v>EPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
EPA-600/2-82-005
February 1982
Research and Development
Technology
Assessment of the
Vertical Well
Chemical Reactor
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EPA-60P/2-82-005
February 1982
TECHNOLOGY ASSESSMENT
OF THE
VERTICAL WELL CHEMICAL REACTOR
Jeremiah J. McCarthy
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
Project Officer |
Robert P.G. Bowker
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY,
CINCINNATI, OHIO 45268 i
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publication.
Mention of trade names or commercial products does not constitute endorsement or
recommendation for use.
n
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FOREWORD
The U.S. Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health and
welfare of the American people. Noxious air, foul water, and spoiled land are
tragic testimonies to the deterioration of our natural environment. Thecom-
plexity of that environment and the interplay of its components require a
concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem solution;
it involves defining the problem, measuring its impact, and searching for solu-
tions. The Municipal Environmental Research Laboratory develops new and im-
proved technology and systems to prevent, treat, and manage wastewater and solid
and hazardous waste pollutant discharges from municipal and community sources,
to preserve and treat public drinking water supplies, and to minimize the
adverse economic, social, health, and aesthetic effects of pollution. This
publication is one of the products of that research and provides a most vital
communications link between the researcher and the user community.
This report assesses a promising new technology which utilizes the well
known wet combustion process to treat high strength organic wastes. Because of
its unique configuration, the technology labeled the vertical well chemical
reactor (VWCR) has the potential to oxidize these wastes more safely and more
economically. The report describes how the VWCR works, analyzes its development
to date, evaluates the technology it uses and makes preliminary cost and energy
estimates. The report also compares the VWCR to equivalent technologies and
makes an assessment of its potential national impact.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory ;
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ABSTRACT
The vertical well chemical reactor (VWCR) is designed to oxidize high
strength organic wastes using wet combustion principles. The reactor vessel
consists of two stainless steel concentric tubes suspended in a well and
surrounded by a heat exchange jacket. Provisions for air injection to support
combustion are included.
VWCR concentric tube configuration uses little space compared to above
ground wet oxidation vessels and promotes efficient heat exchange. Waste
pressurization from the weight of the liquid above results in safer and more
economical operation. Full-size reactors are expected to descend as much as
6000 feet and operate at temperature and pressures exceeding 650°F and 2200 psi
respectively. An important potential benefit from VWCR operation is energy
recovery from autothermal oxidation of the high strength wastes. :
Bench scale COD reduction experience using a batch laboratory reactor has
been similar to that obtained from the pilot plant, supporting the use of bench
scale treatability studies to model expected COD reduction rates. COD
reductions of waste sludges have approached 50 percent on pilot scale. As much
as 80 percent COD reduction is expected for full-scale where higher temperature
and pressures can be attained. The poorly understood interacting effects of
metal solubility, adsorption and desorption has not permitted a definitive
explanation about the fate of metals in the VWCR. More work needs to be done in
this area. In other independent but related work, detoxification of specific
organic compounds by wet oxidation suggests the VWCR is a viable method for
treatment of toxic wastes.
Much of the pilot scale effort has concentrated on solving structural,
mechanical, and other operational problems. Construction is expected to begin
in late 1981 on a full-scale demonstration plant at Longmont, Colorado. Design
and operation of this plant will address the major problems encountered during
pilot scale operation. Important among these are pit corrosion scale
formation, and leaking joints.
In summary, the VWCR is a potentially desirable treatment process for
stabilization of organic wastes when significant sludge volume reduction is
required, where stringent requirements for sludge disposal exist, when destruc-
tion of toxic materials or pathogenic organisms is necessary, or where
potential energy recovery from high strength wastes is good. A major goal
remaining is to demonstrate VWCR steady-state operation at full-scale. This
experience will not only produce operational information and characterize
certain process variables, but will better define actual operating costs for
various strength wastes so they can be more realistically compared to competing
processes.
IV
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CONTENTS
Foreword "" 1""
Abstract r
Figures. : .* Yl
Tables V11
1. Technology Description ; 1
Introduction.." 1
Detailed Description 2
2. Development Status. 6
General ; 6
Bench and Laboratory Scale Research ; 6
P i 1 ot Seal e Re search 9
Full-Scale Facilities 12
3. Technology Evaluation 22
Process Theory 22
Process Capabilities 23
Basic Process !« 23
Configurations 24
Water Composition 24
External Treatment ;..;... 25
Design Considerations ; 26
Energy Considerations 28
Operation and Maintenance Requirements , 33
Costs *-.. 33
4. Comparison with Equivalent Technologies .; 39
5. Assessment of Natipnal Impact , 47
6. Conclusions and Recommendations 50
References 52
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FIGURES
Number
1
2
3
4
5
Page
7
8
9
10
11
12
13
14
15
16
Typical Vertical Well Chemical Reactor Profile 2
Typical Vertical Well Chemical Reactor Cross Section 3
Temperature Versus Detention Time for a Typical
VWCR Reactor 4
Typical COD Reduction Versus Reaction Temperature and ;
Time Using Laboratory Reactor Data 7
Effect of Reaction Temperature on Average Solids Reduction
for Five Sludges After 60 Minutes Using Laboratory Reactor
Data 8
Average Percent of Various Metals in the Effluent
Solids from Five Municipal Sludges 10
Proposed Longmont VWCR Process Flow Schematic 15
Hypothetical Waste Treatment Plant Flowsheet Usinq
the VWCR 17
Schematic Process Flow Diagram of the Treatment
Train Proposed for Montrose., Colorado 20
Preliminary Plot Plan of the Treatment Train
Proposed for Montrose, Colorado
20
Waste Strength Required for Thermal Self-Sufficiency
Under Certain Conditions 31
Net Heat Production Expected for the Longmont VWCR 32
Typical Direct Construction Costs for Thermal
Treatment Plants 42
Typical Power Costs for Thermal Treatment Low Wet
Oxidation Plants 42
Typical Operating and Maintenance Labor Costs for
Thermal Treatment Plants 44
Typical Material and Supply Costs for Thermal
Treatment Plants 44
VI
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TABLES
Number Pa9e
1 Waste Stabilization Trends Using Laboratory i
Reactor Data ----- , ...................................... 9
2 VWCR Pilot Plant and Laboratory Batch Reactor COD
Reduction Data... ...................................... "11
3 VWCR Pilot Plant Solids Removal Data ........ . .......... 11
4 VWCR Pilot Plant!:Solids Settleability Data...... ....... 12
5 VWCR Pilot Plant'iMetal Concentration Data ...... . ....... 13
6 VWCR Pilot Plant<;0ff-Gas Analysis .......... -.»... ....... 14
7 Major Equipment and Unit Operations Proposed for the
Wastewater Treatment Plant at Montrose, Colorado ....... 19
8 Details of the Montrose, Colorado VWCR Design .......... 21
9 Heating Value of Various Materials and Fuels,, .......... 29
10 VWCR Capital Cost Estimates for Treating Sludge ......... 34
i
11 VWCR Annual O&M Cost Estimates for Treating Sludge
from Various Size Wastewater Treatment Plants ........... 35-36
12 Preliminary Life Cycle Cost Estimate for a VWCR
System Containing an Eight-Inch Diameter Reactor ........ 38
13 Wet Oxidation Categories.... ................ . ...... . ---- 39
I >
14 1980 Needs Survey Technical Summary Extract.1. ........... 48
vn
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SECTION 1
TECHNOLOGY DESCRIPTION
INTRODUCTION !
The Vertical Well Chemical Reactor (VWCR) is designed to oxidize high
strength wastewaters and sludges utilizing wet combustion principles. If
sufficient air (oxygen), temperature, and pressure are present, organic sub-
stances can be oxidized in a liquid state. The oxidation reaction proceeds
exothermically and if organic content of the material is high enough and heat
losses are controlled, combustion may be thermally self-sufficient. Auto-
thermal oxidation is a desirable although not required benefit of the VWCR
process. Sludge, with its concentrated organics and therefore higher heating
value, is the preferred wet oxidation medium but wastewaters containing organic
materials with high oxygen demands may also be considered.
1! ' >i,
Configuration of the VWCR is unique for its purpose. Wastewater flows down
the center tube of two concentric vertical tubes and returns in the annular
space. Use of this vertical tube configuration has multiple purposes. Tube
diameter and length are designed so that sufficient reaction time and pressure
desired during fluid waste oxidation can be attained. Pressure is developed
naturally by the hydrostatic 'liquid head above the waste flowing down the tube.
Heat resulting from the exothermic combustion reaction maintains much if not all
of the downhole temperature required to sustain the reaction. Any required heat
is input by a fluid heat exchanger. Conversely, excess heat can be recovered to
the ground surface for use as an additional energy source.
The VWCR configuration utilizes little space and can be placed in existing
well shafts when feasible, its concentric configuration minimizes reactor heat
losses. As the surrounding earth approaches equilibrium with a continuously
operating VWCR, operation will be less affected by waste quality changes or
outside (climate) influences because the surrounding earth will act as a thermal
buffer. Heat loss to the surrounding rock for a rock conductivity of 0.4
BTU/ft-hr-°F is estimated to decrease by 66% after one year (1). With the
exception of the high pressure pumping requirements needed for the heat exchange
fluid and moderate pressure requirements for air compressors, the natural
hydrostatic head in the well eliminates high pressure pumps or fluid containment
vessels and their associated pressure controls. Natural pressurization is
fixed at any point however and depends on the weight of liquid above it. VWCR's
vertical configuration and compactness make downhole accessibility difficult
should temperature and pressure measuring devices or other well components need
to be unplugged, inspected,; or replaced. Thus mechanical reliability and
maintenance of the VWCR system is an important consideration.
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DETAILED TECHNICAL DESCRIPTION (1)
Figure lisa process flow diagram showing a vertical section of the VWCR.
Figure 2 shows a cross section of the reactor. Neither is to scale. These
figures are from the first full-scale design which will extend approximately
6000 feet into the ground.
REACTOR INFLUENT (DOWNCOMER)
START OF REACTION ZONE
3SO'F
HEAT EXCHANGER OH.-
BOTTOM OF REACTOR
TEMPERATURE VARIES
LONGMONT
VERTICAL TUBE REACTOR PROFILE
NOT TO SCALE
Figure 1. Typical vertical well chemical reactor profile.
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STAINLESS STEEL UPCOMER
STAINLESS STEEL DOWNCOMER
VWCR TYPICAL CONSTRUCTION MATERIALS
Fluid Contacting Tubes: Series 300 SS
Reactor Casing: A.P.I, black iron
Expected Tube Life: 10-15 years
Insulation: Ceramic fiber wrapped
with vinyl-backed
fiberglass and
secured with vinyl
cloth tape.
ROCK
VWCR OPERftTI_QN_RANGES
Well Depth: 1500 to
>5000 ft
Bottom Hole
Pressure: >1500 psi
Reaction
Temperatures: 350 to >650°F
Reaction
Times: 30-60 min
HIGH TEMPERATURE CERAMIC INSULATION
CEMENT' GROUT
STEEL WELL. CASING
STE!:L REACTOR CASING
(PRESSURE VESSEL)
STAINLESS STEEL AIR LINE
HEAT EXCHANGER OIL
STEEL HEAT EXCHANGER LINES
Figure 2. Typical vertical well chemical reactor
cross, section.
The VWCR system can be placed into a conventionally cased oil or gas well.
