United States
Environmental Protection
Agency
Municipal Environmental Research EPA-600/2-80-118
Laboratory August 1980
Cincinnati OH 45268
Research and Development
Review of
Alternatives for
Evaluation of
Sewer Flushing
Dorchester Area
Boston
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application; of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related;fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8, "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION, TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equiprrjent, and methodology to repair or present en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161. i
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EPA-600/2-80-118
August 1980
REVIEW OF ALTERNATIVES FOR EVALUATION
OF SEWER FLUSHING
DORCHESTER AREA—BOSTON
by
Herbert L. Kaufman
Fu-hsiung Lai
Clinton Bogert Associates
Fort Lee, New Jersey 07024
Contract No 68-01-4617
Project Officer
Daniel K. O'Brien
Municipal Facilities Branch
Water Division
Region I, Boston, Massachusetts 02203
Technical Advisor
Richard P. Traver
Storm & -Combined Sewer Section
Wastewater Research Division
Municipal Environmental Research Laboratory (Cincinnati)
Edison, New Jersey 08817
MUNICIPAL FACILITIES BRANCH
WATER DIVISION
U.S. ENVIRONMENTAL PROTECTION AGENCY
BOSTON, MASSACHUSETTS 02203
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Facilities
Branch, Region I and the Municipal Environmental Research Laboratory,
U.S. Environmental Protection Agency, and approved for publication.
Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor
does mention of trade names or commercial products constitute endor-
sement or recommendation for use.
ii
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FOREWORD
The Environmental Protection Agency was created because of in-
creasing public and government concern about the dangers of pollution
to the health and welfare of the American people. Noxious air, foul
water, and spoiled land are tragic testimony to the deterioration of
our natural environment. The complexity of that environment and the
interplay between its components require a concentrated and inte-
grated attack on the problem.
Research and development is that necessary first step in problem
solution. It involves defining the problem, measuring its impact,
and searching for solutions. The Municipal Environmental Research
Laboratory develops new and improved technology and systems for the
prevention, treatment, and management of wastewater and solid and
hazardous waste pollutant discharges from municipal and community
sources. This work is to facilitate the preservation and treatment
of public drinking water supplies and to minimize the adverse econo-
mic, social, health, and aesthetic effects of pollution. This publi-
cation is one of the products of that research; part of a most vital
communications link between the researcher and the user community.
The Municipal Facilities Branch of Region I administers the
grant program that provides financial assistance to communities for
the planning, design and construction of wastewater treatment works
to meet the objectives of the Federal Water Pollution Control Act and
monitors the operation of such treatment works. It is through their
sponsorship this report has been prepared.
The application of sewer flushing heavily deposited lines during
dry weather periods to alleviate first flush effect and combined sew-
er overflows when used in conjunction with additional methods of
structural control has been estimated to be a cost-effective method
of urban runoff pollution abatement.
William R. Adams, Jr., Regional Administrator
Region I, Boston, Massachusetts
Francis T. Mayo, Director
Municipal Environmental Research Laboratory
iii
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ABSTRACT
Alternatives employing sewer flushing were developed for the
Dorchester Area of Boston, and their cost effectiveness compared with
the decentralized combined sewer overflow (CSO) storage/treatment and
disinfection facilities proposed as Eastern Massachusetts
Metropolitan Area (EMMA) Alternative 1. Thirty-three alternatives
were evaluated. These alternatives included sewer flushing, offline
storage, in-pipe storage, storage/treatment facilities, and a com-
bination of the above. A study objective was to determine if addi-
tional expenditures to develop sewer flushing techniques and devices
were indeed appropriate.
Available information contained in the past and ongoing studies
was used to obtain watershed and sewer characteristics, and to esti-
mate rate of solids deposition in sewers. The feasibility and effi-
ciency of sewer flushing was based on literature review including a
recently completed report ™'in which extensive sewer flushing data
at four small sewer segments in the Dorchester Area were obtained and
interpreted.
Continuous simulation runs using 16 years (1960-1975) of hourly
rainfall data from May through November were made to determine the
level of CSO pollution control obtained. The Corps of Engineers'
STORM program was modified to include continuous simulation of solids
and organic material deposited in sewers during dry days, the removal
of those deposits by dry day sewer flushing and wet-weather flow, and
the storage and treatment effects of a CSO storage/treatment facility
on the wet-weather discharge. STORM was also modified to do con-
tinuous simulation over a part year period instead of the entire year
to allow flexibility for water quality study in areas where the re-
creational season may be of concern.
The study concluded that (1) the CSO storage/treatment facility,
proposed as EMMA Alternative 1 designed for a one-year storm, would
remove about 50 percent of the BOD and suspended solids in the CSO
and is the highest cost alternative of all considered; (2) the capa-
city of the conveyance and pumping facilities in the original plan
can be reduced by 80 percent and cost reduced by about half while
maintaining the same level of pollution control; (3) sewer flushing
can be an adjunct to, but can not substitute for, structural alterna-
tives; (4) use of storage available in large sewers in conjunction
with sewer flushing could reduce the cost to about 7 percent that of
EMMA Alternative 1; and (5) for all alternatives considered, BOD
removals equal to those of EMMA Alternative 1 could be achieved at
less cost than equal SS removals. Prototype demonstrations of sewer
iv
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flushing, using automatic devices should be pursued actively, espe-
cially in large combined sewers.
This report was submitted in fulfillment of Contract No. 68-01-
4617 by Clinton Bogert Associates under the sponsorship of the U.S.
Environmental Protection Agency, Region I, through Anderson-Nichols,
EPA's Region I Mission Contract Contractor responsible for adminis-
trative project management. Work was completed as of May, 1979.
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TABLE OF CONTENTS
Page
Disclaimer. ii
Foreword iii
Abstract iv
List of Figures ........ix
List of Tables xi
Acknowledgements xiii
I. Prologue 1
II. Conclusions 4
III. Recommendations ...8
IV. Study Background 9
V. Study Objective and Scope 11
VI. The EMMA Alternative 1 12
VII. Description of the Study Area 17
VIII. Computer Model Requirements 23
Improved STORM Program-Features 24
IX. Model Input Data 35
Long-Term Meteorological Data. ..35
Watershed Characteristics 39
Dry-Weather Flow and Pollutant Loadings 41
X. Development and Evaluation of Alternatives....45
Development of Sewer Flushing Alternative...45
Development of Storage Alternative 46
Evaluation of Alternatives 49
vii
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TABLE OF CONTENTS (continued)
XI. Cost Estimates and Alternative Comparisons....64
Cost of Storage 64
Cost of Sewer Flushing 68
Operation and Maintenance Cost 68
Cost of Alternatives 75
Optimal Number of Flushing Stations 77
Solids Handling Consideration 80
XII. Flushing - Possible Limitations and
Advantages 81
XIII. References 85
viii
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LIST OF FIGURES
Number
2
3
4
5
10
11
12
13
14
15
16
Page
Satellite regulation facilities and collection
systems in EMMA Alternative 1. 13
One year 6-hour design storm hyetograph 14
Dorchester Bay branch areas 18
Major processes modelled by STORM 25
Major processes modelled by the improved
STORM 26
Pollutant removal in a storage unit as a
function of detention time 31
Cumulative probability plots of rainfall
duration and antecedent dry periods ..37
Cumulative probability plots of rainfall
amount and intensity
Locations of heavy deposition segments and
flushing stations
Total deposition versus number of segments
ranked by deposition rate.
