United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-87/107 Apr. 1988
&ERA Project Summary
Treatment of Municipal
Wastewaters by the Fluidized
Bed Bioreactor Process
0. Karl Scheible and Gary M. Grey
A 2-year, large-scale pilot investiga-
tion was conducted at the City of
Newburgh Water Pollution Control
Plant, Newburgh, NY, to demonstrate
the application of the fluidized bed
bioreactor process to the treatment of
municipal wastewaters. The reactor
was 1.2 m wide, 1.8m long, and 6
m high, with a typical active bed
volume of 9 m3. The bed medium was
quartzite sand with a nominal diameter
of 0.5 mm. Oxygenation was accom-
plished by a device installed below
grade (providing approximately 14m
static head); the oxygen source was on-
site liquid oxygen tanks. The waste-
water feed was primary effluent drawn
from the main plant.
The experimental effort investigated
the ability of the process to treat
municipal wastewater to secondary
levels. Process design elements such as
removal rates, temperature effects,
oxygen transfer rates, oxygen require-
ments, sludge production, and bed
maintenance and control were evalu-
ated. Additionally, the studies evalu-
ated the stability of the process under
high hydraulic peak loads typical of
combined sewer systems.
The plant primary effluent had an
average 5-day biochemical oxygen
demand (BOD) of 103 mg/L, a chem-
ical oxygen demand (COD) of 287
mg/L, and an average total suspended
solids (TSS) of 109 mg/L. An organic
loading rate of 0.2 to 0.3 gm BOD/gm
volatile suspended solids/day is sug-
gested in order to meet secondary
treatment levels. Suspended solids
removals are relatively low through the
reactor; the results indicate that final
clarification would be required for the
plant to meet a 30 mg/L TSS limita-
tion. The oxygenator exhibited good
performance (greater than 80% oxygen
transfer) at gas to liquid ratios less than
0.04.
The system exhibited good stability
under high short-term peak hydraulic
loadings. Maximum to average raw
flow ratios up to 5 could be handled
without bed washout. Special reactor
modifications to yield an expanded
upper section allowed a ratio as high
as 10 to 1 without loss of the bed. The
system exhibited relatively quick recov-
ery to normal treatment efficiency after
the hydraulic surges.
Applications appropriate to the pro-
cess are for secondary treatment and
as a roughing treatment process in
plant upgrades.
This Project Summary was devel-
oped by EPA's Water Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
Fluidized biological reactors represent
a newly developed and innovative tech-
nology that combines the advantages of
activated sludge and trickling filtration.
It attempts to combine the efficiency of
the suspended growth system with the
stability and efficient operation of the
attached growth process. Within the
fluidized bed reactor, a fixed film of
biologic growth is allowed to develop on
the surface of a granular medium, such
as sand. Wastewater, which has been
oxygenated, flows upward through the
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medium at velocities sufficient to expand
the bed. This expansion, or fluidization,
occurs when the drag due to the upward
fluid velocity is greater than the down-
ward force due to gravity. The fluidization
separates the particles and allows
microorganisms to attach and grow on
all surfaces of the medium. Thus, the very
large surface area available can produce
high volatile solids concentrations, often
several times greater than in suspended
growth reactors. The attachment of
microorganisms to the medium also
reduces subsequent requirements for
wastewater clarification.
A portion of the effluent stream is
recycled; the ratio of recycle to raw flow
affects both column expansion and
oxygen transfer, and often serves as a
process control parameter. In the major-
ity of reported studies, the reactors had
been operated on constant flow regimes,
with recycle ratios ranging from zero to
three for municipal sewage. The recycle
mode can also provide stability in appli-
cations with high influent hydraulic
variability.
Control of the biomass film thickness
and sludge wasting are accomplished by
a sand-biomass separation device. This
generally operates by shearing the
biomass from the sand, separating the
sludge from the sand, then returning the
sand to the reactor.