Two concentric tubes serve as the reactor vessel and are constructed from 300
series stainless steel. The reactor tubes are surrounded by a heat exchange
iacket containing a liquid which independently adds or removes heat as needed to
maintain the required reaction temperature range. Air is injected at several
downcomer locations along the waste fluid path at depths ranging from 150 to 800
feet depending on influent COD. The multiple air injection depths are used to
accommodate varying influent waste strengths. A maximum air compressor
pressure of 500 psig is recommended. Air assists fluid flow through the reactor
and provides the oxygen needed for combustion. When waste strength oxygen
demand (COD) exceeds air supply capability, the waste is diluted with effluent
or other low COD strength wastes. Insulation to minimize heat losses from^the
VWCR to the surrounding earth completes the basic VWCR reactor design. Typical
projected VWCR operation ranges and common construction rnaterials are also
summarized in Figures 1 and 2.
Waste fluid is injected into the downcomer tube at the earth's surface.
Air is injected at several points down the reactor. As the waste stream and air
flow down the tube, they undergo natural pressurization due to the hydrostatic
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head above. Thus fluid pumps need to be primarily designed to overcome surface
friction and pressure head at their influent or injection point. They do not
need to develop the high pressure actually experienced at the bottom of the
reactor. At some depth (typically 1500 to 2000 feet) temperature of the waste
fluid increases to 350° due to heat transfer from the upcomer effluent to the
downcomer influent and wet oxidation effectively begins. Temperature increases
with depth however it can never be allowed to exceed the boiling temperature at
any^point or the fluid will flash into a vapor. Maximum allowable temperature
varies with pressure existing at any point and approximately follows the
saturated vapor curve for water. The fluid flows down the center tube and up the
annulus and oxidation proceeds until either the organic material or dissolved
oxygen are depleted or until hydrostatic pressure and temperature decrease
below, those necessary to support combustion. Upflowing oxidized waste is
gradually cooled as it transfers heat to the downflowing fresh waste. Any
ISmS4, "? may u68"!* from the exotheic oxidation reactions is removed
from the reaction zone by the exchange jacket. Excess heat is thus available for
nfLcc 9J°Und surf,ace- The ^at exchange jacket can also supply heat when
2SS?iy' -+T- ecxn?Ple, during startup. Effluent fluid temperature is
generally within 50p of the influent temperature. A temperature profile for a
typical reactor as a function of detention time is shown in Figure 3
700 7
600 -
OOWNCCMER
UP COMER
REACTOR BOTTOM
1 300
25 30 35 40 45
0 5 10 15 20
Figure 3. Temperature versus detention time for a
typical VWCR reactor.
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The VWCR is designed with no moving parts below the .ground surface and
needs no high pressure vessels above ground like those used for conventional wet
oxidation methods. Its components below the surface are the tubes containing
the waste, heat exchange fluids and air, and the associated temperature
thermistors and pressure measuring devices. The tubes are subject to potential
scaling and corrosion characteristic of high pressure, high temperature
combustion reactions. VWCR operation efficiency, in addition to design and
waste composition considerations, is dependent on the reliability and accuracy
of downhole measuring equipment and on the efficiency of heat exchange between
the fresh and oxidized waste.
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SECTION 2
DEVELOPMENT STATUS
GENERAL
The Vertical Well Chemical Reactor process is being developed and tested by
the Vertical Tube Reactor Corporation, Englewood, Colorado. Their research on
oxidation of municipal wastewater sludge has been partially sponsored by EPA's
Municipal Environmental Research Laboratory (MERL) (2). The study forms part of
MERL s municipal sludge conversion research effort. In particular this
process, while maintaining all the advantages of wet oxidation, appears to be
more cost effective than conventional wet oxidation methods because expensive
high pressure equipment is minimized (high pressure vessels, pumps, and
compressors).
BENCH AND LABORATORY SCALE RESEARCH
_ Research on the development of the VWCR process began in 1973 with the
design and fabrication of a 2.7 liter stainless steel laboratory batch reactor
The laboratory reactor employs the same thermodynamic principles upon which a
full-scale VWCR process is based and is designed to oxidize organic materials
over a wide range of COD concentrations, temperatures, pressures and reaction
t 1 HIS S
Under EPA/MERL sponsored research since July 1979, raw primary and di-
gested secondary sludges from several municipal wastewater treatment plants
have been oxidized in the laboratory batch reactor (2). Sludge used in the
laboratory testsjias usually been diluted to collect basic treatability data on
various strength wastes. The laboratory reactor is also limited with respect to
the amount of oxygen it can supply to satisfy waste oxygen demand. For a typical
laboratory run, 1.3 liters of the sample material is placed inside the reactor
CODreui>ementre ai> iS USed t0 pressur1ze the vessel and satisfy sample
Batch reactor tests using municipal wastewater have primarily been run on
wastewater from Montrose, Colorado. Montrose wastewater is made up of a candy
factory wastewater portion (which presently contributes about 21 percent of its
total organic load) and a domestic wastewater portion. Based on a 0.6 BOD/COD
(iPnJin8 menSrn^)s ?6 BOD °f the industrial wastewater averages about
IJ1240 ["9/1 co°) and ranges from 120-1500 mg/1 (200-2500 mg/1 COD). The
/im murncipa1 wastewater presently averages 320 mg/1 and ranges 166-482
mg/ i
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For organic wastes without a significant COD refractory component, it has
been found that the extent of COD reduction is a function of operating con-
ditions rather than specific waste make-up. Figure 4 summarizes general COD
removal experience using the laboratory reactor oxidizing both wastewater and
sludge (3). Figure 5 summarizes average solids reduction of.five municipal
wastewater sludges collected in the Denver metropolitan area and treated in the
laboratory reactor (1). Solids and BOD reduction experience has been more
sporadic than COD reduction experience. One possibility which may affect BOD
reduction results is that BOD refractory compounds which initially do not exert
a BOD demand may be partially oxidized to non-refractory compounds which do
exert a demand, thus effectively masking true BOD reduction percentages.
o
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Q-
.0
UJ
ce.
o
o
cj
LU
03
100
90
80
70
60
50
40
30
20
10
0
60 MINUTES REACTION TIME
30 MINUTES REACTION TIME
15 MINUTES REACTION TIME
I
400
450 500 550
REACTION TEMPERATURE, °F
600
650
Figure 4. Typical COD reduction versus reaction temperature and time using
laboratory reactor data.
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Ul
o:
Ul
O-
y
o
100
90
80
70
60
50
40
30
20
10
TOTAL SUSPENDED,SOLIDS
TOTAL VOLATILE SUSPENDED SOLIDS
TOTAL VOLATILE SOLIDS
TOTAL SOLIDS
-f-
400
450 500 550
REACTION TEMPERATURE, °F
600
l
650
Figure 5.
Effect of reaction temperature on average solids reduction for
five sludges after 60 minutes using laboratory reactor data.
iah,o +runds a£out COD and BOD Deduction via wet oxidation using the
laboratory batch reactor are summarized in Table 1. These trends generally
SfSi"*1*^?0^^?!.^ b^ Hurwitz et a1" at the Metropolitan Sanitary
District of Greater Chicago in 1965 usiTig-Tonventional wet oxidation above-
ground pressure vessels (4). Hurwitz made pilot plant and full-scale studies of
EJcf,lLiOXh rnn°n of, ?y.eral fewage sludges for various degrees of oxidation as
measured by COD, volatile solids, and BOD removal.
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TABLE 1. WASTE STABILIZATION TRENDS USING LABORATORY REACTOR DATA
-...-' - -- '-!=^=- . . . *~ "~"
The extent of COD reduction significantly increases with temperature up to
at least SCOT.
Increasing batch reaction time from 1/2 to one hour effects about10 per-
cent greater COD removal at 40QOF and approaches no difference at 650°F.
The particulate (ash) COD decreases to almost zero about 600°F confirming
expectations of an inert ash. ;
The portion of effluent BOD remaining (as a fraction of total COD)
increases with reaction temperature indicating the refractory reactor
effluent is mostly biodegradable for the wastes tested.
Selected metal analyses have been made on laboratory reactor effluents to
determine whether the metals remain in the soluble or particulate fractions.
Preliminary results suggest that it varies among the metals tested. Figure 6
summar "esythe results of metal transformations for five sludges in a 30-minute
and one hour run at 65QOF in the laboratory reactor (1). J^e metal s are reported
as the percent of total effluent metal concentration within the effluent solids.
The poorly understood interacting effects of metal complexing, solubility,
absorption and desorption at. high temperatures does not permit a definitive
explanation of metal transformations within the reactor ,at this time. It is
possible that some metals may at times be plating out on the reactor walls and
at other times be redissolved into solution. ;
In earlier work, Sommers and Curtis studied the effect of wet oxidation on
selected nutrients and metal concentration levels in the
.
of primary and waste activated sludges (5). In general they ^"^ *"?,"!*
oxidation decreased total nitrogen content of the sludge solids^but had no
effect or increased the phosphorus and metals content, U>pper, zinc, nickel,
cadmium and lead were measured. They discussed the desirability of land appli-
cation of sludge if nitrogen is reduced and metal content remains the same or
increases. They also discussed the desirability of recycling if nutrient or
metal build-up causes removal or operational problems.
PILOT SCALE RESEARCH
A VWCR pilot plant has operated intermittently since 1977 at Lowry Bombing
Range, approximately 20 miles east of Denver, Colorado. The pilot plant reactor
consists of a 1-3/4 inch diameter downcomer and a 2-inch diameter upcomer, both
of 304 L stainless steel encased in a 2-1/2 inch reactor casing of schedule 40
American Petroleum Institute (API) pipe. The reactor air line heat exchanger
lines and insulation are all suspended in a 5-inch diameter standard API well
casing. The pilot VWCR extends to a depth of 1500 feet below the ground surface
(1) Raw or digested sludge for oxidation in the VWCR has been obtained from
nearby Aurora and Englewood wastewater treatment plants. The sludge is hauled
to the pilot plant site in 1500 gallon loads. A 6000 gallon sludge storage tank
receives the sludge. Well water is mixed with the sludge to obtain the desired
COD concentration for a day's run.
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«=c
fc
20 .
CD
ZN
Figure 6. Average percent of various metals in the effluent solids
from five municipal sludges.
Most of the pilot scale effort has concentrated on solving structural
mechanical, and other operational problems (2). Of the many enqineeHna
problems encountered, four major ones have resulted in proposed Sees or
additions for future reactors. The problems are pit corrosion and a scalp
le'S^oi ?f 'T^ "Pe P1^"*. reactor-heSt exchange interface ad
slctiSn. 6 subjects are ^scussed in the Technology Evaluation
RQ.a ThS 4?Uot VWff reactor was last reinstalled in the ground in June 1980
Because of its small size and limited depth, it has not been possible to Achieve
reaction temperatures above SlQOp or demonstrate autogenous oxidation with
^^^^
heat-pressure regimes oxidizing municipal sludges. The first was
6"15 at,- 40°-440°F with reactor maximum pressures at
10
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Table 2 presents a summary of sludge COD reduction data from various pilot
plant tests and compares them to batch laboratory reactor removals experienced
under similar operating conditions. Table 3 presents a total solids and total
volatile solids removed data for two composite sludges oxidized in the pilot
plant in March 1981. Table 4 presents the results of a settleability test made
on a pilot plant sludge effluent sample in March 1981.
TABLE 2. A COMPARISON OF VWCR PILOT PLANT AND LABORATORY BATCH REACTOR COD
REDUCTION DATA (1)
=====*
Date
7/24-25
1980
9/11-22
1980
11/06-21
1980
12/07-16
1980
12/20-23
1980
3/19-23
1981
3/20/81
3/23/81
Flow
qal /mi
4.0
4.1
4.2
4.5
4.5
4. -5
4.5
4.5
Reaction
Time
n min
30
25
25
20
20
28
28
28
r^ - -
Max
Temp
OF
440
400
420
,440
420
,500
500
510
Inf.