Potential storage locations.
Effect of off-line storage on pollutant re-
moval, high deposition rate.
.38
.48
.50
.51
.58
Effect of off-line storage on pollutant re-
moval, low depositions rate. 61
Pumping station construction cost 66
Capital cost of storage in EMMA study 69
Storage reservoir man-hour requirements. 71
ix
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LIST OF FIGURES (continued)
Number
17
18
19
Storage reservoirs - miscellaneous supply cost
(ENR 2200) ..... . ............................. •
Storage reservoirs - energy requirements .....
Raw wastewater pumping man-hour requirements
Page
71
72
73
20
21
22
23
24
Raw wastewater pumping - miscellaneous
supply cost (ENR 2200) ...... .
Raw wastewater pumping - energy requirement
Wall shear stress in circular pipes,
flow =0.5 cfs ........ .
Wall shear stress in circular pipes,
flow =1.0 cfs .....
Wall shear stress in circular pipes,
flow =1.5 cfs
73
74
.82
83
.84
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LIST OF TABLES
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Page
Physical Description of Alternatives ., 2
Summary of Facilities and Costs for Alter-
native 1. ..15
Dorchester Bay Branch Data 20
Cumulative Pipe Length of Sewer System (feet) 22
Input Data Related, to CSO Plant and Sewer
Flushing ,
.28
Input Data of Watershed Characteristics 29
Average Annual Statistics of Quantity and
Quality Analysis.. .33
Annual Precipitation, May through November. 36
Land Use and Percent Impervious From COM 40
Monthly Evaporation Rates 42
Diurnal Variation of Dry-Weather Flow. 44
Average Daily Pollutant Concentration of Domestic
Waste water , 44
Separate Sewer Segments Ranked by Deposition
Rate , 47
Combined Sewer Segments Ranked by Deposition
Rate. 47
Total Deposition of Segments Ranked by Deposi-
tion Rates 49
Alternative Definition and Pollution Control
Effectiveness, High Deposition Rate.. , ...53-^54
xi
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LIST OF TABLES (continued)
Number
17
18
19
20
21
22
23
24
25
26
Page
Pollutant Removal Efficiency of Alternatives,
Low Deposition Rate.
,55
Optimal Combination of Pumping and Storage
Capacities for Pollutant Removal • 60
Equivalent BOD Abatement Alternatives 62
Equivalent SS Abatement Alternatives * • • 62
Open Cut Sewer Cost ($/linear foot) 65
Summary of Off-line Storage Costs •
Estimated Cost of Automatic Sewer Flushing
(ENR 2800)
Cost Summary of Alternatives Based on BOD
Removal (ENR 2800)
Cost Summary of Alternatives Based on SS
Removal (ENR 2800)..., •
Equivalent SS and BOD Abatement Alternatives
with Sewer Flushing
67
.70
,76
.78
,79
xii
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ACKNOWLEDGEMENTS
The support of the project by the U.S. Environmental Protection
Agency, Region I, under a contract with Anderson-Nichols, Boston, an
EPA Mission Contractor responsible for administrative project
management, is acknowledged and appreciated.
Mr. Daniel K. O'Brien, Project Officer, Municipal Facilities
Branch, U.S. EPA Region I; Mr. Richard P. Traver, Technical Advisor,
Storm and Combined Sewer Section, Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency; and Mr. Gary B.
Saxton, Project Manager for Anderson-Nichols, have provided timely
comments and assistance. The critical review by Mr. Richard Field,
Chief, Storm and Combined Sewer Section is much appreciated.
For Clinton Bogert Associates, this project was directed by Mr.
Herbert L. Kaufman, Partner-in-charge. Dr. Fu-hsiung Lai, Associate,
served as Project Engineer. Mr. Ivan L. Bogert, Partner, and Mr.
John H. Scarino, Principal Associate, provided valuable.criticism and
review.
Dr. William C. Pisano, President, Environment Design & Planning,
Inc., was most cooperative and gave generously of his knowledge and
experience.in computer modeling and sewer flushing. Dr. Pisano also
shared freely his information on the characteristics of the Dorches-
ter sewer system obtained during the Process Research, Inc. (PRI)
study and recent EPA Demonstration Grant studies. Finally, Dr.
Pisano and his staff, through an extraordinary effort, completed the
draft of their report on sewer flushing in time for use in this study
and made themselves fully available for discussions.
Thanks are also due Mr. John R. Elwood, Supervising Sanitary
Engineer, Environmental Planning Office, the Metropolitan District
Commission (MDC), the Commonwealth of Massachusetts, who assisted in
providing information from various Engineers serving the MDC and
provided the operation and maintenance cost data of the Cottage Farm
facility. Mr. Donald G. Wood of that office was also very helpful.
Finally, the authors are grateful to Metcalf & Eddy, Inc. (M&E),
Camp Dresser & McKee, Inc. (COM), and Hydroscience, Inc., who were
cooperative in providing basic data used in this study. M&E provided
data related to the design of storage/treatment facilities; COM sup-
plied the watershed map, boundary and characteristics, and dry-weath-
er flow metering data. Hydroscience provided a copy of the magnetic
tape containing the rainfall records used in the study.
xiii
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SECTION I
PROLOGUE
A report, released in September 1978 and co-sponsored by the
OMB, EPA, NSF and CEQ, criticized the EMMA Alternative 1 for CSO pol-
lution abatement as being structurally intensive. As a result, EPA
Region I through Anderson-Nichols, an EPA Mission Contractor, engaged
Clinton Bogert Associates to compare CSO pollution abatement tech-
niques, including sewer flushing, "and to make this comparison and
hence determine if additional expenditures to develop sewer flushing
are appropriate."(a)
The comparisons were made for the Dorchester Area of Boston
draining to existing CSO outfalls 49, 50 and 67 (EMMA Report, Volume
7). Included in the comparisons were capital, operating and mainte-
nance costs totaled in terms of present worth.
