UNOX DESIGN INFORMATION
FOR
CONTRACT DOCUMENTS
PREPARED FOR
ENVIRONMENTAL PROTECTION AGENCY
TECHNOLOGY TRANSFER PROGRAM
DESIGN SEMINAR
FOR
WASTEWATER TREATMENT FACILITIES
FEBRUARY 29 - MARCH 1, 1972
NEW YORK, NEW YORK
2v3
METCALF A EDDY,
IC.
ENGINEERS
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UNOX DESIGN INFORMATION FOR CONTRACT DOCUMENTS
By
Ariel A. Thomas, P.E.
President
Metcalf & Eddy of New York, Inc.
60 East 42nd Street
New York City
Presented to the Environmental Protection Agency Region II
Technology Transfer Design Seminar, in New York City, on
February 29j March 1 and 2, 1972.
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UNOX DESIGN INFORMATION FOR CONTRACT DOCUMENTS
This presentation is designed to provide consulting
engineers with design data, and municipal and regulatory agencies
with information with which they can evaluate designs. This
information was obtained from many sources, including research
data from Middlesex County (New Jersey) Sewerage Authority;
Washington, D.C.j and Batavia, New York, and design data from
the Middlesex County Sewerage Authority and the New Rochelle
(Westchester County) wastewater treatment plants which are com-
pletely or almost completely designed. The data presented are
the best available but could be changed markedly when experience
is gained in daily continuous operation.
A. The "Unox Activated Sludge Process" is a patented modifica-
tion of the conventional activated sludge plug flow process
which uses gaseous oxygen rather than air to maintain dis-
solved oxygen in the mixed liquor (See Figure 1 which is a
schematic of the process). It can meet a much higher mixed
liquor oxygen demand than any aerated modification. It can
meet the oxygen demands of a more concentrated mixed liquor
and of a higher volumetric BOD loading in the oxidation
tanks. It may give the same BOD removals at higher F/M
ratios. It effectively oxidizes dissolved BOD. It may have
problems in removing suspended solids from mixed liquors in
the final settling tank, especially in cold weather. It was
METCALF & EDDY
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OXYGEN
GAS
WASTE GAS
<—RETURN
SLUDGE
PUMP
OXYGEN
SOURCE a
STORAGE
FINAL
SETTLING
TANKS
UNOX INFLUENT
RAW OR SETTLED WASTEWATER
COVERED
OXYGENATION
TANKS
WITH MIXERS,
OXYGEN
COMPRESSORS
AND SPARGERS
MIXED LIQUOR
WASTE UNOX
UNOX SLUDGE EFFLUENT
FIG. I UNOX PROCESS
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METCALF ft EDDY
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more stable than the completely mixed and step-bio modifica-
tions of the activated sludge process in pilot plant opera-
tion with the MCSA wastewaters which are at least 50 percent
industrial wastes.
Figure 2 is a schematic section of the pilot plant used
at the Middlesex County Sewerage Authority. It is typical
of a prototype four-stage oxygenation tank and final set-
tling tank.
B. Design Data for the Oxygenation Tanks are presented below:
1. BOD removals, approximately 90$.
2. Tank volume based on 160 lbs of BOD^/lOOO cu ft.
Unox believes that this can be increased to 215 lbs,
or higher.
3. Mixed liquor suspended solids, 6500 mg/L.
4. Mixed liquor volatile suspended solids, 5500 mg/L.
5. F/MLVSS ratio, O.465. Unox believes this can be
much higher.
6. Mixed liquor dissolved oxygen is targeted at
3 to 9 mg/L.
7- Purity of applied gaseous oxygen.
Cryogenic 99-5%
PSA 90.0$
8. Oxygen in vest gas, 50$.
9. Applied oxygen utilized, 90$.
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Oxygen fron Liquid
Storage Tank
Automatic O2
Feed Regulator
Compressors and Drives
Oxygen Analyser
Dry Flow
Meter
Clarlfler
Dry Flow
Meter
£ -2.
Influent
Effluent
Variable Speed Sludge
Return Puap
Waste
FIG. 2 MIDDLESEX COUNTY SEWAGE TREATMENT PLANT
"UNOX" PILOT FACILITY SCHEMATIC
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10. Mixing Equipment and Sparger:
10.1 Sparger and mixer on same shaft.
10.2 Shaft is hollow and carries compressed
oxygen to sparger.
10.3 Sparger and mixer turn at a constant speed
which will keep contents of oxygenation
tanks mixed.
