United States
                     Environmental Protection
                     Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
                     Research and Development
EPA/600/S2-87/074 Jan. 1988
&EPA          Project Summary
                    Evaluation of the Two-Zone
                    Wastewater Treatment  Process  at
                    Norristown,  Pennsylvania

                    Stephen R. Weech, Vernon T. Stack, and Graham Orton
                      A comprehensive  demonstration
                     study  evaluated a novel biological
                     wastewater treatment concept called
                     the Two-Zone  process*. Two-Zone
                     utilizes a combination reactor/clarifier
                     to incorporate biological treatment and
                     liquid/solids separation in a single tank.
                     The lower segment of the tank serves
                     as the  biological reactor  zone, and the
                     upper segment is used for clarification.
                     Oxygen requirements are satisfied by
                     injecting high-purity oxygen gas into a
                     recirculating stream of mixed liquor that
                     passes through a below-ground oxygen
                     dissolving device and then back into
                     the  reactor section of the treatment
                     unit.
                      Total secondary system volume  re-
                     quirements for biological reaction plus
                     secondary clarification are 40% to 50%
                     lower with the Two-Zone process than
                     with conventional activated sludge. This
                     makes  Two-Zone a good  candidate for
                     upgrading the capacity of existing treat-
                     ment facilities. Either aeration tanks,
                     secondary clarifiers (preferably rectan-
                     gular),  or both can be  retrofitted as
                     single-tank reactor/clarifiers in a given
                     plant.
                      This  Prelect 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
                     ordering Information at back).

                     Introduction
                      Economic and practical considerations,
                     such as limited land area  for siting new
                     * Mention of trade names or commercial products
                      does not constitute endorsement or recommenda-
                      tion for use.
or upgraded treatment facilities, encour-
age investigation of alternatives to con-
ventional strategies for wastewater
treatment. One alternative is the Two-
Zone process  developed by Canadian
Liquid Air, Ltd. (CLA) of Montreal, Quebec.
The Two-Zone process is a novel activated
sludge  process that combines aerobic
biologicJl reactor and secondary clarifier
functions in one tank for retrofit into
existing plant tankage to increase capacity.
A key feature of the Two-Zone process
that allows integration of the two func-
tions into a single tank is the oxygenation
of the recycled biomass with pure oxygen
in an external transfer device. Within the
tank, oxygenated recycle sludge is blended
with influent wastewater and  passes
upward through the sludge blanket into a
clarification zone prior to displacement
as effluent. A collector mechanism en-
sures the  movement of heavy solids
across the tank floor and provides scum
removal at the surface of the clarification
zone.
  A 21.9-L/sec (0.5-MGD) demonstration
of the Two-Zone process was carried out
at the Borough of Norristown, Pennsyl-
vania. The project originated from a
mutual  desire of CLA and U.S. EPA  to
demonstrate the Two-Zone process at a
site in the United States. CLA had con-
ducted extensive research and develop-
ment on the process in Canada and
demonstrated its feasibility.  EPA  was
interested in the process because of its
potentially optimum use of basin geometry
and space and its possible retrofit into
existing tankage to upgrade wastewater
treatment facilities.
  The project objectives were to:
  • demonstrate the capabilities of the
    Two-Zone process to treat municipal

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    wastewater  in facilities developed
    through modification (retrofitting) of
    a portion  of an existing  aeration
    basin,  a modification that accom-
    plishes secondary treatment in a
    smaller tank volume than required
    by conventional biological processes,
    and
 • establish an improved understanding
    of the performance capabilities and
    stability characteristics of  the Two-
    Zone process over a range of loading
    conditions.