Thirty-two of the more promising alternatives to EMMA Alterna-
tive 1 that were compared are described briefly in Table 1. EMMA
Alternative 1, as considered for adoption by the MDC, comprises a
conveying conduit and pumping station capable of delivering the peak
flow from a one year return frequency storm to a combined sewage
treatment plant (CSTP). The CSTP provides both primary treatment
with basins (sized to allow 15 minutes detention at the peak flow)
and storage when the combined sewage volume does not exceed the
detention basin volume. At the end of the rainfall event, combined
sewage and sludge retained in the basins would be returned to the
interceptor for treatment. Five of the alternatives evaluated
proposed reducing the pumping station and conveying conduit capacity
by 75 to 80 percent of that proposed in EMMA Alternative 1 and
achieved essentially equivalent pollutant removals. It also appears
that sewer flushing can improve pollutant removals with this type of
facility. Four alternatives considered the effects of sewer flushing
alone at various time intervals. These alternatives indicated
pollutant removals significantly lower than those achieved by EMMA
Alternative 1.
Other alternatives evaluated the pollutant removals to be
achieved by various amounts of storage both with and without sewer
flushing and the effects of high and average sewage pollutant
strengths. Finally the use of storage capacity available in existing
pipes by flow routing was evaluated.
(a) from "Project Overview"
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TABLE 1. PHYSICAL DESCRIPTION OF ALTERNATIVES
Sewer
Flushing In-
terval (dry days)
No. of
Flushing
Stations
Storage
Capacity
(mgd)
CSTP
Pumping
Capacity
Conduit
Size
(ft)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1
2
7
1
1
1
28
28
28
28
28
28
28
28
28
28
28
28
5
12
104
5
12
104
_ *
—
—
—
5.1
5.1
6.8
6.8
13.1
13.1
7.7
No
No
No
No
No
No
No
No
No
No
Yes
-
-
-
-
15.0
15.0
15.0
15.0
15.0
15.0
497.4
7.7
5.7
5.7
7.1
5.1
7.3
6.0
7.5
10.0
9.1
10.0
9.5
10.0
5.5
5.3
4.9
9.3
9.2
9.0
6.2
5.9
7.7
Yes 497.4
Yes
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
No
100.0
Yes 100.0
15.0
15.0
15.0
15.0
Small
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
15.0
10.5
10.5
5.0
5.0
Yes 125.0
Yes 115.0
Yes 100.0
7.0
6.0
10.5
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The effectiveness of the alternatives were measured by two cri-
teria: obtaining (1) BOD removals and (2) SS removals equal to EMMA
Alternative 1. The results of this comparison follow:
Case
No.
8
10
12
17
13
18
14
19
15
20
16
Alternative
Description
EMMA Alt. 1
EMMA Alt. 1 (Modified)
Storage - Strong Sewage
Case 12 - Strong Sewage
Storage & Flushing (b)
Strong Sewage
Case 13 - Strong Sewage
Case 12 - Normal Sewage
Case 12 - Normal Sewage
Case 13 - Normal Sewage
Case 13 Normal Sewage
Existing Pipe Volume
Criteria
-
-
BOD
SS
BOD
SS
BOD
SS
BOD
SS
BOD
Present
Worth
($ x 10 6)
46.89
26.46
26.31
30.58
24.79
32.04
27.03
30.58
26.94
32.62
03.30
used for storage w/Flushing
21 Existing Pipe Volume used
for storage w/Flushing
(b)Flushing at 28 locations.
SS
14.27
Further studies indicate that flushing at no more than 12 nor less
than 5 locations may be cost effective.
Note: Both strong and normal sewage were considered because of
their different solids deposition rates.
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I
SECTION II
CONCLUSIONS
1. EMMA Alternative 1, as presently considered, removes about 50
percent of the BOD and SS in the Combined Sewer Overflows (CSO).
It is the highest cost alternative of all considered, principal-
ly due to sizing conveying and pumping facilities for flows
expected from a one-year return frequency storm. ,
2. EMMA Alternative 1 could be modified and its cost reduced to
more reasonable levels, while maintaining its performance. This
could be accomplished by reducing the capacity of the connecting
conduit and pumping station to 20 percent of that considered in
EMMA Alternative 1.
3. Sewer flushing alone can not match EMMA Alternative 1 in
pollution abatement levels attained. Daily sewer flushing at 28
strategic locations affecting about 45 percent of the solids
deposited would reduce SS 'discharge by about seven percent and
the BOD by 17.6 percent. If flushing at a 2-dry-day interval is
employed, approximately 5.2 percent of the SS and 14.1 percent
of the BOD would be removed. If the flushing interval is
extended to seven dry days, only about 1.2 percent of the SS and
4,0 percent of the BOD would be removed.
4. Sewer flushing, if combined with storage, should substantially
reduce the cost of achieving equal SS and BOD removals as com-
pared to EMMA Alternative 1. If compared to the modified EMMA
Alternative 1 (See Conclusion 2), the combined sewer flushing
and storage alternative with strong sewage should cost less for
equal BOD removals. With normal sewage, this advantage is
largely lost. If equal SS removals are desired, a combined sew-
er flushing and storage alternative does not appear cost-effec-
tive for either strong or normal sewage.
5. Use of storage made available in existing pipes in the
Dorchester area by flow routing, in conjunction with sewer
flushing, could reduce present worth costs to about seven per-
cent of EMMA Alternative 1 for equal BOD removals, and ,to about
30 percent, for equal SS removals. If compared:to EMMA
Alternative 1 (modified) the respective percentages are 12.5 and
54.0.
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9.
10,
Based on the locations of moderate to heavy sewer solids depos-
its identified in past studies by others, the maximum number of
sewer flushing stations appears to be 28. These locations
should affect 45 percent of the pollutants deposited during dry
weather flows. Eleven are located in combined sewer segments
and 17 in separate sewer segments. Additional sewer flushing
stations become increasingly marginal since the solids flushed
per station reduce rapidly. However, as few as five and no more
than 12 stations flushing those sewer reaches with the heaviest
deposits, appear to be optimum.
For equal flows and pipe slopes, the wall shear stress may be
almost independent of the pipe size. If this is proven, it
should be as feasible to flush large sewers as small sewers and
would simplify greatly the maintenance required to achieve the
economy in pollution abatement that appears possible with in-
line storage.
