FINE PORE DIFFUSER CASE HISTORY
FOR FRANKENMUTH, MICHIGAN
by
Thomas A Allbaugh and S. Joh Kang
McNamee, Porter & Seeley, Inc.
Engineer/Architects
Ann Arbor, Michigan 48108
Cooperative Agreement No. CR812167
Project Officer
Richard C. Brenner
Water and Hazardous Waste Treatment Research Division
Risk Reduction Engineering Laboratory
Cincinnati, Ohio 45268
RISK REDUCTION ENGINEERING LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
Development of the infonnation In this report has been funded in part by the U.S.
Environmental Protection Agency under Cooperative Agreement No. CR812167 by the
American Society of Civil Engineers, The report has been subjected to Agency peer and
administrative review and approved for publication as an EPA document. Mention of trade
names or commercial products does not constitute endorsement or recommendation for use.
ii
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FOREWORD
Today's rapidly developing and changing technologies and Industrial products and
practices frequently carry with them the Increased generation of materials that. If Improperly
dealt with, can threaten both public health and the environment. The U.S. Environmental
Protection Agency (EPA) is charged by Congress with protecting the Nation's land, air, and
water resources. Under a mandate of national environmental laws, the Agency strives to
formulate and implement actions leading to a compatible balance between human activities
and the ability of natural systems to support and nurture life. These laws direct EPA to
perform research to define our environmental problems, measure the impacts, and search for
solutions.
The Risk Reduction Engineering Laboratory is responsible for planning, implementing, and
managing research, development, and demonstration programs to provide an authoritative,
defensible engineering basis in support of the policies, programs, and regulations of EPA with
respect to drinking water, wastewater, pesticides, toxic substances, solid and hazardous
wastes, and Superfund-related activities. This publication is one of the products of that
research and provides a vital communication link between the researcher and the user
community.
As part of these activities, an EPA cooperative agreement was awarded to the American
Society of Civil Engineers (ASCE) In 1985 to evaluate the existing data base on fine pore
diffused aeration systems in both clean and process waters, conduct field studies at a number
of municipal wastewater treatment facilities employing fine pore aeration, and prepare a
comprehensive design manual on the subject. This manual, entitled "Design Manual - Fine
Pore Aeration Systems," was completed in September 1989 and is available through EPA's
Center for Environmental Research Information, Cincinnati, Ohio 45268 (EPA Report No.
EPA/625-1-89/023). The field studies, carried out as contracts under the ASCE cooperative
agreement, were designed to produce reliable information on the performance and operational
requirements of fine pore devices under process conditions. These studies resulted in 16
separate contractor reports and provided critical input to the design manual. This report
summarizes the results of one of the 16 field studies.
E. Timothy Oppelt, Director
Risk Reduction Engineering Laboratory
ill
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PREFACE
In 1985, the U.S. Environmental Protection Agency funded Cooperative Research
Agreement CR812167 with the American Society of Civil Engineers to evaluate the existing
data base on fine pore diffused aeration systems in both clean and process waters, conduct
field studies at a number of municipal wastewater treatment facilities employing fine pore
diffused aeration, and prepare a comprehensive design manual on the subject. This manual,
entitled 'Design Manual - Fine Pore Aeration Systems," was published in September 1989 (EPA
Report No. EPA/725/1-89/023) and Is available from the EPA Center for Environmental
Research Information, Cincinnati, OH 45268.
I
As part of this project, contracts were awarded under the cooperative research agreement
to conduct 16 field studies to provide technical input to the Design Manual. Each of these
field studies resulted in a contractor report. In addition to quality assurance/quality control
(QA/QC) data that may be Included in these reports, comprehensive QA/QC information is
contained in the Design Manual. A listing of these reports is presented below. All of the
reports are available from the National Technical Information Service, 5285 Port Royal Road,
Springfield, VA 22161 (Telephone: 703-487-4650).
1. "Fine Pore Diffuser System Evaluation for the Green Bay Metropolitan Sewerage
District" (EPA/600/R-94/093) by J.J. Marx
2. "Oxygen Transfer Efficiency Surveys at the Jones Island Treatment Plants,
1985-1988" (EPA/600/R-94/094) by R Warriner
3. "Fine Pore Diffuser Fouling: The Los Angeles Studies" (EPA/600/R-94/095) by M.K.
Stenstrom and G. Masutani
4. "Oxygen Transfer Studies at the Madison Metropolitan Sewerage District Facilities"
(EPA/600/R-94/096) by W.C. Boyle, A. Craven, W. Danley, and M. Rleth
5. "Long Term Performance Characteristics of Fine Pore Ceramic Diffusers at
Monroe, Wisconsin" (EPA/600/R-94/097) by D.T. Redmon, L. Ewing, H Melcer
and G.V. EUefson
6. "Case History of Fine Pore Diffuser Retrofit at RIdgewood, New Jersey"
(EPA/600/R-94/098) by J.A. Mueller and P.D. Saurer
7. "Oxygen Transfer Efficiency Surveys at the South Shore Wastewater Treatment Plant
1985-1987" (EPA/600/R-94/099) by R Warriner
8. "Fine Pore Diffuser Case History for Frankenmuth, Michigan" (EPA/600/R-94/100)
by T.A. Allbaugh and S.J. Rang
9. "Off-gas Analysis Results and Fine Pore Retrofit Information for Glastonbury,
Connecticut" (EPA/600/R-94/101) by RG. Gilbert and RC. Sullivan
10. "Off-Gas Analysis Results and Fine Pore Retrofit Case History for Hartford,
Connecticut" (EPA/600/R-94/105) by RG. Gilbert and RC. Sullivan
iv
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11. "The Measurement and Control of Fouling in Fine Pore Diffuser Systems"
(EPA/600/R-94/102) by E.L. Bamhart and M. Collins
12. "Fouling of Fine Pore Diffused Aerators: An Interplant Comparison"
(EPA/600/R-94/103) by C.R BaJllod and K. Hopkins
13. "Case History Report on Milwaukee Ceramic Plate Aeration Facilities" i
(EPA/600/R-94/106) by LA. Ernest
14. "Survey and Evaluation of Porous Polyethylene Media Fine Bubble Tube and Disk
Aerators" (EPA/600/R-94/104) by D.H. Houck
15. "Investigations Into Blofoullng Phenomena In Fine Pore Aeration Devices"
(EPA/600/R-94/107) by W. Jansen, J.W. Costerton, and H. Melcer
16. "Characterization of Clean and Fouled Perforated Membrane Dlffusers"
(EPA/600/R-94/108) by Ewlng Engineering Co.
