V-/EPA
United States
Environmental Protection
Agency
Municipal Environmental Research \
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-82-057 August 1982
Project Summary
Full-Scale Evaluation of
Activated Bio-Filter
Wastewater Treatment Process
Kerwin L. Rakness, James R. Schultz, Robert A. Hegg, Jan C. Cranor, and
Richard A. Nisbet
The City of Helena, Montana, utilizes a
relatively new biological treatment con-
cept called the activated bio-filter (ABF)
process* for secondary treatment of its
municipal wastewater. The four major
components of the ABF process are a
redwood media trickling filter-type tower
called a bio-cell, a bio-cell recirculation
system, a short-term activated sludge
aeration tank, and a conventional secon-
dary clarifier.
A field evaluation study was initiated
at Helena with the primary objective of
developing year-round, full-scale operat-
ing and performance data on the ABF
process. The experimental program was
conducted so that organic and hydraulic
loadings imposed during the winter
months approached the manufacturer's
recommended design criteria. Other
objectives included defining process
energy requirements and sludge produc-
tion values.
Three different process loading regi-
mens were investigated over a 17-month
time frame. These regimens correspond-
ed roughly to (1) one-half of the manu-
facturer's recommended loadings on
both the tower and the aeration tank, (2)
one-half of the recommended loading on
the aeration tank and the full recom-
mended loading on the aeration basin,
and (3) the full recommended loadings
on both the tower and aeration basin
"Mention of trade names or commercial products
does not constitute endorsement or recommenda-
tion for use.
(winter months). Because of available
wastewater flows and plant facility
operating constraints, the desired 50
and 100 percent loadings in reality aver-
aged closer to 40 and 80 percent of the
manufacturer's recommended design
criteria.
Excellent overall treatment perfor-
mance was observed throughout the
study. Phase-average final effluent BOD5
and total suspended solids (TSS) con-
centrations ranged from 14 to 24 mg/L
and from 10 to 27 mg/L, respectively. In
general, effluent residuals increased
with increasing process loadings. EPA's
monthly-average, 30 mg/L secondary
treatment requirement was exceeded
during one 5-week stretch for TSS only
because of operational procedures.
Potential savings were indicated in the
energy requirements of the ABF process
as compared with those of the conven-
tional activated sludge process. The
ability to handle short-term peaks in the
excess sludge production rate was
determined to be a critical factor in the
performance of ABF sludge treatment
and disposal facilities.
This Project Summary was developed
by EPA's Municipal Environmental Re-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that are more fully documented
in a separate report of the same title (see
Project Report ordering information at
back).
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Introduction
Biological wastewater treatment op-
tions are generally classified as either
suspended growth processes or attached
growth processes. The ABF process is a
modification thereof combining charac-
teristics of both types of processes.
Marketed by Neptune Microfloc, Inc.,
the process can be designed to achieve
carbonaceous removal, i.e., secondary
treatment only or carbonaceous removal
plus nitrification. This project evaluated
a full-scale municipal ABF installation in
a secondary treatment application.
The ABF process consists of a tower
or bio-cell, a bio-cell recirculation sys-
tem, a short-term aeration basin, and a
secondary clarifier. The bio-cell contains
redwood media stacked in a tower over
which primary effluent is distributed in
similar fashion to other stationary at-
tached growth processes. In addition,
settled sludge from the secondary clari-
fier is returned to mix with primary efflu-
ent and bio-cell underflow in the recircu-
lation wet well, resulting in a "suspended
growth" mixed liquor that is also contin-
uously distributed over the redwood
media. Originally, the bio-cell" was the
only biological unit in the ABF process.
Later, a modification was incorporated
that utilizes short-term aeration of the
mixed liquor following the bio-cell, as at
Helena. Intermediate clarification be-
tween the bio-cell and aeration basin is
not provided.
Limited full-scale performance infor-
mation was available for the ABF process
operated at Neptune Microfloc's rated
design conditions. This 2-year research
study was undertaken at Helena to
expand the existing data base.
Plant Description and
Experimental Schedule
The Helena plant serves a population
of about 28,000 and has no major indus-
trial contributors. The plant's design
flow rate is 22,700 m3/day (6.0 mgd),
and the plant was operating at about
11,350 m3/day (3.0 mgd) during the
study.