Available prototype data was obtained from flushing*12- and 15-
inch sewers. For these small sewers, flushing volumes of ap-
proximately 50 cubic feet injected at a rate of approximately
0.5 cfs would effectively flush solids in the sewers. Shear
stress computations indicate a greater flow rate would be re-
quired to flush and keep pollutants in suspension in larger
sewers with slopes equal to or less than 0.003. However, larger
sewers generally carry greater sewage flow than the smaller sew-
ers, therefore, solids transport capability of large sewers may
be greater. • J
Trunk sewers and larger combined sewers normally follow valley
bottoms, while the smaller separate and combined lateral sewers
are located on the valley sides. Hence, the smaller sewers ge-
nerally have steeper slopes than larger sewers. An urgent need
exists to determine the optimum methods of flushing in larger
sewers. Based on experience in Detroit, such a study should
provide highly valuable results. /
Sewer flushing during dry days is more effective in reducing BOD
than SS.in CSO. The resuspended heavier solids tend to resettle
in downstream sewers. In the Boston area for the period from
1960 through 1975, the number of days with zero precipitation
averaged 165 from May through November, or about two dry days in
every three. To dry weather flush with the required frequency,
automatic installations appear essential. If flushing devices
are operated after 24 dry hours, the number of flushes would be
between 165 (assuming flushing every dry day) and 70 (assuming
flushing every consecutive 48 dry hours), with BOD removal
ranging between 17.6 and 14.1 percent and SS removal between 7.0
and 5.2 percent. The reduction in removal efficiency is rather
small. A longer interval between sewer flushing would not
impact significantly on pollutants flushed by wet weather
flows...
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11. Sewer flushing alternatives are an adjunct to, but are not a
substitute for, structural facilities to obtain the same pollu-
tant reduction in the CSO's as EMMA Alternative 1. With a
strong sewage, the same amount of BOD removal as the EMMA Alter-
native 1 can be achieved by (1) 7.1 million gallons of off-line
storage; or (2) 5.1 million gallons of off-line storage supple-
mented by daily sewer flushing. The daily flushing/storage al-
ternative has the lowest total cost of the three. The modifi-
cation to the EMMA Alternative 1 would cost about seven percent
more than the lowest cost alternative of sewer flushing and
storage, indicating that the EMMA proposal, if optimized, could
be a viable alternative.
12. With a strong sewage, the same amount of SS removal as in EMMA
Alternative 1 can be achieved by: (1) 10 million gallons of
off-line storage, or (2) 9.1 million gallons of off-line storage
together with daily sewer flushing. Of the three pollution
abatement schemes, the optimized EMMA Alternative 1 may be about
15 percent less in total cost than the storage alternative with
the estimated cost of storage used herein. The sewer flushing/
storage alternative appears to cost slightly more than the non-
flushing storage alternative. If the sewage strength is reduced
by 45 percent, sewer flushing becomes less effective and that
alternative costs more to achieve the same degree of pbllution
control.
If dry-^weather sewage strength is reduced by about 45 percent,
the optimum EMMA proposal for the same amount of BOD; removal
might be less costly. However, facility planning should in-
vestigate more thoroughly the probable cost of storage.1
The most economical alternative would be to exploit the storage
in large sewers near the two outfalls. As much as 7.5 million
gallons of potential storage appear to be available. This low
structural alternative would require several flow regulating de-
vices and frequent flushing of the sewer to maintain effective
storage capacity. The possible savings justify the funding of a
sewer flushing demonstration project for large sewers to permit
evaluation of its efficacy.
15. Cost of sewer flushing based on full-scale operating experience
is not available. Estimates are based on automatic flushing
equipment and need verification by operation of prototype de-
vices and field demonstration. Cost of storage, although more
available, varies widely depending on the type, size and facili-
ties included. The EMMA study report included only gross cost
estimates of primary treatment facilities of known volumes.
This study used the EMMA cost estimates to determine the cost of
detention basin storage. These costs appear high for off-line
storage and hence tend to favor the EMMA alternatives. They
13.
14.
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should be investigated on a specific site basis in the ongoing
facility planning. Storage alternatives may be economically
more attractive than set forth herein.
16. The U.S. Army Corps of Engineers' STORM computer program re-
quired modification to include continuous simulation of dry day
sewer solids deposition, wet day solids removal, and- dry day
sewer flushing effects. The output of the program indicates the
amount of combined sewage quantity and quality diverted to the
main treatment plant (STP) and the new CSTP, pollutants removed
in the CSTP, deposited solids in sewers resuspended by combined
sewage flow, and combined sewage pollutants from dry-weather
flow, etc. These data are useful in the development and evalu-
ation of alternatives.
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SECTION III
RECOMMENDATIONS
Prototype demonstrations of sewer flushing should be pursued ac-
tively especially in large combined sewers. Sewer flushing may
be a cost-effective adjunct if integrated with structural alter-
natives .
Automatic sewer flushing devices, which require minimal mainte-
nance, should be developed and demonstrated for operational
reliability. Better cost information should be developed in
conjunction with field demonstrations for comparison with the
cost of other viable CSO pollution abatement alternatives.
There is economical incentive to explore and utilize potential
storage which may be available in existing large sewers. In the
Dorchester area, use of this storage may be all that is required
for effective CSO pollution abatement. This alternative, cou-
pled with sewer flushing^ has been demonstrated as practical in
the Detroit sewer system. Flow regulating devices are available
for this purpose.
The computer simulation model "STORM", should be farther ex-
panded to permit analysis of more than one drainage !area at a
time and to include additional treatment processes. This would
allow the development and evaluation of pollution abatement
schemes for each individual subarea of a watershed taking into
consideration its particular drainage and pollution characteris-
tics while meeting the gross objective of reducing the amount of
pollutant discharged to receiving waters from the watershed.
Additional verification of the quantity and quality of dry
weather flow deposits should be undertaken. This verification
should concentrate on those sewer segments with the relatively
greater deposits.
The use of dry-weather flow by backup and release for flushing
should be confirmed as being practicable.
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SECTION IV
STUDY BACKGROUND
In March, 1976, the Metropolitan District Commission (MDC) pre-
sented a comprehensive plan for Wastewater Management in its report,
Wastewater Engineering and Management Plan for Boston Harbor -
Eastern Massachusetts Metropolitan Area (EMMA)d). The report iden-
tified CSO as a major source of pollution. The plan included 13 de-
centralized CSO pollution abatement facilities, 10 proposed, 2 exist-
ing and one under construction. The necessary collector sewers were
planned to divert wet weather flows to these facilities. The total
estimated capital cost for the CSO pollution abatement facilities, at
Engineering News Record (ENR) Index equal to 2200, is $279 million.
