It is difficult to correlate excess sludge production for a wide variety of wastewaters, since
there are other determining factors to consider when attempting to predict production figures. It
is necessary to account for

®@ The fraction of volatile suspended matter entering the unit that is nonbiodegradable
@ The temperature effect on the degree of endogenous respiration
@ The quantity of organic material oxidized, not just the BOD, removal

These points are in essence the effect of the COD/BODg ratio and, of course, the source of the
waste itself. In every instance where Unox and air-activated-sludge systems have been operated
side by side, the net solids production from the Unox system has been significantly lower.

Solids-Liquid Separation

The importance of solids-liquid separation to the waste-treatment field cannot be over-
emphasized. Most often the success of a waste-treatment plant depends on the success of the
solids-liquid separation equipment. This statement is especially true for the activated-sludge
process, where efficient performance of the final settling tanks is imperative because the solids
must be recycled to the aeration basins to sustain the process, and where any solids escaping
separation will impair the quality of the effluent.

The performance of the secondary gravity clarifier used in biological-treatment processes,
such as in the activated-sludge process, is related to both the physical and chemical nature of the
sludge and to the hydraulic characteristics of the clarifier. For the activated-sludge process to be
successful, the clarifier must serve the dual function of clarifying the liquid overflow and thicken-
ing the sludge underflow. The clarification capacity of the unit is related to the initial settling
velocity of the sludge. Conventionally, the area required for clarification of a suspension with
hindered settling is estimated such that the vertical liquid-rise rate in the clarifier is less than the
solids-subsidence rate over the operating MLSS range. In other words, the allowable overflow rate
in this case can be calculated by:

180 (ISV)
R=
SF
where
ISV = initial (zone) settling velocity of the mixed liquor, ft/hr
OR = allowable overflow rate, gpd/ft?
SF = safety factor applied

The area required for sludge thickening, on the other hand, is related to the solids flux or mass
loading that the clarifier would be able to handle under gravity. An analysis of the solids flux in a
batch-gravity settling tank was given by Kynch. Dick proposed a similar analysis to determine the
limiting solids flux in a continuous-gravity thickener or clarifier. In the analyses of both Kynch and
Dick, the following basic assumptions were made:

@ At any point m the dispersion within the gravity settling tank, the velocity of fall~i.e., the
settling velocity of a particle—depends on the local concentration of particles.

16


Chapter VI
ECONOMIC CONSIDERATIONS

The economics of oxygen use in the activated-sludge process relative to a conventional air-
activated-sludge process derives from the relative process differences discussed in earlier sections
of this report. The process and operational advantages notwithstanding, the essence of the
economic decision is that sufficient savings in investment and operating costs must be shown
within the wastewater-treatment plant as a system to justify the selection of oxygen. Thus, it is
necessary to consider such costs as sludge handling and disposal, as well as the direct costs of
installing and operating the activated-sludge part of the waste-treatment plant.

DIRECT FACTORS

The direct economic factors governing the economics of oxygen versus air for the activated-
sludge process include the following:

Higher mixed-liquor solids under aeration can be maintained when using oxygen as the aeration
gas, without occurrence of oxygen mass-transfer limitations. Concentrating the active biomass in a
smaller volume reduces concrete tankage requirements, and is a chief reason for expecting reduced
costs from such a system. This effect can be extremely important when land is limited and when
extensive piling work is necessary.

Higher oxygen-transfer efficiencies are made possible using oxygen as the aeration gas. These
efficiencies lower equipment requirements for oxygen dissolution, with an attendant reduction in
requirements for auxiliary equipment, such as electrical switchgear.

Power savings generally result from the higher oxygen mass-transfer efficiencies experienced
in pure-oxygen processes. The power required to generate oxygen for the biological process added
to that required for oxygen dissolution generally is less than that required to provide oxygen to a
conventional activated-sludge process with air as the oxygen source.

INDIRECT FACTORS

: The economics of oxygen are also governed by indirect considerations, such as sludge handling
and disposal and odor control.

The improved settling characteristics of oxygen sludge result in the achievement of a much
thicker clarifier-underflow concentration than is achievable with conventional aeration systems
at the same overflow rate. This effect allows the maintenance of higher mixed-liquor concentra-
tions, but also provides advantages in sludge handling and disposal, since the waste solids from the
secondary clarifier are available in a significantly decreased volume of liquid.

37


Chapter V
UNOX-SYSTEM SCOPE OF SUPPLY

GENERAL

In general, the scope of supply for a Unox system includes oxygen-dissolution equipment,
oxygen-generation equipment, and appropriate controls and instruments to mate the two sub-
systems in an efficient, reliable, and safe operating mode. The scope of supply does not include
concrete work, deck covers, or, in many cases, installation of the foregoing equipment. All
engineering work necessary to design and provide a workable Unox system is performed in coop-
eration with the customer and/or his consulting engineer.

The Unox-system dissolution equipment will be, in general, either surface aerators or sub-
merged turbines, depending on economic considerations. Generally speaking, where deep tanks
offer an advantage over shallow, and where the waste stream is large in quantity and high in
strength, a submerged-turbine system will be preferable. Therefore, again in the general case,
it can be expected that the greatest number of Unox systems will employ surface aerators while the
largest sized applications may well use submerged turbines. Because of the complexity inherent
in large jobs, it is often the'policy to offer these Unox systems on “‘installed by UCC” terms, while
the smaller jobs can be more cost effective on uninstalled terms. As has been mentioned earlier,
the cryogenic-oxygen supply is often more cost effective for large plants, and the PSA for smaller
ones. The most common combinations will therefore be surface aerators with PSA oxygen and
submerged turbines with cryogenic oxygen. The following paragraphs outline a typical scope of
supply foi each of these combinations with the surface-aerator-PSA combination preceding the
submerged-turbine, cryogenic-oxygen combination

SURFACE-AERATOR-PSA SYSTEM

Dissolution Equipment

The number of units specified in the design will be supplied. Each will be designed to be
assembled as a complete unit ready for installation through openings in the tank covers. Each
assembly will consist of the following:

®@ Motor, speed reducér, and lube system

© Shaft and aerator blade

@ Mounting skid and shaft seal

All equipment and materials are designed to provide long, maintenance-free service. The
speed reducers are specified to have an overall AGMA service factor of 2, while other.components

critical to the continuous operation of the assembly are designed for service factors of up to 5.
Gearing and bearings of praven quality are used throughout the speed-reducer assembly. The

33


Appendix A
SPECIFICATIONS FOR FINAL SETTLING TANKS*

1. Overflow rate of maximum day is 1,200 gal/ft?/day.

2. Solids loading at Middlesex County Sewerage Authority (MCSA) will be 34 lb/ft2/day of
tank area for design flow of 120 mgd with mixed-liquor concentration at 5,500 mg/l. This loading
increases to 55 Ib/ft?/day at maximum loading.

3. Sludge-volume index must be looked at carefully. Sludge-volume index is the volume in
milliliters occupied by settled mixed liquor containing 1 gram of dry solid. The sludge-volume
index for a 2,000-mg/l mixed liquor that settles to 25 percent in 30 minutes is 125; however, the
sludge-volume index of an 8,000-mg/] mixed liquor that does not settle at all in 30 minutes is 125.
Obviously a good sludge-volume index for conventional activated sludge may be very poor for Unox
sludge, which has a mixed-liquor concentration in the final tank influent two to six times greater.

4, Settleability rates are a much better method for comparison. Figure A-1 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 rate theoretically maintains the same mixed-liquor concentrations in the oxygena-
tion tanks. This theory holds 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
reduces the period of retention of the liquid and the sludge and increases the overflow rate and
solids-loading rate, with the possibility 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.

*Prepared by Ariel Thomas, Metcalf & Eddy, Engineers, New York, N.Y.

43


Table 11-4 —Unox-system pilot-test data, performance summary for locations C and D

Location C
Component Location D,
Phase I Phase |
Retention ime . Hours 107 22(1 43.2)
Recycle ratio . | Percent 22 31
Sewage temperature . °F 72 67
MLSS. . : mali 4,300 6,200
VsS/TSS : 079 0.77
Organic loading | Pounds BOD per 147 208 143
| 1,000 Ft3 day
Biomass loading | Pounds 800 per | 0.51 0.85 048
pound MLVSS/day |

Influent characteristics \

cop... mg/l | 460 392 454

BOD, ... . mg/l | 17 145 209

Suspended solids malt 143 120 126
Effiuent characteristics

cop... : . - | mail 124 181 90

BODg . - . : . mg/l 14 19 "

Suspended solids . : . mal! 36 3 30
Removals

cop. : Le Percent 73 54 80

BODs, . . . | Percent 91 87 95

Suspended solids . Percent 75 74 76
Clanfier overflow rate . | gal/tt/day 640 1,085 | 529 (360-800)
Clartfter mass loading - . Pounds SS/tt?/day 40 53 326
Clarifier underflow concentration Percent 23 30 23
Sludge-volume index : . - 47 a 56
Pounds VSS per pound BODs removed

Accumulation . - 052 | 083 064

For disposal . - 0.33 | 062 050
Pounds Og per pound BODs removed 1.32 123 106
Pounds 05 per pound COD removed . : 064 | 073 oss
COD/BOD ratio - 26 27 217
Solids retention ime : 375 142 3.36

oxygen to support metabolism without destructively high energy inputs to the mixed liquor. Figure
IJ-2 presents the data correlating F/M removed against phase-average biomass loading for pilot-plant
results in tables II-3 to II-7. The slope of the line represents the percentage BOD, removal.

The range of secondary-system removals varied from 87 to 97 percent for all programs pre-
sented as a group for phase-average biomass loadings as high as 1.4. It is evident that the process
can be sustained reliably and can achieve high substrate removal at the design-average biomass-
loading range of 0.5-0.8. Figure [1-2 also shows that the process will perform well during diurnal or
peak organic-load periods (represented by phase-average data, as high as 1.4) when biomass loadings
will exceed the design range for some period of time, provided that oxygen is not limiting. This
factor is important in maintaining consistently high-quality treatment.

Figure II-3 is a comparable presentation of the data based on the organic-loading parameter
pounds BOD, per 1,000 cubic feet oxygenation tankage per day.
Oxygen Requirements

The total oxygen requirements in the activated-sludge system are related to the oxygen required
for synthesis and to that associated with endogenous respiration. Since the Unox system is a closed

10


MLSS %

(mg/) RSS
10000}- sk
8000}- ab
6000}- 3b
isd
° OXYGEN
4000- 2b
SLUDGE OXYGEN
SLUDGE
2000+ air ie
AIR
SLUDGE SLUDGE
1 L 4 =e 1 i 4 i
0300 400 G00 80010001200 7400 0700 4006008001860 720014601600

OVERFLOW RATE, GPD/FT?

Figure 11-8. Typical clarifier performance for air and oxygen sludges {at 30% R/Q).


Substantial evidence in many operating programs continues to indicate that less total pounds
of dry solids will be produced with an oxygen process than with conventional aeration-activated
sludge. This result will also affect the economics of sludge handling and disposal.

The excellent flocculating characteristics of oxygen sludge, which account for improved
settling characteristics in a secondary clarifier, also contribute to improved performance in sludge-
handling equipment, such as centrifuges and vacuum filters.

Odor control is frequently an important consideration in selection of oxygen for secondary
treatment. Since nitrogen is rejected from the gas feed to the process before using the oxygen, and
since 90 percent of the oxygen is consumed compared with about 5 percent for conventional air-
activated-sludge processes, the total gas volume vented from an oxygen system is normally less
than 1 percent of the volume vented from a conventional air process. This gas is vented at a single
point rather than from the entire surface area of the tank and, therefore, can be collected for further
odor treatment if desirable.