10.4 Mixer is ship-type propeller.
10.5 Oxygen compressor for oxygenation tanks
are centrifugal, with suction throttling.
10.6 For MCSA, mixer hp is 6000 and compressor
hp is 5100, of which 1900 is standby. All
of the mixer horsepower is connected and in
operation at all times. MCSA expects that
total power for mixing and dissolution will
be less than 0.l6l KWH per lb of oxygen
dissolved.
10.7 Oxygen compressor suction pipes are subject
to condensing and freezing of moisture in
the oxygen gas stream in cold weather.
10.8 Special lubricants must be used if oxygen
comes in contact with lubricant.
10.9 The mixer shaft rotates in a liquid seal
which is cast into the top slab.
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10.10 Spare mixer motor, speed reducer, shaft,
propeller, and sparger must be stored on
site.
10.11 Standby compressors must be Installed for
each oxygenation stage or, sometimes, one
for two stages.
11. Normal oxygen pressure under covers of oxygenation tanks
is 2 inches of water.
12. Tank covers designed for 100 lbs of live load and
H inches of vacuum.
13. Oxygen feed into the first pass is controlled by pres-
sure under the covers in the first pass. The approxi-
mate set point is 2 inches.
14. The pressure in the first pass is controlled by the rate
of oxygen use and the purity of the gases vented to the
atmosphere from the fourth pass. If the oxygen content
is more or less than 50 percent, then the vent valve
closes or opens, as necessary, to bring purity back to
50 percent.
15. Dissolved oxygen in each pass is controlled by the rate
of discharge of compressed oxygen gas to that pass.
The compressors can be controlled automatically or
manually using DO meters.
16. Waste gas must be discharged into a stack approximately
15 feet high so that the waste oxygen will have a chance
to mix before it reaches the ground.
METCALF ft EDDY
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17- BOD^ applied to the oxygenation tanks varies during the
day, daily, weekly, monthly, and with growth. The
amount of oxygen required to meet the BOD^ demand varies
with the volumetric BOD^ loading. As the loading in-
creases the amount of oxygen required rises at a decreas-
ing rate. See Figure 3> "Oxygen Demand Curve". You will
note that, at the design loading of 160 lbs of BOD^
per 1000 cu ft, 1.8 lbs of oxygen is required to remove
1 lb of BOD^. As the loading increases to 255> the
amount of oxygen required to remove 1 lb of BOD5 drops
to 1.1 lbs.
Design Data for Final Settling Tanks are presented below:
1. Overflow rate of maximum day is 1200 gal/sq ft/day.
2. Solids loading at MCSA will be 3^ lbs/sq ft/day of tank
area for design flow of 120 mgd with mixed liquor concen-
tration at 5500 mg/L. This increases to 55 lbs/sq ft/day
at maximum loading.
3. Sludge volume index must be looked at carefully. Sludge
volume index is the volume in ml occupied by settled
mixed liquor containing one gram of dry solid. The
sludge volume index for a 2000 mg/L mixed liquor which
settles to 25 percent in 30 minutes is 125- However,
the sludge volume index of an 8000 mg/L mixed liquor
which does not settle at all in 30 minutes is 125-
Obviously, a good sludge volume index for conventional
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3.0
0.5
0 100 125 150 160 175 200 225 250 275 300
Z
m
Q BOD LOADING (lbs./1000 ft3)
TJ
CP
m
O
O
FIG. 3 OXYGEN DEMAND CURVE
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activated sludge may be very poor for Unox sludge which
has a mixed liquor concentration in the final tank
influent 2 to 6 times greater.
4. Settleability rates are a much better method for compar-
ison. Figure 4 gives some indication of MCSA Unox sludge
initial settling velocities.
5- Return sludge concentration varies from 1.5 to 3-0
percent. MCSA is designed for 2.2 percent.
6. Normal return sludge rate is 33 percent of influent
wastewater flow. Maximum rate is 100 percent.
7- Union Carbide has preferred to have the rate of return
sludge proportional to the Unox influent. This theoret-
ically maintains the same mixed liquor concentrations in
the oxygenation tanks. This is true at the moment the
change is made because the return sludge concentration
has not changed. However^ an increase in mixed liquor
flow of the same concentration reduced the period of
retention of the liquid and the sludge and increases the
overflow rate and solids loading rate with the possibil-
ity that solids separation will be reduced and the mixed
liquor concentration will decrease. Union Carbide is
having second thoughts on keeping the return sludge
proportional to the Unox influent.
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METCALF & EDDY
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1000
TIME, MIN.
