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
-------
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
/
-
-t-
•
\
Wet
Well
Oxygenator
Figure 1. Flow diagram of Norristown Two-Zone system
2
-------
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 \
r* l I
^
^ u u
3'-11" 2'-4"
10" 10"
Recycle Sludge
Suction Header
(Concrete Encased}
/
' ^rb^^^r
i
\
J5'-0"
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-
-------
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.
-------
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
o
I
5
70
60
50
\\
V
\
1
1
^
\
\
^^.1
>^
^X-»
)
—
X ->
\
-^x
'
)
J^-A^/
o
<
\
X
\
\
>
Note: Curve x = Curve o
Multiplied by Curve A
~^A
A
V
\
\
O
k
\
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
-------
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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