A schematic of a typical ABF system is
depicted in Figure 1. The Helena plant
flow diagram is shown in Figure 2. Of
special note is the fact that the plant
does not have any sludge treatment or
disposal recycle streams, thereby elimi-
nating this potential adverse effect on
wastewater efficiency. The original
Helena plant upgrade, incorporating just
the bio-cell towers, bio-cell recirculation
system, and secondary clarifiers, was
Bio-Cell
**<~*~* Tank g
Flow Control
& Splitting
Recirculation
Flow
Primary
Effluent
Bio-Cell
Lift Station
Final
Effluent
Return Sludge
Waste Sludge
Figure 1. Schematic of typical activated bio-filter system.
completed in 1 975. Subsequently, BOD5
and TSS secondary treatment effluent
limitations were violated, during cold
weather months (November-May). The
addition of the short-term aeration basin
in 1978 resulted in consistent permit
compliance from then on. Research pro-
ject data collection began in December
1978 per the schedule shown in Table 1.
The Helena ABF bio-cell consists of
two towers, structurally and functionally
independent. Valves are available to
operate one tower at a time. The red-
wood-slat media dry specific surface
area is 46 m2/m3 (14 ft2/ft3). A larger,
undetermined operating specific surface
area occurs because of biomass buildup.
The flow distribution system uses fixed
"vari-flow" nozzles manufactured by
Neptune Microbloc, Inc.
The short-term aeration basin is divided
into two compartments connected by an
overflow opening. During Periods C and
D, one aeration compartment was re-
moved from service to approach design
detention times with existing waste-
water flow rates. Aeration is provided by
three positive displacement blowers
through a grid system of static tube
diffusers. The identically-sized blowers
were provided with various sized pulleys
to change air supply possibilities.
Final "clarifier settled sludge (suction
sludge in Figure 1) is returned by gravity
to the recirculation wet well. Waste
sludge (hopper sludge in Figure 1) from
each clarifier is diverted to a separate
wet well and pumped to the primary clari-
fier for co-thickening with raw sludge and
removal to the sludge handling system.
Process Operational Controls
Special emphasis was made to achieve
optimum process control to ensure that
observed performance was not being lim-
ited by operational constraints. Numerous
operational variables exist for the ABF
process. Four controls that were believed
to relate most directly to performance
were selected for monitoring and adjust-
ment during the research program: (1)
volume of direct bio-cell mixed liquor
recirculation, (2) aeration basin dissolved
oxygen (DO) level, (3) secondary clarifier
return sludge flow rate, and (4) system
suspended sludge mass controlled by the
sludge wasting rate. Direct bio-cell recir-
culation is an operational control typical
of trickling filter systems. The other three
are controls typically associated with
activated sludge systems.
Control of the direct bio-cell recircula-
tion rate was based on Neptune Micro-
floc's recommended bio-cell hydraulic
loading range of 41 to 224 L/min/m2
(1.5 to 5.5 gpm/ft2). To stay within this
range, limited recirculation adjustments
were necessary. When both bio-cell
towers were in service (Period A), the
direct recirculation rate averaged 92
percent of the primary effluent flow rate
and provided an average hydraulic load-
ing of 81 L/min/m2 (2.0 gpm/ft2). No
direct recirculation was used during all
other periods when only one bio-cell
tower was in service because the bio-
cell hydraulic loading remained in the
satisfactory range of 98 to 106 L/min/m2
(2.4 to 2.6 gpm/ft2) with just the mixed
primary effluent and return sludge
to the tower.
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Influent
Legend
Wastewater
Grit Chamber No. 1
Comminutor w/Bar
1 Screen Backup
Suction Sludge
Hopper Sludge
~~** Bio-CeU'Recirculation
Combined Primary &
Secondary Sludge
Grit Chamber No. 2
pi Primary \
y Clarifier ,
i
t
/ j~v Waste
r\ {->~Sludgt
r^ Wet
ij Well
Re
»» W
w
*l
«* 1
,-« £-
"'
1 Maz-o-rator
Sludge
Storage
(2)
Suction
Sludge
dr.
et
ell
'!
1
^iM^H^
-c
T"
Bio-Cell
Recirculation
r
Hopper
Sludge
!