One advantage of the fluidized bed
process is its reduced land area require-
ment, due to its ability to carry a high
biomass inventory. Typical volumetric
organic loading rates of 3 to 20 kg BOD/
m3-d and mass organic loading rates -of
0.2 to 2.0 kg BOD/kg VSS-d (where VSS
is volatile suspended solids) have been
reported. Typical rates for conventional
activated sludge treatment are 0.3 to 2
kg BOD/m3-d and 0.2 to 0.6 kg BOD/kg
VSS-d, respectively.
In this project a large-scale pilot study
was conducted at the Newburgh, NY,
Water Pollution Control Plant to inves-
tigate the application of the fluidized bed
bioreactor process to the treatment of
municipal wastewaters. The study was
directed to the system's ability to achieve
secondary treatment levels of 30 mg/L
TSS and BOD, and its stability under the
hydraulic variability associated with a
plant receiving combined sanitary and
storm wastewaters. A large-scale Dorr-
Oliver Oxitron* pilot plant was set up on-
* Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
site and continuously monitored for
treatment performance; specific studies
and tasks were also directed to assessing
the unit operations that comprise the
process.
Program Objectives and Scope
of Work
The study included the installation of
a fluidized bed bioreactor pilot system.
This system was operated over a period
of approximately 18 months, receiving
settled primary effluent from the New-
burgh Water Pollution Control Plant.
Sampling of influent, effluent, and the
reactor was conducted to assess per-
formance under variable loading condi-
tions and to evaluate bed stability.
Specific task elements addressed bed
maintenance requirements, solids set-
tleability, oxygenation capacity, and bed
wasting and sand-biomass separation
procedures.
The objectives of the program are as
follows:
1. To demonstrate the ability of the
fluidized bed bioreactor process to
achieve secondary treatment levels
(30 mg/L TSS and BOD);
2. To assess the stability of the
process and its ability to handle the
highly variable hydraulic loads
characteristic of a combined
sanitary-stormwater collection
system during rainfall runoff
events;
3. To evaluate the operation and
maintenance requirements of the
fluidized bed process as applied to
the treatment of a municipal treat-
ment plant primary effluent;
4. To define and establish the major
process parameters that influence
the design and operation of the
fluidized bed reactor, such as
hydraulic and organic loading
rates, oxygen transfer and utiliza-
tion efficiencies, biomass-sand
separation efficiency, bed control,
sludge production, and wasting
requirements; and
5. To develop cost information for the
fluidized bed process relative to its
application to municipal waste-
waters.
Description of Facilities
The fluidized bed pilot plant facility was
located at the Newburgh Water Pollution
Control Plant, Newburgh, NY. The city is
approximately 100 kilometers (60 miles)
north of New York City. The treatment
plant is a conventional secondary acti-
vated sludge facility, treating domestic
and industrial wastes, and discharging
to the Hudson River. The plant services
a population of approximately 20,000;
industrial inputs are primarily dye mills
and commercial laundry wastewaters.
Description of the Pilot Plant
The Oxitron pilot plant was a skid
mounted, field assembled plant designed
by Dorr Oliver, Inc., Stamford, CT, to
demonstrate the capabilities of fluidized
bed reactors. This same system had
previously been used to demonstrate
nitrification of domestic wastewaters at
the Greenwich Wastewater Treatment
Plant in Greenwich, CT. The facility was
situated in an area of the Newburgh
Water Pollution Control Plant that had
been set aside for future expansion.
Figure 1 presents a process schematic
of the pilot plant showing all major unit
operations. The 1,900-L influent head
tank located near the primary effluent
channel was consistently fed by one of
two submersible pumps. Primary effluent
was pumped to the tank at a rate
generally in excess of 1,500 L/min.
Excess flow passed through an overflow
pipe back into the primary effluent
channel. A float valve was provided to
activate a low level alarm and automat-
ically shut off the pilot plant system if
the level of the head tank dropped as a
result of interrupted feed supply. A 25-
cm-diameterPVC pipe supplied feed from
the head tank to the influent pump
located in the pilot plant building. The
influent pump was an Aurora, 23-cm, 15-
hp centrifugal pump with a pressure
relief bypass loop. Normal operating
pressure of this pump was 2.8 to 3.2
kg/cm2. Maximum flow was approxi-
mately 950 L/min, depending on system
pressure.
