were designed for downflow operation, and employ a nozzle-type underdrain system for rapid spent carbon removal. The contactors were assumed completely housed. The costs exclude backwash and surface wash pumping facilities, the initial carbon charge, and carbon handling equipment outside of the building. Conversion of Sand Filters to Carbon Contactors.—An inexpensive method of providing carbon contact is afforded by removing the sand media and replacing it with activated carbon. Generally, the underdrain and support gravel design can be retained without modification. The installation of a spent carbon collector and transport system and a return system for regenerated carbon are the only additions required. Extensive piping replacement and alteration of existing filter rate controls and instrumentation are usually necessary if the application rate is to be increased beyond the original filtration rate. Multiple Hearth Granular Carbon Reactivation.—Multiple hearth reactivation uses a multiple hearth furnace operated under closely controlled conditions of temperature, oxygen, and moisture content. The required furnace size is a function of the required reactivation frequency, carbon dosage (which is a function of the nature of the organics adsorbed), allowable hearth loading, and anticipated downtime. The costs that were developed include the basic furnace, cooling fans, spent carbon storage and dewatering equipment, regenerated carbon handling system, quench tank, exhaust scrubbing system, and all necessary instrumentation. The furnace was assumed to be housed. Operation and maintenance cost curves are based on continuous operation, and adjustment must be made for operation times falling below 100%. 140
J. Environ. Eng. 1983.109:139-156.
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Infrared Granular Carbon Reactivation.—Granular carbon can be reactivated by using infrared energy to generate heat. The principal advantage of infrared reactivation is the ability of the furnace to be rapidly put into or taken out of operation without furnace damage or excessive operational cost. The carbon moves through the furnace on a conveyor belt, and reactivation time is varied by changing the conveyor speed and varying the depth of the carbon on the belt. The cost curve includes the furnace, spent carbon and dewatering facilities, quench tank, afterburner and scrubber, all required electrical equipment and controls, and an enclosure for all equipment. The operation and maintenance cost curves assume operation 100% of the time, and adjustment must be made for lesser operation times. Fluid Bed Granular Carbon Reactivation.—This type of reactivation uses hot gases both to fluidize and reactivate carbon. No inert heat source, such as sand, is required. Reactivation rates as high as 70 lb/ hr/sq ft (0.095 kg m" 2 s -1 ) can be accomplished. The cost curve includes spent and reactivated carbon storage, carbon dewatering equipment, the fluid bed reactor, fluidizing air blower, quench tank, particulate scrubber, instrumentation, and controls. All facilities were assumed to be housed. The operation and maintenance cost curves are developed on the basis of 100% operation, and adjustment must be made for less operational time. Off-Site Regional Carbon Reactivation.—Small water treatment plants employing activated carbon may find reactivation at a large plant (or a regional reactivation facility) more economical than purchasing virgin carbon to replace spent carbon or providing on-site reactivation. The construction costs include only granular carbon dewatering-storage bins. The costs do not include the cost of reactivation at the regional site. The operation and maintenance curves include the cost of hauling to the regional reactivation location, but do not include the cost of reactivation. DEVELOPMENT OF COST EQUATIONS
The original data set has proven extremely useful, however, for optimization and preliminary design studies they are also slightly cumbersome. In order to make these data more tractable, equations for preliminary estimation of the following form were developed: OM = Kj USRT" PRtPPFDHR'iNTGeDSEL/UN*fr™D!c7MI t
1
m TDH G
(1)
and CC = K2USRT CCI UN n o (2) in which OM = annual operating and maintenance costs, in dollars per year; CC = annual capital cost, in dollars per year based on 8% interest rate and 20-yr amortization period; USRT = design parameter; PR = power cost, in dollars per kilowatt-hour; PPI = producers price index divided by 100; NTG = cost of natural gas, in dollars per standard cubic foot; DHR = direct hourly wage rate, in dollars per hour; DSEL = cost for diesel fuel, in dollars per gallon.; UN = number of units in a process; TDH = total dynamic head, in feet; G = velocity gradient, in feet per second per foot; MI = distance, in miles; CCI = Engineering News Record construction cost index divided by 100; and Ku K2, a, b, c, ... m = constants. 141