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ABSTRACT
Frankemnuth Is a community of 4,000 people In central Michigan. About 25-30% of the
flow and 50-70% of the BOD load to the wastewater treatment plant are contributed by a
brewery. In January 1986, conversion from a stainless steel broad band coarse bubble
diffuser system to fine pore aeration was completed In all of the six existing aeration tanks.
The Frankenmuth retrofit was designed with tn-sttu wastewater oxygen transfer efficiencies
(OTE) at average air flow and peak flow based on off-gas tests at other locations. These values
were adjusted to account for the significant high strength industrial component of the influent
wastewater. The design OTE at 2 scfin/diffuser was only two-thirds that used at 1
scfim/dliffuser (ocSOTEs of 16.9% and 11.0%, respectively). In spite of lower than expected
OTEs, the Frankenmuth retrofit to fine pore diffusers was an economic success. The actual
capital cost of installation was slightly more than estimated during the evaluation period, but
the projected energy savings appeared to be slightly greater as well.
OTEs were measured by off-gas testing on selected aeration cells on 13 different days
between April 1987 and May 1988. Some of the off-gas tests were carried out on consecutive
days before and after gas cleaning of the diffusers. No relationship could be developed
between gas cleaning and OTE.
The rate of diffuser plugging and fouling at Frankenmuth Is significant. The plant staff has
employed different methods of determining when cleaning should be done since the fine pore
equipment was installed. These have included cleaning when the dynamic wet pressure (DWP)
reached 16-18 in. w.g., cleaning with small doses of gas every 2 weeks, and operating at higher
air rates than required for oxygen demand to hopefully inhibit biological growth on the
diffuses. The first two methods employed in 1986 and 1987 appear to have been successful
in maintaining acceptable levels of DWP and system performance. Operating at elevated air
flows (January through June 1988) was probably successful in limiting DWP but resulted in a
significant increase In system operating cost. The hydrogen chloride gas used was
approximately one-third of a pound per diffuser per year during the evaluation state.
The condition of the fine pore diffusers was monitored over the long term by measuring
DWP and pressure drop across air distribution orifices in test diffuser assemblies. Four
diffuses were placed in one of the six aeration cells, and measurements were obtained at 1 to
2 week intervals. No relationship could be developed between DWP and OTE. However, gas
cleaning was effective in controlling diffuser DWP.
This report was submitted in partial fulfillment of Cooperative Agreement No. CR812167 by
the American Society of Civil Engineers under subcontract to McNamee, Porter & Seeley, hie.
under the partial sponsorship of the U.S. Environmental Protection Agency. The work reported
herein was conducted over the period of 1986-1988.
vl
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TABLE OF CONTENTS
Page
Foreword ......................... ............
Preface ................................
[[[ iv
Abstract ...............................
[[[ . ..................... vl
Figures and Tables [[[
Summary ...............................................
Conclusions [[[ 0
*....*....... ««. ........................ .........fc.l>>s<>sB><>^B>Ba £
System Description and Performance [[[ 3
Aeration System Retrofit Design ......... ....................... . ............................... 3
Plugging/Fouling Potential ....................... . .......................................... 8
Equipment Procurement and Specifications [[[ !0
Performance of the New Equipment .............................. . ........................... n
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LIST OF FIGURES AND TABLES
1 Aeration Tank Arrangement [[[ 4
2 Total Plant Energy Use vs. BOD Applied ........................... . ................................. 12
3 KWH/lb. BOD vs. Time ....................... [[[ 13
4 Cell 5 DWP vs. Time [[[ . 17
5 Off-Gas Collections Plan [[[ 18
6 Alpha SOTE vs. Time [[[ ; 22
1 Aeration Retrofit Energy Analysis Predesign Estimates ...... . ..................... ; ........... 6
2 Fine Pore Aeration System Retrofit Design Parameters ............................ . ........... 7
3 Summary of Plugging/Fouling Potential Test Results .............................. . ........... 9
4 Electrical Energy Use [[[ 14
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SUMMARY
The Frankenmuth retrofit was designed with in waste oxygen transfer efficiencies at average
air flow and peak flow based on off-gas tests at other locations. These values were adjusted
to account for the significant high strength industrial component of the influent wastewater.
The design OTE at 2 scfm/diffuser was only two-thirds that used at 1 scfm/diffuser (11.0%
and 16.9% alpha SOTE, respectively).