The raw wastewater feed was then
passed through a rotary strainer to
remove large solids and protect the
reactor distributor nozzles from plugging.
The strainer was backwashed twice per
day or more frequently as needed.
Pressure gauges on both sides of the
strainer were used to monitor its per-
formance. Generally, under normal
operation conditions, a pressure drop of
under 0.2 kg/cm2 was maintained.
-------
VORTEX
Flow
Meters
(to flow
recorder)
Self-priming Centrifugal Pump
(rudder impeller)
D.O. Probe
(Controls
Oi feed)
Submersible
Pump (2) \
/ //DSM
/^-Screen
^-' (Solids/Sand
Separation)
Waste
Biomass
Waste
Tank
Primary
Effluent
Channel
Rotostrainer
Backwash
Waste
Oxygenator
Pit
(29' Depth—below grade)
Figure 1. Process schematic of the fluidized bed pilot facility.
Feed then passed through a Keystone
control valve and Ball Vortex flow meter.
The control valve could be manually or
automatically operated. The automatic
mode was used to provide diurnal flow
variation similar to that of the Newburgh
treatment plant flow. A signal from the
Newburgh plant flow meter was used to
drive the pilot plant control valves. The
flow meter was a circulating cage type
that proved to be very reliable. Occasion-
ally, the flow meters were electronically
calibrated and balanced. The meters
were also connected to remote flow
recorders and totalizers at the control
panel.
The raw wastewater, combined with
the recycle waters, then flowed to the
oxygenator where oxygen gas was
injected to satisfy oxygen requirements
for aerobic biological treatment. The
oxygenator provided gas/liquid contact
to maximize dissolution of oxygen into
the influent wastewater. At the inlet to
the oxygenator, the combined influent
and recycle streams were mixed under
pressure with gaseous oxygen. The
oxygenator was below grade to provide
approximately 14 m of static head (the
difference between the top of the reactor
and the bottom of the oxygenator pit).
This pressure enhanced oxygen transfer
and allowed dissolved oxygen levels of
up to 60 mg/L to be achieved. The
internal design of the oxygenator
ensures that gas/liquid contact is max-
imized for efficient oxygen transfer. The
gas recycle loop allows recirculation of
undissolved oxygen.
The biological support medium was
quartzite sand with a median particle size
of approximately 0.50 mm. At start-up,
the reactor was charged with 4 m3 of
medium. Sand was also added to the
reactor throughout the study to compen-
sate for attrition losses.
The reactor was a 1.2 m by 1.8 m steel
tank, 6.1 m high. The tank was comprised
of four flanged sections, the uppermost
and lower two sections had clear acrylic
windows on one side to allow visual
observation. A scale was added to the
windows to indicate bed height. The
zero point of the scale was 32 cm from
the bottom to compensate for the dis-
tributor piping and plates. The distributor
plates directed the flow upward; note that
the nozzles were pointed downward. Two
1.8-m-long, multi-V-notch weir plates
were located 5.5 m from the bottom of
the tank. Effluent dissolved oxygen was
measured by a Beckman dissolved oxy-
gen analyzer.
A portion of the pilot plant effluent was
recycled to the reactor to reduce the
strength of the incoming wastewater
when necessary, and to maintain bed
fluidization during low flow periods and
during influent flow interruptions. The
recycle tank split the reactor overflow
into a recycle stream and an effluent
stream. The reactor overflow was intro-
duced into the recycle tank through a
tangential inlet to set up a circulating
pattern within the tank. This action
imparted an outward velocity to any solid
particles entering the recycle tank.
-------
Centrifugal forces push the sand parti-
cles, in the event that they should
overflow the reactor, to the periphery of
the tank where they settle out into the
conical section of the tank. A bottom
drain was provided to allow for cleaning
this accumulation from the tank.