The annual operating and maintenance costs is estimated at $3.9
million. While the EMMA plan appears to have represented advanced
concepts at the time it was prepared, it has been criticized as being
structurally intensive in light of developing knowledge. In June,
1978, U.S. EPA awarded a Step 1 construction grant to the MDC for
preparation of a CSO control facilities plan in the Boston Metropoli-
tan Area. The planning area includes about 25,000 acres and has a
population of about 900,000.
The EMMA study considered three general approaches to abate CSO
pollution: sewer separation, a deep tunnel plan and decentralized
treatment facilities. The sewer separation alternative was not found
cost-effective and probably not practical, especially in downtown
Boston. The deep tunnel plan is a centralized approach and requires
an early, very large capital commitment. The decentralized plan was
favored because it "would continue present remedial practices". Such
a plan would permit "staged implementation in accordance with cri-
teria and needs of each immediate area and provides flexibility for
inclusion of future technologies in treatment beyond that presently
provided." y
The EMMA study compared a totally decentralized plan (Alterna-
tive 1) with Alternatives 2 and 3 which combined the decentralized
concept with the deep tunnel centralized concept. Alternative 1 had
the lowest capital cost of $279 million with $299 million and $307
million, respectively, for Alternatives 2 and 3 based on the ENR
Index of 2200. The annual operation and maintenance cost for the
three alternatives were estimated as $3.9, $3.7 and $3.8 million,
respectively. The costs for all alternatives appear to be about
equal considering the range of error expected in preliminary work.
-------
Although non-structural control, such as street sweeping and
sewer flushing, were discussed in the EMMA study report, "as an im-
portant contributor to water pollution control and should be incor-
porated as part of any abatement program", such measures are :not con-
sidered as part of an alternative. Unlike street sweeping, which is
practiced in almost every urbanized area for aesthetic reasons, sewer
flushing is not a common practice, although the idea is not hew. As
the result of several recent U.S. EPA research/development/demonstra-
tion projects (2, 3, 4)^n which sewer flushing data were collected and
techniques demonstrated, the possibility of sewer flushing as a vi-
able contributor to CSO pollution abatement should be investigated.
In Boston, several proposals have been made to refine computer model-
ing techniques, develop automatic flushing equipment, and demonstrate
medium to large scale operating prototypes for sewer flushing eval-
uation. These proposed studies require one to two years to complete
and would cost from $150,000 to $500,000. This study was undertaken,
based on the state-of-the-art,information, to explore the use of
sewer flushing either as an adjunct to reduce the cost of structural-
ly-oriented alternatives, or as an independent technique.
10
-------
SECTION V
STUDY OBJECTIVE AND SCOPE
The major objective of this study, as directed by the U.S. EPA,
Region I, was to estimate the cost of an alternative, employing sewer
flushing which would provide the same degree of pollutipn abatement
as EMMA Alternative 1 for the Dorchester Bay area. Capital and
operating and maintenance costs of the selected alternative were to
be compared with those for EMMA Alternative 1. The costs were to be
presented at the current ENR Index.
The scope of the study included:
1. literature review on sewer flushing research, demonstra-
tions and application;
2. pertinent .data compilation and review on the Dorchester Bay
area;
3. implementation of computer models;
4. evaluation of the pollution control efficiency of the EMMA
Alternative 1;
5. development of alternatives employing sewer flushing and
their pollution control efficiency using a computer model;
and
6. development and comparison of cost estimates of alterna-
t ives.
This study uses five-day Biochemical Oxygen Demand (BOD) and
Suspended Solids (SS) as the pollution abatement parameters. These
two parameters can be modeled and field verified with reasonable
accuracy. Coliforms have not been satisfactorily modeled for field
verifications.
11
-------
SECTION VI
THE EMMA ALTERNATIVE 1
As shown in Figure 1, EMMA Alternative 1 consolidates combined
sewer overflows into 13 groups. Sewage in each group is collected
and treated prior to discharge to receiving waters. These groups
include 10 proposed, 2 existing and one facility under construction.
The Cottage Farm Detention and Chlorination Station and the Somer-
ville Pretreatment Facilities are existing. The Charles River Chlo-
rination-Detention-Pumping Station is under construction. The
Cottage Farm and Charles River facilities include collection con-
duits, treatment and storage tanks, pumping facilities and outfalls.
The Somerville facility includes screening and chlorination facili-
ties with chlorination achieved in the outfall conduits. The pro-
posed facilities are sized based on a storm of one-year severity and
six-hour duration. Its rainfall hyetograph is shown in Figure 2.
This intermediate pattern "design" storm has a total rainfall of 1.78
inches and a 10-minute peak rainfall intensity of 2.63 inches per
hour. The design flow rate and volume were estimated using the Storm
Water Management Model (SWMM). The model was not calibrated.
The collection conduits were sized to carry the peak design
flow. The tank, which consists of two basins, receives flow from the
collection conduits. Flow is delivered first to one basin. As this
basin is filled, a floating scum and oil baffle rises with the water
level to capture such materials. Flow may enter the second basin
from the first basin or may be delivered directly to the second
basin, permitting retention of the first flush in the first basin.
When both basins are filled, overflows are screened before discharge
to the receiving waters. The flow is chlorinated upstream of the
tanks. The tank is designed to provide 15 minutes detention for the
peak design flow. Each facility will have pumps, either before or
after the tank, capable of pumping the peak design flow. At the end
of a storm, water and solids retained in the tanks will be diverted
to the main treatment plant (STP) through the existing interceptors.
Table 2 shows a summary of facilities and costs estimated by
Metcalf and Eddy (M&E) for EMMA Alternative 1.
To compare the cost-effectiveness of EMMA Alternative 1 with al-
ternatives employing sewer flushing, the area tributary to Facility
No. 9 was selected since:
12
-------
0 4500 "9000 I3SOO
SCALE IN FEET
LEGEND
COMBINED SEWER AREAS
EXISTING DETENTION FACILITIES
PROPOSED REGULATION FACILITIES
PROPOSED COLLECTION CONDUITS
LIMIT OF TRIBUTARY AREA
REPRODUCED FROM EMMA. REPORT VOL. 7
Figure 1. Satellite regulation facilities and collection systems
in EMMA. Alternative 1
13
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15
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1. EMMA Alternative 1 is totally decentralized, i.e., each
facility is independently sized with the same hydrologic
and cost parameters. Study of one tributary area provides
data and insight for projection to other areas while: keep-
ing the efforts required for data development reasonable;
2. the area has been fully studied in the past and most, if
not all, of the necessary hydrologic, watershed and pollu-
tional information is available; and
3. the most extensive sewer flushing data (deposition poten-
tial, flushing volume tnode and effectiveness) available
were obtained in the study area. ',
Facility No. 9 consists of a 4000-foot long, 126-inch diameter
collection conduit, a pumping station with capacity of 770 cfs, a
primary treatment tank with capacity of 0.68 million cubic feet (5.08
Mgal) and a 300-foot long outfall sewer. Information provided by
M&E indicates the collection conduit and outfall pipe are to be
supported on piles, except for 800 feet which is to be jacked.