In spite of the lower than expected OTE's, the Frankenmuth retrofit to fine pore diffusers
was an economic success. The actual capital cost of installation was slightly more than
estimated during the evaluation period, but the projected energy savings appeared to be
slightly greater as well. This results from over-estimating the efficiency of both the coarse
bubble and fine pore dif fusers during the pre-design evaluation.
The rate of diffuser plugging and fouling at Frankenmuth is significant. The gas cleaning
procedure has been practiced at varying intervals as shown in Table 4. The hydrogen chloride
gas used was estimated to be approximately one pound per diffuser per year during the
evaluation stage. Actual gas use has averaged about one-third that amount. !
The plant staff has employed different methods of determining when cleaning should be done
since the fine pore equipment was installed. These have included cleaning when DWP reaches
16-18 w.g., cleaning with small doses of gas every two weeks, and operating .at higher air
rates than required for oxygen demand to hopefully inhibit biological growth on the dif fusers.
The first two methods employed in 1986 and 1987 appear to have been successful in
maintaining acceptable levels of DWP and system performance. Operating at elevated air
flows (January through June 1988} was probably successful in limiting DWP, but resulted in a
significant increase in system operating cost as shown in Table 5.
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CONCLUSIONS
The off-gas test data clearly shows the effect of changing "alpha" values from inlet end
to outlet end of a plug flow aeration tank. Comparing alpha SOTE data for cells 5 and 6
(which always operated in series) found in Tables 7a and 7b shows increases in alpha
SOTE ranging from 15% to 103%, for operation at reasonably similar conditions.
No relationship could be developed between gas cleaning and oxygen transfer efficiency
at Frankenmuth.
No relationship could be developed between dynamic wet pressure (pressure loss across
the diffusers) and oxygen transfer efficiency at Frankenmuth. '.
Gas cleaning is effective in controlling the diffuser dynamic wet pressure at
Frankenmuth.
Operation at higher than necessary air flows to control DWP does not appear to have
been economically successful for Frankenmuth.
Ceramic fine pore diffusers are economically viable for plants with relatively high
plugging/fouling potential.
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SYSTEM DESCRIPTION AND PERFORMANCE
Frankenmuth is a central Michigan community of approximately 4,000 people. It is a well
known tourist attraction in the State featuring German decor, gift shops and candy stores,
and two large restaurants featuring family-style chicken dinners which feed in excess of
10,000 people daily. Frankenmuth is also the home of a G. Heileman Brewery which produces
Carling Black Label, and other Heileman products. The City Wastewater Treatment Plant
processes all the wastes from the City, and from the brewery as well.
During the study on aerator performance, approximately 25-30% of the flow and 50-70% of
the plant influent BOD came from the brewery. The restaurants probably contribute
significantly to the plant loading, but there are no long term records to quantify the effect.
In the future, brewery waste will be pretreated using an upflow anaerobic sludge blanket
system.
The process flow scheme on the plant site consists of a manual bar screen, raw sewage
pumping, a square aerated grit tank, two rectangular primary settling tanks, aeration tanks,
two fifty-foot diameter final settling tanks, and disinfection with chlorine. Primary and
waste activated sludges are combined and anaerobically digested, dewatered with vacuum
filters, and hauled offsite to disposal. In-plant recycle streams are returned to the raw
sewage piumping station.
The plant's NPDES permit requires 30-day average effluent BODg and SS to be less than 30
mg/1. There is no limit on ammonia nitrogen in the effluent.
AERATION SYSTEM RETROFIT DESIGN
The aeration tankage at Frankenmuth consists of six individual aeration cells^ each measuring
44 feet by 22 feet with 15-foot side water depth. The total volume of all 6 cells is 87,000
cubic feet (651,000 gallons). The cells are interconnected as shown in Figure 1. The tank
configuration allows considerable operating flexibility, ranging from modified contact stabili-
zation by running up to 4 of the 6 cells in series for return sludge re -aeration followed by the
remainder of the cells (2 to 4) for aeration, to conventional plug flow activated sludge with
all 6 tanks operated in series. The plant has also been run with Tanks 1 and 2 re-aerating
return sludge, and Tanks 4 and 3, and 5 and 6 operating as two parallel two-tank plug flow
aeration trains. Normal operation is with Tanks 1 and 2 (and possibly tank 3) re -aerating
return sludge, and the remainder of the tanks arranged in series as aeration tanks.
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The air diffusion equipment initially installed was stainless steel broad band coarse bubble
diffusers mounted on galvanized steel headers. Four multi-stage centrifugal blowers were
provided, each with a nominal capacity of 6,500 cfm. The existing system was not always able
to keep up with the oxygen demand of the plant's primary effluent flow. This was at least
partly due to the fact that corrosion had caused a significant number of the diffusers to break
off.
The existing diffusion equipment needed to be replaced because of the corrosion failure, so
this project was not a typical retrofit predicated on recouping the investment in new
equipment from energy savings. Before actual detailed design was begun, an economic
analysis was performed comparing the replacement of the existing system with new stainless
steel coarse bubble dif fusers and stainless steel headers and with a retrofit to ceramic fine
pore dif fusers in full floor coverage, but it did not include consideration of keeping the
existing system in service. The results of this pre-design analysis are shown in Table 1. The
capital costs of the two systems were judged to be approximately the same. It was estimated
that the energy savings available with the fine pore system as compared to the coarse bubble
system could repay the investment in new equipment in approximately six years, so the fine
pore system was selected.