System Description
  As  illustrated  in the flow diagram of
Figure 1, the Two-Zone  process is rela-
tively simple  and straightforward.  It
basically consists  of a  reactor/clarifier
with sludge recycle through an oxygena-
tion unit where pure oxygen (either liquid
oxygen stored  and vaporized at the plant
or on-site generated gaseous oxygen) is
added. The recycled sludge must receive
and transport  all of the oxygen required
by the process. The required recycle flow
rate is typically three  to six times the
forward influent flow. The major compo-
nents are:
  • a  baffled inlet chamber section in
     which the influent flow and recycle
     sludge are blended;
  • a reactor/clarifier section equipped
     with a sludge scraper to help main-
     tain solids in suspension in the re-
     actor zone, ensure delivery of any
     heavier solids to the discharge side
     of the reactor/clarifier,  and remove
     scum from the surface of the clarifi-
     cation zone;
  • a pickup header to remove the sludge
     from the reactor clarifier;
  • a sludge recirculation pump;
  • a  Dorr-Oliver below-ground oxygen
     transfer unit (oxygenator) and associ-
     ated dissolved oxygen (DO) control
     equipment;
  • distribution  headers in  the inlet
     chamber to blend recycle sludge with
     influent wastewater and  distribute
     flow into the reactor/clarifier;
  • an overflow weir; and,
  • a skimmer and scum disposal pump
  At Norristown, a 9.1 -m by 9.1 -m (30-f
by 30-ft) segment of an existing aeratior
tank was  isolated  for conversion to th<
Two-Zone demonstration system. Critica
dimensions within the  reactor/clarifiei
tank are shown in Figure 2.
  Good distribution across the width  o
the tank required distribution headers foi
the incoming primary effluent flow anc
the recycled sludge flow. To dissipate
kinetic energy from  the orifices  in the
recycle  sludge  distribution header, the
orifices were aimed upward into the inlei
chamber.  At the  bottom  of  the inlei
chamber,  the cross-sectional  area was
increased to provide slower velocities anc
ensure the release and escape of gases
The opening at the bottom of the baffle
wall served as a distribution orifice across
the width of the tank for introduction  o1
blended flow into the reactor/clarifier
The orifice velocity was about  9 cm/sec
(3.5 in./sec) at maximum flow.
  A skimmer pipe was located atop the
baffle wall to remove accumulated scum
                                                                                           DO Analyser
                                DO Controller

                      Excess Sludge
                                    Recycle Line from Oxygenator
         Influent

        Scum Wasting
                                                                                                       DO Probe
                                                                           Rotometer
                                                                      Oxygen
                                                                      Supply
ry
ell
\
fr >
Oxy
I*
gen
Control
Valve

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                                                                                                              Wet
                                                                                                              Well
                                                                                                    Oxygenator
 Figure 1.    Flow diagram of Norristown Two-Zone system

                                     2

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Top of Wall



Force Wall
i



Influent
Distribution
Header — ^
i

5'-6"


1
6" \
~ t

Recycle
Sludge
Distribution
Header
(Concrete
Encased)
, X

Inlet
Chamber Effluent
i Surface Baffle Troughs
1 f~~*)f Skimmer Pipe x / \^

/ 1


r-6"

py ~* • t
•* — Rubber Seal
Clarification Zone
-« — Baffle Wall
— ft
m 11
r-0"
Reaction Zone _, _„
4 \

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3'-11" 2'-4"
10" 10"


Recycle Sludge
Suction Header
(Concrete Encased}

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t ' '


30'-0"
* It *
Plan View Overall Dimensions were 30' -0" x 30' x 0"


Note: 1 ft = 0.305 m
Figure 2.   Longitudinal section view of Norristown Two-Zone reactor/clanfier.
Flow across the top of the baffle wall into
the clarification zone was prevented by a
rubber seal connecting the top of the wall
to the skimmer pipe.
  The capacity selected for  the sludge
recycle  pump was related  to system
hydraulics. The controlling hydraulic pa-
rameter was the discharge rate from the
inlet chamber into the reactor/clarifier.
Based on its experience, CLA established
the maximum flow  rate  through the
opening at the bottom of the baffle wall
as 1.24 mVmin/m (100 gal/min/ft)  of
tank width. Thus,  the total forward flow
into the  reactor/clarifier (influent flow
plus recycle sludge flow) was limited  to
11.3 mVmin  (3,000  gal/min). Sludge
recycle pump maximum capacity, there-
fore, was nominally set at  11.3 mVmin
(3,000 gal/min).
  Influent flow limitations were based
primarily on overflow rate  limits for the
clarification zone.  The  average influent
flow rate was nominally selected at 1.31
mVmin (0.5 MGD), which corresponds
to a surface overflow rate  of 23 m3/
day/m2 (555 gal/day/ft2). The maximum
influent flow rate was nominally selected
at 2.27 mVmin (0.86 MGD) based on a
minimum 4 to 1 ratio of recycled sludge
flow to influent flow, which would cor-
respond to an  overflow  rate of  39
mVday/m2 (960 gal/day/ft2).
  The  oxygen transfer capacity of the
oxygenator was selected on the basis of
the maximum daily total BOD5 (TBOD)
concentration reported for the Norristown
raw wastewater (385 mg/L). At a  maxi-
mum diurnal peak flow of 37.7 L/sec
(0.86 MGD) and  an  assumed  oxygen
consumption rate  of  0.8  kg/kg  TBOD
applied, the desired maximum oxygena-
tion  rate was set  at  45.5 kg/hr (100
Ib/hr). The  minimum oxygenation rate
was  estimated at 9.5 kg/hr (21 Ib/hr),
corresponding to a minimum influent flow
rate of 13.1  L/sec (0.3 MGD), an assumed
influent TBOD concentration of 150 mg/L,
and an oxygen consumption rate of 1.35
kg/kg TBOD applied.
  Process instrumentation provided read-
outs and recordings of wastewater flow,
sludge recycle flow, and DO concentra-
tions in the recycled sludge before and
after oxygenation. The DO concentration
before reoxygenation was used to modu-
late the oxygen feed rate to the recycled
sludge. The influent flow to the process
was adjusted using a weir that provided a
desired  flow split from the  Norristown
primary clarifiers. Thus, the feed to the
process had approximately the same vari-
ability and  diurnal pattern  as the flow
through the Norristown main plant.

