United Statee
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Researched Development
EPA/600/S2-86/043 June 1986
&EPA Project Summary
Wet Oxidation of Municipal
Sludge by the Vertical Tube
Reactor
Jay L. McGrew, George L. Hartmann, Christina B. Cassetti, George E. Barnes,
and Atal E. Eralp
Evaluations were made of pilot-plant and
bench-scale vertical tube reactor (VTR)
systems for treating municipal wastewater
sludges. The VTR system is designed to
oxidize high-strength liquid organic wastes
using wet combustion principles. The
reactor vessel is a very long U-tube in
which the waste to be oxidized flows
down one leg (downcomer) and returns,
through the other (upcomer). Oil well drill-
ing techniques are used to install the reac-
tor in the ground. The downcomer pro-
vides for air injection to support combus-
tion, and the upcomer is surrounded by a
heat-exchange jacket that provides the
thermal energy necessary to initiate and
maintain temperatures appropriate for the
oxidation reactions.
A pilot-scale VTR system consisting of
a 1,500-ft by 2-in.-diameter reactor was
operated over a period of time using muni-
cipal sludge to accumulate engineering
data and operating experience. The infor-
mation obtained was compared with that
from an existing laboratory bench reactor.
Pilot-plant and bench-reactor data were
correlated over the range of conditions af-
forded by the pilot-scale system. These
correlations were sufficiently reliable to
allow use of the laboratory reactor for ob-
taining data over ranges of operating con-
ditions approximating full-scale.
The pilot-scale system achieved chem-
ical oxygen demand (COD) removals of up
to 50%, and the laboratory reactor (using
municipal sludge from several sources and
simulating deeper reactors with conse-
quently higher temperatures and pres-
sures) obtained COD removals of up to
80%.
At similar temperature and residence
time, the bench reactor results showed
reasonable correlations with pilot-plant
operating results and thus confirmed the
prediction that a full-scale VTR would also
remove 50% of municipal sludge COD.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report order-
ing information at back).
Introduction
The vertical tube reactor (VTR)*'"1" is
designed to treat and dispose of municipal
and industrial wastes by wet-air oxidation
(wet combustion). The products of this
process are easily dewatered ash and a
supernatant containing low-molecular-
weight organics.
The VTR process is especially applicable
to wastes that are difficult to dewater or
that have sufficiently high organic content
to maintain a thermally self-sustaining
(autogenous) reaction. Because oxidation
occurs in the presence of liquid, it is not
necessary to supply energy for the latent
heat of vaporization. Though most com-
bustion processes require dewatered
sludge to achieve thermal self-sufficiency,
considerably smaller concentrations of or-
ganic matter are adequate for wet
combustion.
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
-(U.S. Patent No. 4,272,383. Developed by the Vertical
Tube Reactor Corporation, a subsidiary of Applied
Science and Engineering, Inc., Englewood, Colorado.
The process is currently being marketed by Ver
Tech™ Treatment Systems, 12000 Pecos Street,
Denver, Colorado 80234.
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Oxygen must be added to the wet com-
bustion system in stoichiometric propor-
tions at a rate that will not impede com-
bustion. As the waste reacts with oxygen,
heat is produced. Municipal wastewater
sludge has a heat of combustion of ap-
proximately 3300 cal/gram of COD. So
typical untreated municipal sludge with a
COD of 10,000 mg/L has an overall
heating value of approximately 33 cal/
gram. For reference, fuel oil has a combus-
tion value of approximately 11,000 cal/
gram. Thus sludge must be processed at
very high overall thermodynamic efficiency
if an autogenous process is to be achieved.
With sufficient conditions of tempera-
ture and pressure, wet oxidation proceeds
until the organic removal rate decreases to
zero and the percentage of organics re-
moved remains constant. The organics re-
maining at this point are termed refractory
organics. At low temperatures of 212° to
400 °F, this removal plateau or equilibrium
is not reached for hours. Above 575 °F, it
is reached in a matter of minutes. The rate
of oxidation will increase up to the critical
point of water (705 °F). 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 occuring below 300 °F.
Temperatures of 660 °F or more are re-
quired for 80% COD removal.
Conventional wet combustion techno-
logy involves pumping waste and air into
a pressure tank and heating the mixture
to the desired reaction temperature. The
need for high-pressure tanks, pumps, and
compressors results in high capital cost
and expensive, energy-consuming opera-
tions. However, corrosion-resistant pres-
sure vessel design, high-pressure liquid
pump technology, and the state of devel-
opment of heat exchangers are such that
wet oxidation of municipal sludge in con-
ventional pressure tanks is technically
feasible. But maintenance and safety con-
siderations, capital and operating costs,
and the requisite high level of plant site
technological skill needed for continuous
operation are such that conventional pres-
sure tank wet oxidation is not currently an
economically attractive disposal method
for municipal sludge.