DATE
INITIAL
INITIAL SOLIDS SETTLING
CONCENTRATION, VELOCITY
MG/L FPH
2/25-26 4200
wk of 5/16 5400
wk of 6/6 6400
12/8/70 7200
6 4
4 9
2 9
1 6
1000
u
<
800
600
Q
3
O 400
O
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D. Oxygen is produced by either the Cryogenic Process or the
P.S.A. Process.
1. The Cryogenic Process is more economical for supplying
more than 30 to 50 tons of oxygen per day.
2. The P.S.A. Process is more economical for less than
50 to 30 tons of oxygen per day.
E. The Cryogenic Oxygen Process has the following characteristics:
1. It can be supplied by at least 3 companies on a perform-
ance specification with quality requirements. It can
best be considered as a "black box" piece of equipment
except as its characteristics affect design of the
wastewater treatment plant. This is similar to perform-
ance specifications for blowers, pumps, etc.
2. Cryogenic oxygen is produced by compressing and cooling
air to a liquid and by separating oxygen and nitrogen by
differential evaporation in the range of -300°F. Oxygen
boils at -297°F.
3. Cryogenic oxygen plants can be turned down to approxi-
mately 2/3 of full capacity which, by coincidence,
happens to be a normal turndown for large compressors.
If all of the 2/3 plant capacity cannot be used, then
the oxygen must be wasted to atmosphere through a stack.
4. A full standby for the primary compressor should be pro-
vided. At NCSA, for a 400-ton-per-day plant, the
centrifugal compressor with adjustable inlet guide
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METCALF & EDDY
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vanes will be driven by an 8000 hp electric motor. The
demand charges for running two compressors while chang-
ing from one to the other was so great that one compres-
sor will be taken off line before the second one is
started. This will result in up to two hours of lost
oxygen production.
5. A full standby should also be provided for the turbine
expander which is another large motor.
6. A cryogenic plant can produce liquid or gaseous oxygen
(LOX or GOX) or various combinations of LOX and GOX,
depending on how it is designed. The specifications
for the MCSA plant will give the LOX and GOX capacities
Shown on Figure 5- Each ton of LOX producted will
reduce GOX by about four tons.
7. A cryogenic plant takes 1 to 3 days to start up, depend-
ing on whether liquid oxygen is available or not. It is
not practical to meet variable demands by starting and
stopping cryogenic units.
8. Dividing a cryogenic demand into two or more plants can
help to economically meet the demands of a wastewater
treatment plant at the beginning and end of the design
period if it is serving a rapidly growing service area,
but will not be useful for unpredictable variations.
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METCALF & EDDY
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II ^ II
\
V
MAX. P
DEMAN
OWER .
\
\
\
\
IIaII
/^DESI
f POW
6N
IT "A"
/ rtB"
"D" /
\/
MIN. P
DEMAN
OWER
ID
450 400 350 300 250 200
GASEOUS OXYGEN PRODUCTION (TONS / DAY)
150
FIG. 5 LIQUID AND GASEOUS OXYGEN PRODUCTION
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9- All pipes and equipment containing, processing or trans-
porting liquid oxygen or nitrogen are very cold and must
be well insulated otherwise ice will build up from the
freezing of condensed atmospheric water vapor at any
points not insulated.
10. Cryogenic gas oxygen will cost municipalities approxi-
mately $8 per ton (capital, operation and maintenance)
at capacity production. Liquid oxygen costs approxi-
mately $35 per ton.
F. P.S.A. Oxygen Generators produce oxygen of 88 to 90 percent
purity. PSA stands for Pressure Swing Absorption. Air at
60 to 90 psi is discharged into an absorption unit. Nitrogen
is absorbed and oxygen is discharged from the unit to the
oxygenator. After a few minutes, the pressure is released
and the unit is flushed with air at low pressure and then
with oxygen from another unit, and then receives air at high
pressure to repeat the cycle. Three or four absorbtion units
are used to produce oxygen so that oxygen is produced con-
tinuously. In comparison with cryogenic units, the oxygen
is less pure, but the unit turn-down is limited only by the
compressor capacity. At New Rochelle, we plan to use three
compressors, one of which is a standby, so that we can reduce
the oxygen production to 25 percent of the nominal capacity
of 15 tons per day by operating one compressor at half
capacity. These units are more economical in the smaller
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METCALF A EDDY
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capacities. Because of their turn-down characteristics and
the partial load under which most oxygen generators will
work, the PSA units are more economical at higher oxygen
capacities than would be expected by comparisons made for
capacity production. We considered PSA generators to take
some of the variable load at MCSA but decided against this
on a cost basis.