(
"~|
Tower
i
""^ Aeranon
L ^ Basin
Chlorine
Oxidizer
L
( ]Seco/7rfar/
I 1 I Clarifiers (2)
] Sludge to
i Landfill
OO
Sludge to
Land Application
Chlorine
Contact
Effluent to
Prickley
Pear Creek
Figure 2. Helena ABFplant flow diagram.
The aeration basin DO concentration
was maintained within Neptune Micro-
floe's recommendations of 1 to 3 mg/L
Ft changing the speed or number of
blowers used. The return sludge flow
rate (R) was adjusted throughout the
day, relative to wastewater flow (Q)
variations, to maximize the MLSS con-
centrations in the aeration basin. Addi-
tionally, the daily average return rate
was adjusted to maintain a preselected
R/Q. Neptune Microfloc recommended a
50 percent R/Q. During the project, opti-
mum solids distribution between the
aeration basin and secondary clarifiers
was obtained by maintaining the R/Q
between 50 and 60 percent, supporting
this recommendation.
To maintain a consistent inventory of
suspended sludge solids in the Helena
ABF system, special consideration had
to be given to sludge wasting procedures.
The relationship between MLSS concen-
tration and mean cell residence time
(MCRT) is shown in Figure 3. Prior to
Week 29 of the project, selected amounts
of sludge were wasted each day in an
attempt to maintain a preselected sus-
pended sludge inventory under aeration.
During Period A, this approach was sat-
isfactory in achieving a relatively uniform
MLSS concentration. During Period B,
however, with a smaller aeration basin
volume in service, wide fluctuations in
the MLSS concentration occurred and
the approach to accomplishing sus-
pended sludge inventory control was
modified. The modification involved
having the operators waste to achieve a
preselected MCRT rather than a pre-
selected sludge inventory. Using this
procedure, less variation was observed
in the MLSS concentration, as illustrated
by Period C in Figure 3.
Process Performance
ABF process performance is dependent
upon the individual and interrelated cap-
abilities of the bio-cell, aeration basin,
and secondary clarifier. The Helena ABF
system achieved a sludge volume index
that varied between 50 and 1 50 ml/gm
and never developed the bulking sludge
characteristics that are often associated
with activated sludge processes. As
such, clarifier performance never be-
came a limiting factor in achieving good
overall BOD5 and TSS removals from the
system.
Neptune Microfloc recommends a
design organic loading rate for the bio-
cell of 3.2 kg BOD5/day/m3 (200 Ib/day/
1,000 ft3) and an aeration basin sized to
achieve a system applied food-to-micro-
organism (FA/M) loading of 1.43 kg
BOD5/day/kg MLVSS (based on a pri-
mary effluent BOD5 (FA) of 1 50 mg/L and
an MLVSS (M) concentration of 3,000
mg/L). These design conditions will yield
a nominal (i.e., excluding sludge recycle
flow) aeration detention time of approx-
imately 45 min. The actual loadings
evaluated during the four data collection
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periods were less than Neptune Micro-
floe's recommended values, as illus-
trated in Figure 4. The highest combined
loadings achieved during the project
averaged 79 percent of the recommend-
ed bio-cell organic loading and 75 per-
cent of the recommended system F^/M
loading during Period C. Nominal aera-
tion detention times averaged 106,
103, 59, and 118 min, respectively,
during Periods A, B, C, and B'.
A summary of Helena ABF process
performance for Periods A, B, C, and B'
is presented in Table 2. Weekly-average
effluent BOD5 variations are plotted in
Figure 5. Effluent quality decreased
slightly as process loadings increased
with the exception of TSS removal dur-
ing Period B. For this reason. Period B
was retested (Period B'). Period B'
results better reflect system capabilities
at Period B loadings because operational
procedures did not limit system perfor-
mance during Period B' as they did in
Period B. Operational procedures also
did not limit system performance during
Periods A and C.
Helena plant performance was evalu-
ated based on its ability to meet federal
secondary treatment standards. Weekly-
and monthly-average effluent concentra-
tions were not to exceed 45 and 30 mg/L,
respectively, for both BOD5 and TSS.