Evaluation Program
  Not all of the  originally planned test
conditions  could  be  evaluated on this
project because  of various equipment,
main plant, and process difficulties that
required time and funds to correct. The
evaluation  program, therefore, was
fashioned primarily by the interruptions
that occurred. For the period of February
16 through December 15, 1982, six ex-

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perimental run periods have been defined.
The general scope and test conditions for
these six runs are summarized in Table 1.

 Process Performance

 Reactor/Clarlfler
   Run-averaged operating and perform-
 ance data for the six experimental runs
 are  summarized  in Tables  2 and 3,
 respectively.  Overall performance was
 excellent at wastewater detention times
 varying from  3.3 to 7.0 hr. The only
 effluent TBOD or total  suspended solids
 (TSS) average value over 30  mg/L was
 TSS for Run No. 2 (the highest loaded
 test phase) at 34 mg/L. The wastewater
 detention times given in Table 2 are based
 on the total reactor/clarif ier volume (344
 m3 = 12,150 ft3) and system influent flow
 (excluding sludge recycle). The relative
 proportions of detection time in the re-
 actor and clarifier zones varied from run
 to run and can be estimated by dividing
 the sludge blanket depth by 4.0 m (13.5
 ft), the tank sidewater depth (SWD).
   Additional performance data based on
 more  extensive  test runs at  lower
 wastewater temperatures (10 to  15°C),
 longer  sludge  retention times (SRT's),
 and lower food-to-microorganism (F/M)
 loadings  would  have  been  desirable.
 Nevertheless, it is believed that the in-
 formation obtained is  representative  of
 Two-Zone process capabilities for opera-
 tion in the 1 - to 3-day SRT range. Firm
 projections of Two-Zone  performance
 under lighter load conditions (i.e., longer
 SRT's) than experienced in this demon-
 stration cannot be made without more
 data.