The estimated capital cost of the proposed ceramic fine pore diffuser system included the
diffusers and in-tank piping, new air drop pipes in each tank, in-place gas cleaning, two new
smaller blowers and new air inlet filters. The diffuser cost was based on a total of 2400
diffusers. The analysis assumed no salvage value for the existing equipment not incorporated
into the new work. Capital costs for the proposed new coarse bubble equipment needed to
replace the deteriorated existing equipment included new stainless steel drop pipes, in-tank
headers and diffusers. The diffuser cost estimate was based on a total of 2000 stainless steel
broad band coarse bubble units.
The analysis assumed an "alpha" value for the fine pore system of 0.50. and for the coarse
bubble system of 0.90. "Beta" was assumed to be 0.99 in both cases. Complete design
parameters are shown hi Table 2.
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Table 1
Aeration Retrofit Energy Analysis
Predesign Estimates
Primary Effluent Flow (mgd)
(1)
(1)
(2) ,
Primary Effluent BODg (mg/1)
Primary Effluent BOD (Ibs/day)
Actual Oxygenation Rate (AOR) (Ibs/day)
In-Waste Oxygen Transfer Effluent^
Average Air Flow Required (scfm) ^
Average Brake Horsepower
Estimated Annual Energy Cost ($)
Estimated Annual Operation & Maintenance ($)
Total Estimated Annual Cost ($)
Estimated Annual Savings ($)
Ceramic
Fine Pore
1.4
512
5977
6575
11.9%
2214
80
26,140
3,000
29,140
29,675
Stainless
Coarse Bubble
1.4
512
5977
6575
7 . 2%
3641
180
: 58,815
58,815
(1) Flows and loads based on average for November 1983 through October 1984.
(2) Assumes 1.1 Ibs oxygen required per Ib. BOD applied.
(3) At tank average D.O. of 2.0 mg/1.
(4) Average per kWh cost approximately $0.05.
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Table Z
Fine Pore Aeration System Retrofit
Design Parameters
Average Peak
Primary TZifluent Flow (mgd) 1.8 Z.7
Primary Effluent BOD. (mg/1) 515
Lb. BOD per 1000 cf per day . 86 147
Lb. BOD5 per day 7730 11,600
Lb. O2 Req'd/lb. BOD Removed 1.1 1.1
Tank Avg. D.O. (mg/1) Z.O 0.5
Average MCRT (days) 10
NPDES Permit Limits, BODg (mg/1) 30 45,
NPDES Permit Limits, SS (mg/1) 30 45
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A net energy savings of approximately $32,000 per year was expected using the fine pore
diffusers system. The net total savings was estimated to be $29.000 per year by subtracting
the estimated $3.000 annual cost of additional maintenance required for the fine pore system.
The anticipated operating range of air flows with the proposed fine pore'diffuser system was
from approximately 800 scfrn (the estimated mixing limit) to approximately 4000 scfm
required to satisfy the peak oxygen demand to aeration. The estimated annual average air
flow was expected to be approximately 2200 scfm.
The minimum operating air flow of the existing blowers (surge point) was approximately 2100
cfm. Evaluation of the blower performance curves indicated that the operating efficiency in
the range near surge was very poor when compared to the efficiencies possible with new
smaller units. It was determined that to ensure that the anticipated energy savings were
actually realized at the plant, new blowers sized to operate with the new equipment would
have to be part of the project. As a result, two new units with nominal capacity of 2200 scfm
were included as part of the new facilities. Only the blowers were replaced. Blower bases.
motor starters, valves, flexible connectors, etc., were re-used with the new equipment.
Plugging/Fouling Potential
Before final design of the retrofit was begun, a test header with four ceramic plate-type
diffusers was installed near the inlet end of Aeration Cell 5 to monitor the potential for
plugging and fouling, and to ensure that fine pore diffusers were compatible with the waste.
The dynamic wet pressure (DWP, pressure drop across the diffuser) was monitored daily for a
period of approximately 10 weeks to develop an estimate for the plugging and fouling rate.
At the end of that period, the test header was cleaned by injecting hydrogen chloride gas in
with the air supply.' --.-..-.-.__ ............:......
The observed fouling rate was significant. DWP was observed to increase more than 1 inch
w.g. in as little as one day. However, short term increases in air flow per diffuser (air
"bumping") reduced DWP, and the gas cleaning was effective in reducing DWP as well. The
actual air flow rate during the "bumping" was not measured. Initial DWp: readings were
approximately six inches w.g. DWP's as high 24.5 inches were recorded. The DWP following
gas cleaning was approximately 9.0 inches w.g.. compared to the initial readings of 5-6 inches
w.g. A weekly summary of the plugging/fouling test results is shown in Table 3. Daily data is
shown in Appendix A, As shown, the DWP fluctuated throughout the test period, dropping
abruptly for no apparent reason at times.
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Table 3
Summary of Plugging/Fouling Potential Test Results
Diffuser
Date
8/27/84
9/3/84
9/10/84
9/15/84
9/17/84
9/24/84U)
10/l/84(2)
10/8/84
10/14/84
10/22/84
10/3 1/84 (3)
1 1/4/84 (4)
1
Orifice DWP
5.5
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
6.0
15.0
15.0
24.5
17.5
17.0
11.5
17 ;0
11.0
10.0
14.5
9.0
2
Orifice DWP
6.0
5.5
6.0
6.5
6.5
6.0
5.5
6.5
6.5
6.0
6.0
6.0
6.0
15.5
15.0
23.0
17.0
17.0
11.5
17.0
11.0
10.0
14.0
9.5
3
Orifice DWP
6.0
6.0
6.0
6.0
5.5
5.0
5.0
5.0
5.0
6.0
6.0
6.5
5.0
15.5
15.0
22.5
18.0
12.0
12.0
19.0
12.0
10.0
13.5
9.5
4
Orifice DWP
7.5
: 6.5
7.5
7.0
6.5
5.5
6.0
5.5
4.5
6.0
5.0
7.0
5.5
15.0
14.0
21.0
17.5
18.0
12.0
18.0
13.0
10.0
14.5
9.0
Air flow per diffuser approximately 1.8 - 2.0 scfm.