J. Environ. Eng. 1983.109:139-156.
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The constants are determined by the regression of random design variable inputs against the costs calculated by the original equations. The equations derived from this analysis are shown in Table 1, which includes the appropriate design parameter for both the operating and maintenance and capital cost and the various equation parameters over the ranges for which the equations were developed. Capital costs were derived for specific interest and amortization rates and conversion to a different base is a relatively simple calculation. Overhead items, such as interest during construction and engineering cost, are included in the TABLE 1.—Equations
Capital
USRT Range
Process name (1) Gravity GAC contactor8 concrete Gravity GAC contactor* steel Pressure carbon* contactors Convert filter to GAC contactor Regional GAC regeneration transportation and storage Regional multihearth bc regeneration Multi-hearthc GAC regeneration Infrared carbon'' regeneration furnace Fluidized bed" GAC regeneration
K2 (2) 350 cu ft-10,600 cu ft (140 sq ft-28,000 sq ft) 6,280 cu ft-14,100 cu ft (41,400 sq ft-628,000 sq ft) 390 cu ft-2,260 cu ft (340 sq ft-2,200 sq ft)
i m k (4) (5) (3) 470 0.38 1.0 5.6
n (6) 1.0
0
JZL _
V (8)
—
0.85 1.0
1.0
— —
370 0.38 1.0
1.0
— —
6.5 0.88
0.91 1.0 1.0 1.0
— — — — — —
27 sq ft-1,510 sq ft
60
0.44 1.0
1.0
— —
27 sq ft-1,510 sq ft
0.60
0.44 1.0
1.0
— —
370 sq ft-70,000 sq ft 1,000 sq ft-20,000 sq ft (30,000 lb/yr-3,000,000 lb/ yr)
2,400 lb/day-60,000 lb/day 6,000 lb/day-24,000 lb/day
100 0.61 0.99
— — —
6,800 0.23 0.99 1.0 —
—
"Capital cost based on volume per individual contactor, O&M cost based on b Multiply capital cost by percent of usage. 'Capital and O&M cost based on square feet of single furnace effective hearth d Capital and O&M cost based on pounds per day of total carbon weight. 'Capital and O&M cost based on pounds per day of carbon for single weight. 'No O&M costs. 142
J. Environ. Eng. 1983.109:139-156.
capital cost estimates. Note that, in some cases, the design parameter is an exponent itself. The CCI is based on the October, 1978 value for the Engineering News Record Construction Cost Index (265.38).
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CALCULATING UNIT COSTS
These equations can be used to make various calculations including estimating unit costs for various designs (2). To illustrate the use of these equations, assume the design parameters shown in Table 2. These assumptions are for post filter absorbers. A set of design asfor GAC Systems Cost
Operating Cost
(9) 100 6.2 200
0.76
b (11) 0.28
0.15
d (13) 0.48
0.90
0.40
0.11
0.39
0.78
0.32
0.20
0.33
a (10)
f
0.00018
f
i
(12)
f
0.20
1.0
(14)
i
f
0.63
/ (15)
g (16)
! 0.15
f
0.29
0.50
1.0
0.28
0.58
1.0
0.30
1.0
220
0.80
42,000
0.81
0.045
410
0.86
0.72
0.069
0.14
960
0.62
0.064
0.19
0.25
total filter surface area.