 Oxygen Transfer Device
   Liquid oxygen was trucked in and stored
 on-site to feed pure gaseous oxygen to
 the sludge recycle stream. A control valve
 modulated the oxygen feed rate based on
 a selected DO level in the sludge flow
 leaving the reactor. The oxygen transfer
 device was a Dorr-Oliver oxygenator. The
 main transfer chamber  of the oxygenation
 unit was located  in a  pit 15 m (50 ft)
 below the surface level of the Two-Zone
 reactor where  the static pressure  aided
 oxygen transfer.  The   oxygen  transfer
 objective for the unit was 90%. A limited
 examination of the oxygen transfer char-
 acteristics of the oxygenator indicated
 that the 90% objective  was feasible.
   A characteristic  of the  Dorr-Oliver
 oxygenator, and other devices that may
 generate supersaturation quantities  of
 oxygen with respect to  atmospheric pres-
 sure, is that the excess oxygen tends to
Tabtol.
Test Conditions for Demonstration Runs
Run
No.
1
2
3
4
5
6
Dates (1982)
3/2
3/31
7/1
8/11
10/14
11/10
- 3/30
- 4/16
- 7/9
- 9/25
- 11/9
- 12/7
Days of
Data
22
12
8
26
23
21
Average
Influent
Flow Rate*
22.3 (0.51)
28.9 (0.66)
13.6(0.31)
15.8 (0.36)
19.7(0.45)
21.9(0.50)
Average
Sludge
Recycle Rate*
119.2(2.72)
118.3(2.70)
82.8 (1.89)
120.0(2.74)
161.2 (3.68)
131.0(2.99)
Average
Wastewater
Temp. (°C)
12.9
13.7
21.6
24.5
20.7
18.5
' L/seC (MOD)
Table 2.    Operating Data Summary for Demonstration Runs
                                                  Run No.
Parameter
Reactor Zone MLSS. mg/L*
Reactor Zone MLVSS. mg/L*
Sludge Blanket Depth, m
ft
Clarifier Overflow Rate,
rrf/day/m2
gal/day/ft2
Clarifier Solids Loading,
kg/day/m2
Ib/day/ft2
Sludge Volume Index, mL/g
Initial Settling Velocity, m/hr
ft/hr
F/M Loading, kg TBOD/day/kg
MLVSS
Wastewater Detention Time, hr
Volumetric Organic Loading, **
kg TBOD/day/m3
Ib TBOD/day/IOOO ft3
SRT, days
Net Sludge Wastage
kg TSS/kg TBOD removed^
1
3135
2460
2.3
7.6

24.6
604

488
100
58
—
—

0.64
4.3

1.57
98
2.1
0.95

2
3418
2687
2.3
7.7

32.1
789

557
114
92
—
—

0.77
3.3

2.16
135
1.5
1.07

3
3641
2742
0.7
2.4

15.0
369

381
78
57
9.8
32

0.41
7.0

1.63
102
1.5
2.01

4
5366
4215
1.4
4.7

17.6
431

806
t65
58
4.9
16

0.33
6.0

1.35
84
3.7
1.27

5
4222
3454
2.0
6.6

22.0
539

840
172
51
5.2
17

0.40
4.8

1.39
87
2.6
1.26

6
3112
2334
1.7
5.6

24.2
594

527
108
54
6.4
21

0.95
4.4

2.13
133
1.2
1.37

 * Based on calculated values.
** Calculated on basis of reactor zone volume as determined by sludge blanket depth.
 t Excludes effluent TSS.
Tab/e 3.    Performance Data Summary for Demonstration Runs
                                                  Run No.
Parameter
Influent TBOD (mg/L)
Effluent TBOD (mg/L)
TBOD Removed (percent)
Influent SBOD* (mg/L)
EffluentS80D(mg/L)
SBODS Removed (percent)
Influent TCOD** (mg/L)
Effluent TCOD (mg/L)
TCOD Removed (percent)
Influent TSS (mg/L)
Effluent TSS (mg/L)
TSS Removed (percent)
Effluent DO (mg/L)
1
159
21
87
50
5
90
327
69
79
198
23
88
3.8
2
169
28
83
41
4
90
333
78
77
223
34
85
3.7
3
85
15
82
21
3
86
285
48
83
17O
18
89
4.6
4
117
9
84
47
2
96
318
42
87
151
11
93
4.9
5
136
19
86
47
2
96
321
56
83
143
21
85
3.2
6
161
24
85
43
5
88
394
69
82
199
24
88
3.3
   Soluble BODs.
   Total chemical oxygen demand.