Orifice == Pressure drop across distribution orifice.
DWP = Pressure drop across diffuser.
All valuess are inches water gauge
Values are weekly. Daily data is in Appendix A.
(1) Before air bumping.
(2) Af ter air bumping.
(3) Before gas cleaning
(4) After gas cleaning.
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It was decided that the plugging and fouling phenomenon could be controlled and that the
potential energy savings were significant enough that design and installation of the retrofit to
fine pore diffusers should proceed.
EQUIPMENT PROCUREMENT AND SPECIFICATIONS
The new diffused aeration equipment was specified to be ceramic disc-type diffusers in full
floor coverage with in-place hydrogen chloride gas cleaning. Two new multi-stage 200 Hp
centrifugal blowers, nominal capacity 2200 scfm, were specified to replace two of the
existing blowers. Two existing (250 Hp) blowers were left in place to provide standby capacity
for anticipated peaks. Because of the age of the existing air piping system, in-line air filters
to be placed immediately upstream of the air drop into each cell were specified requiring 97
percent removal of particles 0.3 microns and larger to protect the new equipment from air
side fouling. These were not installed. New elements for the existing inlet filters capable of
removing 20 micron and above particles were installed instead at the City's request. This
relaxation of the air filtration requirements was requested by the City because the inside
lining of the existing air piping system was determined to be in excellent condition, thereby
minimizing the potential for rust particles to plug the diffusers. City personnel also felt that
the maintenance cost savings from the less expensive filter elements outweighed the danger
of plugging from airborne particulate matter smaller than 20 microns,
A total of 2400 diffusers, 400 evenly distributed per aeration cell, were specified based on
anticipated actual oxygen requirements under the design peak condition. This provides one
diffuser for every 2.42 sf of tank bottom area. The anticipated maximum air flow rate per
diffuser at peak load was approximately 1.7 scfm. A minimum air flow of approximately
1,750 cfrn was anticipated based on review of past plant operating records. The air flow per
diffuser at this rate would be 0.7 scfm. There is one air drop pipe into each aeration cell. Air
flow to each cell is controlled manually with a butterfly valve.
Construction was begun in December 1985. The installation work was performed by the
treatment plant staff, with the aeration equipment manufacturer providing technical
assistance as required. The new equipment was placed in service in January 1986. The total
project capital cost for equipment was $160,000. The plant staff invested approximately 800
man hours in the installation and startup for the new equipment. The total project cost,
including installation and engineering, was approximately $190,000.
10
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PERFORMANCE OF THE NEW EQUIPMENT
Figure 2 shows a graph of total plant energy use versus pounds of BOD treated. Figure 3
shows a plot of kWh per pound of BOD treated. As shown, a marked decrease in energy
consumption was experienced in the two years (1986 and 1987) after the retrofit was
completed. Data in 1988 indicates that savings may no longer be present. However, the plant
staff has chosen, beginning in January 1988, to operate at higher air flow rates than
previously to lessen the necessity for gas cleaning and. they believe, enhance treatment
efficiency. This has resulted in higher operating levels of D.O. and consequent increased
power usage. The efficiency of the primary settling tanks has also been increased, resulting
(as shown is Table 4) in lower BOD loading to the aeration system. The 1988 data is not
necessarily indicative of a loss in energy savings potential.
Table 4 shows total pounds of BOD treated and total plant electrical power use from
December 1984 through May 1988. It also shows kWh per pound of BOD treated. Based on the
average monthly BOD from December 1984 through December 1985 (295.939 Ibs./month) and
the average kWh/lb. BOD for that and succeeding years, estimated power savings are shown
in Table 5.
The annual cost of practicing gas cleaning has varied considerably. Gas cost for Frankenmuth
is approximately $1.55/lb. Labor costs, including payroll taxes and insurance, average
approximately $20.00/hour. During the first 12 months when cleaning was done (4/86 - 5/87).
the total gas used was 963 Ibs.. and the total labor required was 45 man-hours. The cost was
approximately $2,400. During the last 12 months reported, 152 Ibs. of gas and 8.5 man-hours
were used for a total cost of approximately $400. A summary of gas use and cleaning labor is
shown in Table. 6. These figures would indicate that the preliminary economic evaluation,
which estimated cleaning costs at $1.25/year/diffuser, was fairly conservative.
The plant; consistently meets its NPDES permit for effluent BOD in spite of the fact that
wide daily fluctuation in BOD load are experienced. Appendix B contains a summary of some
plant operating parameters for the period since the retrofit was completed. Significant
additional decreases in energy consumption for aeration, and in the cost and frequency of gas
cleaning are expected following completion and startup of the brewery pretreatment system.