The collection conduit connects the existing regulator near
overflow No. 50 and to that near NO. 49. M&E has estimated, using
SWMM, that the one-year storm peak runoff rate for the area tributary
to the upstream regulator is 535 cfs^ •*-'. The additional peak runoff
rate from the area tributary to the downstream regulator is 520 cfs.
The collection conduit provides modulating storage which permits
reducing the peak flow to the CSTP to 770 cfs. The cost of the
collection conduit and outfall was estimated at $7.8 million. The
costs of treatment tanks and pumping stations were estimated as $12.8
million and $13.6 million, respectively. The estimates were based on
the ENR index of 2200. The total capital cost of the facility was
$34.2 million. All the above costs include an allowance of 25
percent for engineering and contingencies. Land costs are not
included.
16
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SECTION VII
DESCRIPTION OF THE STUDY AREA
The tributary area to the Facility No. 9 is entirely within the
boundary of Dorchester. The area is approximately bounded on the
north by Columbia Road, on the west by Pennsylvania Central Railroad,
on the south by Wilmington Avenue and Ashmont Street and on the east
by the Dorchester Bay. Figure 3 is a map delineating the study
area.
Several pollution control studies have been made in the Dorches-
ter area; the EMMA Study by Metcalf & Eddy. {M&E) in 1976(-L) the
Process Research, Inc. (PRI) Report in 1975,^ ' and the Camp, Dresser
& McKee (COM) study of Water Quality Improvement of Tenean and Malibu
Beaches in 1972 ^6'. COM is preparing a CSO facilities plan for the
study area. These available studies and data provided by COM and M&E
were reviewed to compile the information for this study. The PRI
Report, which studied the Dorchester sewer system in detail, con-
tained data as to solids deposition locations and rates in both the
separate and combined sewer areas, dry weather flow rates, population
and potential storage locations. Information in the PRI Report was
adjusted or supplemented by information reported in other studies.
There are three combined sewer overflows in the study area
designated as Nos. 50, 67 and 49 (Figure 1) in the EMMA study and
Nos. 100, 101 and 103 respectively in the COM study. CSO Nos. 50 and
67 serve the same drainage area. At low tide, only No. 67 functions.
During high tide the capacity of the conduit connecting the Nos. 50
and 67 is reduced and both may discharge combined sewage. For this
study, CSO Nos. 50 and 67 are treated as one overflow.
While the boundaries of the area described in the above reports
differed somewhat, it was possible to determine the boundaries suffi-
ciently for purposes of this report.
In CDM's ongoing CSO study, the two subareas tributary to CSO's
Nos. 50 and 49 have a total area of 1,735 acres (508 and 1,227 acres,
respectively). This compares to M&E's tributary area of 1,580 acres.
The PRI report has 37 branch areas, 17 of which can be aggregated to
form an area that is essentially equivalent to those used in the, on-
going CSO study. The total sewered area of these 17 subareas is
1,613 acres, the number used in this study.
17
-------
ELTON ST,
DORCHESTER AVE.
RAMSEY ST.
SAVIN HILL
HANCOCK ST,
BAY ST,
;5^w:^
v&e^/C s»^
COMMERCIAL/
POINT <~
NEPONSET
AVE.(N)
PIERCE AVE,
Figure 3. Dorchester Bay Branch area
18
-------
Table 3 summarizes pertinent data for each branch area as ob-
tained from the PRI report. The location of each branch area is
shown in Figure 3. Of the total 1,613 acres sewered, 716 acres have
combined sewers and 897 acres have separate sanitary sewers. The
total population based on 1970 census data is 55,844 or 34.6 persons
per acre. The dry weather flow rate is based on an average of 150
gallons per capita per day (gpcd), derived from dry weather flow
monitoring during the period from February to May 1974.
The PRI report presented estimates of the daily solids deposi-
tion rate in each branch area, using a shear stress method, for every
sewer pipe in the area. This estimate was based on detailed schema-
tics and pertinent pipe characteristics such as pipe size, shape,
roughness and slope obtained from street and sewer maps. The collec-
tion system was segmented into a sewer element network with each ele-
ment averaged about 200 feet. The cumulative upstream sewer length
for each sewer element was obtained from sewer element connectivity.
Using an average population density of 19 persons per 100 feet of
sewer and an average suspended solids generation rate of 0.2 Ibs/
capita/day, the average daily dry weather flow rate and solids load-
ing for each sewer element was estimated. Maximum daily flow rate
was related to average daily flow rate by an empirical formula with
population as a variable. The daily maximum shear stress of a sewer
element was computed from the maximum daily flow rate and assumed to
prevail over a daily 24-hour period. The fraction of solids deposi-
tion in a given pipe element during a dry day was computed from the
maximum pipe shear stress. The amounts deposited were dependent on
the shear stress during peak flow and the amount deposited upstream.
Applying this deposition model to Dorchester as well as to several
other Massachusetts urban areas provided a data base from which
Pisano 'developed regression equations relating solids deposition
potential with sewer length, slope, size and per capita water use.
Recent fi^l4 studies using measured sewer strengths and sewer deposit
samplings^ ' show a fairly good comparison between the predicted and
actual deposition.
Based on a dry weather sewage with solids strength of 0.2 .Ibs
per capita per day, the total daily deposition of dry weather flow
solids was estimated as 724.6 Ibs, of which 526.6 Ibs was deposited
in separate sanitary sewers and 198.1 Ibs in combined sewers. This
is equivalent to 6.5 percent of the daily solids generated.
As shown in Table 3, of the 198.1 Ibs deposited daily in the
combined sewers, 76.9 Ibs were deposited in trunk sewers, or about 41
percent of the total. Comparatively, 119.3 Ibs out of 526.5 Ibs
deposited daily in the sanitary sewers are found in the trunk sewers,
or about 23 percent of the total. Trunk sewers receive flows from
lateral sewers and convey sewage to the downstream interceptors.
Trunk sewers are generally larger and laid on a flatter slope than
lateral sewers.
