United Statee
                     Environmental Protection
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
                     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
                       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).

  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
 "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.

  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
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
  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,
  The PPR was large enough  to  provide
essential concept verification and engine-
ering data. Designed to process up to 8

 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
           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)
            33F - 100F
            Reactor Effluent (Upcomer)
                                                      5F to 20 F Above Influent
         Start of
      Reaction Zone

  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
400F than at 500 or 600 F. At 650F,
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

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

600 .

500 .

400 .
|    300  J


    200  .
     100 .
                                          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.

                                                                              and pressures can be attained and where
                                                                              the reaction time would be longer than 30
                                                                              min because of the longer VTR.

                                                                                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
                                                                                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.

Table 1.    Correlation of Data from Pilot- and Bench-Scale Reactors
                                     Pilot-Plant Reactor Data
                                            Laboratory-Bench Reactor Data

October 1979
July 23, 1980
July 24-25, 1980
November 1980
December 1980
March 1981
22.0 + 9.4%
24.6 + 6.3%
35.9 14.0%
^35.9 14.0%
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
                               400 F
                                          500 F
                                                     600 F
                                                                 650 F
                                                                                 400 F
                                                                                             500 F
                                                                                                        600 F
                                                                         650 F
 Type of Sludge:
  Primary W/Food Processing
  Domestic Waste
 Average & Standard Deviation
21%       20%
30%       44%
20 + 9%    4414%


                       79 5%
35%       63%        79%        83%
37%       65%        78%        81%
 -         53%        69%        78%

15%        -         61%        80%
16%       55%        7O%        77%
26+12%   596%     71 7%    802%
                                                                                      U. S. GOVERNMENT PRINTING OFFICE: 1986/646-116/20855



    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
Center for Environmental Research
Cincinnati OH 45268
   PERMIT No. G-35
Official Business
Penalty for Private Use $300