Effluent left the recycle tank by over-
flowing the center launder. Recycle flow
was withdrawn from the tank through
a submerged outlet in the conical tank
section, and was pumped through a
Keystone butterfly flow control valve, a
Ball Vortex flow meter, and combined
with the influent raw wastewater before
entering the oxygenator. A check valve
was located in the recycle line to prevent
backflow during periods when the recy-
cle pump was shut down.
A ratio control station was installed in
the pilot plant to allow pilot plant flows
to vary as the Newburgh wastewater
treatment plant flow varied. The operator
had the option of operating the pilot plant
under constant influent and recycle flow
or an automatic mode where a signal
from the Newburgh flow meter controlled
the pilot plant flow. The automatic mode
could be operated in two ways: constant
recycle with varying influent flow,
providing a variable hydraulic flux; or
constant total flow, with recycle and
influent flow varying to provide a con-
stant hydraulic flux.
The bed wasting operation is critical
for effective bed control and mainte-
nance of thin biofilms. A rubber-lined
centrifugal pump transported the sand
and biomass to a gravity screening device
(Dorr-Oliver DSM) and provided the
shearing force necessary to dislodge the
biomass from the sand media. With the
aid of spray water, sheared biomass
passed through the screen and sand
media was retained and returned to the
reactor. The waste biomass was col-
lected in the waste tank and pumped to
the main plant's recycle pit after volume
measurement and sampling for TSS and
VSS.
Small-Scale Pilot Plant
As previously stated, the maximum
feed flow from the reactor influent pump
was approximately 950 L/min, depend-
ing on the operating pressure of the
reactor. The lowest available flow (to
maintain fluidization) was approximately
380 L/min (176 L/min per m2 based on
influent flow). This provides a maximum
to average flow ratio of 2.5:1. One goal
of the project was to test the system
under higher maximum to average ratios
than was possible with the large pilot
system. Dorr-Oliver supplied a smaller
pilot plant to evaluate stormwater
impact. This smaller unit is shown on
Figure 2. The reactor consisted of two
sections of clear acrylic pipe. The lower
section was 15 cm in diameter, while the
upper section was interchangeable. Two
upper sections were used, one 15-cm-
diameter section to provide a straight
column, and one 23-cm section to
provide an expanded column.
Influent to the column was taken from
the pilot plant distributor and went
through a booster pump and rotometer.
This configuration permitted a maximum
hydraulic loading of 2,100 L/min per m2,
corresponding to a ratio of 12:1 when
using the pilot plant minimum influent
hydraulic flux for fluidization.
A small DSM screen was set up to
accomplish wasting from the small unit.
Wasting was done manually.
Rdtometer\\
Drain
From Main Reactor
Distributor
Influent Pump
Figure 2. Schematic of small pilot unit used
during a portion of the studies.
Experimental Program
The experimental phase of the project
covered a total period of 19 months, from
September 1983 through March 1985.
The reactor was operated under constant
raw wastewater conditions from Sep-
tember 1983 through September 1984.
During the final portion of the study,
December 1984 through March 1985,
the raw flow was automatically adjusted
to be proportional to the Newburgh
plant's raw flow rate. The recycle, in this
case, was automatically adjusted in order
to maintain a constant flux through the
reactor. Operating periods and number
of samples taken were as follows:
1. 09/15/83 to 11/10/83, 28
samplings;
2. 02/06/84 to 03/08/84, 31
samplings;
3. 03/21/84 to 04/09/84, 15
samplings;
4. 04/16/84 to 05/22/84, 28
samplings;
5. 06/26/84 to 07/24/84, 22
samplings;
6. 08/16/84 to 08/21/84, 4
samplings;
7. 08/29/84 to 10/09/84, 31
samplings;
8. 12/19/84 to 01/04/85, 8
samplings;
9. 01/21/85 to 02/08/85, 10
samplings;
10. 02/14/85 to 02/20/85, 4
samplings; and
11. 02/28/85 to 03/14/85, 7
samplings.