VTR Process
The VTR process was developed to eli-
minate the problems associated with con-
ventional wet oxidation while providing
reaction pressure, temperature, mixing,
and reactant retention time for autoge-
nous wet combustion. In the VTR, two
2
very long concentric tubes are suspended
in a conventionally cased deep well. The
waste liquid and air are injected into the
center tube (downcomer) at the earth's
surface at low pressure. As the waste
stream and air flow down the tube, the
mixture is heated by counter-flow heat
exchange with the upflowing oxidized
fluid. As the downflowing waste ap-
proaches the bottom of the tube system,
the temperature and pressure become suf-
ficiently high to initiate the oxidation reac-
tion. The tube lengths and diameters are
appropriately sized to allow an adequate
retention time in the high temperature/
pressure reaction zone to achieve a high
degree of reaction completion.
The oxidized water ultimately flows up
the annulus between the two tubes (up-
comer) and is cooled by the transfer of
heat to the downflowing fluid. As the fluid
ascends, the pressure also decreases as
the result of decreasing hydrostatic head.
Thus the pressurization and depressuriza-
tion of the fluid waste and heat exchange
are accomplished in a highly efficient,
nearly reversible thermodynamic process.
Oxygen needed for combustion is sup-
plied by the injection of air into the down-
comer at several depths typically ranging
from 150 to 800 ft, depending on influent
COD. When waste oxygen demand (COD)
exceeds air supply capability, the waste is
diluted with effluent or other low-COD-
strength wastes.
A heat-exchange jacket surrounding the
outer tube from a short distance below the
wellhead to the bottom of the U-tube cir-
culates a heat transfer fluid (heated when
necessary in a burner at the wellhead) to
preheat the reactor and to add or extract
heat as required to maintain appropriate
temperatures during reactor operation. In-
sulation to minimize heat losses to the sur-
rounding rock complete the basic VTR
design. Schematic views of a typical full-
scale reactor are shown in Figures 1 and 2.
In actual operation, as the waste stream
and air flow down the tube, they undergo
natural pressurization due to the hydro-
static head above. Thus fluid pumps need
to be designed primarily to overcome sur-
face friction and pressure head at their in-
fluent or injection point. They do not need
to develop the high pressure actually ex-
perienced at the bottom of the reactor. At
some depth (typically 1,500 to 2,000 ft),
temperature of the waste fluid increases
to 350 °F because of heat transfer from
the upcomer effluent to the downcomer
influent, and wet combustion effectively
begins. As the fluid flows through the
reactor, oxidation proceeds until either the
organic material or the dissolved oxygen
are depleted, or until hydrostatic pressure
and temperature decrease below those
levels necessary to support combustion.
Upflowing oxidized waste is gradually
cooled as it transfers heat to the down-
flowing fresh waste.
A temperature profile for a typical reac-
tor as a function of retention time is shown
in Figure 3. Any excess heat which may
result from the exothermic oxidation reac-
tions is removed from the reaction zone by
the heat exchange jacket. Excess heat can
be available for use at the ground surface.
Effluent temperature is expected to be
within 5 °F of the influent temperature.
The VTR has no moving parts below the
ground surface and needs no high-
pressure equipment above ground like that
required for conventional wet oxidation
methods.
The concentric U-tube arrangements of
the VTR affords very high heat recovery
from the effluent to the influent waste
stream. The entire reactor is, in effect, a
counterflow heat exchanger, thus greatly
reducing heat losses normally encount-
ered in wet oxidation systems. In addition,
as the surrounding earth approaches equi-
librium with a continuously operating VTR,
operations will be less affected by waste
quality changes or above-ground (climate)
influences because the surrounding earth
will act as a thermal buffer.
The VTR requires little real estate. At the
same time, its vertical configuration and
its compactness make downhole acces-
sibility difficult should temperature- and
pressure-measuring devices or other well
components need to be unplugged, in-
spected, or replaced. Thus mechanical
reliability and maintenance of the VTR
system are of critical importance.
Pilot-Plant Reactor Tests
The main objective of the pilot-plant re-
actor (PPR) study was to operate the
system on a continuous basis over an ex-
tended period to monitor, measure, ana-
lyze, and otherwise evaluate all significant
operating parameters and reaction infor-
mation during operation.