SCOPE OF SUPPLY

The scope of supply of oxygen equipment for use in an activated-sludge process is small
relative to the equipment requirements for a wastewater-treatment plant. Union Carbide’s scope
of supply includes the oxygen-generation plant, the equipment for oxygen dissolution, and the
instrumentation and controls to integrate this equipment into the activated-sludge process. Union
Carbide does not supply concrete tanks or covers, clarifiers, liquid pumps and controls, or any
sludge-handling and disposal equipment, but prefers to work with the consultant to integrate the
Union Carbide equipment into the equipment best supplied and constructed under the control of
the consultant. Given this method of operation and the economic effects discussed earlier, it 1s of
little or no value to discuss the cost of Union Carbide-supplied equipment only. Rather, it is
necessary to consider the overall economic consequences to the waste-treatment facility of a
decision to install oxygen for the activated-sludge part of the plant. In practice, this economic
comparison is performed by the consultant as an integral part of his process evaluation and selection
service to his client. Union Carbide participates by providing economic data covering its own scope
of supply for each specific case. With such a procedure the total economics are rarely known to
Union Carbide until the overall evaluation-and-selection process is complete and published. A few
such evaluations are available and are summarized in the following section.

COMPARATIVE ANALYSES

Middlesex County, N.J.

A 300-day pilot-plant program was conducted prior to process selection of Unox by the
Middlesex County Sewerage Authority and the consultant Metcalf & Eddy. The economics
were prepared comparing complete-mix, air-activated sludge with oxygen for a 120-mgd plant.
Table VI-1 shows the results as published.

38


INTRODUCTION

The recent and accelerating emphasis on protection of the environment has necessitated the
rapid development of improved technology to aid in pollution control. The use of oxygen in place
of air in the activated-sludge process is one recent advancement in this basic process. The potential
of this development, in terms of higher quality treatment from existing plants and construction of
new facilities at reduced cost, has resulted in extremely rapid acceptance by municipalities and
industry.

The primary distinguishing feature of the Unox system (see fig. 1) is that high-purity oxygen
is the source of oxygen for the micro-organisms in the aeration basin, as opposed to air as the source
in conventional activated-sludge systems. The concept of using oxygen in this manner is not new;
many investigators have performed work since the early 1940’s to demonstrate the feasibility of
a high-purity-oxygen activated-sludge process. The major deterrent to oxygen use in the processes
envisaged by these early investigators was economics, with objections centering mainly around the
cost of oxygen production and the problem of sufficient consumption of the high-purity oxygen
to achieve a cost-effective system. This lack of an economically attractive system notwithstanding,
enough experimental work was performed by early workers to indicate many desirable features
for systems using a higher purity source of oxygen than for air-activated-sludge processes.

INFLUENT

RAW OR SETTLED WASTEWATER

OXYGEN

x

CHIGEN SOURCE &
STORAGE

MIXED LIQUOR
IN

COVERED
OXYGENATION
TANKS
WITH MIXERS,
OXYGEN
COMPRESSORS,
AND SPARGERS

WASTE GAS

RETURN
SLUDGE

FINAL
PUMP SETTLING
TANKS.

WASTE EFFLUENT
SLUOGE

Figure 1 Unox process


Chapter IV
PROCESS SAFETY

OXYGEN GAS

In a Unox system, oxygen is delivered to the system in a relatively pure form, and contact with
the mixed liquor takes place in a closed system. For these reasons, a few routine safeguards are
necessary to insure a totally safe wastewater-treatment system.

Oxygen itself is not dangerous; it is colorless, odorless, and tasteless, and it supports combus-
tion. A flame burning in pure oxygen combusts at a more rapid rate than in air because of the
absence of nitrogen as a diluent. It is acommon fallacy that the combination of oxygen and
combustible material will spontaneously ignite. The lower explosive limits (LEL) of all commonly
found hydrocarbons are almost exactly identical in an oxygen or air atmosphere. Furthermore,
for ignition to take place a spark must be present; autoignition does not occur. Thus, the safe-
guards built into a Unox system are not to protect against oxygen gas, which is not dangerous by
itself, but to prevent a buildup of combustibles in the gas space in the covered aeration basin.

OXYGEN AND THE UNOX SYSTEM

LOX is stored at a few inches of water pressure in highly insulated steel tanks. Approximately
5 days of storage are provided. The heat gain into the storage tanks is met by evaporation of
approximately 0.2 to 0.4 percent of the LOX per day.

The LOX storage tanks should be surrounded by a dike that 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 will tend more to stay on the ground and not mix.

Oxygen enters the oxygenation tank, where a gage pressure of 1-4 inches of water is main-
tained, and mixes with the wastewater. After entering the oxygenation tank, the high-purity gas
(90-100 percent oxygen) passes through a series of wastewater-treatment stages and is vented to
the atmosphere with approximately a 40-50-percent oxygen concentration. The process gas in the
aeration tank presents a safe environment because it is at low pressure (several inches of water),
ambient temperature, and is saturated with water vapor.

All equipment and materials having to do with the storage and handling of oxygen are selected
carefully for oxygen compatibility. In this connection, the mechanical, electrical, and control equip-
ment are located in the open atmosphere, avoiding the high-oxygen concentrations under the tank
cover. Because of the low pressures in the process, it is highly unlikely that an oxygen leak could
create a hazard to equipment or personnel by creating high-oxygen concentrations.

31


@ The local concentration of particles is uniform in the radial or horizontal direction or layer.
@ Wall effects can be ignored.
® The particles are of the same type.

With the foregoing assumptions, the settling process occurring in the thickening zone of a
gravity settling tank can be described using a continuity equation (a solids mass balance in the verti-
cal direction) without knowing the details of the forces acting on the particles.

Assume now that one can express the settling velocity, V’,, solely as a function of solids
concentration, C;, by the following equation:

V; = AC;"

where A and 7 are constants obtained from settling data.

This equation implies that a plot of V; versus C; will form a straight line on log-log paper with
a negative slope. Some of the settling data presented by Dick seem to support this assumption. To
obtain data for Unox systems, a Plexiglass settling column 5% inches ID by 8% feet in length was
constructed with a stirring mechanism and sample taps along the length of the column, and was used
to obtain settling data in many of the demonstration plants. The results of these tests are shown in
figure II-6. The band of data for Unox sludges covers a wide range of waste streams in many
locations at varying temperatures and biomass loadings. Activated sludge developed in raw degritted
wastewater will settle somewhat better than a sludge developed on primary effluent, so the right-hand
side of the data band represents raw-wastewater activated sludges or low-VSS/TSS (volatile suspended
solids to total suspended solids) sludges. Activated sludges generated from primary effluent wastes
will settle with characteristics more typical of the left-hand side of the data band. With these data
it is possible to predict closely the expected settling characteristics for a given waste and, therefore,
to develop a consistent design for both the biological reactors and the clarifier.

The interaction of the clarifier with the reactor is very critical to the design and reliable
operation of any activated-sludge process. The design flexibility is shown elearly in figure II-7,
which presents the relationship between clarifier-overflow rate and system-solids concentrations.
The figure indicates the trade-offs available to the designer in selecting low clarifier-overflow rates
to achieve very high MLSS and recycle-sludge concentrations, or in selecting higher overflow rates
with somewhat decreased MLSS and recycle-sludge concentrations. The same figure is also useful in
evaluating plant operation as a function of hydraulic cycles, as clarifier-overflow rates change with a
corresponding change in system MLSS and recycle-sludge concentrations. Such fluctuations are
important design considerations. Figure II-7 is drawn at a constant recycle fraction of 30 percent.
A discrete band exists for any recycle fraction. MLSS will increase with increasing recycle ratio
(R/Q), while recycle-sludge concentration will decrease.

There is a limited ampunt of initial-settling-velocity data, especially data covering a wide range
of solids concentrations for air-activated sludge in the open literature. Katz et al. present a plot
of settling rate versus initial solids concentration for an activated sludge without giving any more
details. Dick et al. report settling data obtained in three activated-sludge plants, but again no details
are given about the plants themselves. Dick also uses a “‘typical’’ plot of settling velocity versus
concentrations in his illustration problems when he discusses thickening in secondary clarifiers.
From these sources we have come up with a range of settling velocities versus concentration rela-
tionships that might be representative of the air-activated:sludge-zone settling characteristics. Where
the initial settling velocity is plotted against the initial solids concentration for both air and Unox
sludges, the Unox sludges have higher initial settling velocity over the entire range of the sludge con-
centrations of interest (fig. II-6). The general effect on system performance is shown in figure II-8,
where MLSS and recycle concentrations are given for typical air and oxygen systems as a function
of clarifier-overflow rate.

17


sludge will leave the secondary system if no primaries exist. Other important quality parameters
are temperature (average and range), pH, alkalinity, TSS and VSS level, alpha and beta values for
the sewage, nutrient content, and the expected concentration of heavy metals or other components
potentially toxic to the secondary-system biomass. The above parameters are also to be supplied
by the customer’s engineer. Specifications for final settling tanks are listed in appendix A.

Another quality parameter best determined by pilot-plant operation is the oxygen consumption
requirements in terms of the amount required for biomass synthesis and endogenous metabolism,
both of which occur during the removal of the pollutant material. In many cases, the waste stream
does not exist presently, or it is known that additional streams will alter its character in the near
future, so that pilot-plant results would not necessarily be representative of those to be expected in
full-scale operation. It is in situations such as these that the very large amount of Unox-system
pilot-plant data on a variety of waste streams becomes especially useful. As can be seen from figure
II-4, the oxygen consumption requirements are related to the F/M ratio.

The same discussion used in regard to the oxygen requirement also can be used to describe the
mixed-liquor settleability and the biokinetic characteristics of the wastewater, which also must be
known and can be determined best from pilot-plant data. These characteristics, too, can be estimated.
based on broad pilot-plant experience and the known source of the wastewater (see fig. II-6 for
settleability).

The effluent quality required is defined by the customer or his engineer, and is usually based
on receiving-stream standards or State regulations for removal. Parameters such as BOD,, suspended
solids, TKN, and phosphorus usually are involved. This required effluent quality must, of course,
be known by the designing engineer, as it dictates the extent of the required treatment. In the
extreme, it may not be possible to attain the required effluent quality with a secondary system, in
which case some form of additional treatment would be required.

The remaining required information pertains to specific site limitations and to prior and
subsequent unit operations that the customer’s engineer is planning to specify. Required site-
limitation information includes whether the job is to convert existing tankage to upgrade the plant
capacity or to install new tankage. Also, it must be known whether land area is limiting, and what
limitations must be placed on tank depth and area due to water table level and piling requirements.
The prior and subsequent unit operations are important in that they influence the character of the
waste fed to the secondary system. Most sludge-handling systems have supernate streams that are
returned to either the primary or secondary systems and, depending on the system in question, these
streams may have a noticeable effect on the character of the waste entering the secondary system.

The secondary clarifier is of extreme importance in the operation of a secondary treatment
system. The designer of the secondary system must know the overflow rate, depth, feed mechanism,
takeoff mechanism, and sludge-return mechanism that the customer’s engineer intends to install.
The Unox system will perform well with the same clarifiers that are used for a conventional air
system; but since the clarifier itself has such an important effect on the expected underflow concen-
tration and the MLSS concentration, its design must be known so that the Unox system can be
properly integrated with it.

OXYGENATION TANKAGE DESIGN

The first major step in preparing a process design for a Unox system lies in the selection of what.
may be termed “independent variables.” These variables may be called independent because selection
of one does not reduce automatically the freedom of selection of the others due to the interrelation-
ships in the process. The basic independent design variables are

23


L 1 L 1 1 1 =
200 400 600 800 1000 1200 1400
Overflow Rate, gal/day/sq.ft.

Figure 11-7, Predicted secondary-clarifier performance of Unox systems
treating primary effluent (at 30% R/Q).

19


The pumps 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-storage tanks. The PSA system cannot produce LOX. Therefore, the LOX
storage must be refilled from time to time by purchased LOX, which can be delivered in rail tank
cars or trucks (at $35-$40 per ton).

The possible presence of combustible materials in wastewater from gasoline or oil spills and
industrial-plant upsets is well known to treatment-plant operators. Although such spills may occur,
dangerous volatile hydrocarbons are not expected to reach the Unox system for several reasons.
These materials are only sparingly soluble in water and tend to be stripped easily from the waste-
water by contact with the atmosphere. Volatile materials such as gasoline, therefore, normally
would be stripped from the wastewater in the sewer lines before entering the treatment plant. At
the treatment plant itself, these volatile materials would be removed further in existing comminutors,
screen chambers, grit chambers, or primary clarifiers before entering the Unox system.