Experimental Results
Average results for the 11 periods of
operation are shown in Table 1. Reten-
tion times, based on bed height, but
assuming empty bed volume, ranged
between 20 and 40 min. High volatile
solids inventories and use of pure oxygen
permit these short contact times. BOD
removals, based on total influent BOD,
but soluble effluent BOD, were above
70% with the exception of one time
period. BOD removals were based on
-------
Table 1. Reactor Performance Summary Table§
Period Number
Dates
Number of Weeks
Hydraulic Data
Raw Flow (L/min)
Recycle Flow (L/min)
Total Flow (L/min)
Ratio Recycle/Raw
Hydraulic Flux (m/min)
Retention Time*
- RawFlow (min)
Biochemical Oxygen Demand-5 Day (BOD)
BOD Influent Img/L)
SBOD Influent (mg/L)**
BOD Effluent (mg/L)
SBOD Effluent (mg/L)
BOD Loading Rate (kg/d/kd VS)
BOD Loading Rate (kg/d/m3)
SBOD Loading Rate {k$/ d/kg VS)
SBOD Loading Ra'j (kg/d/m3
BOD Remove (Percent)-\
BOD Ft movalRate (kg/ d/kg VS)\
BOD ' emovalRate (kg/d/m3)
SBC > Removal Rate (kg/ d/kg VSjtt
SB D Removal Rate (kg/d/m3)
5 upended Solids (SS)
Total Suspended Solids (mg/L)
- Influent TSS
/A - Effluent TSS
Volatile Suspended Solids (mg/L)
- Influent VSS
- Effluent VSS
TSS Removal (percent)
Bed Characteristics
Total Solids (kg)
Total Solids (gm/L)
Total Volatile Solids (kg)
Total Volatile Solids (gm/L)
Calculated Film Thickness (um)
Total Bed Height (m)
Percent Fluidization
1
9/15/83-
11/10/83
9
335
664
998
2.0
0.46
25. J
138.4
59.5
87.6
32.2
0.44
7.72
0.19
3.4
77.2
0.37
6.05
0.11
1.46
116.4
71.1
87.3
56.6
39.0
5750.0
565.0
149.0
16.8
28.3
3.9
157.0
2
2/6/84-
3/8/84
5
305
580
885
1.9
0.41
29.0
71.7
44.0
38.5
19.6
0.26
3.42
0.16
2.1
71.1
0.21
2.81
0.11
1.37
65.8
44.4
44.3
31.2
32.5
4540.0
493.0
123.0
13.3
123.0
4.1
240.0
3
3/21/84-
4/9/84
4
332
641
962
1.9
0.44
29.9
37.8
23.3
17.3
10.4
0.11
1.31
0.07
0.83
60.4
0.09
1.01
0.04
0.44
50.5
29.5
33.3
22.1
41.5
4332.0
420.0
119.0
11.5
78.0
4.6
298.
4
4/16/84-
5/22/84
6
341
488
834
1.4
0.39
27.9
56.1
28.5
44.5
12.8
0.20
2.56
0.11
1.41
72.7
0.16
1.86
0.06
0.70
64.4
47.8
50.5
37.5
25.8
4662.0
475.0
128.0
12.8
79.5
4.4
262.0
5
6/26/84-
7/24/84
5
311
625
936
2.0
0.43
27.1
71.9
33.2
42.6
15.9
0.36
3.55
0.16
1.6
74.8
0.29
2.95
0.10
0.83
75.9
33.2
54.9
24.1
56.3
4060.0
420.0
92.00
9.6
145.0
3.9
299.0
6
8/16/84-
8/21/84
1
417
647
1064
1.6
0.49
22.3
125.5
34.3
45.8
8.5
0.63
7.53
0.18
2.1
87.4
0.14
2.63
0.05
1.60
44.5
38
33.5
30.0
15.0
5034.0
554.0
115.0
12.6
15.0
4 3
200.0
%Note that calculated values are accomplished on single points, and then averaged.