Three dif:Eusers in each cell were equipped for monitoring DWP and the pressure drop across
the air distribution orifice in the holder. Readings have been recorded on a reasonably
consistent basis since the equipment was installed. Figure 4 shows a plot of the average DWP
11
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Table 4
Electrical Energy Use
MONTH BOD(lbs) KW(hrs) KW/LBS
BOD(lbs) KW(hrs) KW/LBS
DEC. 84
JAN. 85
FEB. 85
MAR. 85
APR. 85
MAY 85
JUN. .85
JUL. 85
AUG. 85
SEPT 85
OCT. 85
NOV. 85
DEC. 85
AVERAGE
STD. DEV.
JAN. 86
FEB. 86
MAR. 86
APR. 86
MAY 86
JUN. 86
JUL. 86
AUG. 86
SEPT 86
OCT. 86
NOV. 86
DEC . 86
AVERAGE
STD. DEV.
266810
254340
211230
283961
330421
326600
312043
402340
386795
328032
318772
183000
242867
295939
61385
258458
240308
320626
349158
343273
259378
314973
384139
292973
291326
286486
265777
300573
41263
242256
209316
227424
220832
229072
248848
252144
255440
223612
237312
238960
257088
196112
233724
17626
163152
161504
174688
177984
182989
156085
169744
163152
181280
163152
194464
191099
173274
11914
0.91
0.82
1.08
0.78
0.69
0.76
0.81
0.63
0.58
0.72
0.75
1.40
0.81
0.83
0.20
0.63
0.67
0.54
0.51
0.53
0.60
0.54
0.42
0.62
0.56
0.68
0.72
0.59
0.08
JAN. 87
FEB. 87
MAR. 87
APR. 87
MAY 87
JUN. 87
JUL. 87
AUG. 87
SEPT 87
OCT. 87
NOV. 87
DEC. 87
AVERAGE
STD. DEV.
JAN. 88
FEB. 88
MAR. 88
APR. 88
MAY 88
JUN. 88
AVERAGE
STD. DEV.
210740
298960
278445
366940
315184
365205
274125
359589
261220
254339
287606
321225
299465
46374
246794
172741
173479
209258
248001
238507
214797
32133
154912
174688
184576
187872
191168
186224
207648
254950
202704
171392
271467
215888
200291
32390
176336
176336
169744
197760
199408
192816
185400
11643
;
0.74
0.58
0.66
0.51
0.61
0.51
0.76
0.71
0.78
0.67
0.94
0.67
0.68
0.12
0.71
1.02
0.98
0.95
0.80
0.81
0.88
0.11
14
-------
Table 5
Estimated Energy Cost Savings
1985 1986 1987 1988
Assumed pounds BOD treated per Month 295-939 295,939 295,939 295.939
Average kWh/lb. BOD 0.83 0.59 0.68 0.88
Average Monthly Power Cost
(at $0.05/kWh) $12.281 $ 8.730 $10,062 $13,021
Average Monthly Savings . $ 3,551 $ 2.219 (-740)
Annual Savings $42.612 $26.628 (-8,880)
»
vs. time for the three diffusers in Cell 5 for the period between February 1987 and December
1987. The times at which gas cleaning was performed are also indicated.
OFF-GAS OXYGEN TRANSFER TESTING
The Frankenmuth plant was selected by the ASCE Committee preparing this manual to
undergo an extensive set of off-gas oxygen transfer tests. The purpose of this additional
testing was to evaluate the effects on in-waste oxygen transfer efficiency of plugging and
fouling, and of gas cleaning.
Off-gas testing was performed .using techniques developed by Ewing Engineering and
described in the literature, analysis equipment constructed in 1982 by Ewing Engineering, and
a 2'-0" x lO'-O" fiberglass off-gas collection hood. The data was gathered by placing the
collection hood in four locations bracketing the center of the aeration cell being tested as
shown in Figure 5, except for data collected in May 1988. This was done in an attempt to
make the data as consistant and reproducible as possible. In May 1988, eight hood locations
spaced along the entire length of the tank were used. The four additional locations are shown
by dashed lines in Figure 5. This alternate sampling scheme was used in an attempt to
quantify the change in "alpha" from inlet end to outlet end of the tank. Only average values
of the four or eight collection hood locations are reported in Table 7.
Each set of off-gas testing was conducted during two or three consecutive days. At least
some test conditions from the first day were duplicated as closely as possible on subsequent
days to identify radically changed conditions which might cause the test results to be
misleading.
15
-------
Table 6
Gas Cleaning Costs
Month
Hours Worked
1986
April(1)
May
June
July
August
September
October
November
December
1987
1988
January
February
March
April
May
June
July
August
September
October
November
December
(2)
Jetnuary
February
March
April
May
June
July
14.00
10.50
4.25
2.00
4.75
0.00
.00
.50
,(1)
2.
1.
1.50
0.00
1,
3.
4.
2.
1,
1,
,50
.00
.60
.85
.00
.00
0.00
2.25
1.00
0
2.25
0
0
1-00
1.00
1.00
0
0
HCL (Ibs)
332(1>
142
114
51
121
0
46
32
31
0
51
43
75
88
50
29
0
47
17
0
22
0
0
22
18
16
0
0
(1)
(2)
Demonstration and training period so values may not be representative.
Conscious effort by plant staff to minimize cleaning effort by operating at elevated
air flow rates during 1988.
A summary of the cumulative results of the off-gas test program are shown in Table 7a and
7b. Complete test results are in Appendix C. The initial plan for the off-gas testing was to
operate the plant with cells 1 and 2 re-aerating return sludge, and cells 4 and 3, and 5 and 6
16
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operating as two parallel plug flow aeration trains (See Figure 1). One train was to serve as a
"control", and the other train was to be gas cleaned at different levels of DWP to determine
if the effects of DWP and gas cleaning could be quantified in terms of oxygen transfer
efficiency.