The reactor of this project consisted of
a 1.75-in.-diameter downcomer in a 2-in.-
diameter upcomer. The reactor, air line,
heat exchanger lines, instrumentation, and
insulation were all suspended in a 5-in.-
diameter well casing. The PPR extended
to a depth of 1,000 ft below the ground
surface in a 9,000-ft-deep abandoned oil
well located 25 miles east of Denver,
Colorado.
The PPR was large enough to provide
essential concept verification and engine-
ering data. Designed to process up to 8
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L/min of municipal sludge with a COD of
2,000 mg/L, the PPR would operate at a
maximum downhole temperature of
500 °C at 1000 ft, with a retention time
of 20 to 30 min to accomplish up to 50%
COD removal.
To achieve the downhole pressure of
680 psi required by the increased tem-
perature of 500 °F, air and sludge inlet
pressures to the reactor were increased to
400 psi, necessitating the installation of
a larger compressor. These modifications
in effect simulated the reaction conditions
of a 500 °F reactor 3,000 ft long — con-
ditions approximately those expected with
a full-size reactor. The COD reductions in
this test series fell within the expected
range of 40% to 55%. The results of PPR
. experiments are summarized in Table 1.
The PPR was operational for four test
periods between July'1979 and March
1981. Although a considerable amount of
Air
Heat Exchanger Line (On) f—
Heat Exchanger line (Out)
time was spent in identifying and remedy-
ing physical problems, enough data were
collected to evaluate the process and to
demonstrate the correlation between
bench-scale reactor and PPR experiments.
Detailed accounts of the problems en-
countered and their solutions in reactor
design and process control are given in the
final report.
Laboratory-Bench-Reactor Tests
The laboratory-bench-reactor (LBR) ves-
sel was fabricated from stainless steel pipe
3.3 ft long with an inside diameter of 2.0
in. Through one end of an otherwise seal-
ed vessel was a 1/4-in, stainless steel tube
for monitoring pressure and providing en-
try and exit for sample and air. Electrical
resistance rods and cooling tubes were ex-
ternally attached to the vessel. To control
heating and cooling rates, the electrical
resistance rods were powered by a motor-
Reactor Influent (Downcomer)
33°F - 100°F
Reactor Effluent (Upcomer)
5°F to 20° F Above Influent
Start of
Reaction Zone
350°F
Heat Exchanger Oil
Bottom of Reactor
Temperature Varies
Reactor Casing
(Pressure Vessel)
Well Casing
Cement Grout
Surrounding Rock ^
Not to Scale
Figure 1. Typical VTR profile.
driven, variable-voltage transformer. Room
temperature tap water flowed through the
cooling tubes and was shut off or metered
by a needle valve to control cool-down
rates. Thermocouples to monitor temper-
ature were welded on the vessel's outer
surface at several locations. The reactor
vessel and attached thermocouples, elec-
trical resistance rods, and cooling tubes
were insulated. This arrangement was
mounted to permit 180 ° rotation in a ver-
tical plane at 7.5 sec/oscillation to provide
internal mixing of sample and air.
The objectives for LBR testing were to
establish the oxidation characteristics of
sludges from different sources and to sim-
ulate as closely as possible the pilot-scale
VTR conditions with the LBR to obtain
treatability data that predicts the COD re-
moval rates expected in a full-scale VTR.
Five types of sludges identified as digested
sludge, blended sludge, primary sludge, pri-
mary sludge with food processing waste,
and domestic waste were used in LBR ex-
periments. The sources and character-
istics of these sludges are detailed in the
main report.
Table 2 summarizes the COD reduction
data for LBR runs by sludge type. In gen-
eral, as the reaction temperature increases,
the COD reduction increases. Increasing
reaction time from 30 to 60 min also re-
sults in greater COD removals. Typically,
larger differences in COD removals occur
between 60- and 30-min LBR runs at
400°F than at 500° or 600 °F. At 650°F,
the COD removal between 60- and 30-min
reaction times is virtually the same. This
effect is probably due to the production
of low- to medium-molecular-weight com-
pounds that require elevated temperatures
rather than increased reaction times to
undergo further oxidation.
Correlation Between LBR and PPR
COD Reduction Data
The COD reductions obtained from LBR
and PPR runs are compared in Table 1 cor-
responding to reaction for similar times
and temperatures. Data from PPR runs in
October 1979 are low compared with
those for 15- and 20-min LBR runs per-
formed at 445 °F. The remaining COD
reductions for PPR runs fall within the
standard deviation of COD reductions
established by multiple LBR runs at the
temperatures and times listed in Table 1.