143
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h (17)
(18)
f
(19)
t
TABLE 2.—Design Parameters for Post Filter Adsorption Value (2)
Parameter
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0) Activated carbon cost Activated carbon loss per reactivation cycle Natural gas cost Electric power cost Construction cost index Producers price index Direct hourly wage rate Amortization rate Amortization period Loss in adsorptive capacity Design capacity Empty bed contact time Reactivation frequency
$0.65/lb ($1.43Ag) 7% $0.0013/scf ($3,679 x 10"7SM3) $0.04/kWh 325.0 243.8 $ll/hr 8% 20 yr 0% 70% 18 min Every 2.4 months
TABLE 3.—Assumptions for Separate Post-Filtration Systems Design Capacity, in Millions of Gallons per Day (Cubic Meters per Day) 10 100 150 1 5 Item (4,000) (20,000) (40,000) (400,000) (600,000) (4) (5) (6) (2) (3) (1) Number of contactors 6 12 40 3 60 Diameter of contactors, in feet 12 12 20 8 20 (3.7) (6.1) (meters) (2.4) (3.7) (6.1) 13 14 Depth of contactors, in feet 13 13 14 (meters) (4.0) (4.0) (4.0) (4.3) (4.3) Volume of GAC per contactor, 653.1 1,469.5 1,469.5 4,396.0 4,396.0 (18.5) in square feet (cubic meters) (41.6) (41.6) (124.4) (124.4) Minimum empty bed contact time, in minutes 18 18 18 18 18
sumptions is required for the physical configuration of the filters themselves (Table 3). These assumptions are in accordance with the empty bed contact time (EBCT) constraint of 18 min shown in Table 2. The appropriate equations from Table 1 are as follows: ACQ = 5.6 USRTS USRT°'85 CCI 10 UN 10 (3) 90 040 011 039 OMs = 6.2TJSRT^ PR PPI DHR (4) 4 10 10 ACQ, = 0.60 USRT^ CCI UN (5) 81 0045 a28 058 10 OMm = 42000USRT^; PR DHR NTG UN (6) in which ACQ = annual capital cost calculated at 8% for 20 yr for gravity steel contactors; OMs = annual operating and maintenance costs for gravity steel contactors; ACQ, = annual capital cost calculated at 8% for 20 yr, for multihearth furnaces; OM,„ = annual operating and maintenance costs for multihearth furnaces; USRTS = design and operating parameters for gravity steel contactors; in cubic feet; USRTm = design 144
J. Environ. Eng. 1983.109:139-156.
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and operating parameters for multihearth regeneration furnaces. All of the other variables are as defined for Eqs. 1 and 2. Note that for these technologies the design and operating parameters are the same, which is not the case for all of the equations in Table 1. The operating parameter range and units is given in parentheses in the USRT column if it differs from the capital cost parameter. The design parameter for contactors is cubic feet, while the design parameter for furnaces is in square feet of hearth area. A conversion factor of 70 lb/day/sq ft (0.095 kg m"2s_1) of hearth area was used, and a minimum hearth area of 27 sq ft (2.50 m2) was assumed. Additional costs required are those costs associated with the initial carbon charge which will be treated as a capital cost and the cost associated with carbon replacement. These equations are as follows: CF = (WT)(UNC)(AM)
(7)
CR = (WR)(UNC)(CRB)(REGEN)
(8)
in which CF = annual capital costs for initial carbon fill, in dollars per year; WT = total initial weight of carbon, in pounds; UNC = unit cost of carbon, in dollars per pounds; AM = capital recovery factor; CR = annual cost for carbon replacement, in dollars per year; CRB = carbon loss per cycle; and REGEN = number of regenerations per year. The unit costs for the various sized system assumed in Tables 2 and 3 are shown in Table 4 according to a subsystem categorization, including contactors and reactivation. CHOICE OF TECHNOLOGIES
The equations in Table 1 can also be used to evaluate different types of technology (1). In the following sections, two examples of the use of these equations are given. One is a comparison of concrete vs. steel contactors, and the other is a comparison between the various kinds of furnaces available to reactivate carbon. TABLE 4.—Total Production Cost for GAC by Subcategory, in Cents per Gallon Design Capacity, in Millions of Gallons per Day (Cubic Meters per Day) Item
(D Contactor cost, in cents per cubic meter Reactivation cost, in cents per cubic meter Initial carbon cost, in cents per cubic meter Carbon replacement cost, in cents per cubic meter Unit cost, in cents per cubic meter
1 (3,780) (2) 6.98 (1.80) 27.01 (6.97) 1.39 (0.36) 5.23 (1.35)
5 (18,900) (3) 5.52 (1.42) 11.68 (3.01) 1.25 (0.32) 4.71 (1.22)
10 (37,800) (4) 5.43 (1.40) 8.48 (2.19) 1.25 (0.32) 4.71 (1.22)
100 (378,000) (5) 4.52 (1.17) 3.25 (0.84) 1.25 (0.32) 4.70 (1.21)
150 (567,000) (6) 4.48 (1.16) 2.80 (0.72) 1.25 (0.32) 4.70 (1.21)
40.62 (10.48)
23.16 (5.98)
19.88 (5.13)
13.72 (3.54)
13.22 (3.41)
145
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Contactor Costs The cost curves for gravity concrete contactors are (9)
ACQ = 470 USRTc°i38CCl10UN10
(10) OMc = 100USRT°f PR PPI DHR in which ACQ = annual capital cost calculated at 8% for 20 yr for gravity concrete contactors; OMc = annual operating and maintenance cost, in dollars per year; USRTcl = design parameters for concrete contactors in cubic feet; and USRT^ = operator parameter for concrete contactor in square feet. Eqs. 3 and 4 give the costs for steel contactors. Fig. 1 compares total operating and maintenance and construction cost for concrete and steel contactors over a range of 5,000 cu ft-10,000 cu ft (141.64 m-283.29 m2). Obviously, steel contactors provide the cheapest alternative for reactivating carbon.