Further, the plant was to achieve 8 5 per-
cent overall removals of the raw waste-
water BOD5 and TSS concentrations on
a monthly-average basis. Although not
designed for nitrification, the ABF sys-
tem was monitored to determine if nitri-
fication occurred. At no time did the
system nitrify at the loading conditions
evaluated.
All standards were met during Periods
A and B'. During Period B, all standards
were met except for one 5-week stretch
when TSS effluent limitation violations
occurred because of operational proce-
dures. All standards were met in Period
C except the 85 percent removal require-
ments during a 10-week stretch when the
influent raw wastewater concentrations
were unusually low. It was concluded
from these results that all federally de-
fined standards can be met at the ABF
loadings evaluated if good process
control is exercised.
Process Energy Requirements
Energy to operate the Helena ABF
system was consumed for pumping to
the bio-cell and for oxygen transfer and
mixing in the aeration basin. Calculated
ABF process energy requirements for
Table 1. Experimental Schedule for Helena Project
Period
A
B
Plant
Modifications
C
D
Date
12/1/78 to
2/22/79
2/23/79 to
7/12/79
7/1 3/79 to
8/2/79
8/3/79 to
1/10/80
1/1 1/80 to
2/28/80
No. of
Weeks
12
20
3
23
7
Description
Total bio-cell and total aeration
basin in service
Half bio-cell and total aeration
basin in service
Half bio-cell in service. Aeration
basin down for modification.
Half bio-cell and half aeration basin
in service
Half bio-cell and half aeration basin
in service. Primary clarifier out of
service most of the time.
B'
3/21/80 to
7/10/80
16 Half bio-cell and total aeration
basin in service.
48
44
t 4°
f~ 36
o
x 32
co
CO
^ 28
24
20
-v 16
I 72
r^
5
Period A
'MLSS
Period B
MCRT Control
Initiated
Period C
2 6 10 14 18 22 26 30 34 38 42 46 50 54 58
Dec. Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec. Jan.
1979 1980
Figure 3. Weekly variations in MCRT and MLSS concentration.
Period A, B, and C loadings versus theo-
retical energy consumption for a con-
ventional activated sludge process are
graphically illustrated in Figure 6. Similar
treatment and oxygen transfer efficien-
cies were assumed for all systems in
making the calculations. The energ^
requirements depicted for the ABF pro
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Neptune
Microfloc
Recommendation
Period A
(12 weeks)
Bio-Cell Loading
(Ib BODs/day/1,000 ft3}*
System F*/M
(kg BODs/day/kg MLVSS)
Period B
(20 weeks)
Period C
(23 weeks)
Period B'
(16 weeks)
*lb/day/1,000 ft3 x 0.016 = kg/day/m3
Figure 4. Actual process loadings evaluated compared to manufacturer's recom-
mended design loadings.
Table 2. Process Performance Summary
Parameter
Period A Period B Period C Period B1
BOD5
Raw fmg/Lj
Primary (mg/L)
Secondary (mg/L)
Overall Removal (%)
TSS
175
130
14
92
148
128
21
86
152
124
24
84
164
112
19
88
Raw (mg/L)
Primary (mg/L)
Secondary (mg/L)
Overall Removal (%)
130
73
10
92
194
111
27
86
214
109
22
90
196
102
17
91
cess indicate a potential energy savings
when compared with those of the con-
ventional activated sludge process.
Several factors must be considered,
however, when a design comparison is
made:
Aeration basin oxygen transfer effi-
ciency
A higher efficiency, e.g., due to
equipment selection, will favor
the conventional activated
sludge process.
A lower efficiency, e.g., due to
elevation or equipment selection,
will favor the ABF process.
Bio-cell organic loading
A lower bio-cell loading reduces
aeration basin oxygen demand
and energy consumption, e.g..
Period A versus the other periods.
Bio-cell equipment selected
A smaller media depth will reduce
bio-cell pumping and energy re-
quirements.
Unit process layout
Provision of gravity return sludge
flow from the secondary clari-
fiers to the recirculation wet well
will increase the bio-cell pumping
total dynamic head.