The first set of tests (April 1987) were performed in this matter. However, shortly after
these tests were completed, the plant staff determined that in order to maintain compliance
with their NPDES permit it would be necessary to have more than 2 cells re-aerating return
sludge. The plant flow scheme was changed so that RAS went to Tanks 1, 2 and 3 in series.
Return sludge and all primary effluent were then directed to Cell 4. then to 5 and 6.
Testing done in June. September and December 1987, was in this revised configuration. The
plant was reverted to the original parallel flow scheme in February 1988 and remained so for
tests conducted in March and May of that year. These operational changes precluded the
possibility to have a parallel operation for comparison purposes.
Alpha SOTE vs. time is plotted for each each cell in Figure 6. A review of the off-gas test
results will show that the observed oxygen transfer efficiency varies significantly from one
set of test results to another. A review of the materials in Appendix C shows significant
variation from one hood location to another at some times. This is possibly the result of
short circuiting within an aeration cell caused by the feed point location and lack of "cross
mixing" in a tank with fine pore diffusers in full floor coverage. Several sets of tests were
run at different locations before and after gas cleaning. In most instances, an additional set
of "after" readings were collected on the following date for comparison. This was not the
case in September 1987 when the cleaning was done on the second day of testing (9/10/87).
Based on the data collected, no demonstrable effect can be observed on oxygen, transfer
efficiency as a result of the decreased DWP following gas cleaning. The data from one set of
tests to the next is so variable that it is not possible to analyze the effect of DWP on OTE
except by comparing values before and after cleaning. In some instances, immediately
following gas cleaning, a slight decrease in OTE was observed. However, in each of these
cases when additional testing was done the following day, the OTE had rebounded to be
approximately the same as it was before the cleaning process took place. Lino case did gas
j
cleaning appear to increase OTE. However, it is important to note that it was effective' in
limiting the increase in DWP. Table 8 shows DWP before and after gas cleaning for each set
of comparative off-gas tests, and the amount of HC1 used for each cleaning.
21
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The detailed system design was based on an in-waste alpha SOTE of approximately 16-9% at
an air flow of 1 scfm per diffuser. and approximately 11.0% at 2 scfm per diffuser, based on
off-gas test results from another treatment plant. Actual in-waste oxygen transfer efficiency
as measured by the off-gas testing is in general lower than was anticipated in the design
stage, in some instances less than 6 percent. This indicates that alpha was actually lower than
expected. The actual observed air flow rates per diffuser are higher than those expected to
be required even for the peak design load to the plant, averaging more than 2 scfm per
diffuser.,
Attempts were made, using the off-gas data to establish a relationship between oxygen
transfer efficiency and air flow rate per diffuser using linear regression analysis. However,
there is no apparent statistically significant correlation between the two, at least at
Frankeniauth. As expected, oxygen transfer efficiency improves as alpha changes between
the influent and effluent ends of the aeration portions of the system as shown by comparing
data from the same dates taken in Tank 5 versus that collected in Tank 6. At all times during
the testing Tanks 5 and 6 were operated in series with flow passing from 5 into 6, and then
into the final settling tanks. Data collected in Tanks 4 and 3 in April 1987 and in May 1988
also represent series operation with flow from. Tank 4 to Tank 3 and then to the final settling
tanks. Other data collected in Tanks 4 and 3 were for periods when Tank 3 was operating as
the last of 3 stages of return sludge reaeration and Tank 4 as the first of 3 stages of aeration.
24
-------
APPENDIX A
Summary of Plugging/Fouling Potential Test Results
25
-------
CITY OF FRANKENMUTH - WASTE TREATMENT PLANT
FINE BUBBLE PRESSURE READINGS
DATE
-27-O4
8-28-81
8-29-81
8-30-81
8-31-841
9-01-841
9-02-84
9-03-84
9-01-84
9-05-81
9-06-84
9-07-84
9-08-81
9-09-81
9-10-81
.9-11-84
9-12-84
9-13-84
9-11-84
9-15-84
9-16-84
9-17-84
9-18-84
9-19-84
9-20-84
9-21-84
9-22-84
9-23-84
9-24-84
9-25-84
9-26-84
9-27-84
9-28-84
9-29-84
9-30-84
GREEN
5.5
5.5
5.5
6
6
6
6
6
6
6
6 .
6
6
6
6
6
6
6
6
. 6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
ORFICE
BLACK
6
5.5
7
6
6
5.5
5.5
5.5
5.5
5.5
6
6
6
5.5
6
6
6
7
7
6.5
6.5
6.5
6.5
6.5
6.5
6.5
6
w
6.5
ft
V
ft
V
6
5.5
6
g
6 5
** ^
LOSS
BLUE
6
5
5
c
j
5
5.5
5.5
6
5.5
5.25
6.5
5.5
5.5
6
A
v
ft
O
5.5
5.5
ft i.
o O
6
5.5
5.5
6
5
5.5
5
5.5
5
5.5
5--
DYNAMIC WET PRESSURE
GRAY
4.5
5.5
ii
H
5.5
6
6
6.5
6
6
6.5
6
7
7.5
7c
.5
6
6.5
7
7
6.5
5.5
5.5
5.5
5
it c
1.5
5
5^
.5
5r-
.5
5-
4.5
6.5
6,- .
GREEN
T--
8.5
8.5
13
12.5
13
25.5
15
16.5
17.5
18
18.5
16.5
14.5
15
19
16
11
23
21.5
21
17.5
18.5
16.5
13
16.5
17
15
17
12.5
14
15.5
10.5
15.5
17
BLACK
...