The PSA generator using four absorption units uses
24 to 30 valves., most of which are operating every 2 to 5
minutes. The dependability of these valves is of paramount
importance -- if one valve malfunctions, the entire unit
shuts down. We have considered the possibility of provid-
ing an extra absorption unit which could be used to replace
a malfunctioning unit. However, the interrelationship of
the four units and the 24 to 30 valves make this difficult.
Standby is provided from LOX storage.
G. Oxygen Storage is usually provided for liquid oxygen. Users,
such as steel mills, with violent variations in demand,store
oxygen gas at high pressures. This was not economical at MCSA.
1. Liquid oxygen is stored at a few inches of water pressure
in highly insulated steel tanks. Approximately 5 days
of storage is provided.
2. The heat gain into the storage tanks is met by evapora-
tion of approximately 0.2 to 0.4 percent of the liquid
oxygen per day. At MCSA,where 2000 tons of oxygen will
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METCALF & EDDY
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be stored, approximately 5 tons of LOX will be evapora-
ted each day. This, if purchased, would cost approxi-
mately $175 per day, or approximately $63,000 per year.
3. LOX will be produced by the cryogenic plant when the
full capacity of the plant is not required for gaseous
oxygen. This LOX will be discharged to the LOX storage
tanks along with a small amount of gaseous oxygen which
will evaporate in the insulated pipe lines to counter-
act the heat gain in these lines. The cryogenic plant
will have capacity not only to make up the LOX evaporated
in storage and pipe lines but also capacity to make up
for some of the LOX used in emergencies.
4. The LOX which evaporates in the storage tanks and pipe
lines will be discharged into the GOX pipes feeding the
oxygenation tanks.
5. LOX storage is provided for unforeseen failures of the
cryogenic plant and also for part of the routine 10-day
shutdown for maintenance, which occurs every two years.
6. The liquid oxygen must be evaporated and raised to
ambient temperature before it can be used in the oxygen-
ation tanks. Approximately 172 BTU is required to
evaporate a pound of LOX and raise it to 70°F. Since
it is desirable to be able to get GOX out of the storage
tank on short notice, a fossil-fuel boiler would have
to be maintained at operating temperature at all times.
Heat could be provided by electric heaters. We are now
METCALF & EDDY
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considering the use of final effluent to evaporate the
LOX. Using 2600 gpm, the temperature drop in the water
is estimated to be in the range of 15°F. This would be
less than half a degree from the entire plant effluent.
We have not decided if the danger of water freezing in
the heat exchangers is critical.
7- The pumps which are used to move the LOX are critical.
They must be kept as cold as the LOX. LOX could be
moved through the evaporators and to the oxygenation
tanks by maintaining a small pressure in the LOX stor-
age tanks.
8. If oxygen is produced by PSA, the LOX storage must be
refilled from time to time by purchased LOX which can
be delivered in rail tank cars or trucks.
9- The oxygen industry is set up to deliver LOX so that
there is no concern that they will be able to supply
400 tons of LOX per day to MCSA for indefinite periods,
if this should ever become necessary.
H. Dangers using pure oxygen are not great.
1. GOX strongly supports combustion so that any materials,
such as clothing, which may become saturated with
oxygen will burn furiously. It is important, therefore,
that precaution be taken until the oxygen is dissipated.
2. GOX explosion limits with explosive materials are the
same as if the explosive materials were in air. No added
danger.
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3- If compressors and pipe lines are inside, then ventila-
tion with 6 changes per hour should be used.
4. We doubt if explosive concentrations of materials in the
raw wastewater will build up in the oxygenation tanks,
but we have provided explosion meters and facilities
for purging the gas space with air. We do this by
valving the compressor suction so that air can be com-
pressed and discharged through the spargers.
5- LOX will detonate if it comes in contact with hydro-
carbons. Therefore, no asphalt should be used in the
vicinity of LOX storage or pipe lines.
6. The LOX storage tanks should be surrounded by a dike
which will hold the entire contents of the tanks. In
the extremely unlikely chance that the LOX storage tank
ruptures or leaks, the LOX will evaporate rapidly and be
discharged into specific gravity of 32, compared with
air at 29, so that it is only slightly heavier than air
and will dissipate easily. In case of a large spill the
GOX will be very cold and would have more of a tendency
to stay on the ground and not mix.
For MCSA the use of the Unox Process is expected to save
approximately 15 percent in project costs and annual costs.
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