The presence of volatile hydrocarbons in the oxygen-rich gas space of the Unox-system
aeration tank is not a sufficient condition to constitute a hazard; a source of ignition is also required.
The Unox-system components are designed to eliminate any potential ignition sources. As dis-
cussed earlier, no electrical components are installed under the tank cover, and no metal-to-metal
contact of moving parts is present.

Even though both an ignition source and a fuel in concentrations exceeding the LEL are
required for a hazard to exist, it is the policy in designing a Unox system to eliminate both. To
this end, combustible gas analyzers are employed in the aeration tankage to monitor continuously
the process gas for the presence of combustible material. Should an approach to an LEL limit
occur, the analyzer activates the necessary controls that cause the gas space to be purged with air,
thereby preventing a buildup of the combustible vapors to a level greater than 50 percent of the
LEL. The purging continues until the gas and liquid have carried the combustible material out of
the system.

Greases and oils are also commonly present in both municipal and industrial wastewaters.
These substances have low volatility at wastewater temperatures and pass through the system with
as much bio-oxidation and decomposition as are normally obtainable in a biological treatment
process. Greases and oils are not combustible under these conditions.

82


Lbs BODs,/1000 CP-Day

7tbs. MEVSS-Day

sR

Ibs 80D,

350

150

100

50

Range o! Secondary System
BOD, Removat = 87-377

aw
a
Za
ia
a
1 1 4 1 1 _
rn 7 Fo
Figure I|-2. Unox system, organic removal as a function of biomass loading.
fe
7
aw
aa
4s
fos
awa
See
7
47
SEC
—f a Range of Secondary System
Be “tabyfemovat sears
o/
J las
AB
&
ao
e
ca
“
L 1 1 i 4 1 —_t.
50 100 150 200 250 300 350 400

BODg,/1000 CP-Day

Figure |1-3. Unox system, organic removal as a function of organic loading.

14


Table II-1.-Completed pilot-plant operations

Duranion. Wastewater type Component Consultant
Northeast Lo. 6 | Rawdegritted Domestic
Batavia, N.Y 6 Raw degritted | Darry-product processing,
1 domestic
Mid-Atiantic cee 5 Primary effluent 50% industrial-chemical
producing, domestic
Midwest... . a 3 Primary effluent Grain processing, meat-
Intermediate trickling-filter packing, domestic
effluent, intermediate
clarifier effluent
Mid-Atlantic : 6 Primary effiuent Pulp and paper mill waste, |
Kraft process waste
Cincinnati, Ohio foo 8 Primary effluent 40% industrial, domestic Malcolm Pirme
Midwest bee 4 Primary effluent Various industries to 60%
| Q, domestic i
Grand Island, NY 3 Primary effluent | Domestic i
Mid-Atlantic i 4 Primary effluent Chrome plating, dye produc- |
I ing, exotic alloy manutac- |
i turers, domestic |
Southeast | 3 Primary effluent ) Brewery waste |
Loussville, Ky / 9 Primary effluent, primary | Ossulleres, dasry produet, Consoer & Townsend
i effluent with industrial | slaughterhouses, chemical |
1 H y manufacturers, domestic i
Miami, Fla | 3; Rew degritted | Domestic ! Miami
Middlesex, N J P41 | Raw degritted | Pulp and paper manutacturers, ; Metcalf & Eddy
| \ food processors, chemical
i { manufacturers, plastics, |
! | domestic |
Southeast i 2 Raw degritted | Textile, poultry process, i
: i i domestic \
New Orleans, La | 6 | Raw degritted | Brewers, seatood processing, | W.S Nelson
j | | poultry processing, :
| i | domestic :
Northeast ! 8 Raw degrtted , Pulp and paper manutacturers, |
\ : + plastics producers, }
' | * domestic f
West \ 5 | Prmary effluent Canning-process water, 50% — |
: industiiai, domestic j
Northeast H 9 | Primary effiuent i 30% orgame dye producing, — |
1 ' if domestic |
Northeast | 7 Primary effiuent 1 60% combined industrial, |
| \ i domesne
Northeast 9 | Primary effiuent | 60% industrial, 40% domestic
Northeast | 3 | Primary effiuent i Domestic, brewery i
Northeast | 3 | Pumary effluent | Domestic, industral !
West | 3 | Primary effiuent ; Domestic, canning waste t
Midwest | 6 | Primary effiuent ) Industriai-chemical, dairy, |
1 i | brewery, food, domestic
Northwest . | 5 | Primary effluent | Domestic, canning waste H
Charleston, W. Va j 3 i Primary effluent ; Petrochemical, domestic Union Carbide
Taft, La | 9 Lagoon effiuent | Petrochemical , Union Carbide
Southeast 6 | Primary effluent | Domestic, brewery, 1
| | nitefication i
Northeast (4 | Raw degritted | Chemical producers, coke j
| | producers, refineries, |
| domestic
Midwest 4 | Primary effluent | Domestic



bottom velocities throughout the tank, mixing should be adequate. If it is found (as will often be

the case with a weak waste) that sufficient bottom velocity will not be maintained, additional
horsepower must be added; the system then is said to be mixing limited. Brake-horsepower require-
ments for dissolution and mixing in a Unox system typically run from 0.08 to 0.14 hp per 1,000
gallons of mixed liquor under aeration, depending on the waste strength, the degree of mixing
limitation, the feed-oxygen purity used, and the mass-transfer capability of the dissolution equipment.

SUSTAINED PEAK LOADS

Most waste-treatment plants experience, a few times during the year, waste loads that can be
described as the “sustained organic peak loading.”’ This loading is the maximum organic loading
(pounds of BOD; per day, includes the combination of flow and concentration) which the plant
sees for a sustained period, usually in excess of 4 hours. As 4 or more hours are sufficient to
“upset” the system for substantially longer than the period of occurrence if essentially zero-DO
conditions prevail in the oxygenation tanks, the oxygenation system should be designed to maintain
1-2 mg/l of DO in the system in the first stage during these peak-organic-loading periods. The Unox
system has several inherent characteristics that allow it to handle these peak conditions. At peak-
load conditions, Unox systems are designed to maintain approximately 6 mg/l DO in the mixed
liquor. In the event of an unusually severe, short-duration peak, the ability to transfer about 30
percent more oxygen into the mixed liquor is achieved if the DO level decreases to 1-2 mg/l. Also,
because LOX is available for backup purposes with the same supply capacity as the installed plant,
it is possible to double the oxygen-plant flow to the oxygenation tankage. Assuming the oxygen
plant is designed to supply enough oxygen for the sustained peak condition with 90 percent utiliza-
tion, it is therefore possible to decrease the utilization to less than 50 percent. This decrease results
in a much-increased average oxygen-gas-phase concentration with a resultant increase in dissolution
capacity. Although this mode of operation is not economic over extended periods of time, for the
short term it can be quite effective.

Both of these methods for increasing dissolution capacity function by increasing the driving
force for mass transfer. The first reduces the mixed-liquor DO, while the second increases the
oxygen-saturation concentration in the mixed liquor. Neither option is available to an air system,
since the design DO level is usually already at 1 or 2 mg/l and the oxygen concentration of air is
fixed.

As the load on a plant increases, the food-to-biomass ratio increases at least proportionately.
If only the BOD;-concentration increase is responsible for the peak condition and the flow remains
the same, the MLVSS level will remain substantially the same and the food-to-biomass concentration
will indeed increase by the same amount as the organic load. If, as is more commonly the case, the
increase in organic load is a result of both flow and concentration increases, or of flow increases
alone, the clarifier-overflow rate will increase and the MLSS level will decrease. This effect results
in a food-to-biomass ratio that is increased by some amount greater than the increase in organic
load. Due to the ability of the Unox system to obtain good removals over such a wide range of
food-to-biomass ratios (see fig. II-2), this increase in food-to-biomass ratio will not be detrimental
to system performance as long as the required oxygen can be supplied. As can be seen from figure
II-4, however, an increase in food-to-biomass ratio can result in a substantial decrease in the oxygen
required per pound of BOD; removed as more biomass is formed and less endogenous respiration
occurs.

With the capability to transfer more oxygen than at the design point, and considering that a
decrease in consumption ratio usually occurs at the sustained peak condition, the Unox system has
the capability to handle organic loads from 2 to 2% times the design organic loading while still
maintaining completely aerobic conditions in the aeration tankage.

29


No

UPPER

COLUMN TURBINE |

DRIOX i
LIQUID v

OXYGEN
STORAGE _

MAIN

neath penser 8
HEATER 4)
VAPORIZER j :
WASTE No 1
°
A

OXYGEN SURGE

TANK

AFTER '
COOLER
‘
SUCTION \
FILT
Aoust LOWER
COLUMN

REVERSING HEAT EXCHANGER

AIR .
COMPRESSOR COLD BOX

MAKEUP COMPRESSOR

COOLING WATER

Figure Il1-1. Flow diagram of a cryogenic oxygen-generating system.

quality requirements; it can be considered best as a “black box” piece of equipment, except as its
characteristics affect design of the wastewater-treatment plant. The performance specification is
similar to those for blowers, pumps, and so forth.

Cryogenic oxygen plants can be turned down to approximately two-thirds full capacity,
which; by coincidence, happens to be a normal turndown for large compressors. If all of the
two-thirds plant capacity cannot be used, then the oxygen must be wasted to atmosphere through
astack.

A full standby for the primary compressor should be provided. At the Middlesex County
Sewerage Authority (in New Jersey), for a 400-ton-per-day plant, the centrifugal compressor with
adjustable-inlet guide vanes will be driven by an 8,000-hp electric motor. The demand charges for
running two compressors while changing from one to the other was so great that one compressor
will be taken offline before the second is started. This practice will result in up to 2 hours of lost
oxygen production, A full standby should also be provided for the turbine expander, which is
another large motor.


Chapter |
UNOX-SYSTEM DESCRIPTION

The Unox system uses a covered and staged oxygenation basin (fig. I-1) for contact of oxygen
gas and mixed liquor. High-purity oxygen (90-100 percent volume) enters the first stage of the
system and flows cocurrently with the wastewater being treated through the oxygenation basin.
Pressure under the tank covers is essentially atmospheric, being from 2 to 4 inches water column,
sufficient to maintain control and prevent backmixing from stage to stage. This procedure allows
for efficient oxygen use at low power requirements. Effluent mixed liquor is separated in conven-
tional gravity clarifiers, and the thickened sludge is recycled to the first stage for contact with influent
wastewater.

Mass transfer and mixing within each stage is accomplished either with surface aerators or
with a submerged-turbine rotating-sparge system. In the first case, mass transfer occurs in the gas
space; in the latter, gas is sparged into the mixed liquor where mass transfer occurs from the gas
bubbles to the bulk liquid. In both cases, the mass-transfer process is enhanced by the high oxygen-
partial pressure maintained under the tank covers in each stage. As a consequence of the increased
oxygen-partial pressure, it is feasible to maintain higher dissolved oxygen (DO) levels in the mixed
liquor relative to those achievable with air, and to achieve oxygen dissolution with substantially
lower energy inputs to the mixed liquor than would be required with air. Thus, the effective transfer
efficiency of a mass-transfer device in oxygen service will be greater than its standard transfer
efficiency, whereas the opposite is true with the same device operating in air.

The selection of the number of stages, the number of parallel biological reactors, and the type
of mass-transfer device to employ are variables that depend on waste characteristics, plant size, land
availability, treatment requirements, and other similar considerations. Jn practice, the consultant
will have essential input to the oxygen-system design or to establishing the actual design.

AERATION
VALVE \
OXYGEN 4 Q EXHAUST
FEED GAS \ _f GAS
ascpbe— To sscbe Tsscbe
LC, nt — ——t I —

te an NTN NTN YN] eee
et amen | RS mel eae
Va an yp as, eb
oor ail!