*Based on empty bed volume, at average bed height.
•*SBOD is soluble BOD; SCOD is soluble COD.
ffiOD or COD removal determined by total influent minus soluble effluent.
HSBOD or SCOD removal determined by soluble influent minus soluble effluent.
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Table 1. (Continued)
Period Number
Dates
Number of Weeks
7
8/29/84-
10/9/84
7
a
12/19/84-
1/4/85
3
9
1/21/85-
2/8/85
3
10
2/14/85-
2/20/85
1
11
2/28/85-
3/14/85
3
Hydraulic Data
Raw Flow (L/min) 382 413 375 384 466
Recycle Flow (L/mm) 622 636 665 511 448
Total Flow (L/min) 1005 978 913 915 914
Ratio Recycle/Raw 1.6 1.5 1.8 1.3 1.O
Hydraulic Flux (m/mm) 0.46 0.45 0.42 0.42 0.42
Retention Time' 24.3 24.1 27.1 23.6 19.9
- RawF/ow (min)
Biochemical Oxygen Demand-5 Day (BOD)
BOD Influent (mg/L)
SBOD Influent (mg/L)'"'
BOD Effluent (mg/L)
SBOD Effluent (mg/L)
BOD Loading Kate (kg/d/kd VS)
BOD Loading Rate (kg/d/m3)
SBOD Loading Rate (kg/d/kg VS)
SBOD Loading Rate (kg/d/m3
BOD Removal (Percent ft
BOD Removal Rate (kg/d/kg VStf
BOD Removal Rate (kg/d/m3)
SBOD Removal Rate (kg/d/kg VS)tt
SBOD Removal Rate (kg/d/m3)
Suspended Solids (SS)
Total Suspended Solids (mg/L)
- Influent TSS
- Effluent TSS
Volatile Suspended Solids (mg/L)
- Influent VSS
- Effluent VSS
TSS Removal (percent)
Bed Characteristics
Total Solids (kg)
Total Solids (gm/L)
Total Volatile Solids (kg)
Total Volatile Solids (gm/L)
Calculated Film Thickness (fim)
Total Bed Height (m)
Percent Fluidization
121.1
54.3
66.5
17.1
0.49
6.86
0.20
2.9
86.9
0.36
5.27
0.13
1.84
138.6
65.3
98.2
47.6
53.0
5238.0
548.0
136.0
14.2
14.5
4 3
205.0
128.6
62.1
95.0
33.5
0.89
8.00
0.44
3.9
73.9
0.32
2.95
0.11
1.09
126.3
88.8
95.8
65.7
29.7
5865.0
543.0
97.0
8.9
88.8
4.6
200.0
151.9
72.5
83.7
26.2
0.56
7.80
0.27
3.8
82.6
0.43
5.98
0.17
2.24
154.8
97.0
122.1
78.0
37.3
4938.0
449.0
150.0
13.6
98.9
4.7
279.0
111.5
59.5
55.0
22.0
0.59
7.74
0.33
4.3
79.3
0.27
5.65
0.09
0.41
116.8
80.3
83.8
69.8
31.3
4919.0
534.0
114.0
12.5
66.8
4.2
212.0
120.6
59.1
50.6
24.0
0.54
8.28
0.26
4.1
80.5
0.49
6.55
0.15
2.38
94.6
62.0
73.0
47.7
34.0
5350.0
523.0
158.0
15.9
118.4
4.3
221.0
§/Vofe that calculated values are accomplished on single points, and then averaged.
"Based on empty bed volume, at average bed height.
**SfiOD is soluble BOD; SCOD is soluble COD.
ffiOD or COD removal determined by total influent minus soluble effluent.
ttSBOD or SCOD removal determined by soluble influent minus soluble effluent.