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come out of solution when the pressure
is reduced. Release of supersaturated
oxygen can be rapid if the stream being
oxygenated contains solids to serve as
sites for nucleation.  Facilities  using
devices  that produce  supersaturation
should, therefore, be designed to maintain
the  highest possible  pressure on  the
oxygenated stream as it is returned to the
process  tank.  Specifically, the stream
should be returned at or below the bottom
level of the reactor.
  Return of the sludge  recycle stream to
the bottom of the reactor at Norristown
would have required cutting through two
aeration  basin walls and was not accept-
able to the Borough staff. Therefore, it
was necessary to bring the sludge recir-
culation  line ever the  end wall of the
reactor,  resulting in release  of oxygen
from solution. Part of the released oxygen
was vented to the atmosphere through a
valve on  top of the horizontal run of pipe
from the  oxygenator to the reactor. Addi-
tional released  oxygen  escaped from
behind the force wall before the blend of
recycled  sludge and wastewater was in-
troduced to the reactor.
  A series of tests evaluated how much
of the oxygen transferred to the recycle
sludge by  the oxygenator was actually
reaching the reactor zone. DO concentra-
tions were measured  at the discharge
from the  oxygenation device and the inlet
to the reactor zone.
  Oxygen  transfer  efficiencies for  the
range of  sludge  recycle flow  rates
examined  are plotted  against the con-
centration  of oxygen fed to the sludge
recycle stream from the cryogenic supply
in Figure 3. Although approximately 85%
oxygen transfer was accomplished by the
oxygenator at  recycle stream oxygen
concentrations in the range of 20 to 50
mg/L, overall process oxygen  transfer
dropped  to 68% to 80% over the same
range. The difference  was  due to  ef-
fervescence of supersaturated oxygen
through  the vent valve and behind the
force wall  as depicted by the curve of
triangular dots  in  Figure 3. At oxygen
doses where  supersaturation  did  not
occur, no  loss  was  incurred  due to
effervescence.
  The measurement of oxygenator outlet
DO was made near the  horizontal sludge
recirculation piping, i.e., about 5.5 m (18
ft) above the bottom  of the  Two-Zone
reactor/clarifier. At this elevation, some
of the oxygen transferred by the oxygena-
tor had already been released from solu-
tion. If the oxygen in solution had been
measured at an  elevation equivalent to
the bottom of the reactor/clarifier, it is
believed  the transfer efficiency of  the
oxygen transfer device alone would have
been nearer the 90% design  level than
the 85% value measured.

General Design
Recommendations
  The  Two-Zone  process was initially
targeted  for retrofit into existing treat-
ment  plants as a means of  increasing
hydraulic loading capabilities while main-
taining secondary treatment  standards.
Another promising area is as a first-stage
treatment system. In this application, the
Two-Zone process could  be  retrofitted
       wo
                                         into an existing treatment plant to reduce
                                         carbonaceous loading to a subsequent
                                         treatment system. First-stage applications
                                         could  be  used either to reduce organic
                                         loading on an existing single-stage pro-
                                         cess to improve subpar performance or to
                                         allow the existing process to meet more
                                         stringent effluent criteria, such as newly
                                         mandated nitrification standards.  As a
                                         first-stage treatment unit, Two-Zone could
                                         be retrofitted  into existing plants or in-
                                         stalled in new plants as a component of a
                                         two-stage system.
                                           When evaluating the potential use of
                                         the Two-Zone  process,  the  designer
                                         should consider the  following  general
       90
       80

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        70
        60
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y
— O Oxygen Transfer by the Oxygenator
— 4 Oxygenator Effluent Oxygen that
Reached the Process
— X Overall Transfer of Oxygen to the Process
I III
       40 	

          0       10      20      30     40       50      60

                    Oxygen Concentration in Recycle Stream /mg/L)


Figure 3.    Oxygen transfer performance.

                                         5
                                                                   70      80

-------
application guidelines prior to selection
of the process:

  • Unless Two-Zone is to be used in a
    first-stage application, flow equaliza-
    tion should be provided whenever
    the peak-to-average hydraulic load-
    ing ratio, including plant recycle flow
    streams, exceeds 2.4. Load equaliza-
    tion should also be provided when-
    ever  the peak-to-average  carbon-
    aceous  loading ratio, including
    recycle loads, exceeds 2.5.
  • The Two-Zone process should  be
    preceded by primary treatment  to
    ensure that heavy solids do not enter
    the Two-Zone reactor and foul the
    oxygenator. Influent flow to a Two-
    Zone  system  should be  passed
    through  a fine screening device for
    the same reason.
  • Strong wastewaters with an average
    process influent TBOD level in excess
    of 200 mg/L and situations requiring
    nitrification within the Two-Zone
    process  should  be avoided. Actual
    oxygen dosages required  to meet
    oxygen demand in these situations
    may result in  undesirable flotation
    of the biomass within the  reactor.
    Due to the associated high oxygen
    demand,  nitrification  within the
    Two-Zone process should be avoided
    whenever possible.
  • Provisions should be made to chlo-
    rinate or otherwise  sterilize the
    waste sludges  from  the Two-Zone
    reactor in the  event of a Nocardial
    bloom. Two-Zone  demonstrated  a
    propensity for concentrating these
    organisms in the scum waste stream.
    In some situations, these organisms
    could disrupt other plant operations.
  • If a Dorr-Oliver oxygenator  is used
    with future Two-Zone installations,
    provisions should be  made to back-
    flush the oxygenator. Design require-
    ments for backflushing the oxygen-
    ators should be obtained from the
    manufacturer.  Due to the  high
    volumes  of flow required for back-
    flush, these discharges should be
    hydraulically equalized prior to rein-
    troduction to the treatment plant.
  • If waste sludges are to be aerobically
    stabilized prior to disposal,  the de-
    signer should  confirm  that the
    oxygenation system for the  aerobic
    digestion process  has sufficient
    capacity  to cope with the higher
    oxygen demands of  the Two-Zone
    sludge. This recommendation also
    applies to  aerated sludge  holding
    tanks and  is a direct result of the
     relatively  small amount of en-
     dogenous respiration that occurs in
     the Two-Zone process.
  •  The treatment plant should be staffed
     with technically qualified personnel.
     The Two-Zone process  requires
     regular monitoring by personnel with
     a good technical background in bio-
     logical treatment, physics,  and a
     fundamental knowledge of the oper-
     ation of pure oxygen systems. The
     owner must also be able to provide a
     well-staffed, on-site laboratory for
     process monitoring.