COD
ma/1
350
100
740
600
880
1,424
1,118
1,063
Eff.
COD
mg/L_
248
73
548.
468
695
784
657
567
P.P. COD
Reduction
*
29
:27
26
:22
: 21
: 45
: 41
t
47
Lab COD
Reduction
* .
28
18
24
22
22
51
-
51
*Estimated time that sludge was above 350°F while flowing through pilot
plant.
TABLE 3. VWCR PILOT PLANT SOLIDS REMOVAL DATA (1)
Total Solids* Total Vol. Solids*
3/19/81
3/20/81
Inf.
2070
2000
Eff.
958
736
% Rem. Inf. Eff. % Rem.
54 1140 661 42
64 1040 -
Total Sus.
Inf.
1630
1180
Eff.
488
300
Solids*
% Rem.
70 '.
75
Total Vol.
Sus. Solids
Inf. Eff. % Rem.
790 452 43
950
^Influent and effluent concentrations are in mg/1.
11
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TABLE 4. VWCR PILOT PLANT SOLIDS SETTLEABILITY DATA (1)*
Total Solids
Effluent
Settled Effluent
% Removed
mg/l
1480
878
41
Total Sus.
Total Vol. Solids Solids
mg/l
681
502
26
mg/l
600
141
74
Total Vol.
Sus. Solids
mg/l
166
52
69
Results from one grab sample of VWCR reactor effluent which was placed
into a 2.0 liter graduated cylinder and allowed to settle for 2 hours The
decant from this cylinder was then poured out carefully and analyzed for
solids determination. An additional 1.0 liter of reactor effluent was also
placed into a 1.0 liter Imhoff cone. After 2 hours the settled solids had
compacted to 2.5 percent of the cone volume.
c,m if Resents results of the metal analyses from VWCR pilot plant
samples of municipal sludge. Pilot plant reaction was 20 minutes and bottom
reactor temperature was 400-44QOF. Table 5 indicates that most of the iron
total chromium and lead effluent metal concentration is in the ash and copper
is approximately 30% soluble. Chromium VI, cadmium and nickel concentrations
were too nearer below detection limits to make any conclusions about their
tl k !^1S a.sli9nt reduction in both iron and lead which suggests they
might be tied up in downhole scale formation.
Shown in Table 6 are the results from a reactor effluent off-qas samole
taken December 23,.1980. Gas samples were analyzed by combined gas9chromato-
graph/mass spectrometer analysis. No organic components were detected at the
detection limit of 5 ppm.
In summary pilot plant performance with respect to COD reduction, solids
removal, metal concentrations and off-gas make-up are reportedly as expected
considering the reaction time, pressure, and temperature limitations of the
pilot reactor. COD reduction experience using the batch laboratory reactor
has been close to that obtained from the pilot plant under similar operating
conditions, supporting the use of batch reactor to model pilot plant treata-
DI nuy expectations.
FULL-SCALE FACILITIES
EPA|s Office of Research and Development has recently received a ore-
hn? HC,dnH°r f°%fede;al assistance from the City of Longmont, Colorado to
build and evaluate a full-scale demonstration VWCR (1). -The facility proposed
S!!SSVf * CR ^stem complete with all downhole and above' ground
Dlant 5 2? ? H0-^6^ ^V S]Udge from the Lon9mont wastewater treatment
plant and selected industrial sludges from the surrounding area.
12
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13
-------
TABLE 6. VWCR PILOT PLANT OFF-GAS ANALYSIS (1)
Nitrogen
Oxygen
Argon
Carbon Dioxide
Percent by Volume
The Longmont wastewater treatment plant (WWTP) is a secondary biologi-
cal process with flow equalization and two stage anaerobic sludge digestion
The treatment process includes mechanical screening and shreddinq aerated
grit removal, primary sedimentation, redwood media roughing filters, rotating
Si?]0?,1"1. contactors, secondary clarification and chlorination. Longmont
WWTP flow is expected to average 7.5 mgd in 1982.
Treated plant effluent is discharged to the St. Vrain River which is a
tributary to the South Platte River. Anaerobically stabilized sludge is
spread on agricultural lands in the vicinity of the WWTP.
1np9 1^c-!1.1t1eDs1 at-the Lon9mont WWTP are nearing design
able
reactafinHo a* ^nffnont includes an 8-inch nominal diameter
reactor 6,000 feet deep which will allow for operation of the process over the
£ I nnS96 °n V^etures from 50QOF to 65QOF with influent sludge strength!
of 5,000 mg/1 to 10,000 mg/1 COD and reaction times from 25 to 100 minutes
* P1"1-* I111 be d11uted to achieve th1s relatively loi
i ?eS1?n 1S based on a 1985 diluted slud9e flow volume of
innm! onTs h.per da^ *° the VWCR at an average COD concentration of
10 000 mg/1. This results in 12,500 Ibs COD processed each day. Oxidation
cSS^dT/edL'rTd^ \°- be ab7°Ut- ?5 PSrCent 9"'vin9 an estimated 9400 bs o?
SI I + -, fy> FlQUre 7 gives the P^cess flow schematic for the VWCR
demonstration plant proposed for Longmont, Colorado (1).
The overall objective of the Longmont program is to demonstratP
a a1ndecon°mic feasibility of a full-scale 9 VWCR lor cation Jf
sludge and selected industrial wastes (1). The program involves
three basic phases which will last at least two years. pr°9ram involves.
14
-------
LONGMONT
PROCESS FLOW SCHEMATIC
SLUDGE
I GRINDER SLUOGE PUMP J
SLUDGE CONTROL BUILDING -^
VTR SETTLED EFFLUENT
RETURNED TO HEAOWORKS
DRYING, .BEDS
ASH PUMP
Figure 7. Proposed Longmont V-WCR process flow schematic.
15
-------
. . Phase l» the construction phase, includes design, construction and
initial operation of the VWCR at Longmont. In particular, sludge and air
flow, plumbing, heat control and instrumentation will all be checked out to
insure they meet design criteria.
Phase II, the operation and evaluation phase, is designed to provide
longterm reliability and operative information in processing municipal sludge
and to evaluate the impact of VHCR operation on Longmont wastewater treatment
plant operations. Full-scale treatability results will be compared to those
predicted in the laboratory; computer model thermodynamic predictions will be
verified or reevaluated, optimum operating conditions for processing Longmont
municipal sludge will be established. Optimum operating conditions are those
which effect the desired COD reduction, a minimum COD recycle load, and net
positive heat production. In general, important engineering and operational
data will be developed that was not possible at pilot scale due to size and
design limitations. Evaluating the extent and subsequent control methods for
reactor scaling and corrosion using nitric acid is an important task to be
done. The biological treatability of VWCR effluent will also be studied using
bench scale aerobic and anaerobic treatment units.
j ?has? IH wil1 1nvesti9ate treatment and disposal of complex organic
industrial wastes. In addition, an energy recovery system will be sized -in
order that excess combustion heat from the wet oxidation process can be
covnerted to electricity for treatment facility use. During all phases
system components will be monitored for energy efficiency, operation and
maintenance characteristics, and material durability. An extensive sampling
program will monitor oxidation efficiency as well as the fate of selected
metals and complex industrial organics as appropriate. The preapplication is
under review as of this writing. Grant award and initial construction is
anticipated during the fall of 1981.
In general, full-scale facilities for VWCR plants must consider the
following major unit operations or equipment:
Preliminary treatment: to protect pumps, remove large objects
measure flow and pretreat as necessary. '
Equalization Basin: to allow a constant pumping rate to the VWCR
to receive recycle flow and to act as a buffer against large
variations in wastewater quality and quantity.
. Vertical Well Chemical Reactor: for wet oxidation.
Heat Exchange Unit and Boiler: to maintain required temperatures
for wet oxidation or to remove excess heat for productive use
(includes water conditioners, high pressure pumps and boiler
make-up requirements if heat exchange fluid is water). :
. Acid Wash System: to protect the stainless steel reactor tubes
against the effects of corrosion and deposition.
16
-------
. Air Compressors: to provide air required for waste transport and
oxidation, and pressure for air injection into the reactor.
Foam Separation Tank: to allow both solids and gas separation from
the VWCR effluent (a surge gas stand pipe may proceed the tank).
. Final Biological or Chemical Polishing Operations: to meet permit
requirements of wastewater effluent or to treat sludge supernatant
(which may be recirculated back to the head of the plant).
Solids Dewatering Facilities: to prepare the VWCR solids for final
disposal. ;:
. Air Treatment Equipment: if needed to scrub odorous off-gases.
A hypothetical waste treatment plant flow sheet is shown in Figure 8.
Unit operations before and after the VWCR will be a function of the incoming
waste characteristics (sludge or wastewater) and disposal requirements.
RECYCLE (OPTIONAL)
INFLUENT
VERTICAL WELL
CHEMICAL REACTOR
ACID WASH AS NEEDED
pH CONTROL
CHEMICALS
USED ACID
NEUTRALI-
ZATION AND
SEPARATION
EFFLUENT
SUPERNATANT
Figure 8. Hypothetical waste treatment plant flowsheet using the VWCR.
17
-------
While no full-scale facilities exist, the 201 Facility Plan for the city
of Montrose, Colorado recently included consideration of a wastewater treat-
ment plant design utilizing VWCR/aerobic fluid bed filter (AFBF) unit
operations (3). Montrose wastewater is primarily domestic, supplemented with
an organic loading (10-20%) from a candy factory discharge. During the
facility planning process, four final wastewater treatment alternatives were
considered for a design flow of 2.88 mgd. These were activated sludge with
anaerobic sludge digestion and landfill disposal of sludge; use of an
oxidation ditch; use of the VWCR/AFBF; and use of a deep well biological
reactor with aerobic sludge digestion and disposal. The list of major
equipment and unit operations for the VWCR/AFBF alternative is outlined in
Table 7. The schematic process flow diagram and preliminary pilot plant for
this alternative are shown in Figures 9 and 10.
Details of the VWCR design for Montrose, along with the expected influent
and effluent quality of the reactor are summarized in Table 8. Although the
VWCR/AFBF process was selected by the consultant and approved by the city as
the most cost effective innovative design, failure of Montrose to obtain full
innovative funding at 85% and lack of assurance from the State of Colorado
that the city would be eligible for 100% payback funding should the process
fail precluded choice of the process in the final selection.
18
-------
TABLE 7. MAJOR EQUIPMENT AND UNIT OPERATIONS PROPOSED FOR THE WASTEWATER
TREATMENT PLANT AT MONTROSE, COLORADO (3) . . "
Pretreatment Facilities
a. Bar Screens
b. Grit Chambers
c. Influent Flow Meter
d. Return Flow Meter
Equalization Tank/Met Well ;
a. Equalization Tank
b. Wet Well
c. VWCR Feed Pumps
Vertical well Chemical Reactor
a. Feed Pumps
b. Air Compressors
c. Reactors :
d. Heat Exchangers r
e. External Boiler
f. Discharge Standpipe
g. Flow Meters i
h. Acid Cleaning Facilities
Anaerobic Fluid Bed Filter
a. Filters
b. Flow Meters
c. Effluent Pump ,
d. Recycle Pumps
Ash Ponds
Prechlorinated Facilities
a. Mixing Compartment
b. Mixing Pump
c. Chlorinator
Wastewater Monitoring
a. Composite Samplers
b. pH Recorder
c. Parshall Flume
d. Flow Recorder-Totalizer
Other Facilities
a. Control building ;
b. Standby Power Generator
c. Fence
d. Outfall Sewer
Number (each)
2
2
1
1
1
1
4
see above
:3
2
included as part of reactor
1
1
2
included as part of reactor
4
4
1
3
Part of equalization tank
1
2
(influent/effluent)
19
-------
AIR
COMPRESSOR
AIR
RELEASE
GRIT
REMOVAL
GAS TO BURNER
INFLUENT
BAR
SCREENS
AERATED
GRIT
CHAMBER
.EFFLUENT
MONITORING
STATION
Figure 9. Schematic process flow diagram of the treatment train
proposed for Montrose, Colorado.