Figure |-1. Schematic diagram of Unox system with surface aerator, showing three stages.


The Linde Division of Union Carbide Corporation began active experimentation with the use
of pure oxygen in the activated-sludge process during the mid-1960’s. In 1969, bench-scale and
pilot-plant investigation culminated in a full-scale demonstration of the Unox system at Batavia,
N.Y., under Federal Government sponsorship.!»2

At the conclusion of the Batavia demonstration, Union Carbide recognized the necessity of
demonstrating the widespread applicability and highly desirable performance characteristics of the
Unox system on a variety of wastewaters. Accordingly, 10 demonstration-size pilot plants were
constructed and have been operated at approximately 30 locations in the United States, covering a
broad range of wastewater characteristics.

The economic use of oxygen in the activated-sludge process requires the availability of
relatively inexpensive, high-purity oxygen gas in tonnage quantities. This capability had been
developed in the 1950’s in the form of efficient large tonnage cryogenic air-separation plants
to produce high-purity oxygen gas. A substantial distribution network also exists in the United
States for bulk quantities of liquid oxygen. Thus oxygen use in the activated-sludge process was
feasible from an oxygen-supply standpoint, even before the process development to use oxygen
supply effectively over a broader range, through the use of pressure-swing adsorption to produce
reliable high-purity oxygen in small quantities.

Ww


Lb. 0,/Lb, BOD, Removed

—_ L 1 i L i i 1
0 0.2 o.d 3.6 0.8 1.9 1.2 14 1.6
Lbs. BOD. ,/Lbs. MLVSS-Day
Figure 1-4. Unox system, oxygen consumption as a function of F/M.
1.0F
ah

S
Qa 6
2 7
a
=
a Vip
2 4b iy
g MAMMA
4 Wi

LM)

0 n n L 1 1 1 a
t) 1 2 3 4 5 6

SRT, Days

Figure II-5. Unox system, solids accumulation as a function of solids retention time.

15


Chapter Il
OPERATING DATA AND EXPERIENCE

OPERATING EXPERIENCE AND RESULTS FROM SPECIFIC PROGRAMS

The design of Unox systems today is based upon published technology for design of conven-
tional activated-sludge processes and upon a very large amount of data accumulated in pilot-plant
demonstrations and Union Carbide-funded development programs. Operating experience with
oxygen-activated-sludge systems commenced with pilot programs before the Government-funded
Batavia demonstration work. Since the conclusion of the initial work at Batavia, a comprehensive
pilot-plant program has been maintained using a total of 10 pilot plants. These pilot plants have
been and are being used throughout the United States (and in certain foreign countries) to demon-
strate the Unox-system capabilities to consultants, municipalities, and industries, and to obtain
design data for the specific waste stream being studied. As of this time, programs have been com-
pleted in a total of 30 locations encompassing nearly 18 years of operating experience (table II-1).
Programs are currently underway in eight additional locations (table II-2).

A typical pilot-plant program will involve setting up a pilot plant at the customer’s site and
assisting the customer’s personnel in the operation of the plant. All analytical work normally is
performed by the customer at his plant laboratory. Most pilot-plant programs have extended over
a 4-6-month period, although some have been conducted for as long as 1 year. Each program
normally includes one phase of operation at steady-state flow at approximately the biomass loading
anticipated for a system design. Following this phase, the plant is operated on a diurnal flow cycle
to simulate the anticipated hydraulic pattern expected at the site. Subsequent operational phases
will include sustained operation at high biomass loadings or high hydraulic flows to simulate more
severe conditions than normally expected in the actual design. During each operating phase a full
complement of data is taken in the plant by regularly sampling influent, effluent, mixed liquor,
recycle, and sludge-waste streams and by regularly reading all plant-operating indicators 24 hours
per day, 7 days per week.

Two types of pilot plants are used in these applications. The majority of the plants are mounted
within 40-foot warehouse van trailers, so that they can be easily transported around the country
and simply placed into operation. The others are somewhat smaller portable plants that can be
relocated quite easily but require a building for weather protection, as well as slightly more exten-
sive installation service than is required for the mobile plants. The mobile plant has a nominal
capacity of up to 43,000 gpd, whereas the portables operate at up to 7,500 gpd.

The mobile unit is a completely enclosed, secondary wastewater-treatment system with an
external secondary clarification unit. All tankage, instrumentation and controls, pumps, and a
small laboratory are contained within the 40-foot warehouse van trailer. A schematic diagram of
a three-stage unit is presented in figure II-1.

As indicated, the oxygenation tankage in the mobile pilot facility is divided into four sections,
or stages, by means of baffles, and is covered to provide a gastight enclosure. The liquid and gas
phases flow concurrently through the system. Raw wastewater, recycle-sludge, and oxygen gas are
introduced into the first stage. The pilot unit has the flexibility to be operated as a two-, three-, or


Chapter Ill
PROCESS DESIGN

REQUIRED PROCESS INFORMATION

The proper design of any secondary waste-treatment process requires certain basic information
concerning the waste to be treated; the Unox system is not an exception. In general, this information
can be classified as follows:

@ Wastewater quantity
© Wastewater quality
@ Effluent quality required

© Definition of the other unit operations or existing facilities in the treatment plant and their
relationship to the secondary system

Table III-1 generally shows the required data. The following paragraphs elaborate on each of
the foregoing categories and outline the most likely sources of the information. It will be apparent
that it is not always possible to obtain all the required information; therefore, it is usually necessary
to make some judgments and, consequently, to build some conservatism into the design to account
both for inadequate and potentially inaccurate data. The degree of conservatism required, however,
can be assessed better by a recognition of those parameters which affect process performance and
the resultant appreciation for the quantity of missing information.

Wastewater quantity is, of course, characterized by the rate of flow. Since this rate varies
with time it is necessary to have information on the daily diurnal fluctuations as well as on the
average and peak flows during the year. This information is generally an extrapolation of existing
information into the future and in some instances, where no facilities now exist, for a given facility
it is a forecast based on experiences at similar facilities located elsewhere. The information is
supplied by the customer’s engineer and is used, along with process characteristics, to ascertain
a design flow and corresponding peak and minimum flows.

The phrase ‘‘wastewater quality” is used herein to describe characteristics of the wastewater
that result from the components it contains. The term includes such familiar parameters.as BOD,
and COD. These parameters measure the oxygen requirements of the wastewater. The diurnal
variations and annual average and peak values should, therefore, be known in order to quantify
accurately the instantaneous and average oxygen demands in the secondary system. These values
are also supplied by the customer’s engineer and, along with the design flow, serve to define a
design loading usually expressed in pounds of BODs per day. Another parameter that helps to
characterize the wastewater is the source. Is it municipal, or partially or totally industrial, and
what is the industry involved? Also important, especially as it affects the expected settling charac-
ter of the mixed liquor and the quantity of waste activated sludge for disposal, is the treatment
preceding the secondary system. If the waste is only degritted the MLSS will settle better and will
contain a lower volatile fraction than if the waste has recéived primary settling. Also, more waste

21


EPA-625/4-73-003a Revised

Oxygen Activated

Sludge Wastewater
Treatment Systems

Design Criteria and Operating Experience

EPA Technology Transfer Seminar Publication



1.

2.

Appendix B
SPECIFICATIONS FOR OXYGENATION TANKS*

BOD removals are specified approximately 90 percent.

Tank volume is based on 160 pounds of BOD; per 1,000 cubic feet (average); Unox

engineers believe that this base can be increased to 215 pounds or higher.

3.

4.

10.

MLSS specifications are 5,500 mg/l.

MLVSS specifications are 5,000 mg/l.

. F/MLVSS ratio is 0.510; Unox believes that this ratio can be much higher.

. Mixed-liquor DO is targeted at 3-9 mg/l.

. Purity of applied GOX is as follows: cryogenic, 95 percent minimum; PSA, 90 percent.
. Oxygen in waste gas is specified at 50 percent.

. Applied oxygen consumed is 90 percent.

Mixing equipment and sparger:
10.1 Sparger and mixer are on same shaft.
10.2 Shaft is hollow and carries compressed oxygen-rich gas to sparger.

10.3 Sparger and mixer turn at a constant speed, which will keep contents of oxygena-
tion tanks mixed.

10.4 Mixer may be ship-type propeller or pitched-blade turbine.

10.5 Oxygen compressors for oxygenation tanks are centrifugal, with suction throttling.

10.6 For Middlesex County Sewerage Authority (MCSA), mixer is 6,000 hp and com-
pressor is 5,100 hp, of which 1,900 is standby. All of the mixer horsepower is con-
nected and in operation at all times. MCSA expects that total power for mixing
and dissolution will be less than 0.161 kW-h per pound 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 part of shaft-and-mixer-skid
assembly.

*Prepared by Ariel Thomas, Metcalf & Eddy, Engineers, New York, N.Y.

45


CONTROL OXYGEN

VENT
VALVE — PRESSURE SIGNAL
AERATION
1 AGITATOR at i
' \ COVER
oe = \ Jo 1h CLARIFIER
1 T] n
Reba! Nb=z UT xredbsz
wasTewaTeR | [Ce

FEED ==”,

(IC OO C)ror | 7 ee

com

——> = =e

_ll alt

RETURN ACTIVATED SLUOGE

in
WASTE
ACTIVATED SLUDGE

Figure II-1. Schematic diagram, three-stage Unox system.


Table }-2.—Current pilot-plant operations

Location Wastewater type Component

Biue Plains, Washingfon, D.C. . . Primary effluent Domestic

1
}
i

water

|
Southwest . : see eee Holding pond effluent Petrochemical
Midwest... 00. we Se eee Raw degritted | 30% combined industrial,

| 70% municipal
West. «2... Se eee Primary effluent | Combined industrial, municipal
Northeast Primary effluent | Domestic
Southeast ....... see . Stabilization pond effluent Organic, chemical waste
Northeast . bee nee eee . Secondary effluent | Domestic, nitrification
Northeast... ee eee ones | Coke oven waste
Northeast. cee ee . cea Raw degritted { Combined municipal, industrial
West... eee . i" and paper mill process

a

four-stage system. The number of stages employed may vary, depending on the specific application.
Each of the stages is a completely mixed unit with the overall four-stage system approximating a
plug-flow-type reactor. The wastewater and the biomass return are mixed upon entering the first
stage. The pilot-plant wastewater-feed and sludge-recycle pumps are calibrated, variable-speed units.
Their operating speeds are used to monitor the influent and recycle flows.

During operation, high-purity oxygen gas is fed into the first-stage gas space; the gas pressure
is maintained at about 1 inch of water above the atmospheric. The oxygen-enriched gas flows
through interstage gas passages and is vented to the atmosphere through a volumetric flowmeter.
The slight pressure drop between successive stages is sufficient to prevent backmixing of the oxygen-
enriched gas. The staged gas-phase oxygen compositions are measured periodically with a paramag-
netic oxygen analyzer, As the oxygen-enriched atmosphere passes through the system, small dia-
phragm compressors in each stage pump the gas down the hollow mixer shaft and through the rotating
sparger at a rate sufficient to maintain a DO concentration of approximately 7 mg/]. The oxygen-gas
feed 1s measured volumetrically and controlled automatically by the gas-phase pressure in the first
stage.

Effluent from the biological reactor flows to a center feed clarifier, which contains a peripheral
effluent weir and a plow-type scraper with a center takeoff for settled solids.

In addition to the monitoring of gas and the liquid flows by appropriate metering and recording
equipment, several basic parameters are measured to determine the system performance. Composite
samples of the bioreactor influent, the clarifier effluent, and the sludge recycle are formed from grab
samples taken frequently. Wastewater-flow and gas-flow measurements are taken to correspond to
each of the composites formed.

Grab samples of the mixed liquor from stages 1 and 4 are taken three times daily for solids and
settling tests. Solids are wasted from the clarifier semicontinuously in an attempt to approximate
a full-scale system. The volumes wasted are measured, and grab samples of the solids recycle are
taken for solids analysis. Grab samples are also taken from stages 2 and 3 once each day for solids
analysis. Virtually all gf the analytical procedures are carried out as prescribed in the 13th edition
of Standard Methods.*

The results from many of the pilot-plant operations are included in tables H-3 to II-7.