5
9
7
13
13
13.5
16
15.5
17.5
17.5
18
18.5
17
11.5
15
18.5
16
10.5
21.5
23
20
17
17.5
16
12
15.5
16.5
14.5
17
12.5
14
16
11
16
17.5
BLUE
7.5
9
9.5
13.5
13.5
13.5
16
15.5
16.5
17.5
17
19
, 17.5
14
15
18.5
16
! 12
; 21.5
22.5
20
18
17.5
17
13
17.5
17.5
15.5
18
12.5
' 14.5
16.5
11.5
16.5
18.5
GRAY
5.5
9.5
8.5
15
13
12.5
15.5
15
16.5
17
16.5
18.5
15.5
13.5
14
17.5
16
12
20
21
19
17.5
17.5
16.5
13.5
18
18
16
18
12.5
14.5
17
10.5
16
18
26
-------
CITY OF FRANKENMUTH - WASTE TREATMENT PLANT
FINE BUBBLE PRESSURE READINGS
DATE
10-01-84
10-02-84
10-03-84
10-04-84
10-05-84
10-06-84
10-07-84
10-08-84
10-10-84
10-11-84
10-12-84
10-13-84
10-14-84
10-22-84
10-23-84
10-24-84
10-25-84
10-26-84
10-27-84
10-28-84
10-29-84
10-30-84
10-31-84
11-01-84
11-02-84
11-03-84
11-04-84
11-05-84
GREEN
5
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
ORFICE
BLACK
5.5
5.5
5.5
5.5
5.5
6
6.5
7
6
6
5
6.5
6
6
7
7
7
7
7
6.5
5.5
6
6
5.5
5.5
6
LOSS
BLUE
5
5
5
5
5
5
5
4.5
4.5
4.5
6.5
5
6
6
5.5
5.5
6
6
6
6
6
6
6
5.5
5.5
6.5
GRAY
6
5.5
5.5
6
5.5
5
5.5
4.5
5
4.5
4.5
4.5
6
6
6
6
5.5
5.5
5.5
5.5
6
5
6.5
6.5
6.5
7
DYNAMIC WET PRESSURE
GREEN BLACK BLUE GRAY
11.5
16
16.5
17.5
18
17
17
22
12
11.5
11.5
11
10
10
12
12.5
12.5
12
11
9.5
9
14.5
9.5
9.5
9
13
11.5
16
16.5
17
18.5
17
17
19.5
12
11.5
12.5
11
10".
9
11
11.5
11.5
11
9.5
9
9
14
9.5
10
9.5
12.5
12
16.5
17
18
20
18.5
19
18
13.5
13.5
11.5
12
10
9.5
12
13
12
12
1 1
9
8.5
\* ^
13.5
9.5
10
9.5
11.5
12
1 &
15.5
1 ^ W
17
17.5
' I «/
19 5
1 x ^
18
18
18
13.5
-J J
13
13.5
13
1 -J
10
9
11 5
' ' J
13
' J
13
1 J
13
1 w
1 1
10 5
I V ^
9
14.5
8 5
*» J
9 5
j * J
9
10.5
27
-------
APPENDIX B
Plant Operating Records
28
-------
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APPENDIX C
Off-Gas Oxygen Transfer Test Results
33
-------
Off-gas Analysis Equations and Nomenclature
Data Collected in Field;
Mole fraction of carbon dioxide in off-gas
HR Absolute humidity, off-gas and reference gas (air), Ib. water/lb. dry air.
^£ . Oxygen sensor output, millivolts, for off-gas and reference gas.
MLT Mixed Liquor Temperature
C* -C Dissolved Oxygen deficit, C* , is D.O. saturation for ML, C is actual
D.O. <§ test site.
Ag Area of off-gas collection hood, square feet. ',
Pg Absolute Barometric pressure, in Hg
R Rotometer float height, mm
PI Hood pressure, inches water column
P« Vacuum at oxygen sensor, in w.c.
Data To Be Computed;
Symbol Definition
Mole fraction of oxygen in reference gas ,
FQR = 0.2095 (1-HR (29/18))
Mole ratio, oxygen to inerts in reference gas
29/18 s Molecular Wt Air
Molecular Wt HgO
(continued)
34
-------
Symbol
Definition
OG
OG
OTE,
TF
OTE
SP
OTE
TF
Mole fraction of oxygen in off-gas
FOG = FOR * MOG/MR
Mole ratio of oxygen to inerts in off-gas
OG
OG
- FOG - FCO, * HOG (29/18)
ft
OTE at field conditions
OTE
'TF
RR " ROG
R
R
OTE corrected to standard conditions
OTE,,
OTE
SP20
'TF
U.024)2°-MLT
Specific OTE, i.e., OTE per unit (mg/1) of driving force.
Mean weighted field oxygen transfer efficiency.
OTE _ (OTEn
TF
TF
OTE
SP
X
(continued)
Mean weighted specific oxygen transfer efficiency.
OTEsp =
Flow rate correction factor to air flow rate for oxygen depletion, mixed
liquor temperature, pressure and humidity.
35
-------
Symbol
Definition
1 -0.21
293
°'5 n r, 0.5
P3- P1+P2l
27
(273 + MLT)
29.92
1- 0.007 (29/18)
Qj
Q,
Corrected air flow rates for small (1) and large (2) rotameters.
0.0235 R -( 0.34
mm
0.0909 Rmm + 1.85
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