PRECHLORINATION
PRETREATMENT-
EQUALIZATION-
PUMP STATION-
CONTROL BLDG."
VWCR-
ANAEROBIC FLUID
BED FILTERS
ASH POND
4J
EFFLUENT MONITORING-
RECYCLE PUMP STATION
ASH POND
Figure 10. Preliminary plot plan of the treatment train proposed
for Montrose, Colorado.
20
-------
TABLE 8. DETAILS OF THE MONTROSE, COLORADO VWCR DESIGN (3, 6)
1. Expected Wastewater Characteristics around the
a. Design Flow: 2.88 mgd
b. Influent: BOD range
BOD av.
COD range
COD av .
Nhhi-N av.
TKN av.
c. Effluent*: BOD range
BOD av .
COD range
COD av.
NH'j-N av.
TKN av
100 - 600
245
150 - 1000
590
27
46
50 - 300
123
54 - 360
212
33
37
reactor:
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
mg/1
*Effluent estimates are based on a maximum 625°F downhole temperature and
34-minutes reaction time as well as influent wastewater characteristics.
VWCR and Auxiliary Facilities:
a. Feed Pumps
b. Air Compressors
c. VTR Reactors
d. Heat Exchangers
e. External Boiler
f. Discharge Standpipe
g. Flow Meters
h. Acid (HMOs) Cleaning
Facilities
Two 1400 gpm at 52 psig, 120' TDH
(60 - 75 HP ea.)
Two 1400 gpm at 25' TDH (15 HP ea.)
Three 500 scfm at 112 psig (25 HP ea.)
Two 18". diam. x 4500' deep
Included with reactors
5 x 106 BTU/hr
24" diam. x 25' high
Two each ',
Included with reactors
21
-------
SECTION 3
TECHNOLOGY EVALUATION
PROCESS THEORY (7, 8, 9, 10, 11)
The VWCR process is a unique application of wet air oxidation. It makes
use of the fact that any burnable substance can be oxidized in the presence of
water at sufficiently high temperatures (flameless combustion) and resulting
high pressure. Oxygen to support the combustion is often supplied by air.
_Wet oxidation is the result of three types of chemical reactions.
Initially, the destruction of solid organic waste is predominantly the result
of heterogeneous (two-phase) oxidation due to direct contact and ensuing
reaction between adsorbed oxygen gas and organic solids. At elevated
temperatures, these solids are quickly reduced to simpler organic colloids
which are solubilized by hydrolysis. Hydrolysis splits the organic polymers
from both colloidal and soluble organic matter into smaller units but will not
destroy them. The only remaining path for ultimate destruction of the organic
matter is liquid phase oxidation, i.e., wet combustion, following the
individual collisions and interactions of dissolved oxygen with solute
elements or compounds in solution.
In wet air oxidation, organic matter can be potentially stabilized to
carbon dioxide and water. Organic nitrogen is converted to ammonia and sulfur
to sulfates. One of the last residual compounds prior to complete oxidation
is acetate. The oxidation reactions are exothermic and release energy as
heat. The amount of heat released is dependent on the make-up of the waste.
If the heating value of the waste is high enough, the temperature required for
oxidation can be supported by the oxidation itself.
Pressure down hole in the VWCR at any point is a function of the weight
of the liquid above it. Maximum allowable temperature is just below the
boiling point of the liquid. Maximum temperature is a function of the
pressure of the liquid and roughly follows the saturated vapor temperature-
pressure curve for water. Detention time is dependent on the depth and
diameter of the VWCR and the velocity of the mixture flowing through it.
For sufficient conditions of temperature and pressure, wet oxidation
proceeds until the organic removal rate decreases to zero and percent organics
removed remains constant. The organics remaining at this point are termed
refractory organics. At low temperatures of 212 to 400° F, this plateau or
equilibrium is not reached for hours. Above 575° p, it is reached in a matter
of minutes. The rate of oxidation will increase up to the critical point of
22
-------
water (705°F) above which it cannot exist as a liquid regardless of pressure.
The height of the plateau also increases with increasing temperature. Thus,
the extent and rate at which a material is oxidized is significantly
influenced by reactor temperature with very little oxidation occurring below
approximately SOOOp. Temperatures of 430°F or more are required for 80% COD
removal.
Oxygen must be added to the wet combustion system in stoichiometric
proportions at a rate that will not impede combustion. Oxygen in excess of
stoichiometric requirements does not accelerate the process and is economi-
cally undesirable because of air compression costs. In practice, excess
quantities of air above stoichiometric requirements may be necessary to
account for differences between ideal and actual combustion conditions. The
COD of the influent material is normally used as a convenient parameter of
oxygen requirements. It is roughly equal to the oxygen utilized in the
combustion process.
In summary, four important parameters control the performance of wet
oxidation units: feed solids concentration, pressure, temperature, and air
supply. The COD test is normally used as a measure of process efficiency. The
average wet oxidation efficiency is 70-90 percent as COD reduction. Some
organic matter in the form of low molecular weight compounds such as organic
acids, aldehydes and acetates will be observed in the effluent. Final oxida-
tion products are highly dependent on the degree of oxidation and the compo-
sition of the waste.
PROCESS CAPABILITIES AND LIMITATIONS
Basic Process (12)
Waste treatment by a VWCR is basically a combustion process utilizing
high temperature/high pressure wet-air oxidation to produce an easily
dewatered ash plus some stabilization of the supernatant. Because it is not
necessary to supply energy for the latent heat of vaporization (since oxida-
tion must occur in the presence of a liquid) wet air oxidation is particularly
applicable for materials like organic sludges which are combustible but
cannot be separated readily from water and which can supply some or all of the
heat required for reaction. While most combustion processes require
dewatered sludge to achieve thermal self sufficiency, considerably less
concentration of the organic matter is adequate for wet air oxidation. As an
extreme, wet air oxidation can be used for sludge conditioning which results
in improved sludge solids separation characteristics but leaves a high
strength supernatant.
i
Corrosion and scale formation in the reactor and heat exchange tubes are
inherent problems of the process. High temperature, pressure and the presence
of oxygen are all conducive to corrosion. Calcium, magnesium and sulfate
ions in the waste can cause deposition problems. Stainless steel depends on
a very thin surface layer of chromium oxide for its stainless properties.
Scale (such as calcium carbonate or calcium chloride deposits) formed on this
surface may deprive the surface of sufficient oxygen to maintain its
protective passive layer properties. When the scale breaks off, it may also
23
-------
remove the protective layer of chromium oxide. Pit corrosion can result,
leading to stress corrosion cracking failures (13). Measures can be taken to
minimize corrosion and deposition such as repassivation. They are discussed
under Design Considerations. The degree of scaling and corrosion which can be
expected in a full scale VWCR has yet to be characterized.
Configurations
Configuration of the VWCR is claimed to be an important advantage of the
process. As indicated earlier, the VWCR takes little space. It is estimated
that the land area required to treat sludge from a 40 mgd wastewater treatment
plant is one-half acre. The concentric tubes are conducive to good heat
transfer because of their proximity. Once the surrounding earth reaches
equilibrium, it may act as a heat envelope and buffer to external temperature
variations. The vertical tube also allows natural pressurization because of
the weight of the fluid column, eliminating the need for high pressure surface
vessels and controls.
The depth of the column -of waste in the vertical tube sets downhole
pressure. Maximum allowable temperature (that temperature just below the
flashing point) and pressure can be considered related according to the
saturation vaporization curve of water. Thus maximum allowable temperature
in the reactor at any given depth is set and in this respect the VWCR is less
flexible than above ground pressure vessels which can mechanically increase
pressure and therefore increase their maximum allowable temperature. Within
certain limits there are trade-offs which can mitigate this situation. They
include heat exchange (removal), dilution of waste, and increasing effective
reaction time in the reactor (Figure 4) via recirculation or lower flow
velocities (while still maintaining adequate velocities to avoid grit
deposition). In addition, much of the oxidation may occur at less than
maximum allowable temperatures. The point to be stressed is that preliminary
treatability studies on the waste to be oxidized must be made at several
operating conditions so that alternative reactor dimensions and plant flow
schemes can be evaluated in the planning phase. The degree and rate of waste
wet oxidation are significantly influenced by temperature, pressure and
oxygen supply.
The compactness of the VWCR makes downhole maintenance difficult. Delays
in pilot operation to date have come from difficulties in replacing components
which for various reasons had to be changed or redesigned (2). Considerations
about VWCR maintenance and accessibility are discussed under Design Con-
siderations.
Water Composition
As indicated previously, the VWCR is especially applicable to wastes
which_are difficult to dewater or which have sufficiently high organic content
to maintain a thermally self-sustaining (autogenous) reaction. Solids content
greater than 20 percent may create problems with mixing and consequent mass
transfer of the oxygen needed for combustion. Municipal wastewater sludges
however, normally contain only 2-10 percent solids.
24
-------
Oxidation using the VWCR may provide a good approach to treatment of
toxic and hazardous wastes, landfill leachates in particular. Wet oxidation
using moderate (527°F) and high (608°F) temperatures has been demonstrated to
be an excellent method to destroy and detoxify certain organic compounds
including various phenols, acrolein, dinitrotoluene and diphenylhydrazine
(14) Destruction of 99.8 + percent of the starting materials was achieved by
wet oxidation at 608QF for 1 hour. Oxidation at 527°F for 1 hour produced over
99 percent destruction of most compounds. In other work, laboratory tests have
shown that organic substances can be completely solubilized and broken down
("reformed") to low to medium molecular weight compounds in supercritical
water (SCW) (i.e., above 7050C and 220 atm) (15). This discovery is the basis
of a U.S. Patent assigned to the Massachusetts Institute of Technology
(M.I.T.) (15) and from which O'Donnell and Rich Enterprises, Inc., of Natick,
MA have devised a new process to treat toxic and hazardous wastes. The MODAR
process, initially developed at M.I.T. for coal degasification, involves
catalytic oxidation of the products of SCW reforming while still under
supercritical conditions. The major advantages claimed for operating in the
supercritical region are:
enhanced solubility of air and oxygen in water (essentially 100%),
which eliminates two-phase flow;
. rapid oxidation of organics approaching adiabatic combustion and
allowing short residence times;
. complete oxidation of organics;
. removal of inorganics which precipitate out rapidly when the
temperature is 450-500°C because of the extremely low solubility
of inorganic salts at these temperatures.
Unlike the reformation step which has been demonstrated to work, oxida-
tion of the reformed products is just beginning to be tested with SCW. The
Incineration Research Branch of the U.S. EPA's Industrial Environmental
Research Laboratory is currently co-sponsoring research to establish opera-
ting parameters for the extraction/destruction of PCB's and 2,4-dinitro-
toluene using conventional above ground high pressure/temperature equipment
(17). The fluid which exists under these supercritical conditions has a
density of 0.2-0.5 g/cm3. Thus some of the natural hydrostatic pressure head
advantage of a VWCR would be lost when operating in the supercritical regime
(only reactor contents near the bottom would be at supercritical temperature
and pressure). The reactor would have to be pressurized.