ABF Sludge Production
The mass of secondary solids directed
to the sludge handling system plus the
mass of TSS contained in the secondary
effluent were defined as the total amount
of secondary sludge produced. Weekly-
average secondary sludge production
rates for Periods A, B, C, and B' are pre-
sented in Figure 7. Greater-than-antici-
pated quantities of secondary sludge
were produced for all periods, and signi-
ficant variations occurred from period to
period. A partial explanation for the large
secondary sludge production rates and
variations may have been higher-than-
normal quantities of pass-through solids
in the primary effluent, which do not
represent sludge grown but, neverthe-
less, sludge that must be wasted. This
factor alone, however, could not totally
account for the large variations noted
among periods.
Because of higher process loadings,
the secondary system sludge production
rate was expected to be somewhat
higher during Periods B and B'than dur-
ing Period A, but the magnitude of the
increases was surprising. At the same
time, the highest process loading condi-
tions evaluated, Period C, resulted in a
lower sludge production rate than for
either Period B or B'. The higher sludge
production rates observed in Periods B
and B' may have been due, in part, to the
process testing arrangement, but it is
unlikely that the arrangement caused
such large variations.
Periods B and B' were both conducted
during the months of March through July.
Investigations at several activated sludge
plants have shown that the sludge pro-
duction rate typically varies from one
season to another. Periods of high and
low sludge production normally last for
several months, and some plants demon-
strate recurring high sludge production
during the months of March through July.
No definite cause has been isolated, but
environmental changes are suspected.
The variable sludge production rates at
Helena may have been more influenced
by environmental conditions than by
process loadings, at least for the loading
ranges evaluated.
This evaluation emphasizes the impor-
tance of sizing the sludge treatment and
disposal facilities for the ABF process,
like any process, to adequately handle
short-term peaks in sludge production as
well as long-term average values. Based
on the Helena data, secondary sludge
treatment and disposal facilities for an
ABF system that should be designed to
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handle on the average about 1.1 kg
TSS/kg (BOD5)R and should be capable
of handling as much as 160 percent of
this production rate for several months
at a time. Operational experience at
Helena indicates that sludge handling is
a high priority item for a well-operated
ABF system.
Summary and Conclusions
The ABF process is an attractive, com-
petitive secondary treatment alternative
because of its operational stability, per-
formance reliability, and energy savings
potential. System design should take
into account the following factors:
1. The potential exists for reducing
energy consumption by more than
25 percent compared with that of
the conventional activated sludge
process.
2. Secondary sludge treatment and
disposal facilities should be de-
signed to handle both the average
and peak rates of sludge produced
(at Helena, an average rate of 1.1
kg TSS/kg (BOD5)R with a peak
rate of 1 60 percent of this value for
several months).
3. Consideration should be given to
increasing the detention time of the
short-term aeration basin beyond
that recommended by Neptune
Microfloc, Inc., especially if the
bio-cell loadings recommended by
Neptune Microfloc are used in
design.
4. The demonstrated stable sludge
settling characteristics should be
considered an advantage to system
performance, but should not be
considered a reason for providing
minimal process control.
5. System operation and maintenance
requirements should be considered
as similar to those of an activated
sludge process.
The full report was submitted in fulfill-
ment of Grant N. R806047 by the City
of Helena, MO, under the partial spon-
sorship of the U.S. Environmental Pro-
tection Agency.
Uj
20 23
Time Frame (weeks)
Figure 5. Weekly variations in final effluent BOD5 concentration.
16
Aeration Energy
Pumping Energy
70 kW
58 kW
45 kW
52 kW
Period A Period B Period C Activated
Sludge
Figure 6. Comparison of calculated
energy requirements for
the ABF process versus
the conventional activa-
ted sludge process.
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Period B
Jy ^ '
Period C
PeriodB'
20 23
Time Frame (weeks)
Figure 7. Weekly variations in secondary system sludge production.
16
Kerwin L Rakness, James R. Schultz, and Robert A. Hegg are with M&l. Inc.,
Consulting Engineers, Fort Collins, CO 80525-; Jan C. Cranor and Richard A.
Nisbet are presently with the State of Montana and the City of Helena, Helena,
MT 59601, respectively.
Richard C. Brenner is the EPA Project Officer (see below).
The complete report, entitled "Full-Scale Evaluation of Activated Bio-Filter
Wastewater Treatment Process," (Order No. PB 82-227 505; Cost: $12.00,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U. S. GOVERNMENT PRINTING Of FICE: 1982/555-092/0466
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Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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