Euclid, Ohio

One evaluation has been released on a plant substantially smaller than the Detroit and
Middlesex jobs. The Euclid, Ohio, design was prepared by Havens and Emerson for a plant
size of 22 mgd. The published economics (see table VI-4) indicated a total capital savings of
20 percent for Unox.

Table V1-4.—Economic evaluation, Euclid, Ohio

Cost component Unox Air-activated sludge
Dollars
Capital... 6.2.2 eee tee 10,000,000 12,000,000
Operating cost per year ........- 1,120,000 1,180,000

Union Carbide Cost Projections

Union Carbide has attempted some economic analysis; however, generalization is difficult
because of the many different circumstances existing in each specific location. The value of land,
piling requirements, and method of sludge disposal, among other factors, always affect the specific
case. The published economics in the EPA report! covering the initial demonstration of oxygen
at Batavia is an attempt at generalization. While the absolute numbers certainly are not applicable
for every specific treatment plant, Union Carbide does find that the general conclusions appear
valid. In general, the economic attractiveness of oxygen improves as plant size increases, because
large-size oxygen generators are more cost effective. At the time of this report, oxygen has been
selected at a number of plants in the 4-6-mgd range, but below that size economic factors have
not been favorable except in the industrial market, where high-strength wastes are being treated.
Union Carbide is currently completing technical and market development work of preengineered
plants in the 1-5-mgd range employing pure oxygen. The projected installed costs for these plants
appear in table VI-5. Costs as presented include all tankage and equipment, as well as installation.

Preengineered Unox plants can be offered with oxygen supply from LOX or from onsite
oxygen generators. The operating costs of these plants are comparable throughout the range, and,
therefore, oxygen systems can be expected to compete favorably with air systems in this plant-size
range.

Table V1-5.—-Projected installed-plant costs

Capacity, mgd Unox Conventional air

Dollars

130,000 130,000
320,000-400,000 370,000-390,000
530,000-700,000 700,000-750,000

40


the first adsorber is regenerated. During regeneration the impurities are cleaned from the adsorbent
so that the bed will be available again for the adsorption step. Regeneration is carried out by de-

pressurizing to atmospheric pressure, purging with some of the oxygen, and repressurizing back to
the pressure of the feed air.

As discussed earlier, the driving force on the oxygen-gas generator is a simple, relatively low-
horsepower air compressor. An obvious and totally acceptable form of overall system backup
would, therefore, be a spare feed-air compressor, which would supply essentially 100 percent backup
capability. A more desirable, effective, and flexible form of oxygen-supply backup, however, can
be obtained through the use of onsite LOX storage in conjunction with the onsite gas generator.
This backup is in the form of from one to several days’ production capacity of LOX storage. This
method not only provides absolutely failsafe backup to the onsite oxygen-gas generator, but it
also adds the capability of instantaneous delivery of substantial additional oxygen capacity to the
aeration tanks to handle peak-load conditions. Such peak or above-average loads otherwise could
be handled only by substantially oversizing the oxygen generator.

The PSA process is more economical for less than 50-30 tons of oxygen per uay. PSA oxygen
generators produce oxygen of 88-90 percent purity. Three or four absorption units are used to
permit continuous oxygen production. In comparison with cryogenic units, the oxygen is less pure,
but the unit turndown is limited only by the compressor capacity. Because of their turndown
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.

The PSA generator employing four adsorption units uses 24-30 valves, most of which are
operating every 2-5 minutes. The dependability of these valves is of paramount importance—if one
valve malfunctions, the entire unit shuts down. The possibility has been considered of providing
an extra adsorption unit that could be used to replace a malfunctioning unit; however, the inter-
relationship of the four units and the 24-30 valves makes this difficult. Standby is provided from
LOX storage.

Both PSA and cryogenic plants have efficient turndown capability, with the PSA units being
capable of turning down with only a small loss in efficiency throughout the entire range of their
production capacity. Although the cryogenic plant cannot turn down as far, it does have the capa-
bility to produce LOX during low-load periods. This liquid can be stored and used to supplement
the plant during periods of high load. Therefore, although the cryogenic plant cannot turn down
all the way, it can be installed at a capacity somewhat less than the sustained peak would demand.

OPERATING-POWER REQUIREMENTS

Given the basic gas-liquid mass-transfer capabilities for the dissolution equipment selected,
the operating horsepower for oxygen transfer can be determined for the oxygen-gas-phase
concentrations, oxygen-consumption requirements, wastewater alpha and beta values, temperature,
and operating DO level. The gas-phase concentration varies throughout the system, with the
highest concentration (highest transfer driving force) present at the front end of the system where
the highest demand exists. Thus, in comparison with air systems, a fourfold to fivefold increase can
be attained in the latter stages where the gas is vented at 40-60 percent oxygen concentration.

Once the dissolution-power requirements have been determined, a check should be made to
be sure that sufficient energy is supplied with the selected dissolution equipment to maintain
proper mixing biomass in suspension. Generally speaking, if the equipment can maintain sufficient

28


10.10 Spare mixer motor, speed reducer, shaft, propeller, and sparger must be stored onsite,

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 are designed for 100 pounds of live load and 4 inches of vacuum.

13. Oxygen feed into the first pass is controlled by pressure under the covers in the first pass.
The approximate 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.
Vent-gas Oy purity can be varied.

15. DO in each pass is controlled by the rate of discharge of compressed oxygen-rich 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.

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 increases the amount of oxygen required rises at a decreasing rate
(see fig. B-1), Note that, at the design loading of 160 pounds of BOD; per 1,000 cubic feet, 1.8
pounds of oxygen are required to remove 1 pound of BODs. As the loading increases to 255, the
amount of oxygen required to remove 1 pound of BODs drops to 1.3 pounds.

Ib Oj FED/Ib REMOVED
2

° 100 125 150 160 175 200 225 250 275 300

BOD LOADING {Ibs /1000 ft3)

Figure B-1. Oxygen-demand curve. U.S. Enviconmental Protection J
Region V, Library
46 230 South Dearborn Street
Chicago, Mlinois 60604


OXYGEN

f Dt @PPSUPPLY
—
<P

NITROGEN
RICH WASTE

COOLING
WATER
SYSTEM

Hoga

AIR AIR
FILTER rose ORIOX BACKUP
WATER
SEPARATOR

Figure 111-2. Pressure-swing adsorption oxygen generator for Unox systems.

All pipes and equipment containing, processing, or transporting liquid oxygen (LOX) 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 point not insulated.

Cryogenic gas oxygen will cost municipalities approximately $8 per ton (capital, operation,
and maintenance) at capacity production. LOX costs approximately $35 per ton.

Cryogenic plants must be shut down for a period of 5-10 days once every 112-2 years for
deriming (cleaning). A cryogenic plant can produce LOX or gaseous oxygen (GOX), or various
combinations of the two, depending on how it is designed. Each ton of LOX produced will
reduce GOX by about 4 tons.

A cryogenic plant takes 1-3 days to start up, depending on whether LOX is available or not.
It is not practical to meet variable demands by starting and stopping cryogenic units.

Dividing a cryogenic demand into two or more plants can help to meet economically 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 such division will not be useful for unpredictable
variations.

The PSA system employs a multibed adsorption process to provide a continuous flow of
oxygen gas. Figure III-2 shows a schematic diagram of the four-bed embodiment. The feed air
is compressed and passed through one of the adsorbers. The adsorbent removes the carbon
dioxide, water, and nitrogen gas, producing relatively high-purity oxygen. While one bed is
adsorbing, the others are in various stages of regeneration. The PSA oxygen generator operates
on a PSA concept in which the oxygen is separated from the feed air by adsorption at high
pressure (30-60 psig) and the adsorbent is regenerated by ‘‘blowdown” to low pressure. The
process operates on a repeated cycle having two basic steps, adsorption and regeneration. During
the adsorption step, feed air flows through one of the adsorber vessels until the adsorbent is
partially loaded with impurity. At that time the feed-air flow is switched to another adsorber and

27


The Unox system has other important benefits. The maintenance of high DO levels with low
energy input to the mixed liquor contributes to a highly aerobic biological mass that flocculates
well. These factors are responsible for enhanced settling characteristics relative to air systems, so that
with conventional clarification designs a much higher level of mixed-liquor suspended solids (MLSS)
(4,000-8,000 mg/l) can be carried in the biological reactor than is normally achievable with air-
activated sludge. This factor is used in design to reduce the required reactor detention time while
still maintaining a design food-to-biomass (F/M) level very similar to common practice in air systems,
High DO levels are also an important factor contributing to lower excess solids production than is
commonly achieved with air-activated-sludge systems at comparable biomass loadings.

Since the oxygen gas fed to the system is devoid of nearly all nitrogen, and since approximately
90 percent of the oxygen gas normally is used, the total gas venting from the system is only about
1 percent of the gas vented from an air-activated-sludge system. Since the reactors are covered,
this gas is vented at one point; thus effective odor control is achieved and the biological aerosol
problem typical of air systems is eliminated.

The process control of the Unox system is very simple. Since the tank is covered, the biologi-
cal reactor acts essentially as a respirometer. Thus, pressure control can be used to control the oxygen
feed rate. A simple pressure sensor is installed in the first-stage tank cover; this device detects changes
in gas pressure resulting from a decrease or increase in oxygen uptake as flow or strength changes.

A signal is relayed to a flow-control valve on the inlet oxygen line, which adjusts oxygen flow to
maintain the desired gas-pressure setpoint under the first-stage cover. Thus, if flow or 5-day bio-
chemical oxygen demand (BOD; ) increases, resulting in a lowering of gas pressure in the first-stage
gas space, the control valve will open causing more oxygen to flow to the system. This simple control
circuit provides real-time response to the BOD; demands placed on the system. It is also possible

to automate the oxygen-production unit to respond to these changes in demand, thereby causing

the system to use only the appropriate power for oxygen generation commensurate with the BOD;
load placed on it.


10. FL]
Hees!
OXYGEN

ae BIOMASS
= ie
aS Jain \Setex
G Biqmass \EE aaa
% ste
é y
210 =
z nets:
5 ASS:
a
2
3
z

04
1000 10,000 100,000

CONCENTRATION,
mast

Figure II-6. Settling characteristics for air and oxygen biomass, initial settling rate
versus concentration.

The data presented on Unox systems were obtained for biomass developed from primary-
effluent wastewaters. The expected settling velocities would be higher than indicated in figure II-6
for biomass developed from degritted wastewater, so that higher MLSS concentrations can be
expected than those shown in figures II-7 and II-8 at comparable overflow rates.

18


provided on or near the tanks. Each compressor is manifolded to deliver oxygen gas to all similar
stages in the tanks. The compressors are designed to the same high standards as the mixing assem-
blies and are instrumented similarly to warn of possible impending problems. Automatic shutdown
of any unit is provided before extensive damage can occur to the machinery. Automatic shutdown
of equipment is a feature afforded only in the Unox system, since process backup is provided by the
other equipment on stream.

Cryogenic-Oxygen-Generation Plant

The oxygen-generation plant is an automatically controlled unit. It is composed of the follow-
ing major components:

© Air-suction filter house

© Centrifugal air compressor, driven by an electric motor, with interstage cooling after each
stage

® Air-surge tank to absorb the cyclic variation in air flow to the reversing heat exchangers

® Air-separation unit or cold box consisting of a column and reboiler, gel traps and super-
heater, and reversing heat exchangers—cryogenic equipment installed in cylindrical casings
faloricated from carbon steel, perlite insulated, and maintained under a slight positive
pressure to prevent moist air from entering the insulation space; cold box factory fabricated
with most internal parts made of aluminum

®@ Cryogenic expansion turbine-—process air admitted radially inward through variable-area
nozzles, expanded in the turbine impellers, and exhausted axially; the impeller shaft directly
connected to the blower; turbine-seal-gas system, the turbine-lube-oil system supplied with
the turbine; oil temperature controlled, and backup for turbine coastdown supplied by an
oil-filled, pressurized accumulator

@ One thaw heater assembly to thaw the cryogenic cold box, including gel traps during
turnarounds

© Cooling tower to supply required intercooler, and aftercooling of the gas leaving the air
compressor

In addition to the foregoing major components, all the instruments, piping, controls, and
instrument panels necessary to make this plant functional are included.