External Treatment
Preliminary treatment to remove large refuse and inorganic solids is
required. Thickening may be desirable to concentrate the waste so it has a
higher heating value. Equalization to act as a buffer and provide uniform
flow to the VWCR is necessary for certain applications.
25
-------
Odors, especially in the case of sludge treatment, may result from pre-
treatment thickening, or as off-gases from the VWCR system itself. Future
pilot operation should answer some of these questions. Pilot plant operation
to date has not resulted in objectionable odors from the VWCR system. Odor
level is dependent to a high degree on the total hydrocarbon content of the
waste. Air pollution control must be addressed when comparing alternatives.
Although effluent solids from the VWCR are sterile and small in volume
the concentration of metals or other suspected toxic materials in the sludge'
as well as their leachability, must be determined so that proper disposal can
be made. This is especially of concern when treating industrial wastes and
requires pilot plant or laboratory reactor study. Figure 8 shows inputs,
effluent, and residual streams for a hypothetical waste treatment plant for
either wastewater or sludge treatment.
DESIGN CONSIDERATIONS
Several mathematical models have been developed to assist in desiqnina
full scale Vertical Well Chemical Reactor plants. Verification of model
predictions should come from demonstration plant operation and later from
full-scale experience. Parameters addressed in model development represent
important design considerations and are summarized below (1, 18).
Hydrodynamic Analysis
Using output from this model and laboratory treatability data for the
given waste at similar reaction times and pressures, various VWCR size and
depth options are considered. Operating air and water horsepower require-
ments can be determined for specific cases. Options may be ranked according
to estimated costs and/or desirability.
Inputs--
Waste COD treatability data
Plant site ambient conditions
. Waste flow rate
VWCR physical dimensions including reactor depth, size, roughness
and heat exchange properties '
Oxidation reaction zone boundaries
Desired reaction temperatures at various depths (controlled by the
heat exchanger)
. Air injection point location and characteristics
. Air compressor characteristics
26
-------
Outputs--
Flow velocities, pressures, and temperatures at downhole locations
along the reactor. Flow velocities allow determination of reaction
time.
Heat Flow Analysis
Output from the model is used when evaluating the energy deficiency or
surplus for a given waste and VWCR reactor in order to establish heat
exchanger design criteria.
Inputs
Reactor physical dimensions
Operating temperatures
Insulation thickness and thermal conductivity
Earth thermal conductivity and temperature profile
Initial reactor temperature profiles
Outputs--
Heat loss to earth surrounding the reactor system as a function of
time and reactor insulation.
Outputs--
Heat Exchanger Analysis :
Output from this model allows calculation of the net energy surplus
expected from (or energy input required to) the heat exchanger fluid serving
the VWCR system. External boiler capacity can then be determined and poten-
tial energy credits estimated. \
Inputs-- 1 ' ,. .
. Heat flow losses to the earth for various operating conditions
. Reaction enthalpy production rates
Heat flow losses from the heat exchanger lines as a function of
reactor physical dimensions, flow rate, temperature, and line
insulation thickness and thermal properties.
Note that when considering the feasibility of a VWCR, a subsurface
geological investigation is necessary both to consider well drilling costs
and to estimate the surrounding earth's thermal and structural properties.
Important parameters which should be addressed include: 1) depth to bedrock;
2) thickness of bedrock; 3) dip of sedimentary rock; 4) identification of
aquifers in the area; and 5) identification of geological structures in the
area. A geophysical survey of available sources of data, including geologi-
cal maps_and cross sections, water, oil or gas well logs may contain most or
all of the information necessary (19). ,
27 '
-------
As indicated previously, precautions must be taken to minimize the
effects of corrosion and deposition in the reactor. A weekly nitric acid
passivation operation (10-20% aqueous nitric acid for 20 to 30 minutes through
the reactor) has been recommended to mitigate the effects of both corrosion
and scaling (13). The success of this approach in a full-scale reactor, the
influence of temperature, and the effect of acid addition operations on
wastewater treatment efficiency is not known at this time. The procedure for
acid addition over long-term operation will be developed during demonstration
plant operation. A related problem discovered during pilot plant testing is
leaking joints in the reactor case. This leaking problem is expected to be
corrected by welding all joints in the reactor case in future full-scale
reactors (20).
Corrosion and deposition tendencies of impurities in the heat exchange
fluid must also be addressed. If it is water, chemical stabilizers or
corrosive inhibitors commonly used for high pressure boiler water control can
be employed. If scale deposits are allowed to accumulate or if the heat
exchange jacket annulus is too small, heat exchange efficiency in the reactor
will decrease and heat exchange fluid pumping head and costs will increase.
The heat exchange jacket annulus has been increased and redesigned to enable
it to be pulled independently to minimize plugging problems.
ENERGY CONSIDERATIONS
The most important energy consideration is the heating value of the
waste. Table 9 gives the heating value of a number of waste materials, fuels,
and pure substances. Ideal combustion reactions for carbon and hydrogen are:
C + 02 = C02 + 14,100 BTU/lb. C
2H2 + 02 = 2 H20 + 61,000 BTU/lb. \\2
In the absence of other data, a value of 6000 BTU/lb. COD oxidized can be
used to estimate the heat value of the waste. This value correlates with
observed heat generation levels of 1200-1400 BTU/lb. air required for
oxidation of most waste materials (8). How it is used is illustrated in the
discussion below which calculates minimum COD for a thermally self-sufficient
sludge:
Consider a 10 mgd plant producing 10 tons per day of dry sludge solids.
Assuming the wet sludge consists of 5 percent solids, then:
10 tons X 2000 Ib. x
ton
are produced daily.
1
4 x 105 Ibs. wet sludge
Assuming further that the wet sludge has the specific heat properties of
water, then to raise one pound of it one degree Fahrenheit requires 1
BTU.
28
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TABLE .9. HEATING VALUES OF VARIOUS MATERIALS AND FUELS
A.
B.
Heating Value and Heat Delivered Per Pound of Air Consumed in Oxidation
(1Q) BTU Lbs. 02 Lbs. Air BTU
Per Lb. Per Lb. Per Lb. Per Lb.
Material Material Material Material of Air
Ethylene 21,460
Carbon 14,093
Acetic Acid 6,270
Oxalic Acid 1,203
Pyridine 14,950
Fuel Oil 19,376
Lactose 7,100
Casein 10,550
Waste Sulfite
Liquor Solids 7,900
Semi-ChemicaJ
Solids 5,812
Sewage Sludge '.'
Primary 7,820
Sewage Sludge
Activated 6,540
42
66
07
0.178
53
26
13
1.75
1.32
.955
1.334
1.191
14.8
11.53
4.6
0.77
10.9
14.0
4.87
7.55
5.70
4.13
5.75
5.14
1,450
1,220
1,365
1,565
1,370
1,380
1,455
1,395
1,385
1,410
1,365
1,270
Heating Value of Typical Sewage Treatment Residuals (21)
Waste Materials Dry Solids Combustibles (%) BTU/lb. of Combustibles
Grease & Scum
Raw Sewage Solids
Fine Screenings
Digested Sludge
Grit
88.5
74.0
84.4
59.6
33.2
16,750
10,285
8,990
5,290
4,000
Comparative Heating Values of Pertinent Fuels (21)
Fuel " Heating Value (BTU/lb of fuel)
No. 2 Oil 19,600
No. 6 Oil 17,500
Natural Gas 22,800
Bituminous Coal 13,600
Wood (air dried) 5,500
Grease & Scum 16,700
Sludge (dry solids) : 10,000
Anaerobic Digester Solids 5,300
Anaerobic Digester Gas 15,400
Municipal Refuse (70% moisture) 4,900
Heat Released on Combustion of Sewage Sludge (21)
Material
Raw Sludge (primary & activated dry solids)
Digester Sludge (from anaerobic digester,
dry solids)
Range (BTU/lb of Material)
6.,500 - 9.500
2:,500 - 5,500
29
-------
In the VWCR, heat loss from the VWCR system to the surrounding earth
effectively reduces the heat available to heat the sludge. Assume 20% of
the heat produced is loss to the surrounding earth. Then,
effective heat value of the waste = 6000 x (1.0-0.2) = 4800 BTU/lb.
COD oxidized.
Influent and effluent temperatures are generally within 5°F during
steady-state operation (1). Assuming a loss of 5°F has to be made up then
the heat required to make this up for 4 X 105 15$. of wet sludae is (0 =
MCAT):
4 X 1<)5 Ibs. X 1 BTU x 5oF = 20 X 105 BTU
The COD required to be oxidized to make up this heat loss (taking into
account heat losses to the earth) is:
20 X 105 BTU
4800 BTU/lb. COD oxidized
416.7 Ib. COD oxidized.
Not all of the sludge COD is satisfied in the VWCR. Assume 75% is
reduced. Then the COD of the sludge which has enough heating value to
make up for the heat losses and oxidation efficiencies is:
416.7 Ib. COD
0.75 oxidation efficiency
555.6 Ib. COD
Thus for steady-state conditions where heat losses just equal heat
production through the VWCR, the COD must be at least:
555.6 Ibs. COD
4 X 105 Ibs. wet sludge
self-sufficiency.
1389 ppm or mg/1 to maintain thermal
The example illustrates how several variables can affect calculations
estimating the thermal self-sufficiency of the oxidized waste. Note that the
example assumed that sludge had the specific heat of water, that influent and
effluent temperatures were within 5°F, that 20% of the heat was lost to the
earth, that only 75 percent of the volatiles were oxidized, and that the wet
sludge was composed of 5 percent solids. If the difference between influent
and effluent waste temperatures was only 2°F, the mg/1 COD required for
thermal self-sufficiency would have been only 556 mg/1. Influent-effluent
temperature differential plays a large part in establishing thermal self-
sufficiency boundry conditions. Figure 11 shows how waste strength (as COD)
required for thermal self-sufficiency varies with temperature differential
(washout heat) and heat transfer efficiency.
30
-------
HEAT TRANSFER EFFICIENCY
70% 80% 90%
2000
1500
1000
ASSUMPTIONS
. heat value of waste is
6000 BTU/lb COO oxidized
. 75% of waste is oxidized
. sufficient air and pres-
sure are present for
oxidation
024 68
INFLUENT-EFFLUENT TEMPERATURE DIFFERENTIAL AT, °F
Figure 11. Waste strength required for thermal self-
sufficiency under certain conditions.
Figure 12 gives the projected energy balance for conditions inside the
VWCR proposed for Longmont, Colorado. Washout heat for an influent-effluent
temperature differential of 5°F is 260,000 BTU/hr (150,000 gal/day X 8.34
Ib./gal X day/24 hr X 1 BTU/lb.-°F X 5°F). Heat loss to the surrounding rock
for the particular conditions, existing at Longmont is projected to be about
430,000 BTU/hr at steady-state conditions. Heat of reaction is 2,350,000
BTU/hr (9400 Ibs. COD/day X 6000 BTU/lb. COD X day/24 hr). Net heat production
is thus 1.66 X 106 (2.35-0.43-0.26) BTU/hr.
At Longmont, heat loss to the surrounding earth is expected to be about
18% (0.43/2.35 X 100). In other words, heat transfer in the reactor is expected
to be about 82 percent efficient. From Figure 11, for a wash-out heat loss due
to a temperature differential of 5°F, and heat transfer efficiency of 82
percent, minimum mg/1 COD required for thermal self-sufficiency is about 1370
mg/1 COD. Longmont expects COD of the waste to be 5,000-10,000 mg/1 COD so that
exothermic conditions should exist.