-------
soluble effluent BOD because the exist-
ing system did not provide adequate
suspended solids removal. Final clarifi-
cation is considered necessary to assure
effluent suspended solids below 30
mg/L. The unusually high BOD removals
experienced in periods 6 and 7 occurred
after the reactor was recharged with
clean sand and are attributed to devel-
opment of a dense, thin biofilm.
A number of short-term runs were
made using the small pilot unit with
expanded top sections shown in Figure
2. Results of a run made on February
20, 1985, are shown in Figure 3. The
maximum hydraulic flux attained in the
lower section of the column was over
2,000 L/min per m2 with the bed being
maintained about 0.5 m below the point
of washout. Although the degree of
pollutant removal was small during high
flux, the bed could be contained in the
reactor and the biofilm was not lost from
the sand particles. The large pilot plant
was run on February 20 at the highest
feed rate possible with available pumping
capacity, but the maximum flux was only
500 L/min per m2. Pollutant removal was
better from this system than from the
small pilot system.
Conclusions
The fluidized bed aerobic bioreactor
process appears technically viable for
application to the secondary treatment
of municipal wastewaters. Improve-
ments must be made to the sand-
biomass separation system for more
effective operation. Estimated installed
and operating costs are in the range of
other biological treatment methods.
Organic loadings between 0.2 and 0.3
kg BOD/kg VSS-day would be required
to yield soluble effluent BOD levels less
than 20 mg/L. Final clarification will be
necessary to assure effluent suspended
solids levels less than 30 mg/L.
The process is particularly suited for
applications in which land is limited, and
where roughing treatment is appropriate.
In this case, the process, at loadings of
0.4 to 0.5 kg BOD/kg VSS-d can be
expected to accomplish greater than 50%
BOD removal. Intermediate clarification
would not be needed if followed by a
conventional biological system for final
treatment.
There is a correlation of effluent
suspended solids concentration with the
TSS and total BOD loading to the reactor.
Impractically low loadings would be
required in order to meet secondary
effluent TSS limits without final clarifi-
Oxitron Pilot Plant
o
u.
1600
1200
800
400
0
r- Total
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Raw
I I I I
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600
400
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r -,
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Time (day)
6 8
Expanded Section Reactor
750
100
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fj 1 — 1 1 r- In fluent
i
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400
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Influent
I
I I
I
Soluble Effluent
. .
i _ i
Sam 10 12pm 246 81012 2am 4 6
Sam 10 12pm 24 6 81012 2am 4 6
Time (day)
Figure 3. Flow and influent/effluent COD, BOD. and TSS data from the February 20. 1985 hydraulic variability evaluation.
7
JUU
200
n
f-1 l—j r- Influent
_ ~ •*" \ ^ _ /- Soluble Effluent
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-------
cation. Although the effluent TSS levels
were adversely affected at Newburgh by
the inefficiencies in the bed wasting
system, final clarification should still be
required. High rate clarification (50 m3/
mVd) is feasible if coupled with chemical
conditioning. Doses of 10 mg/L alum and
1 mg/L polymer were found to be
effective for the Newburgh application.
The process has the ability to undergo
significant short term hydraulic peak
loads and still maintain bed integrity.
Removal effectiveness is, however,
greatly reduced. A straight wall reactor
can operate at a hydraulic flux of 1
m/min without any loss of bed, approx-
imately 2.5 times its normal operating
flux. This is equivalent to a maximum to
average raw flow ratio of 4 to 6, effected
by substituting raw flow for recycle flow.
Modification of the reactor to one with
an expanded upper section (approxi-
mately 2.3 times the cross-sectional area
of the lower section) will significantly
enhance the reactor's ability to undergo
hydraulic surges. A flux of 2.5 m/min
was demonstrated (based on lower
section cross-sectional area), with no
loss of bed. This is equivalent to a
maximum to average raw flow ratio of
10. The reactor organic removal efficien-
cies return to normal relatively quickly
after cessation of the hydraulic surge.
Bed stability can be controlled by the
hydraulic flux and the average particle
density. The particle density will be a
function of the average biofilm thickness;
this should be maintained below 50 //m.