INSTRUMENTATION

An integrated-contro] system is included, which automatically controls the oxygen-gas flow
to match the demand of the oxygen load on the system. The oxygen-generating plant increases
or decreases its output automatically in response to the system.

A main panel is supplied with a vent-gas analyzer and indicator, a combustible-gas analyzer
and recorder, a feed-gas-flow controller, and a central alarm that will sound in case of an impending
malfunction of the unit.

Stage pressure/vacuum relief devices are provided, as are appropriate shutdown switches for
the mechanical equipment.

35


OXYGEN-BACKUP FACILITIES

A Driox storage tank and vaporizer will be provided with capacity for at least 24 hours of
oxygen backup at the rated generation-plant-product automatic flow.

36


© Clarifier overflow rate

®@ Recycle ratio

® Food-to-biomass ratio

@ MLVSS (mixed-liquor volatile suspended solids) concentration

@ Aeration-tank geometry

For conventional gravity secondary clarifiers, the MLVSS concentration is, of course, related directly
to the secondary-clarifier design. The sludge-settling characteristics, the clarifier-overflow rate, and

the recycle fraction directly determine the MLVSS levels attainable in the oxygenation tank, as
discussed earlier.

The food-to-biomass ratio initially is selected at a value at which the system will operate
properly in terms of required removals, both at design and sustained-peak loadings. The range of
food-to-biomass ratios under which Unox systems have been run at high removal efficiencies on a
variety of wastes is shown in figure II-2. The design-level food-to-biomass ratio is selected such that
the sustained peak food-to-biomass ratio lies within the range of proven operation. Typically, the
peaks and design points will be such that the design food-to-biomass ratio is between 0.5 and 0.8.
Following selection of the food-to-biomass ratio at the design point, it remains only to determine
the MLVSS level to have dictated the required aeration-tank volume. This MLVSS level (used as
a measure of the viable organisms in the mixed liquor) is, of course, dependent on the total MLSS
level, which, in turn, is dependent on the clarifier-overflow rate, recycle ratio, type of waste, and
type of pretreatment. Figure IJ-8 shows the expected clarifier-underflow concentrations for Unox
systems as a function of overflow rate at a given recycle ratio. As discussed earlier, the type of
waste and the pretreatment received will dictate where within the band of expected underflow
concentration the case under consideration will fall. Once an R/Q has been specified and a value
of underflow concentration determined, MLSS level is determined by material balance (ignoring
influent solids and the net solids production within the aeration tanks relative to the amount of
solids present in the recycle stream) (see fig. II-7).

The recycle ratio is selected as follows. The higher the R/Q, the higher the mixed-liquor-
solids concentration will be (up to about 100 percent recycle); but the clarifier-underflow concen-
tration will be correspondingly lower and the subsequent treatment of the waste activated sludge
correspondingly more difficult. The recycle ratio should be selected to maximize the MLSS con-
centration, recognizing that clarifier-underflow concentration will diminish as recycle fraction
increases. A value of 0.3 for the recycle ratio typically is selected for a Unox-system design.

Once the MLSS concentration has been determined, the volatile fraction must be ascertained
in order to specify the system volume. Pilot-plant data are the best source for this information;
and, as can be seen from tables II-3 to II-7, values for degritted waste can vary from 0.66 to 0.83,
and for primary-treatment waste from 0.72 to 0.90. In the absence of specific pilot-plant data,
and due to the large quantity of Unox-system pilot-plant data, a good judgment as to the expected
value for the volatile fraction can be made by knowing the waste source.

Knowing the design-point BOD, load, food-to-biomass ratio, and MLVSS level, the detention
time (based on raw flow) is fixed and, given the design flow, so is the oxygenation-tank volume.
With the information in hand concerning land-area restriction, piling requirements, restrictions
imposed by existing tankage, and so forth, the economic tank depth can be ascertained. Thus, the
area of the tankage is determined, and it remains only to specify the number of parallel bioreactors
and the number of stages per bioreactor to have specified completely the oxygenation tankage.
Generally, at least two parallel bioreactors are used with at least three stages per bioreactor.

24


CONTENTS

Introduction
Chapter I. Unox-System Description .

Chapter Il. Operating Data and Experience

Operating Experience and Results From Specific Programs :

Correlation of Results for Design .

Chapter III. Process Design . :
Required Process Information .
Oxygenation Tankage Design
Oxygen Requirements .

Oxygen Supply

Operating-Power Requirements
Sustained Peak Loads . . .
Activated-Sludge-Waste Quantity .

Chapter IV. Process Safety .
Oxygen Gas. . . :
Oxygen and the Unox System .

Chapter V. Unox-System Scope of Supply -
General . . : .
Surface-. ‘Aerator: PSA System
Submerged-Turbine- Cryogenic System
Instrumentation
Oxygen-Backup Facilities

Chapter VI. Economic Considerations
Direct Factors .
Indirect Factors
Scope of Supply .
Comparative Analyses .
References
Appendix A. Specifications for Final Settling Tanks .

Appendix B. Specifications for Oxygenation Tanks

iii

Page


SOLID-LIQUID INTERFACE, mi.

1000

800

600

400

200

1000

~~ 800
=
wi
ra}
zt
¢
o 600
a
#
Z
2
B 400
2
a :
3
® 200 —— + |
i
ho} |
| |
|
| | | |
° 5 10 15 20. ~«225°~C«O
TIME, MIN
INITIAL
INITIAL SOLIDS SETTLING
CONCENTRATION, VELOCITY
DATE CURVE MG/L FPH
2/25-26 [a] 4200 64
wk. of 5/16 iG 5400 49
wk, of 6/6 fel 6400 2.9
12/8/70 p! 7200 16

|

\
h

\

os 1 2 3 4

TIME,HR

Figure A-1. Typical solids-settleability curves.

44


REFERENCES

1 Environmental Protection Agency, Project No. 17050DNW, May 1970.
2Environmental Protection Agency, Project No. 17050DNW, Feb. 1972.

3Standard Methods for the Examination of Water and Wastewater, 18th ed., American Public
Health Association, New York, N.Y., 1971.

AL


Table |1-5.—Unox-system pilot-test data, performance summary for locations E and F

Location E Location F
Component Unit ——
Phase | | Phase I! Phase | Phase 1
Retentiontime.. 6... eee Hours 18 18 1.7 (1.1-2.8) 1.3:(1.2-1.5)
Recycle ratio . . i Percent 28 26 27 26
Sewage temperature | °F 70 78 87 87
MLSS..... mall 5,600 | 7,350 6,200 5,950
VSS/TSS . | 066 0.74 0.75 0.83
Organic loading . | Pounds BOD per | 181 193 162 196
1,000 ft3 /day {
Biomass loading ..... bese . | Pounds BOD per 0.76 0.66 055 0.64
pound MLVSS/day

Influent characteristics \

cop an we. | mali 1 308 377 380 398

BOD, : re 1] | 210 229 184 170

‘Suspended solids . bee mg/l | 191 | 236 183 172
Effluent characteristics | |

cop .. a vee ee | magi | 64 | 66 67 63

BOD, . . . cee . - | mg/l | 12 : 12 6 5

Suspended solids 5... . mg/l | 1%) 8 7 13
Removals t {

cop .. Percent | 79; 82 82 | 84

BOD, . ... bones Percent | 94 | 95 97 | 97

Suspended solids | Percent | 98 | 88 91 92
Clarifier overflow rate | gal/ft2/day | 650 650 | 730 (420-1,060) | 890 (780-1,000)
Clarifier mass loading _— | Pounds SS/ft2/day | 37 50 41 52
Clarifier underflow concentratio! . | Percent a5 i 32 27 23
Sludge-volume index... . woe de | 79 48 56 55
Pounds VSS per pound BODg removed |

Accumulation Looaee wef | 053 | 0.38 0.31 036

For disposal. . bocce - 045 | 027
Pounds O2 per pound BODg removed. - 081 | 1.03 1.22 1.07
Pounds 09 per pound COD removed . ... - 065 | 072 070 053
COD/BOD ratio. : - 1.47 164 206 232
Solids retention time _ oo de 25 47 5.85 425

jo. —

system, and oxygen-containing gas is monitored for both flow and concentration, it is apparent that
the oxygen consumed per unit of BOD; removed may be determined readily. Figure II-4 presents
a correlation of oxygen consumption per unit BOD; removal as a function of biomass loading for
several plants and types of waste. At high food-to-micro-organism ratios, oxygen requirement per
unit BOD; removal is governed mainly by oxygen required for cell synthesis, since the degree of
endogenous respiration is relatively low as is indicated by higher excess sludge production at high
food-to-micro-organism ratios. At low food-to-micro-organism ratios the degree of auto-oxidation
increases, the oxygen requirement increases, and the quantity of excess sludge produced decreases.

A second, and most important, factor for determining the oxygen requirement is the COD/BOD;,
(chemical oxygen demand to BOD; ) ratio. Increasing the COD/BOD; ratio results in increasing the
oxygen consumption per unit removal, which is opposite to the effect of increasing F/M.

The COD test measures the total oxidation potential of the wastewater substrate. This test
includes biodegradable materials as well as nonbiodegradable or refractory substances. The BOD;
test measures some fraction of the biodegradability relative to the COD. In an activated-sludge
process, the amount of substrate removed biologically generally will exceed the indicated BODs.
This incremental removal in excess of the measured influent BOD, represents biodegradable COD
not entirely detected in the standard BOD, test. Thus, the COD/BOD, ratio is indicative of the
relative (biological) toughness and total organic strength of the waste.


Table 11-7.-Unox-system pilot-test data, performance summary for locations | and J

Location | Location J
Component Unit
Phase | | Phase 111 | Phase 1? | Phase It
Retention time Hours 18 1.85 15 0.75
Recycle ratio Percent 29 42 25 25
‘Sewage temperature oF 65 64 73 64
MLSS. . mail 4,700 | 4200 | 4,000 | 4,200
VSS/TSS ... . 084 | 085 0.72 0.76
Organic loading... 2... ss Pounds BOD per 350 280 117 286
1,000 ft3/day .
Biomass loading... 6.6.6 eee eee eee Pounds BOD per 141 42 067 1.44
Pound MLVSS/day

Influent characteristics.

cop mail 826 825 260 400

BODg .. Loeb e cee e cece scence vee ma/l 415 331 133 143

Suspended solids... 2c eee eee eee eee mai 180 386 160 136
Effluent characteristics.

cop... . mg/l 99 132 57 95

BOD, ........ mg/l 12 23 " 15

Suspended solids mg/l 18 42 19 32
Removals:

cop Percent 88 84 8 76

BODs _ Percent 97 93 92 89

Suspended solids Percent 90 89 87 7
Clarifier overflow rate... .. gal/#t?/day 520 530 588 782
Clarifier mass loading... . .. : Pounds SS/ft2/day | 31.0 26.0 24.4 34.2
Clarifier underflow concentration . Percent 15 13 19 18
Sludge-volume index ....... : 82 85 106 94
Pounds VSS per pound BODg removed:

‘Accumulation bene - 0.67 1.23 0.61 1.10

For disposal . : 0.64 1.08 0.46 0.86
Pounds Oo per pound BODg removed . - 0.85 090 | 71.60 0.98
Pounds 02 per pound COD removed . : : 0.42 0.36 0.86 0.40
COD/BOD ratio... .... wees tee eee ee : 2.00 2.50 1.95 2.80
Solids retention time J. 1.06 0.68 2.44 0.63

1Unclarified trickling-filter effluent.
Partial nitrification occurring.

approach, with all streams entering the first stage, the food-to-micro-organism ratio decreases
rapidly from stage to stage. Therefore, a high degree of stabilization occurs in the latter stages,
resulting in decreased sludge production. It has also been determined that a high-DO environment
will result in a lower sludge yield owing to the highly aerobic character of the biological floc. In
essence, this means that all floc particles are in a working mode and, when placed in a food-limiting
situation, will therefore undergo a higher degree of endogenous respiration or auto-oxidation than
will floc in a low-DO environment. The measurement of sludge production in any biological
process is difficult and requires careful control of wasting schedules and system-sludge-inventory
levels, as well as careful analytical monitoring of the system. If care is not taken, misleading
results easily can be obtained. In Unox pilot-plant programs, extensive solids-production data are
taken regularly.