It is important to keep in mind that these energy examples assume that
sufficient conditions exist in the reactor for oxidation to proceed. That
is, initial start-up heat to bring the reactor up to the temperature required
for oxidation has been supplied; there is sufficient air for oxidation; and
there is sufficient pressure to keep water in liquid phase at the reaction
temperature. An important task in demonstration plant operation will be to gain
31
-------
2500
2000
g 1500
i 1000 -.
500 -
HEAT OF REACTION
NET HEAT PRODUCTION
NET INSULATED HEAT LOSS
~
ROCK HEAT LOSS (K=0.5) FROM AN INSULATED REACTOR (K=0.04)
WASHOUT HEAT LOSS DUE TO AT=5°F
3 10
20 30 40 50
WEEKS AFTER START-UP
Figure 12. Net heat production expected for the Longmont VWCR.
confidence in such assumptions made when designing full-scale facilities for a
particular waste. In any reactor design the thermally self-sufficient COD
concentration will be a function of the specific design, the waste char-
acteristics and the thermal conductivity of the surrounding earth.
Other energy requirements for the VWCR are for feed pumps, air com-
pressors, heat exchange fluid pumps, and miscellaneous smaller requirements,
such as from acid wash and recirculation pumps. Sidestream energy requirements
are in addition to these. For the Longmont, Colorado VWCR, which is designed
to treat sludge generated by a 7.5 mgd wastewater flow, the operating
horsepower requirements are estimated to be 135 hp total: 20 hp VWCR feed pump;
100 hp air compressor; 5 hp heat exchange pump; and 10 hp for other
miscellaneous pumping requirements.
During start-up, there will be an initial large energy (fuel) input into
the heat exchange boiler to bring the reactor up to operating temperature. Just
how much fuel will be needed will be determined from the demonstration plant
studies at Longmont. It is expected that the oxidation reactions will quickly
(within a few days) generate enough heat for oxidation to proceed autogenously
and that net heat production will increase until steady-state conditions are
attained (in about 40 weeks for Longmontsee Figure 12).
32
-------
OPERATION AND MAINTENANCE REQUIREMENTS (22, 23)
Operation and maintenance requirements for the full-s'Jf6 plant at Long-
mont have been estimated based on pilot plant experience. See Figure 3 for the
unIt operatTons comprising the VWCPR system. Major daily ^^f^Zl^
for pump and valve maintenance, coarse screen cleaning, and sludge> disposal to
the drying beds. For the particular case at Longmont, it is estimated that the
7.5 mgd wfstewater flow will result in 7,000 Ibs. of dry suspended solids each
day coming from the primary clarifier to the VTR. These will be reduced 90
percent in the VWCR to 700 Ibs., of dry solids. It is estimated that 80 percent
of these solids will be settled in the foam separator/clarifiergiving 560 Ibs.
of dry solids in a 10 percent concentrated sludge (or "ash"). This works out to
be approximately 3.3 yd3 of thickened sludge to the drying beds each day
(560/[O.J X 62.4 X 27]) from the 7.5 mgd plant. :
Structures and reactor casings are estimated to have a 40 year life
including air and heat exchange lines; mechanical/electrical equipment 1.5
years The stainless steel upcomer and downcomers of the VWCR are conserva-
tively estimated to have a 10;'year life and will have to be replaced over a
normal 20 year life cycle period.
i
Nitric acid quantities and indeed acid wash operation have yet to.be
determined. This is one of the important areas to be investigated at the
Longmont VWCR demonstration plant. The range of high, temperatures and
pressures occuring in the reactor and the unknown buffering capacity of the
wastewater which influences p'H complicate the estimation of free C02 which can
be expected to be in solution. This in turn makes estimates about bicarbonate
and carbonate quantities difficult. Hardness as calcium bicarbonate TUS
soluble. Calcium carbonate is not and may precipitate out or adhere to the
sides of the reactor. At this writing, best estimate of the amount of nitric
acid needed for Longmont is based on the reaction:
CaC03 + 2HN03
H2C03 + Ca(N03)2
The 150,000 gpd of sludge (40,000 gpd) plus dilution water or recycled VWCR
supernatant (110,000 gpd) entering the Longmont VWCR is expected to contain a
total of 350 Ibs. of calcium, From the stoichiometry, for every 40 Ibs. of
calcium, 126 Ibs. of nitric acid is needed. Thus about 1110 Ibs (126/40 X 350)
of nitric acid is needed each day to react with the potential calcium
carbonate which may form. Considering that equilibrium C02 increases with
temperature (and thus less bicarbonates decompose to carbonates) and that not
all calcium carbonate will necessarily adhere to the reactor piping, about 800
Ibs. of nitric acid is estimated to be needed for Longmont. This figure must
be considered a best guess. Nitric acid available on the market is at 67
percent concentration and has a specific gravity of 1.3. Thus 110 gallons of
67 percent concentrated nitric acid (800/[.67 X 1.3 X 8.34]) is estimated to
be needed daily for Longmont, Colorado. \
COSTS (22, 23) \
Capital cost estimates for sludge treatment using a VWCR are summarized
in Table 10. The capital costs have a range which includes the differences in
33
-------
TABLE 10. VWCR CAPITAL COST ESTIMATES FOR TREATING SLUDGE
FROM VARIOUS SIZE WASTEWATER TREATMENT PLANTS (22)
Wastewater Treatment
Plant Size, mgd
-*
7.5
15.0
30.0
100.0
VWCR Plant
Size, mgd
7.5
7.5
15.9
33.0
Reactor
Diameter,
Inches
8
8
12
16
Number of
Reactors
1
2
2
3
Capital
Range,
9 _
4 -
7 -
20 -
- -
Cost
106*
3
C
10
30
«nri ?n nSrpS ?nS" 5°°°;-000 feet deep rector, 60QOF maximum temperature
and 70 percent COD reduction. The capital cost range includes the cost of
additional biological treatment for sidestreams and costs of different
drilling situations. The ENR Construction Cost Index is 3510
ge°,1ogy (affecting well drilling costs), construction cost differences
Co°Unt7 T* the neei1n some cases to include Additional biolog?ca
c frh Sldes.t,re1ams' Odor contro1 equipment is not included in this
estimate. The capital cost estimates are for the generalized plant unit
operations shown in Figure 8. Other assumptions are listed In the Table
a?thnMnhatthhe VWC? ^ S1"Ze, nomenc^ure uses wastewater flow quanlit es
treSt pfant °r " °Xldlz1ng the ^^ generated by the wastewatlr
*c+- Tab]V] !"mm7arjzesjoperation and maintenance (O&M) costs based on those
estimated for the 7.5 mgd VWCR proposed for Longmont, Colorado. See Figure 7
for definition of which unit operations contribute to these costs. Electric
anlie+1C7alCOSJS 1nc/ease.s for the 1ar9er plants are straight line from the
Longmont 7.5 mgd cost estimates. The previous sections on O&M and enerqy
considerations have discussed the basis for some of these costs SS
assumptions are listed in the Table. Note that Longmont does not include
taSk^^rn'oH1!9 7h tfl? S,1defream (6ffluent fr°m the foam separation
tank is returned to the headworks of the plant) and so both biological
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36
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initial cost of providing and installing all VWCR downhole components (ex-
cluding well drilling and casing costs). i
Table 12 gives a preliminary life cycle cost estimate for a VWCR sized
to treat sludge from a typical 7.5 mgd municipal wastewater;treatment. It is
emphasized that the figures given in Table 12 represent only best estimates.
The exclusions discussed for-Table 11 also apply for Table 12. Potential
energy recovery credits are not included. The factors most affecting the
life cycle costs of any VWCR are well drilling costs and sludge composition
(from both energy recovery and corrosion/deposition considerations).
37
-------
TABLE 12. PRELIMINARY LIFE CYCLE COST ESTIMATING FOR A VWCR SYSTEM CONTAINING
AN EIGHT INCH DIAMETER REACTOR (23)*
Project Capital Costs (and expected service life)
Well drilling and casing (40 yr)
Vertical Well Chemical Reactor
- Reactor heat exchange lines, air lines
and casing (40 yr)
- Reactor upcomer and downcomer (10 yr)
Mechanical/electrical equipment (15 yr)
VWCR building (40 yr)
Existing WWTP modifications to accommodate VWCR
Construction Cost
20% contingencies
20% non-construction costs (engineering,
supervision, etc)
$ 600,000
300,000
300,000
300,000
40,000
60,000
$1,600,000
320,000
320,000
Total Capital Cost (PW)
Replacement Costs
Reactor upcomer and downcomer
- $300,000 X .50245 =
Mechanical/electrical equipment
- $300,000 X .35615 =
$2,240,000
151,000
107,000
Total Replacement COSTS (PW) $ 258,000
Salvage Value
Well drilling and casing
20/40 (600,000 X-.25245) =
VWCR heat exchange lines, air lines and casing
20/40 (300,000 X .25245) =
Mechanical/electrical equipment
10/15 (300,000 X .25245) =
VWCR building
20/40 (40,000 X .25245) =
Salvage Credit (PW)
Operation and Maintenance Costs (see Table 11)
$154,250 X 10.49186 =
Equivalent Annual Costs
$ 3,952,000/10.49186 =
$ 76,000
38,000
50,000
5,000
($ 169,000)
$1,618,000
$3,947,000
$ 376,000/Yr
*See Figure 7 for definition of system components.
The VWCR system is sized,to treat sludge generated by a 7.5 mgd WWTP.
Discount rate = 7-1/8 percent; 20 year life cycle period; energy recovery
costs and credits are not included. ENR = 3510.
38
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SECTION 4 :
COMPARISON WITH EQUIVALENT TECHNOLOGIES (24)
i
Wet oxidation can be divided into three categories of oxidation which are
primarily segregated according to the amount of COD reduced. The three
categories are defined in Table 13. The low oxidation category of wet
oxidation is often used interchangeably with sludge conditioning. This is
because low oxidation primarily changes the composition of the sludge to
improve its thickening and dewatering properties. The small reduction in COD
demand is almost incidental. However, unlike sludge conditioning processes,
air for oxidation is specifically added to the process. ,
This comparison uses as its basis the information contained in the EPA
report, "Effects of Thermal Treatment of Sludge on Municipal Wastewater
Treatment Costs" (24). It is recommended reading (24). The report represents
an independent survey of costs associated with the various processes
commercially available for thermally treating sludges normally generated
during the treatment of municipal wastewaters.
Because virtually all .of the equipment for the thermal treatment of
sludge was supplied by Zimpro or Envirotech, the EPA study dealt largely with
the processes and equipment used by those two manufacturers. Most sludge
treatment plants contacted which were manufactured by Zimpro could be
classified as low oxidation units. A few were intermediate oxidation units.
One (Akron, Ohio) was a high oxidation unit. The sludge treatment plants
manufactured by Envirotech were classified as thermal conditioning plants.
TABLE 13. WET OXIDATION CATEGORIES (24)
Oxidation
Category
Low
Intermediate
High
Typical Reduction
in Sludge COD, %
5
40
i.
92 - 98
Temperature
OF
350 - 400;
450 !
675
Pressure
psi
135 - 250
450
2650
39
-------
The cost figures used in this section come from the EPA report and are in
March 1975 dollars (ENR = 2128). Costs are for thermal treatment processes in
general and wet oxidation in particular when it can be subdivided to that
extent. Direct construction, fuel and electricity, manpower, and material
and supplies costs are given and discussed below. Indirect sidestream
treatment and more general factors which should be addressed when considering
various methods for sludge treatment and disposal are given at the end of this
section.