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6
a28
ols
a48
Furnace Costs Another comparison of interest is the cost of reactivation furnaces. The cost equations for infrared regeneration furnaces and fluidized bed regeneration furnaces are given below: AAQ = 100USRT°-61CCI°" 86
(11)
72
OM, = 410 USRT* PR°- PPI
0069
23
DHR
ACQ = 6,800 USRT^ CCI°"UN 62
OM, = 960 USRT^ PR
a64
PPI
019
014
(12)
10
(13) 0 25
30
DHR ' NTG°- UN
10
(14) in which ACQ = annual capital cost calculated at 8% for 20 yr for an infrared reactivation furnace; OM, = annual operating and maintenance cost for an infrared reactivation furnace; USRT, = design and operating parameters for infrared furnaces in pounds per day; ACQ = annual capital cost calculated at 8% for 20 yr for a fluidized bed furnace; OM, =
210.000
180.000
..Concrete Contactor
•
•
150,000
^
120.000
;
90,000
•
60.000
-
-
y
"
^
•
^
Steel Contactor
' "
30.000
y,i_—i 5,000
1 6.000
L 7.000
. B.000
1 9,000
_...! 1(
L
1
ACTIVATED CARBON VOLUME (CU ft)
FIG, 1.—Total Annual Costs Using Gravity and Coneret© Contactors 146
J. Environ. Eng. 1983.109:139-156.
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annual operating and maintenance cost for a fluidized bed furnace; USRT^ = design and operating parameters for a fluidized bed furnace, in lb/day. The annual capital and operating and maintenance costs for multiple hearth reactivation furnaces are given in Eqs. 5 and 6. Fig. 2 shows the annual costs for all three types of furnaces over a comparable reactivation range. From Fig. 2, it is obvious that infrared technology is the cheapest alternative. SENSITIVITY ANALYSIS
The equations shown in Table 1 can also be used for sensitivity analysis (3). An operating and maintenance cost equation for a gravity steel contactor system can be created by combining Eqs. 4, 6, and 8 as follows: TOMs = OMs + OMm + CR
(15)
in which TOMs = total annual operating and maintenance cost for a gravity steel contactor system with a multihearth furnace. From Eq. 15 and assuming the design data in Tables 2 and 3, it is possible to evaluate the effect of changing one variable at a time on TOMs. For instance, the effect of carbon loss, carbon cost, and power cost on operating and maintenance (O&M) cost could be examined. Fig. 3 demonstrates this effort for carbon loss for 10 mgd and 150 mgd (40,000 m 3 /day and 600,000 m 3 /day) systems operating at 70% capacity. As can be seen, carbon loss has an effect on operating and maintenance cost. Total O&M cost is relatively insensitive to changes in carbon and power cost. ^s* ^S"^
Fluid Bed Furnace
160,000
^r
j? Multi-hearth Furnace
120.000 ^ yr
-
S
80,000
40,000
* Infrared Carbon Furnace
.