19
-------
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Recent extensive field sampling in small upstream laterals in
the study area (4) indicated an average solids loading of 0.56 Ibs/
capita/ day. The deposition model described previously implies that
deposition rates are linearly proportional to dry weather solids
loadings. Using the measured loading of 0.56 Ibs/capita/day, the
daily deposition rate in the study area becomes 2,028 Ibs/day, with
1,474 Ibs/day in sanitary sewers and 554 Ibs/day in combined sewers.
In Dorchester, practically all separate storm sewers enter combined
sewers at some point.
Table 4 shows the cumulative trunk and lateral sewer lengths in
the study area. PRI reports 75 percent of the daily accumulations in
the Dorchester collection systems are expected in about 18 percent of
the pipe components or in about 17 percent of the total pipe length.
Deposition data in Table 3 shows that lateral sewers contain roughly
two and a half times the deposits in trunk sewers. However, the
length of lateral sewers is more than four times that of the trunk.
The average solids deposition per foot in trunks is therefore greater
than that in laterals. Almost all combined sewer depositions are
accounted for in sewers with a deposition rate one Ib/day or greater,
while only about 50 percent of the deposits in sanitary sewers can be
accounted for at that deposition rate. The remaining half of the
solids deposition are in sewers with a lower deposition rate. Since
about 75 percent of the total deposits are found in sanitary sewers,
attempting to reach most of those deposits could require a very
extensive flushing program.
According to the PRI Report , roof drains from older dwel-
lings in Dorchester are connected to adjacent sewers. Storm runoff
has been observed in sanitary sewers. Approximately 20 percent of
the representative census tract areas in Dorchester selected for
planimetering are covered by rooftops. This study assumed that
stormwater drains directly to the adjacent sewer without loss.
During a rainfall, the storm runoff entering the sanitary sewers also
serves to resuspend and reduce deposited solids and associated pollu-
tants.
21
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TABLE 4. CUMULATIVE PIPE LENGTH OF SEWER SYSTEM (FEET)*
Branch
Adam St.
Bay St.
Centre St.
Coffey St.
Commercial Pt.
Deer St.
Dorchester Ave.
Elton St.
Geneva St.
Hancock St.
Kimball St.
Neponset Ave (N)
Pierce Ave.
Popes Hill Ave.
Romey St.
St. Marks Road
Savin Hill Ave.
Trunk
Sewer
1,090
2,175
14,017
-
3,120
1,310
2,550
2,180
8,100
4,970
2,875
-
-
-
1,450
6,176
1,400
Lateral Sewers
Sanitary
4,530
5,980
30,272
5,200
-
1,800
-
-
32,418
11,830
-
3,345
3,075
2,155
-
20,416
-
Combined Storm
2,144
2,450
14,213 3,454
-
3,025
-
3,695
4,355
25,279 4,716
19,010
8,255
920
1,985
1,015
815
8,566
5,755
Total
7,764
; 10. 605
i61,956
5,200
6,145
3,110
, 6,245
6,535
70,513
35 .,810
11,130
4,265
5,060
: 3,170
2,265
35,158
7,155
51,413 121,021 101,482 8,170
282,086
* Data obtained from the PRI Report(5)
22
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SECTION VIII
COMPUTER MODEL REQUIREMENTS
Runoff and CSO from frequent small rainfall events, when allowed
to discharge freely to receiving waters, results in major pollution.
Since reduction by source control, or containment and treatment of
pollutants in the first flush is imperative for pollution abatement,
rainfall volume is more significant than precipitation pattern. A
single precipitation event cannot determine the effectiveness of
pollution control for an alternative. Hence, the continuous
simulation approach should be used. The effectiveness of pollution
control is measured in terms of percent of runoff treated, the annual
number of overflows, and the amount of pollutants discharged to
receiving waters.
For a model to determine pollution abatement effectiveness, it
should permit:
1. use of continuous precipitation records to obtain the over-
flow characteristics (both quantity and quality) of the
sewer system over a period long enough to provide statis-
tically significant information;
2. simulation of surface runoff quantity and quality using
physical and land use parameters;
3. including the effect of dry weather flow quantity and qual-
ity during wet weather conditions and pollutant accumulated
in the sewer during dry days ;
4. evaluating the effect of street sweeping, sewer flushing,
storage and treatment on the overflow quantity and quality;
and
evaluating the pollutant removed at a CSO treatment facil-
ity.
5.
None of the existing computer models satisfied all of the
above criteria. Modification of these models was required. Of these
continuous simulation models, STORM^) an(j the continuous version of
SWMM (9)are probably the most generally used storm water management
models.
23
-------
SWMM can use the RUNOFF and STORAGE/TREATMENT Blocks for con-
tinuous simulation of precipitation records at hourly intervals. It
produces comparable output to STORM but is about twice as expensive
to run. SWMM can account for flow routing in gutters and pipes, and
has better storage/treatment routines than STORM. However, these
were not needed in this study. STORM was, accordingly, us;ed as the
base model for evaluation of alternative improvements.
STORM uses a simplified rainfall/runoff relationship,; neglects
the collection sewer system, and assumes a simple relationship be-
tween storage and treatment. The study area is characterized as a
single catchment from which hourly runoff is directed to storage and
treatment facilities. STORM can evaluate various storage/treatment
options for stormwater runoff pollution abatement.
Figure 4 shows major processes modeled by the current version of
STORM available from the Hydrologic Engineering Center of the U.S.
Army Corps of Engineers. The model considers up to 21 land uses in
determining runoff and the amount of street dust and dirt and asso-
ciated pollutants accumulated during dry days. It can reflect the
reduction in pollutant accumulation by street sweeping and determines
the dust, dirt and pollutants (up to six, including suspended solids
and BOD) washed from the watershed by rainfall using empirical func-
tions. The hourly runoff volumes (including municipal sewage at the
time of rainfall) less than or equal to the available capacity can be
routed to treatment facilities. Excess runoff can be diverted to
storage for possible treatment at a later time. Once the storage
capacity is exceeded, the excess runoff becomes untreated overflow.
Treatment capacity, in excess of that required for dry weather flow
treatment, can be used to draw down the volume in the storage facil-
ity. The computations of the treatment, storage and overflow pro-
cesses at a single outflow from the sewer system are performed by
volume and pollutant mass balance. The current version of STORM does
not allow routing combined sewage through a storage tank before dis-
charge nor does it consider quality improvement in the storage facil-
ity.
In this study, STORM was modified to determine the accumulation
of solids and organic material deposited in sewers during dry days,
the removal of these deposits by sewer flushing arid by wet weather
flows, and the effect of a CSTP on the wet weather discharge. Figure
5 illustrates the processes modeled by the improved version of STORM.