Figure II-5 represents a relationship between excess sludge production and solids retention time,
which has been developed from some of the Unox programs. As indicated, sludge production in-
creases as the solids retention time (SRT) decreases (food-to-biomass ratio increases). Higher
biomass loadings result in an increase in cell synthesis and a decrease in endogenous respiration.

The phase-average data points presented have been corrected for changes in system inventory due
to solids accumulation or losses that may be attributed to sludge production.

13


ACKNOWLEDGMENTS

This seminar publication contains materials prepared for the
U.S, Environmental Protection Agency Technology Transfer Program
and has been presented at Technology Transfer design seminars
throughout the United States.

The information in this publication was prepared by E. A.
Wilcox, representing Union Carbide Corporation, Linde Division,
Tonawanda, N.Y., and Ariel Thomas, representing Metcalf & Eddy,
Engineers, New York, N.Y.

NOTICE

The mention of trade names or commercial products in this publication
is for illustration purposes, and does not constitute endorsement or recom-
mendation for use by the U.S, Environmental Protection Agency.

ween rte

Revised January 1974


gearing has a service factor of 2 plus 200 percent overload-rating capabilities. The antifriction
bearings are designed to the AFBMA (B-10) life rating of 75,000-100,000 hours, depending on
the severity of the application. All components submerged in the mixed liquor are made of
stainless steel or other materials proven to give prolonged service in the mixed-liquor environment.

PSA Equipment

The oxygen-generation plant is an automatically controlled unit. It is made up of an adsorp-
tion unit and a compressor unit. The adsorption unit is designed as a complete package containing
adsorbent vessels, valve-and-piping skids, and controls. The vessels contain sufficient adsorbent,
supplied by Union Carbide, to upgrade air to the desired purity. The vessel design is such as will
minimize air losses.

The valve-and-piping skid contains pneumatically operated automatic valves mounted directly
on the skid. The valves selected have been demonstrated to be leaktight after 1-2 million cycles.
In addition, the PSA skid contains all necessary valves and controls required for flow and pressure
control. It also contains local instrumentation.

An air drier is provided to dry the air used for the instrumentation. This drier is mounted
on the PSA-unit skid, is suitable for outdoor installation, and is capable of automatic operation.

The compressor unit is completely assembled on a common skid and ready for immediate
mounting on a concrete foundation. This skid assembly includes a nonlubricated compressor,
electric-motor driver, aftercooler, moisture separator, discharge-pulsation dampener (if required),
inlet air filter, high-discharge temperature- and pressure-shutdown switches, lube system, and all
interconnecting piping and valves. A separate skid contains the cooling system. The equipment
provided is of the highest quality and is designed to operate for many years with only routine
maintenance. It is also suitable for unprotected outdoor installation, and includes any special
protection or provisions necessary to provide continuous year-round operation.

SUBMERGED-TURBINE-CRYOGENIC SYSTEM

Dissolution Equipment

The oxygenation tanks will be fitted with the number of mixing and oxygen-dissolution
assemblies specified in the design. Each mixing assembly consists of an electric-motor-driven
speed reducer, propeller, and gas injection sparger totally integrated and assembled to a base
plate that bolts to the tank cover. All equipment used is designed to provide long, maintenance-
free service. The speed reducers are specified to have an overall AGMA service factor of 2, while
other components critical to the continuous operation of the mixing assembly are designed for
service factors of up to 5. Gearing and bearings of proven quality are used throughout the speed
reducers. The gearing has a service factor of 2 plus 200 percent overload-rating capabilities. The
antifriction bearings are designed to the AFBMA (B-10) life rating of 75,000-100,000 hours,
depending on the severity of the application. All components submerged in the mixed liquor
are made from stainless steel or other materials proven to give prolonged, trouble-free service in
the mixed-liquor environment. The entire mixing and oxygen-dissolution assembly is instrumented
with simple but reliable monitors that will set off an alarm in case of potential problems and/or
shutdown equipment before any component failure has taken place.

The oxygen-gas injection to the mixed liquor is accomplished by low-pressure oxygen com-
pressors. These machines can be located on the tank covers, either outdoors or in a small building

34


(a4

Table 111-1.—Definitional parameters

Wastewater quantity
maximum month-

Wastewater quality

Required effluent

Site limitations

Solids-handling

Secondary-clarifier

T
‘design year quality \ | equipment specifications
Average flow, mad Average BODs, mg/t BODg, mg/l | New or existing tankage Dewatering Overtiow rate at design flow,
te
Maximum 4-hour sustained Maximum sustained (coincides with Suspended solids, mg/! Land area available | Sludge disposal gpd
peak flow, mgd maximum flow), mg/l Depth, feet
N, mg/i Piling required
Design flow, mgd Design BODg, mg/l J Feed
s 9 s sms P, mg/) Maximum tank depth, feet
COD/BODg, all cases Takeoff
8 COD, mg/!

Source
Preceding treatment

Average suspended solids, mg/l
Average volatile suspended solids, mg/t
Temperature, °C

pH

Alkalinity, mg/l as CaCO3

Alpha, beta

Nutrient content, mg/l N, P

Heavy metals

Other toxic components

Sludge return

Circular or rectangular



EPA 625/4-73-003a

OXYGEN ACTIVATED—SLUDGE
WASTEWATER TREATMENT SYSTEMS

Design Criteria and Operating Experience

&

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noes,
0,
" scent”

%

us. Environmental Protection Agesey.

Region V, Library
230 South Dearborn Street

Chicago, ilinvis £08! ad

ENVIRONMENTAL PROTECTION AGENCY e Technology Transfer

August 1973


ee

Renae ore .
Recommended Units Recommended Units
Customary Customary
Description Unit Symbol Comments Equivalents Description Unit ‘Symbol Comments Equivalents
Len metre m Basie SI unit 393 7 in=3.28 fe Velocity
nn Tinear metre per mis 3.28 fps
km a ce n second
mm in. millimetre mm/s 0.00328 fps
um, $939 10291094
‘ km/s 2.230 mph
Area square metre =m? 10.764 sq ft per second
198 sq yd
square kilometre km? 6.384 sq mi = angular radians per rad/s
247 acres second
square millimetre mm 0.00155 sq in.
hectare ha 2.471 acres Flow (volumetric) cubic metre ms Commonly called 15,850 gpm
m2) is a recognized per second the cumec = 2.120 cfm
multiple unit and
will remain in inter- litre per second —/s 15.85 gam
national use.
Viscosity pascal second Pas 0.00672
Volume cubic metre m 35.314 cu ft= poundals/sq ft
1.3079 cu yd
Pressure newton per N/m? 0.000145 Ib/sq in
litre ' Thelitre is now 1.057 qt = 0.264 gal square metre
recognized asthe = 0.81 X 10° acre or pascal Pa
special name for
the cubic decimetre. kilometre per kN/m? 0.145 Ib/sq in,
‘square metre
Mess kilogram kg Basic SI unit 2.20! or kilopascal kPa
gram 9 a a a = 18.439" bar bar 14.5 bisq in,
milligram mg
tonne or t {Honne= 1000 kg OBE tot on) = ‘Temperature Kelvin K Basic Slunt —5F
‘megagram Mg 1Mg=1000k9 1.1023 ton (short) dearee Celsius C The Kelvin and “g — 17.77
us
Time second s Basic SI unit jentical,
day 4 Neither the day nor ‘The use of the
the year is an SI unit Celsius scale is
vyeor year but both are impor- recommended as
tant. itis the former
centigrade scale,
Force newton N ‘The newton is that 0.22481 Ib (weight)
force that produces = 7.233 poundals Work, energy, joule 4 Ajoule= 1 N-m 2.778 X 107
an acceleration of quantity of heat where metres are kw hr=
1 m/s? in a mass measured along 3725 x) LM
of kg. the line of phe = 0.73756
action of i= 90x
‘Moment or newton metre Nm The metre is 0.7375 ft-lbf force N. 10° Btu
torque measured perpendicu- kilojoule Kl 2.778 keh
lar to the line of
action of the force Power watt w Vwatt = 1 Jis
Nota kilowatt k
joule per second —J/s
Stress pascal Pa 0.02089 Ibf/sq ft
kilopascal kPa 0.14485 Ibf/5q in
Application of Units Application of Units
Customary Customary
Description Unit Symbol Comments Equivalents Description Symbol Comments Equivalents
Precipitation, millimetre mm For meteorological Concentration mg/t Tppm
run-off, purposes it may be
evaporation Convenient to meas-
ure precipitation in BOD foading Kilogram per kg/d 0.0624 Ib/eu-ft
‘terms of mass/unit cubic metre day
area (kg/m), per day
1mm of rain’ =
Tkgim2 Hydraulic toad cubic metre m/m2d  Mthisiscon- 3.28 cu ftlsq ft
per unit area; per square metre verted to a
River flow cubic metre m3/s Commonly called 35.314 cfs eg. filtration per day
Per second the cumec rates
in oi Pressed in mm/s
Flow in pipes, cubic metre per m/s (Lnmjs = 86.4
conduits, chan- second 7 m3/m? day).
nels, over weirs,
pumping litre per second = Us 18.85 gpm Hydraulic load cubic metre m3/m3d
per unit volume; per cubic metre
Discharges or cubic metre m3/d—1/s= 86.4m3/d 1.83 X 109 gpm e4., biological per day
abstractions, per day filters, lagoons
y'
cubic metre m3 /year Air supply cubic metre or = m/s
per year litee of free air
per second Us
Usage of water litre per person I/person 0.264 gepd
per day day Pipes
; diameter millimetre mm 0.03937 in,
Density kilogram per kg/m? The density of 0.0624 tb/cu ft length metre m 38.37 in. =
cubie metre - 3.28 ft
Tease Optical units tumen per tumen/m2 0.092 tt
square metre candle/sq ft



ACTIVATED-SLUDGE-WASTE QUANTITY

The two parameters of major importance in sizing of sludge-dewatering and solids-disposal
systems are the pounds of waste solids per day and the concentration of solids in the waste stream.
The former obviously indicates the solids that must be disposed of, while the latter is indicative of
the volume of waste that must be treated. The pounds of solids that must be wasted per day to
prevent accumulation in the system is dependent on several parameters. The biomass is producing
new biomass in the process of removing BOD;. The ratio of the biomass synthesized to the BOD,
removed is known as the yield coefficient. Endogenous respiration is also occurring at a rate
proportional to the number of viable organisms in the mixed liquor, resulting in the destruction
of some of the organisms. Since the endogenous respiration is proportional to that fraction of the
MLVSS that is active biomass, the percentage of active biomass in the mixed liquor will be, in part,
dependent on the stresses put on the system. Thus, maintenance of relatively constant environ-
mental conditions should result in an increased fraction of active biomass and, thus, less overall
sludge production. The maintenance of relatively constant environmental conditions is, of course,
an inherent characteristic of a Unox system.

Other parameters affecting the quantity of solids that must be wasted from the system are the
quantity of nondegradable solids (both volatile and nonvolatile) that enter the system with the
influent sewage, and the quantity of solids that leave the system in the effluent stream. Therefore,
the pounds that must be wasted from the system are the pounds produced by synthesis during
BOD, removal, plus the pounds entering the system as nondegradable-volatile and nondegradable-
nonvolatile solids, minus the solids destroyed by endogenous respiration, minus those lost from the
system in the effluent.