i 't —JL
1 1.500
1 2,000
2.500
3,000
3.500
REACTIVATION REQUIREMENT (lb/day)
FIG. 2.—Total Annual Cost for Reactivation Furnaces 147
J. Environ. Eng. 1983.109:139-156.
A total production cost function can be created as follows:
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PRODc = OMs + ACQ + OMm + ACCm + CF + CR
(16)
A manager interested in minimizing total system cost might choose to purchase the minimum cost activated carbon. Fig. 4 shows total system cost vs. reactivation frequency for a 100 mgd (378,000 m 3 /day) post filter adsorption system (Table 3). Assuming $0.60/lb ($1.32/kg) carbon with the system being reactivated at a frequency of 1.1 months, the total cost is approx tf/16.8/1,000 gal (c/4.32/ m3). If carbon cost is increased from $0.60/lb ($1.32/kg) to $0.80/lb ($1.76/kg), the reactivation period would
2.5
5.0
7.5
10.0
12.5
15.0
17.5
ACTIVATED CARBON LOSS PER REACTIVATED CYCLE (%)
FIG. 3.—O&M Cost versus Activated Carbon Loss for Post Filter Adsorbers
TIME BETWEEN REACTIVATIONS (months)
FIG. 4.—Total Production Cost versus Frequency for a 100 mgd Post Filter Adsorption System (To Convert from Cents per 100 gal to Cents per Cubic Meter, Divide by 3.785) 148
J. Environ. Eng. 1983.109:139-156.
have to increase from 1.1 months-1.5 months for costs to remain constant. If the reactivation period increased beyond 1.5 months as a result of the more expensive carbon, system cost would be lowered. Eq. 16 could also be used to study the tradeoff between carbon loss per reactivation and carbon cost.
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CONTACT TIME AND CARBON USER RATE
Intuitively, one might conclude that the greater the total volume of carbon or empty bed contact time, the more efficient the organic removal process. However, this may or may not be true depending on the characteristics of the organics removed. Fig. 5 shows the relationship between system size, and empty bed contact time for a 100 mgd (4.381 m 3 /s) post filter adsorption system using Eq. 16. From Fig. 5, if the required EBCT at a reactivation frequency of 2 months were increased to 22 min, the reactivation frequency would have to be increased to 2.8 months or greater to achieve a favorable economic trade-off. Therefore, to maintain the same cost if the EBCT were increased by 22%, the period between reactivations would have to be increased by at least 40%. The results appear "counter intuitive." Conventional wisdom leads one to believe that deeper beds are more efficient in their removal of organic material. If, however, the use rate of activated carbon is constant, that is the "pounds of the carbon per millions gallons of water loaded on the bed" remains constant with increasing bed depth, then a shallower bed is in fact more cost effective. The excess activated carbon in the deeper bed is only so much excess inventory, not contributing to system performance until the end of a cycle, but constantly costing more. To understand this nonproportionality relationship properly, one should consider carefully the adsorbent use rate vs. EBCT. Data from pilot columns and field scale studies can be used to calculate the use rate of activated carbon per unit volume to meet a controlling criterion for any empty bed contact time. Adsorbent use rate is calculated by dividing the dry weight of adsorbent for a given bed contact time, by the total volume of water passing through the column until a performance criterion is exceeded. If the adsorbent use rate decreases with the increasing empty bed contact time, then a more than proportional improvement in performance is gained by increasing the EBCT. According to Fig. 6 which shows some typical use rates plotted vs. EBCT, use rate varies considerably among contaminants to be removed. At some point, however, the use rate becomes constant implying a point at which increasing the empty bed contact time no longer increase efficiency of removal. One can calculate use rate per million gallons, for instance, by assuming an activated carbon use rate as described previously and converting to pounds per million gallons. Assuming a use rate of 530 mg/L, the use rate converted to pounds per day is as follows: Use rate (lb/mil gal) = (530 mg/L)(8.34 lb/mg/mil gal/L) = 4,420.20 lb/mil gal
(17) 149
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An equation relating EBCT and C, can be derived from Eq. 16 by dividing the total annual cost, by the millions of gallons produced per year and then dividing the result by 10 to yield cents per 1,000 gal. The equation is as follows:
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PROP. TUC = ; MGY(IO)
(18)
in which MGY = millions of gallons produced per year; and TUC = total unit cost, in cents per 1,000 gallons. The design variable for Eqs. 3 and 4 is in cubic feet. For instance, dividing Eq. 3 by MGY yields
REACTIVATION FREQUENCY (months)
FIG. 5.—Total Production Cost versus Reactivation Frequency for Various Empty Bed Contact Times (100 mgd Post Filter Adsorber)
E -
800
°9