Specific Design
Recommendations

General Sizing Criteria
  Hydraulic loading is the primary variable
controlling the size of the Two-Zone pro-
cess tankage. Minimum surface area
requirements should be based on limiting
the average surface overflow rate to 20.4
m3/  day/m2 (500 gal/day/ft2} or to 48.9
mVday/m2 (1,200 gal/day/ft2) at peak
flow, whichever provides the greater sur-
face  area. Total influent flow, including
anticipated plant recycle streams, should
be used to establish the minimum surface
area  required.
  Based on the Norristown experience,
deeper liquid SWD's will improve oxygen
transfer in the system. The minimum
SWD should not be  less than the 4.1  m
(13.25 ft) used for the Norristown demon-
stration. Due to structural considerations,
it would be anticipated that the practical
limitation on SWD may be about 4.6 m
(15 ft). Only about 0.3 m (1 ft) of freeboard
is  required above the working SWD  of
the tank at peak flow.
  Empirically, the practical  limitation  of
the sludge bed depth, which establishes
the maximum reaction volume, is about
53% of the total SWD for  applications
without flow  equalization.  With flow
equalization,  the practical  bed  depth
limitation might possibly be increased to
70% of the total SWD. These limitations
are  a direct consequence of  observed
expansion and contraction of the sludge
release zone due to changes in influent
flow and oxygen feed rates.

Tank Geometry
  Rectangular  or  square  shapes are
preferred for the process tankage. Circular
tank shapes do not appear to be as well
suited for retrofit of the Two-Zone process.
The actual length and width of the tankage
is controlled in part by system hydraulics.
Specifically, the width of the inlet end of
the tank must be sized to maintain the
total  forward flow  velocity,  including (
recycle sludge flow, at less than 1.24
mVmin/m (100 gal/ min/linear ft).  A
second factor that controls tank width is
the practical  width of available scraper
mechanisms. Generally, standard widths
for rectangular scraper  mechanisms do
not exceed 4.6 m (15 ft). The 9.1 -m (30-
ft) mechanism used at  Norristown was
obtained only with great difficulty as a
special order unit  from  a  manufacturer
who, as a condition of fabrication, would
not provide any guarantees. Dual-drive
scraper mechanisms would not be ac-
ceptable due  to interference  with  the
fluidized bed  crossing the reactor floor
and the potential for developing areas of
deposition. Given  the  success  of  the
Norristown unit, other manufacturers may
be less reluctant to provide similar scraper
widths.

Inlet Chamber
  The inlet chamber  serves to blend the
influent  wastewater with the recycle
sludge to provide a gas release zone and
to distribute the flow evenly across the
tank floor. It is critical that the outlet area
at the  bottom of the  baffle wall  be
designed to reduce the average velocity
of the forward flow into the process to
about 0.14 m/sec  (0.45 ft/sec) in order
to capture  free gas bubbles within  the
confines of the inlet chamber. This rate is
approximately one-half of the anticipated
slowest bubble rise rate (0.3 to 0.6 m/sec
or 1 to 2 ft/sec).