Figure II-5 indicates the pounds of excess volatile solids formed per pound of BOD; removed
as a function of sludge retention time (sludge age) as observed at several Unox-system pilot-plant
locations. The results include the effect of temperature (some plants were run in the South in
summer, with resulting low production, and others in the North in winter, with higher production),
variations in nondegradable volatile solids in the influent from plant to plant, as well as the yield
and endogenous decay effects. Given the desired SRT, the temperature of interest, and the expected
quantity of nondegradable volatile solids in the influent sewage, it is possible to estimate from
figure II-5 the pounds of excess volatile suspended solids that will be produced, Given this estimate,
the expected suspended solids in the effluent, and the ratio of the volatile suspended solids to the
total (see tables II-3 to II-7), the pounds of solids that must be wasted daily can be estimated.

It remains to determine the gallons per day of the waste stream, which can be done given the
information previously presented. The clarifier-underflow concentration was determined in assigning
the mixed-liquor-suspended-solids level. This concentration of the waste stream and the pounds of
solids wasted per day fixed the gallons wasted per day. A highly concentrated waste stream, such as
the one that can be obtained from a Unox-system sludge, can have value in decreasing the costs
associated with sludge handling and disposal.

30


Table V1-1.—E£conomic evafuation, Middlesex County, N.J.

Cost component Unox Air-activated sludge
Dollars
Capital 83,580,000 | 104,020,000
Operating cost per year . . + 7,390,000 | 8,290,000

Detroit, Mich.

The city of Detroit and the consultant Hubbell, Roth and Clark determined economics for
their initial 300-mgd plant installation. For this portion of their ultimate 1,200-mgd requirements
their economic evaluation indicated a 20-percent savings for the aeration-tank portion only. Their
evaluation compared a Unox system with a 2.28-hour-detention-time, air-activated-sludge system
(see table VI-2).

Table V1-2.—Economic evaluation, Detroit, Mich.

Cost component Unox Air-activated sludge
Dollars
Capital 39,500,000 51,700,000
Operating cost per year . 1,599,000 1,911,000

Oakland, Calif.

A pilot-plant program was completed recently at the East Bay Municipal Utility District plant
in West Oakland, Calif. The Unox system was selected for the West Oakland plant after meetings
with the U.S. Environmental Protection Agency, the State Water Resources Control Board, and the
Bay Area Regional Water Quality Control Board. Unox was selected, not only for economic con-
siderations, but because the data indicated that pure oxygen was more reliable and provided a higher
margin of safety in meeting Federal and State standards. The system also will serve as a building
block for additional systems, if additional water-quality standards are imposed on the San Francisco
Bay dischargers in the future. Indicated costs for Unox in comparison with a chemical-treatment
and trickling-filter process are given in table VI-3.

Table V1-3.~Economic evaluation, Oakland, Calif.

Chemical treatment

Cost it
ost componen Unox plus trickling filter

Dollars

Capital bees 47,000,000 56,000,000
Operating cost per year .. 2.2... 4,000,000 4,900,000

39


Specifications for oxygenation tanks are listed in appendix B.

OXYGEN REQUIREMENTS

As mentioned earlier, the oxygen requirements for a Unox system depend on both the
COD/BODs ratio of the feed wastewater and the food-to-biomass ratio under which the system
is to operate. Figure II-4 depicts the quantitative dependence on F/M as determined from the
many pilot-plant programs. Given the value of the COD/BOD, ratio and the food-to-biomass
ratio as previously specified, it is possible to determine the expected consumption ratio (pounds
of oxygen consumed per pound of BOD, removed) for the wastewater to be treated. Given the
design flow, BOD; -feed concentration to the secondary system, and required removal, the pounds
of oxygen consumed per day are determined. If (as is typically the case from economic considera-
tions) about 90 percent utilization of the oxygen is specified, the tons per day of oxygen that must.
be generated at the design point are fixed.

OXYGEN SUPPLY

Following determination of the quantity of oxygen that must be generated to satisfy the
wastewater demands, it is necessary to specify the type of oxygen generator that best will serve
the needs of the plant. Two basic oxygen-generator designs are employed to supply most econom-
ically and practically the wide range in oxygen demands that result from the wide range in size
of wastewater-treatment plants. These designs are the traditional cryogenic air-separation process
for the larger size applications and a pressure-swing adsorption (PSA) system for the somewhat
smaller and more common plant sizes.

The standard cryogenic air-separation process involves the liquefaction of air, followed by
fractional distillation to separate it into its components (mainly nitrogen and oxygen). Figure IJI-1
shows a schematic diagram of this process. The entering air is first filtered and compressed. It is
then fed to the reversing heat exchangers, which perform the dual function of cooling and removing
the water vapor and carbon dioxide by freezing these mixtures out into the exchanger surfaces.

This process is accomplished by periodically switching or reversing the feed air and the waste
nitrogen streams through identical passes of the exchangers to regenerate their water vapor and carbon
dioxide removal capacity. The air is next processed through “cold and gel traps,” which are adsorb-
ent beds that remove the final traces of carbon dioxide as well as most hydrocarbons from the feed
air. It is then divided into two streams, one of which feeds directly to the lower column of tine
distillation unit. The other stream is returned to the reversing heat exchangers and partially warmed
to provide the required temperature difference across the exchanger. This stream is then passed
through an expansion turbine and fed into the upper column of the distillation unit. An oxygen-
rich liquid exists from the bottom of the lower column and the liquid nitrogen from the top. Both
streams are then subcooled and transferred to the upper column as shown in figure III-2. In this
column the descending-liquid phase becomes progressively richer in oxygen until that which collects
in the condenser reboiler is the oxygen-product stream. This oxygen is recirculated continually
through an adsorption trap to remove all possible residual traces of hydrocarbons. The waste
nitrogen exists from the top portion of the upper column and is heat exchanged along with the
oxygen product to recover all available refrigeration and to regenerate the reversing heat exchangers
as discussed in the foregoing.

The cryogenic process 1s more economical for supplying more than 30-50 tons of oxygen per
day. The process can be supplied by at least three companies on a performance specification with

wo
oa


Table 11-3.-Unox-system pilot-test data, performance summary for locations A and B

Location A Location B
Component Unit — ~
Phase | Phase 1! Phase} | Phase I)
Retention time Hours | 4g | 1.6 (0.89-2.1) 18 2.0
Recycle ratio . Percent | 2 i 28 37 030
Sewage temperature . °F | 57 | 66 a4 80
Miss . mg/l ; 6,100 4,000 | 5,060 | 7,010
vss/Tss | 062 | 0.74} 076 | 0.77
Organic loading Pounds BOD per | 165 | 190 | 198 171
1,000 ft3/day | Hl
Biomass loading 6... 0... ieee eves eee eee ... | Pounds BOD per | 0.66 4.02 | 083 | 050
pound MLVSS/day | |
Influent characteristics:
coD mg/l 375 365, 471 467
BODs, beeen eee mg/l 185 200} 238 227
Suspended solids... mg/t 180 136 83 76
Effluent characteristic |
cop ....... mail 8B | 72| 106 81
BODs ae mg/} 21 7 10 8
Suspended solids mail 20 16 27 16
Removals:
cop Percent 77 80 76 82
BOD, . Percent 89 92 95 96
Suspended solids Percent 89 88 64 80
Clarifier overflowrate .... 6. eee : gal/ft2/day 660 | 710 (520-1,300) | 650 582
Clarifier mass loading. ss... sobs te esse +. | Pounds ss/n2/day 45 29 (23-75) | 27.3 | 44.5
Clarifier underflow concentration Percent 2.6 17 19 3.13,
Sludge-volume index ........ - 40 61 65 42
Pounds VSS per pound BODs, removed:
Accumulation. . . beeee eee : 0.718 0.581 | 042 | 0.367
For disposal... 0.2... eee ere ie 0.64 0515 | 033 | 0.272
Pounds Oo per pound BODg removed .. . . voce ee fe 0.89 094} 132 | 1.35
Pounds 03 per pound COD removed . 0.689 0.602} 072 | 077
COD/BOD ratio . Loe eee fe 2.08 183 | 2.42 | 2.06
Solids retention time. 2.2.0.2. ee sede 241 169 | 287 | 5.45

CORRELATION OF RESULTS FOR DESIGN

From the results of the large number of programs already completed and from the design
information available on conventional activated sludge, it is possible to develop some useful relation-
ships of value in design of a Unox system for a specific waste stream. The most important of these
relationships include substrate removal, oxygen requirements, excess solids production, and solids-
liquid separation characteristics.

Substrate Removal

The design of Unox systems is closely keyed to the use of biomass loading as a parameter for
design. Conventional air-activated-sludge processes are typically designed for biomass loadings of
0.3 to 0.5, based on volatile solids in most cases. This range of operation represents an equitable
balance between retention time and sludge accumulation and provides for a stable operating mode.
Unox. systems generally are designed on the same basis, except that the range of biomass loadings
is somewhat higher. Most designs are made at average biomass loadings of 0.5-0.8. When oxygen
is not limiting, it is possible to design at this increased level due to the enhanced ability to transfer


Table 11-6 —Unox-system pilot-test data, performance summary for lacations G and H

1
Location G, Location H
Component Unit Phase 1
Phase 1 Phase I
om
Retention time 2 6. 6. eee ee eee Hours 18 18 22
Recycle ratio... Percent 36 33 33
Sewage temperature Lees °F 70 96 94
MLSS 2 ee ee eee eee eee eee mg/i 4,190 5,300 6,600
vss/TSS 0.79 0.88 0.9
Organic loading Pounds BOD per 145 230 200
1,000 ft3/day
Biomass loading ..... . . . eae Pounds BOD per 0.72 0.89 0.54
pound MLVSS/day
Influent characterrstics.
cop... ... eee ee mg/! 321 893 888
BOD, ....... be . mail 177 274 294
Suspended solids... . . mai! 90 84 79
Effluent characteristics
COD ee ee eee cece mg/l 60 425 357
BOD, . beveeeee . mg/l 13 22 27
Suspendedsolids 0-0-0 eee eee moft 28 49 46
Removals:
cop... Percent 84 52 60
BODs wee Percent 93 92 90
Suspended solids we wee eee wee . Percent 71 42 40
Clarifier overflow rate... . bec ee es eeee gal/ft2/day 650 650 520
Clarifier mass loading _ Pounds SS/ft2/day 30.1 33 34
Clarifier underflow concentration .......-..- ee Percent 1.47 2.0 27
Sludge-volume index ..... 0.2.2.0 fees ee | 64 90 130
Pounds VSS per pound 8005 removed |
Accumulation... 01... eee bees - 0.60 0.36 0.41
For disposal see eee we . noe . - 0.49 0.2 0.26
Pounds O2 per pound BODs removed ........ . . - 0.93 1.28 1.46
Pounds 05 per pound COD removed .. .... ... : 0.71 0.68 0.73
COD/BOD ratio : bees - 181 3.35 3.0
Solids retention time .... . . woe eee . - 2.33 3.52 45

Within any activated-sludge system, the micro-organisms attack all of the organic substrate
available to them—not only BOD,, but also BOD29, BOD, 99, and so forth, almost all of which are
included in the COD measure. Because BOD; is relatively easy to degrade, it is almost completely
assimilated, whereas a smaller portion of the BODgq (hard to degrade) is assimilated, and most of
the BOD, gg (very hard to degrade) will pass through the system unaffected.

As the COD/BOD, ratio in the influent increases, the data from Unox pilot plants indicate
that a higher quantity (poundage) of the COD will be biologically removed, although such removal
is not indicated in the BOD; test. Consequently, more oxygen is required per unit BOD; removed;
however, for COD removal oxygen consumption will remain relatively stable over the design range
of food-to-micro-organism ratios employed.

Figure II-4 represents the band of data that will result from oxygen consumption per unit
BOD, removal when both F/M and COD/BODs ratios are investigated.
Excess Sludge Production

The extent of net or excess solids production is a function of the degree of endogenous

respiration occurring in the oxygenation system. The degree of endogenous respiration is governed
by the food-to-micro-organism ratio. Since the Unox system employs a multistage gas-liquid-contacting

12