Effective control requires maintenance of
adequate sand and volatile solids levels;
the bed must be monitored by direct
sampling and analysis to accomplish this.
The sand concentration should aver-
age 400 to 600 g/L through the bed,
under expanded conditions, with an
average volatile solids concentration of
10,000 to 15,000 mg/L. The flux should
be controlled to maintain a bed expansion
of 50% to 100%.
Maximum oxygen transfer efficiencies
can be accomplished with the oxygenator
at gas to liquid ratios less than 0.03
L/min 02 at standard conditions per L/
min wastewater.
A key element to the successful
operation of the f luidized bed is the ability
to maintain and control optimum reactor
bed conditions. The feed to the reactor
must be kept free of debris; this is
accomplished by screening the primary
effluent (or combined primary effluent/
reactor recycle). The distributor must give
even distribution of flow across the
8
cross-sectional area of the reactor,
without localized velocity gradients! The
distributor ports must be non-clogging;
pressure drops should be monitored
across the distributor and the system
should provide a means to access the
distributor ports for cleaning.
The bed wasting operation is critical
for effective bed control and mainte-
nance of thin biofilms. Centrifugal pumps
with rubber-lined impellers provide good
shearing of the biomass from the sand
and the gravity screening device (Dorr-
Oliver DSM) used in this study provided
good separation. Spray water is essential
for effective operation of the separation
screen.
The bed wasting operation was labor
intensive and subject to significant
equipment problems. The problems
related primarily to pumping the sand/
biomass mixture without clogging, and
materials damage. The operation can
also affect the effluent suspended solids
levels from the reactor because of
inefficiencies in separating the biomass
from the sand before the sand is returned
to the reactor. Improvements to the
existing system would include a sub-
mersible pump to remove the sand/
biomass (and shear the biomass from the
sand), a spray water system to enhance
removal of the biomass by the separation
screen, and baffles on the screen to
prevent flow channelling.
An alternative method would be to
install a system separate from the
reactor. An agitator would shear the
biomass, and a two-stage rotational
separator and gravity settler would
provide for separation of the sand from
the biomass and removal of the biomass
and sand slurries.
Recommendations
The applications appropriate to the
fluidized bed process are as a roughing
treatment system prior to a conventional
treatment system, or as a secondary
treatment system for carbonaceous BOD
removal. It is recommended that the
process be given particular consideration
when there is limited land available,
significant hydraulic variability, and/or a
proximate, cost-effective source of pure
oxygen. Clarification should be provided
if the process must meet secondary
effluent TSS limits.
In cases where high hydraulic varia-
bility is expected, it is recommended that
the modified reactor with the expanded
top section be used. This can increase
the capacity of the system and signifi-
cantly enhance bed stability during short
term hydraulic peak loads.
Further work and subsequent demon-
stration is recommended with regard to
bed wasting and control of bed charac-
teristics. This should address procedures
and equipment for removing bed sand/
biomass, shearing the excess biomass
from the sand, separation of the sand and
biomass, and effective pumping/trans-
port of the sand/biomass and sand
slurries. The efforts should be directed
to reducing the labor requirement and
to designing more effective equipment
for accomplishing these tasks. Consid-
eration should be given to wholly sepa-
rating this process from the reactor.
Additional work is needed to demon-
strate the performance of the bioreactor
during the transient conditions of peak
hydraulic loads. More information is
needed on the system's removal efficien-
cies and oxygen requirements during
these periods. This is necessary to
optimize designs for possible application
to the direct treatment of combined
sewer overflow wastewaters.
0. Karl Scheible and Gary M. Grey are with HydroQual. Inc.. Mahwah, NJ
07430.
Richard Brenner is the EPA Project Officer (see below).
The complete report, entitled "Treatment of Municipal Wastewaters by the
Fluidized Bed Bioreactor Process," (Order No. PB 88-140 280/AS; Cost:
$25.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U. S.GOVKNMENT PRINTING OFFICE: 1988/548-158/67108
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