Sludge Scraper
  Two functions are served by the sludge
scraper mechanism. The first is to prevent
solids deposition by pushing the denser
materials to the recycle sludge suction
manifold located at the  end  of the tank
opposite the inlet chamber. The second
function is to sweep scum and flotation
sludges at the surface of the clarification
zone back to the skimmer pipe located
over the baffle wall of the inlet chamber.
The scraper must  travel in the reverse
direction from the  scraper of a conven-
tional rectangular clarifier.

Sludge Recycle Pumps
  Hydraulic considerations  dictate  the
maximum  size  of  the  sludge recycle
pumps. Although  lower  recycle  rates
might  reduce  the sludge blanket  depth
requirements (by  increasing reaction
time),  low  sludge  recycle rates would
require higher than desirable oxygen feed
dosages  to the  recycle streams. High
oxygen dosages lower the overall  effici-
ency of the oxygenation system, and they

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may lead to uncontrollable flotation of
the sludge  blanket. Therefore, it is im-
perative to provide variable speed pump-
ing capabilities,  with  the  maximum
pumping rate set at the limit of the al-
lowable flow through the outlet area at
the bottom  of the baffle wall  of the inlet
chamber. At least 100% reserve pumping
capacity should be provided by installing
reserve pumps  to cover potential pump
failures.

Oxygenator
  Two oxygenation  devices  have  been
used with the Two-Zone process to date.
In a prior  demonstration  in Vaudreuil,
Quebec, CLA employed a 45.7-m-deep
(150-ft-deep)  U-tube.  An oxygenator
designed by Dorr-Oliver, Inc. was used
with the Norristown project. The  Dorr-
Oliver oxygenator was located at 15.2 m
(50 ft) below the surface level of the Two-
Zone  tank.  At this time, it is not clear
which oxygenator will be used by CLA in
the future.  It is presumed that the Dorr-
Oliver unit would  continue  to be used
due to its higher oxygen transfer efficiency
and lower installation cost compared to a
U-tube.

Oxygen Supply
  From an  operational  viewpoint,  liquid
oxygen supply with on-site bulk storage
would be preferred to on-site  generation.
The economics of either source of oxygen
should be carefully evaluated on a case-
by-case basis during the design phase of
the project.

Outlet Weirs
  From an  empirical viewpoint, average
weir overflow rates should be limited to
about 49.7 mVday/m  (4,000 gal/day/
linear ft) of weir length. Also,  a  scum
baffle is required to limit the excursion of
floated  solids to the process effluent
during potential process upsets.

Waste Sludge and Scum Pumping
  Waste sludge pumps should be sized
based on the maximum quantity of waste
sludge production anticipated. Variable-
speed  waste sludge pumps (including
installed reserve pumps) should be pro-
vided  to  allow  continuous wasting
throughout the operating day.
  A precise basis  for estimating  scum
production  does not exist at this time.
Scum production due to flotation in the
Norristown demonstration unit was sig-
nificantly higher than anticipated during
the project design phase. The pumps
selected should be sized to take the maxi-
mum overflow rate anticipated from the
skimmer pipe at a 2.5-cm (1-in.) immer-
sion level without surcharging the skim-
mer pipe. These pumps should also be
designed to pump dense sludges.