This comparison is more accurately termed a contrast with other
equivalent technologies because the basis for comparison is not always the
same. Differences are noted in the section as much as possible. Two major
differences should be noted: Most above ground thermal treatment processes
thicken the waste to about 4-5 percent in order that oxidation may occur in a
smaller vessel size and the waste have a higher heating value. The VWCR does
not because its configuration does not allow for sufficient air as oxygen to
be added for oxidation of such high strength wastes. In fact, sludge wastes
are normally diluted. The trade-off with thickening cost savings is
potentially longer reaction times and a lower heating value of the waste.
This cost comparison does not address thickening considerations. Secondly
the EPA cost figures are essentially summary costs of low to intermediate wet
oxidation plants. VWCR cost estimates are for fairly high (500-650°F, 75% COD
reduction) oxidation operation. With these differences in mind, a contrast
between the VWCR and other thermal treatment processes is made throughout the
remainder of the section. ;
Figure 13, Curve A, gives average thermal treatment plant costs for
sludge feed pumps; grinders; heat exchangers; reactors; boilers; gas separa-
tors; air compressors, where applicable; standard odor control systems; and
piping, controls, wiring and installation services normally furnished by the
equipment manufacturer. Curve B includes costs for typical building
foundation, and utility needs for the thermal treatment system.
Figure 13 construction costs can be roughly compared to similar sized
VWCR cost estimates by noting that the nominal 7.5 mgd size wastewater
treatment plant at Longmont is expected to generate about 7000 Ibs. of drv
solids per day for the VWCR. Average solids concentrations of the thickened
tn^F^fl 9*tyPiC-al ther,mal treatment Processes is 4.5 percent according
to the EPA report. This results in a thermal treatment plant capacity of about
13 gpm and a construction cost of approximately $520,000 (Curve B). Assuming
capital costs are 30 percent over construction costs and using the ENR index
hf n \"£ nnn0" ^ (3510/2128)> P^ent worth capital cost is estimated to
be $1,115,000. This can be contrasted with the $2-3 X Ifl6 range for the
nominal 7.5 mgd VWCR plant cost estimated in Table 10 for a high degree of
oxidation (about 75% COD reduction). VTR estimates about a $1,400,000 capital
40
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10,000
5000
1000
500
100
50
CURVE 8: INCLUDES BUILDINGS, UTILITIES
AND FOUNDATIONS .
FOR THE THERMAL TREATMENT
SYSTEM ONLY
ENR
2128
5 10 50 - 100 500 1000
THERMAL TREATMENT PLANT CAPACITY, GPM
Figure 13. Typical direct construction costs for thermal
treatment plants (24).
Annual costs versus typical thermal treatment plant sizes for both fuel
and electricity are shown in Figure 14. The curves are for those plants
incorporating air addition (wet oxidation) and not solely sludge condi-
tioning. The curves, however, are for low oxidation conditions.
Fuel is used chiefly as a source of heat to produce steam which will heat
the waste to temperatures adequate to support combustion.^ The amount of fuel
used is influenced by. the temperature to which the reactor contents are
raised efficiencies of the boiler and heat exchange systems, insulation
properties, and the degree of heat producing oxidation which takes place in
the reactor. (In the VWCR proposed for Longmont, fuel is not required after
initial start-up because reaction proceeds autothermally.)
Electrical energy needs are determined by sizes and efficiencies of
driven machinery, such as sludge and boiler water pumps, grinders, thick-
eners, and air compressors. The electrical energy curve includes an allowance
for thermal treatment plant building and site needs such as ^fjing. Unit
costs of $2.80 per million BTU for fuel and $0.03/KWH were used to draw the
curves.
41
-------
1000
3 500
100
50
FUEL WITH AIR ADDITION
-ELECTRICAL ENERGY WITH
AIR ADDITION
ENR = 2128
5 10 50 100
THERMAL TREATMENT PLANT CAPACITY, 6PM
500 1000
Figure 14. Typical power costs for thermal treatment low
wet oxidation plants (24).
annual cost estimate. This can be contrasted to thl $36 500 power cost
estimate for the VWCR at a 7.5 mgd plant size in Table 11. As before^ however
25 ifinn5 ' "Ot the Same because the VWCR is desi'9ned f°r high oxidation SF
and_ 1600 psi pressures compared to the 350-40QOF, 135-250 psi pressures found
nl^% °W^Xlda-tl0n cate^y which Flg^e 14 summarizes. At high pressures
42
-------
Labor for operation and maintenance presents one of the highest areas of
cost in the operation of a thermal treatment plant. The labor costs
summarized in Figure 15 are for preventative and routine repair work. The
labor operation costs comprise time spent reading and logging data on the
process, controlling and adjusting various systems and components, and
laboratory work. Labor maintenance costs include cleaning and repairing
process components, general upkeep of the process? area, checking and
repairing of controls arid instrumentation, and performing preventative
maintenance on a daily, weekly, monthly, etc., basis. They do not include
major overhaul functions, such as reactor cleaning, pipe, tube, pump,
compressor or boiler working parts replacement. Costs for these and similar
items requiring skills of contracted specialists are; included in a later
discussion below on materials and supplies.
1000,
500
VI
=3
O
n:
100-
50
10
OPERATING LABOR COSTS
MAINTENANCE LABOR COSTS
ENR = ; 2128
5 '10 50 100
THERMAL TREATMENT PLANT CAPACITY, GPM
, 500 1000
Figure 15. Typical operating and maintenance labor costs for
thermal treatment plants (24). ;
43
-------
From Figure 15, the annual cost for O&M labor at a typical 13 gpm capacity
oo J^atment plant is approximately $38,000 + $10,000 = $48,000 at ENR
2128 or $79,000 at ENR 3510. This is contrasted to the sum of maintenance,
operator, and laboratory labor ($8800 + $35,200 + $6600) equal to $50,600
estimated for the similar capacity 7.5 mgd nominal sized VWCR in Table 12
Ih w,!rDC?uS1Stunt with the exPectation that labor costs should be lower for
the VWCR than above ground thermal treatment plants because of the latter 's
complex array of high^pressure vessels, heat exchangers, pressure reducing
and control valves, piping, pumps and air compressors. Proper control of
Normal annual costs for materials and supplies required to operate and
maintain the thermal treatment system are shown in Curve A of Figure 16 These
costs include materials and parts such as seals, packing, coating lamps
bearings, grinder blades and other items used in scheduled and normll
c?eSnrCh;m- ^Vlf include operating supplies such as lubricants
cleaning chemicals, boiler feed water, and water treating chemicals. At 13
1E?A %UJ-e 1-6' Curve A shows a $580° annua1 cost at ENR 2128 or $9600 at ENR
colt'f Jn^nS+C?Srnnmd t0 ihe- SS °f che^'cals and maintenance materials
cost ($40S150 + $24,000) equal to $64,150 for the VWCR given in Table 12
1000 T
«c 500
100
50
10
5
CURVE B: NORMAL ANNUAL COST PLUS ALLOWANCE
FOR PERIODIC OVERHAUL-
CURVE A: NORMAL ANNUAL COST
ENR = 2128
5 10 50 100
THERMAL TREATMENT PLANT CAPACITY, GPM
i t
500 1000
Figure 16. Typical material and supply costs for
thermal treatment plants (24).
44
-------
The estimate of materials and supplies for the VWCR 1s made: with least
confidence. This is because at this writing the VWCR has no long-term
operating history and "typical" material and supply inventories and usage
rates are not known. Chemical quantities and prices will greatly influence
VWCR supply costs. Since the estimate in Table 12 is for high temperature and
pressure operation, it is expected that corrosion and deposition control
supply costs would be higher than those suggested in Figure 16, which largely
summarize sludge conditioning and low wet oxidation data. Nevertheless, the
greater than 6-fold difference seems high. ;
In addition to routine maintenance tasks required for typical thermal
treatment plants as described by Curve A of Figure 16, additional costs for
major overhaul work are incurred. This work includes such items as motor
rewinding; major overhauls, of pumps and compressors; major,, non-routine
rehabilitation or replacement of heat exchanger tubing piping and controls;
and refitting of boilers. Such work might be done on an average of 6-7 years
depending on the plant and the component. Curve B of Figure 16 summarizes
typical overall costs including those under Curve A to give the total annual
cost for materials and supplies. At this writing, the cost increment repre-
sented by Curve B cannot be estimated with any degree of confidence for the
VWCR and no contrast is made. ,
Costs associated with the handling and treating of liquors resulting
from thermal processing of sludge can significantly affect the total cost of
treatment. Costs for processing the liquor depends on the method chosen to
handle and treat the liquor, the sewage treatment .process, discharge
requirements and most importantly the characteristics of the liquor itself.
Because wet oxidation with- the VWCR normally is in the high oxidation
category, it is expected that the liquor strength will not be as great as those
from heat treatment processes which do not stabilize the waste to such^an
extent. The characteristics of the liquor expected for any given application
must be determined from pilot or bench treatability tests., Once this is done,
the cost of constructing and operating facilities to handle process liquor
must be addressed. The EPA report outlines an approach for estimating
indirect costs of sidestream treatment. Wastewater treatment plant capacity
to handle high strength recycle liquor is selected as the variable having the
greatest influence on costs 'for liquor treatment. Other variables, particu-
larly BOD and suspended solids concentrations in the raw sewage, also
influence cost. ;
Costs to treat concentrated, high-hydrocarbon streams coming primarily
from gas separators or cove'red decanting tanks represent the second major
sidestream impact on overall treatment costs. The EPA report notes that
commonly, five to ten percent of the total costs for thermal treatment are for
odor control. Again, odors resulting from high temperature and pressure
oxidation in the VWCR are expected to be less because the waste is stabilized
more than at low or intermediate oxidation plants. Odor treatment is not
expected to be such a significant factor. The EPA report estimates costs for
odor control in a similar manner to that done for effluent liquors. Costs are
developed for three typical methods of odor control: incineration, carbon
adsorption and chemical scrubbing. It must be emphasized that odor control
45
-------
systems for use in thermal treatment plants must be selected on the basis of
what is needed to adequately treat the specific off-gas involved.
For proper toxics management, there are common questions which have to be
answered, whichever the wet air oxidation process. These include identifying
the form, stability, and toxicity of the compound after oxidation, estab-
lishing how much is in the liquid and solids fraction, and determining its
Teachability or degradability. Mechanical operation of the VWCR would be
safer than conventional wet oxidation configurations because there are no
high pressure vessels and associated transfer and control equipment. How-
ever, wells drilled through potable water bearing strata must be properly
cased to protect groundwater quality.
A comprehensive comparison with equivalent technologies awaits further
full-scale demonstration plant testing to verify design performance and cost
relationships.
46
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SECTION 5 .
ASSESSMENT OF NATIONAL IMPACT ';
The vertical well chemical reactor employs chemical oxidation to oxidize
organic materials in water solution or suspension. In general,_ the desira-
bility of using the VWCR is influenced by plant size, site-specific geological
conditions, wastewater characteristics and sidestream treatment require-
ments. These considerations are not independent and are discussed below.
Table 14 summarizes some of the technical findings of the EPA 1980 Needs
Survey (26). It estimates that over 6000 domestic wastewater treatment plants
need to be built or upgraded by the year 2000 in order to meet the 1983 goals
of the Clean Water Act. All of these will generate sludge and some wastewaters
will have an industrial component increasing their strength, making them more
attractive for potential wet oxidation using the VWCR.