These results support the use of LBR-
generated treatability studies to model
expected COD reduction rates.
Though COD reductions approach 50%
in the present PPR experiments, as much
as 80% COD reduction is expected for a
full-scale VTR where higher temperatures
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•Stainless Steel Upcomer
Stainless Steel Downcomer
Rock ..
High Temperature Ceramic Insulation
Figure 2. Typical 1/77? cross section.
Cement Grout
, Steel Well Casing
Steel Reactor Casing
"'' (Pressure Vessel)
''X""'" ~._S?a//7/ess Steel Air Line
— Heat Exchanger Oil
Steel Heat Exchanger Lines
700.
600 .
500 .
400 .
| 300 J
1
§ 200 .
100 .
• Downcomer •
Upcomer
Reactor Bottom
Reaction Time
Reactor Entrance
Reactor Exit
i i i i r i i [ i i
70 15 20 25 30 35 40 45 50 55
Time, Minutes
Figure 3. Effect of retention time on temperature.
4
and pressures can be attained and where
the reaction time would be longer than 30
min because of the longer VTR.
Conclusions
The pilot-scale system achieved COD re-
movals of up to 50%, and the laboratory
reactor tests using municipal sludge from
several sources and simulating deeper
reactors with consequently higher temper-
atures and pressures obtained COD re-
movals of up to 80%.
At similar temperature and residence
time, the bench reactor results showed
reasonable correlations with pilot-plant
operating results, confirming the predic-
tion that a full-scale VTR would also
remove 80% COD of municipal sludge
COD.
The pilot-scale test program provided in-
formation on structural, mechanical, and
other operational problems important to
the design of full-scale systems.
In summary, the VTR was shown to be
a potentially successful method for stabi-
lizing organic wastes when significant
sludge volume reduction is required, where
stringent requirements for sludge disposal
exist, when destruction of toxics or path-
ogenic organisms is necessary, and/or
where potential energy recovery from
high-strength wastes is good.
The full report was submitted in fulfill-
ment of Contract No. 68-03-2812 by Ap-
plied Science and Engineering, Inc., under
the sponsorship of the U.S. Environmen-
tal Protection Agency.
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Table 1. Correlation of Data from Pilot- and Bench-Scale Reactors
Pilot-Plant Reactor Data
Laboratory-Bench Reactor Data
Sampling
Period
October 1979
July 23, 1980
July 24-25, 1980
November 1980
December 1980
March 1981
Reaction
Temperature
(°F)
435
440
440
420
433
510
Reaction
Time
(min)
17.5
27
30
10.6
14
28
COD
Reduction
(%)
11.0±6.8%
38.1%
29.7±5.5%
24.9±8.9%
22.0 + 9.4%
42.6±9.4%
Reaction
Temperature
(°F>
445
445
445
445
445
445
500
Reaction
Time
(min)
15
20
30
30
15
15 •
30
COD
Reduction
(%l
21.4±2.0%
24.6 + 6.3%
35.9 ±14.0%
^35.9 ±14.0%
21.4 + 2.0%
21.4±2.0%
47. 2± 10.1%
Table 2. Total % COD Reduction at Various Reaction Times and Temperatures
30-min Reaction Time
60-min Reaction Time
Item
400° F
500° F
600° F
650° F
400° F
500° F
600° F
650° F
Type of Sludge:
Digested
Blended
Primary
Primary W/Food Processing
Waste
Domestic Waste
Average & Standard Deviation
13%
27%
9%
54%
52%
50%
21% 20%
30% 44%
20 + 9% 44±14%
75%
75%
63%
75%
50%
68±11%
82%
8O%
74%
86%
79 ±5%
35% 63% 79% 83%
37% 65% 78% 81%
- 53% 69% 78%
15% - 61% 80%
16% 55% 7O% 77%
26+12% 59±6% 71 ±7% 80±2%
U. S. GOVERNMENT PRINTING OFFICE: 1986/646-116/20855
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JayL. McGrew, George L Hartmann, Christina B. Cassetti, and George E. Barnes
are with Applied Science and Engineering, Inc., Englewood, CO 80110; the EPA
author AtalE. Eralp (also the EPA Project Officer, see below) is with the Water
Engineering Research Laboratory, Cincinnati, OH 45268.
The complete report, entitled "Wet Oxidation of Municipal Sludge by the Vertical
Tube Reactor," (Order No. PB86-183 936/A S; Cost: $16.95, subject to change)
will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, MA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-86/043
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