Conclusions
  The major conclusions of this project,
based on an in-depth evaluation of 10
months of  operating and performance
data, are as follow:
  • The Two-Zone reactor/clarifier func-
    tions physically  as a clarifier, the
    capacity of which is determined by
    the limiting sludge flux condition.
    This condition is controlled by man-
    aging the process sludge inventory.
    The sludge blanket level in the re-
    actor/clarifier is routinely monitored
    and solids wasted to control the
    blanket depth within an acceptable
    range.
  • High rates of sludge recycle in the
    process establish  two  hydraulic
    regimes in  the  sludge blanket: a
    hydraulic volume that is transported
    across the tank quickly (about 10 to
    15 minutes  in the  Norristown
    demonstration  unit)  and a  sludge
    release zone that has a longer  resi-
    dence time. Both zones are intimately
    related  and are actively involved in
    biological stabilization of  organic
    material. Thus, the Two-Zone process
    utilizes tank volume more efficiently
    than conventional  activated sludge
    processes.
  • Characteristic of a sludge maintained
    at high DO concentrations, the Two-
    Zone biomass  exhibited  excellent
    settling characteristics throughout
    the demonstration study. The mini-
    mal variations encountered in sludge
    settling rates did  not  present any
    problems in managing the sludge
    blanket. Management of the sludge
    blanket depth was influenced pri-
    marily by the wastewater influent
    flow rate, particularly diurnal fluctu-
    ations.  Generally,  loss of  process
    sludge over the effluent weir did not
    occur if the average sludge blanket
    depth was kept at 2.7 m (9 ft) or less
    in the  4.0-m (13.25-ft) SWD tank.
    Consequently, sludge management
    practices were tailored to  produce
    an average sludge blanket depth of
    about 2.1 m(7ft).
  • DO concentrations ot the blended
    sludge recycle/influent wastewater
    mixture entering the reactor/clarifier
    had a significant impact on whether
    effervescense occurred or not. If the
   DO concentration exceeded satura-
   tion on an average daily basis, sig-
   nificant flotation of reactor solids was
   usually noted.
•  The Norristown Two-Zone system
   functioned well and  achieved 83%
   to 92% TBOD removal at the average
   F/M  loadings of 0.33 to 0.95 kg
   TBOD/day/kg MLVSS, average total
   tank  (reactor zone  +  clarification
   zone) detention times of 3.3 to 7.0
   hr, and average SRT's of 1.2 to 3."
   days evaluated throughout the study.
   Biological flocculation  of the non-
   soluble substrate (suspended solids)
   became more efficient as the SRT
   increased. Accordingly, effluent TSS
   concentrations of less than 20 mg/L
   could be produced at SRT's greater
   than 2.5 days.
•  Norristown's wastewater  has a
   soluble substrate content of 25% to
   35%. The  Monod  constant for  re-
   moval of soluble substrate was rela-
   tively consistent  throughout the
   project  with an average  value of
   0.0064 and  a range of 0.0052 to
   0.0088 day1 per mg/L MLVSS.
•  Process oxygen requirements were
   low,  averaging 0.23 kg/kg TCOD
   removed  (about 0.5 kg/kg TBOD
   removed) when corrected for oxygen
   consumed by nitrification. The low
   oxygen requirements occurred be-
   cause  the nonsoluble substrate
   components removed  by the Two-
   Zone system apparently  were not
   processed  enzymatically for use by
   the biomass in energy and synthesis
   reactions. The lack of  utilization of
   nonsoluble substrate can be attri-
   buted, at least in  part, to the low
   SRT's under which the system was
   operated.
•  Net  wastage of scum and excess
   sludge  averaged 0.66 kg/kg TCOD
   removed  (1.54 kg/kg TBOD re-
   moved). This sludge production value
   is higher than  expected from pre-
   vious work and is a direct consequ-
   ence of nonsoluble  substrate not
   being metabolized to  any significant
   degree  by  the demonstration unit.
   This high value may be characteristic
   of the Two-Zone process when oper-
   ated at low SRT's, the Norristown
   wastewater itself, or  a  combination
   of both.

•  The good performance of the Norris-
   town Two-Zone system was achieved
   under conditions in which operator

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        attention was purposely maintained
        at  low levels.  Operators visually
        inspected the system at 1- to 2-hr
        intervals  around the  clock. Most
        process  adjustments were  made,
        however,  only during the day shift,
        and then generally only when the
        project engineer or the plant super-
        visor was present.
     The full report was submitted  in ful-
   fillment of Cooperative Agreement No.
   CS807404 by the Borough of Norristown,
   PA,  under the partial sponsorship of the
   U.S. Environmental Protection Agency.
           Stephen R. Weech is with BCM Eastern, Inc., Plymouth Meeting, PA 19462;
             Vernon T. Stack is with Smokey Stack, Inc., Cedars, PA 19423; and Graham
             Orton is with the Borough of Norristown, PA 19401.
           Richard C. Brenner is the EPA Project Officer fsee below).
           The  complete report,  entitled  "Evaluation of  the  Two-Zone Wastewater
             Treatment Process at Norristown, Pennsylvania," (Order No. PB 87-234 506/
             AS; Cost: $30.95, subject to change) will be available only from:
                  National Technical Information Service
                  5285 Port Royal Road
                  Springfield, VA 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
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Official Business
Penalty for Private Use S300
EPA/600/S2-87/074
           0000329
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