At this writing, the smallest domestic wastewater treatment plant which
can economically utilize a VWCR to treat sludge is estimated to be about 3 mgd
(27). Minimum desirable reactor diameter and the requirement for enough
sludge flow to maintain continuous operation of the VWCR determines plant
size (If VWCR operation is not dependent on sludge production and oxidizes
the wastewater directly, the 3.0 mgd figure is not appropriate and the minimum
size plant will largely be a function of wastewater characteristics.) Part B
of Table 14 estimates that 135 to 572 of the new plants expected to be built
by the year 2000 will be 3.3.mgd or larger. Using this information and the 3
mqd size restriction for sludge treatment, it can be estimated that the VWCR
can potentially be considered for sludge treatment at;at least 354 (the
average of 135 and 572) domestic wastewater treatment plants expected to be
built by the year 2000. Existing plants will also be'upgraded, creating
additional needs for sludge disposal facilities. '
An important site-specific consideration is drilling costs. These will
normally constitute the single largest capital outlay item. Drilling costs
are a function of subsurface, geology which must be characterized for the site.
The availability of drilling-equipment and know how must also be determined.
Wastewater treatment sites having unused wells on site or nearby make wet
oxidation of the wastewater or sludge by the VWCR more desirable. The VWCR
should also be considered at sites where land is at a premium and sludge
disposal a problem. And, wet oxidation is well suited for those wastewaters
too toxic for direct biological treatment.
Thermal self-sufficiency will be required in most cases in order that the
VWCR be cost effective. Figure 11 suggests that given an optimum temperature
47
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differential of 2°F, general, wastes should have a COD of at least 500 mg/1 COD
and preferably higher for thermal self-sufficiency. Most domestic waste-
waters are not this strong. Conversely, most waste sludges are much stronger,
increasing the possibility of energy recovery. Thus, while wet oxidation with
the VWCR is most desirable for sludges, high strength wastewaters (containing
possibly domestic and industrial components, such as at Montrose, Colorado)
should also be considered. ;
Treatment of sidestreams affects general wet oxidation economics and,
therefore, desirability of using the VWCR (24). While wet oxidation improves
the thickening and dewatering characteristics of sludge, it also transforms
some insoluble organic substances to soluble materials !in the liquor, a
portion of which may be non-biodegradable. This refractory soluble portion is
highly site-specific and a function of the make-up of the sludge. In
addition, the strength of this portion depends on the volatile matter in the
sludge and degree of oxidation achieved in the reactor. The potential of high
strength liquors with refractory components underscore the importance of
performing treatability studies at various wet oxidation! temperatures and
pressures on waste from the site under consideration to characterize expected
VWCR effluent. j
The other sidestream to be considered is odors from the sludge. Exhaust
gases exiting from the stand pipe, solids separation, or; sludge dewatering'
operations may be odorous and have to be treated. Common air pollution
control alternatives are carbon adsorption, afterburning of volatile gases,
or chemical scrubbing. As before, whether air pollution control is necessary
is highly site-specific and depends on the make-up of the waste and degree of
oxidation. It is not possible to generalize about sidestream treatment
requirements. <
Table 14 summarized expected domestic wastewater plant requirements and
not industrial sector considerations. The potential of wet oxidation to
destroy and detoxify toxic organic materials at moderate to supercritical
pressures was discussed earlier in the wastewater composition section. In
such cases where public health is a major immediate consideration, wet
oxidation with the VWCR represents an attractive possibility.
In summary, the VWCR is a potential treatment technology for organic
wastes when significant sludge volume reduction is required, where stringent
requirements for solids disposal exist, when destruction of toxic materials
and pathogenic organisms is necessary, and where potential energy recovery
from high strength wastes is good. It is assumed that engineering and
mechanical aspects are not limiting operation. A major goal remaining is to
demonstrate VWCR steady-state operation at full scale. This experience will
not only produce operational information, but will better define actual costs
for various strength wastes so they can be more realistically compared to
competing processes. . t
49
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SECTION 6
CONCLUSIONS AND RECOMMENDATIONS
2.
3.
b.
Configuration of the VWCR has both advantages and disadvantages:
a. The VWCR uses little space compared to above ground wet oxidation
configurations.
The concentric tube configuration promotes efficient heat exchange
between influent and effluent streams.
c. The vertical tube configuration allows natural pressurization of
the waste from weight of the liquid above it. The below ground
natural pressurization is safer and cheaper than above ground
mechanical pressurization, however, it is less flexible. Pressure
at any point downhole in the reactor is relatively constant and
therefore, the maximum allowable temperature (not exceeding the
waste boiling temperature) is fixed for any depth, approximately
following the saturated vaporization curve for water.
d. Reactor tube size limits the amount of air which can be added to
support combustion. Standard operating procedure involves sludge
dilution to meet maximum air and temperature limitations.
e. VWCR configuration and compactness make downhole accessibility
difficult. Mechanical reliability and maintenance of the VWCR
system are important considerations.
Appropriate bench or pilot scale treatability tests using the waste to be
oxidized are very important. The degree and rate of waste wet oxidation
is significantly influenced by temperature and pressure. Temperature and
pressure requirements affect VWCR depth and ultimately costs.
COD reduction experienced using a batch laboratory reactor has been
close to that obtained at pilot plant scale under similar operating
conditions. This supports the use of a bench scale reactor to model
pilot COD reduction rates. Experience with the fate of metals or toxics
is less definite.
Specifically:
a.
The poorly understood interacting effects of metal solubility
adsorption and desorption at high temperatures do not permit a
50
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definitive explanation about the fate of metals in the VWCR. More
work needs to be done in this area.
b. Independent but related laboratory scale studies investigating
detoxification of specific organic compounds by wet oxidation has
demonstrated wet oxidation to be an excellent method to detoxify
those materials. Thus, wet oxidation using the VWCR is a potential
method for treatment of toxic wastes. Studies using the VWCR for
treatment of toxic wastes have yet to be done and are needed.
The VWCR is especially applicable to wastes having a high, organic
content so that a thermally self-sustaining reaction can be maintained.
The minimum organic concentration for autothermal conditions will
depend largely on the actual temperature differential between influent
and effluent wastes. i
r t1
When considering the feasibility of a VWCR, a subsurface geological
investigation is necessary to identify aquifers, estimate well drilling
costs, and determine 1:he~ea~rth's thermal "properties.
Sludge stabilization 'trends using a laboratory reactor designed to
simulate VWCR oxidation conditions generally agree with historical
above ground pilot and full-scale wet oxidation, observations which
indicate that as pressure and temperature increase:
a. The rate and extent of COD reduction increases;
b. The particulate waste fraction approaches an inert, readily
settleable ash; and :
\
f
c. The soluble waste fraction becomes more biodegradable.
The VWCR is not yet fully developed in that all process variables
normally expected in full-scale application have yet to be char-
acterized: I
a. The efficacy of the acid wash system to control reactor scaling and
corrosion has yet to be demonstrated; ;
b. Verification of heat transfer and heat flow models which influence
VWCR design and predict energy surplus or deficits is not
complete; and i
c. Operation at steady-state autothermal conditions has not been
done. Such operation will help define long-term operational
variables and maintenance requirements. j
A comprehensive comparison with equivalent technologies awaits further
full-scale demonstration plant testing to verify :design, performance
and cost relationships. It is expected that data gained at the
demonstration plant at Longmont, Colorado, will jprovide such a com-
parison. !
51 l
-------
REFERENCES
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Preproposal to U.S. EPA, Office of Research and Development, "Evalua-
tion of the Vertical Tube Reactor Process for Wet Oxidation of Municipal
Sludge and Industrial Wastes at Longmont, Colorado," April 1981.
Monthly reports, Municipal Sludge Disposal by Vertical Tube Reactor
Process, Contract No. 68-03-2812, Cincinnati,. OH, July 1979 to present.
Roy F. Weston, Inc., "Montrose 201 Facility Plan," (Draft Final)
Montrose, CO, November 12, 1980.
Hurwitz, E., 6. H. Teletzke and W. B. Gitchel, "Wet Air Oxidation of
Sewage Sludge," Water and Sewage Works. Vol. 112, No. 8, August 1965
pp. 298-305. y
Sommers, L. E. and E. H. Curtis, "Wet Air Oxidation: Effect on :Sludqe
Composition," JWPCF, Vol. 49, No. 11, Nov. 1977, pp. 2219-2225.
Montrose, CO, 201 Draft VTR Alternative, VTR Corporation, Englewood,
CO, October 20, 1980. ,
Weber, Walter, J., Physicochemical Processes for Water Quality Control
John Wiley & Sons, New York, NY, 1972. '
Rich, L. 6., Unit Process of Sanitary Engineering, John Wiley & Sons,
Inc., New York, NY, 1963.
u's- EpA, Sludge Treatment and Disposal, Technology Transfer Publi-
cation Center for Environmental Research Information, Cincinnati, OH
45268, EPA-625/4-79-012, October 1978. ;
Teletzke, G. H., Wet Air Oxidation, Presented at the AIChE Symposium on
Developments in Industrial Aqueous Waste Disposal and Control, Houston
Texas, December 1963. ;
Eralp, A. E., unpublished notes on wet air oxidation, 1979.
u-s- EPA» Sludge Treatment and Disposal, U.S. EPA Technology Transfer
Process Design Manual, Center for Environmental Research Information,
Cincinnati, OH, 45268, EPA-625/1-79-011, September 1979.
Nielson, D. H. and Jacknam, A. L., "A Metallurgical Examination of VTR
Welded Tubing," Biomaterials Research Institute, Salt Lake City UT
January 1980. '
52
-------
14. Randall, T. L. and P. V. Knopp, "Detoxification of Specific Organic
Substances by Wet Oxidation," JWPCF, Vol. 52, No. 8!, August 1980, pp.
2117-2130. :
15. Amin, S., R. C. Reid and M. Model!, "Reforming and Decomposition of
Glucose in an Aqueous Phase," Intersoc. Conf. on Environmental Systems,
San Francisco, July 21," 1975.
16. Model!, M., R. C. Reid and S. Amin, "Gasification Process," U. S. Patent
4,113,446, September 129 1978.
17. Olexsey, R. A., Issue, Paper on Supercritical Fluids Processing for
Hazardous Waste, IERL, Cincinnati, OH, June 1980.
18. Vertical Tube Reactor System Computer Models, VTR Corporation, Engle-
wood, CO, May 12, 1980/
19. Letter from City of Montrose, CO, to U.S. EPA, Region VIII, February 27,
I i/oU
20. Interim Report, Municipal Sludge Disposal by Vertical Tube Reactor
Process, U.S. EPA Contract No. 68-03-2812, Cincinnati, OH, October
21.
22.
23.
24.
25.
26.
27.
Bastian, R. K., "Sewage and Animal Manures as a Source of Biomass,"
Prepared for Presentation at the BIO-ENERGY '80 World Congress and
Exposition, Atlanta, GA, April 21-24, 1980. j -
Letter from George Hartmann, Vertical Tube Reactor Corporation, June 4
1981. ;,
Personal conversations with George Hartmann, Vertical Tube Reactor
Corporation, June 1981.
Effects of Thermal Treatment of Sludge on Municipal Wastewater Treat -
ment uosts, EPA-600/2-78-0/3, Municipal Environmental Research Labora-
tory, Cincinnati, OH, June 1978. :
i
"Phase I Report of Technical Alternatives to Ocean Disposal of Sludge in
the New York City-New Jersey Metropolitan Area," Camp, Dresser and
McKee/Alexander Potter Associates, June 1975. ;
1 1 '!' '
Cullen, M. J., C. H. Burnett and J. A. Chamblee, "Total Domestic Waste-
water Costs Pegged at $22 Billion a Year," JWPCF, Vol. 53, No. 5, May
1981, pp. 522-529. ; ! ! *
Personal conversation with George Hartmann, Vertical Tube Reactor
Corporation, May 1981.
53
* U.S. GOVERNMENT PRINTING OFFICE: 1982-559-092/3372
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