IMPROVED STORM PROGRAM - FEATURES ;
Sewer Flushing
Solids and organic materials deposit in sewers during dry days.
The amount of deposition in the sewer at the start of a rainfall
event depends on the amount remaining at the end of the last rain,
the frequency and efficiency of sewer flushing methods, the number of
dry days since the last rain, the sewer slope, and the sewage
24
-------
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strength and quantity. During wet weather flows, the solids and pol-
lutants resuspended become a part of combined sewage and the amount
depends on the initial deposit mass and the driving shear stress
generated by the sewer flow velocity. The daily solids deposition
can be estimated from a field sampling program or by using the SWMM
TRANSPORT Block or by regression equations derived from a follow-up
study of the PRI Report ' ' . The daily deposition rate of organic
material (BOD) can be determined as a fraction of the solid deposi-
tion rate estimated from a field sampling program ™' or by calibra-
tion using field data(l°). The result of both studies shows that BOD
mass equals about 40 percent of the deposited solids.
Sewer flushing methods and efficiency were recently field inves-
tigated in Dorchester under an EPA R&D grant study
(4).
A solids re-
moval efficiency of 40 percent and BOD removal efficiency of 60 per-
cent were attained in a 12 or 15 inch sewer using a flushing volume
of approximately 50 cubic feet, injected at a rate of approximately
0.5 cubic feet per second. These removals were effective for segment
length of up to 1000 feet downstream of the flushing station.
The input data required to determine the effect of flushing is
shown on Table 5 and includes:
1. separate sanitary sewer area;
2. combined sewer area;
3. solids deposition rate in separate sewer area;
4. solids deposition rate in combined sewer area',
5. fraction of solids deposition contributing to BOD;
6. sewer flushing Interval in dry days;
7. fraction of deposition amount in separate sewer area
flushed;
8. fraction of deposition amount in combined sewer area
flushed;
9. removal efficiency of SS by mechanical sewer flushing;
10. removal efficiency of BOD by mechanical sewer flushing; and
11. minimum wet weather flow rate in sewers resulting in com-
plete removal of solids.
Depression Storage Effect Analysis
STORM was improved by introducing three new variables shown on
Table 6; namely, DEPRS, DETIMP and PERNIMP. DEPRS is the depression
storage in inches for the pervious area, DETIMP is the same for the
impervious area, and PERNIMP is the percent of impervious area that
has zero depression storage. PERNIMP was used to determine the
direct runoff to sewers. Table 6 also shows other input data for
STORM.
Combined Sewage Treatment Plant (CSTP)
The EMMA Alternative 1 includes a new interceptor diverting com-
bined sewage to a new CSTP. The plant provides storage, settling
27
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after its storage volume is filled, and chlorination of overflows.
The plant includes two basins. The first basin can serve as a
holding tank and the second as a flow-through tank or both can serve
as flow-through tanks. At the end of a rainfall event, waste
remaining in the basins is returned to the interceptor for treatment
along with the dry weather flow.
Pollutants removed, by settling in the plant after storage was
filled, was modeled after a study presented in Water and Wastewater
Engineering ^11). The fraction of the initial SS loading of a slug
removed is a function of the detention time of that slug, asi shown in
Figure 6. The removal function has the following expression:
% SS removal = 0.68 (1 - a'1'2528 x DT)
where DT is the detention time in hours. The older version of SWMM
used this formula for estimating removal efficiency of a sedimenta-
tion tank. The new SWMM version, while using the same formula, al-
lows coefficients to be varied. Based upon the ratios shown in
Figure 6, the percent removal of BOD is taken as 55 % of the sus-
pended solids removed. The amount of pollutant removed by settling
is small, if any, compared to that removed by the storage phase. At
the design detention period of 15 minutes, the theoretical removals
of BOD and SS by settling are 10 and 18 percent, respectively.
An alternative formula that is used is
(22)
removal = 0.82 e
overflow rate
2780
where the overflow rate is in gallons per day per square feet. For a
detention time equal to or greater than 1 hour, the difference in
removal rates computed by the two formulae is small. Greater differ-
ences occur when the detention time is 30 minutes or less. However,
the accuracy of any formula at such high loading rates is questiona-
ble. Further, since a smaller amount of pollutant is removed at the
higher flows or shorter detention times, either formula should pro-
vide similar results using the long-term rainfall records.
The input data required relative to combined sewage treatment
facilities are:
1. CSTP capacity;
2. total storage capacity of the treatment tank; and
3. storage capacity of the first holding basin.
The storage capacity of the second flow-through basin is the
difference between items (2) and (3) above. By modifying the storage
capacity in the two basins, the model can simulate the following con-
ditions:
30
-------
SUSPENDED SOLIDS
3 4
TIME , hr
REPRODUCED FROM REFERENCE 12
Figure 6. Pollutant removal in a storage unit as a
function of detention time
31
-------
(i) one storage-settling tank with capacity equalling the total
tank capacity; and
(ii) one storage basin and one storage-settling basin.
Facility No. 9 proposes a primary treat-
5.08 Mgal. Simulation over a 16-year
indicates that, for this capacity, one
result in less pollutant discharged than
between storage and storage-settling.
would be operated as one storage-settling
EMMA Alternative 1 for
ment plant tank capacity of
continuous rainfall record
storage-settling tank would
two basins divided equally
This study assumed the CSTP
tank.
Part Year Modeling
STORM was also modified to do continuous simulation over a
period equaling a part of the year instead of 12 months as in the
original program. This allows flexibility for water quality studies
in areas such as the Dorchester Bay where the critical period, as far
as water quality is concerned, is during the recreational season.
This season is from May through November (see Table 5). Statistical
summaries of rainfall, runoff, and combined sewage and CSO quantity
and quality are for that period.
For evaluation of alternatives, the relevant information in-
cluded in the STORM output is shown on Table 7. The data are the
average values over the number of simulation years. Throughout this
study, the "annual" duration encompasses only the period from May
through November unless otherwise specified. The additional data
printed include:
1. total volume and pounds of dry weather flow contributing to
combined sewage but excluding those pollutants resuspended
from sewers during wet weather;
2. total volume and pounds of combined sewage diverted to the
main STP;
3. total volume and pounds of combined sewage diverted to the
CSTP;
4. total volume and pounds, out of (3) above, captured in the
first and second basins of the CSTP for later return to the
interceptor for treatment at the main STP;
5. total volume and pounds of combined sewage overflowed from
the CSTP;
6. total volume and pounds of combined sewage remaining in the
offline storage at the end of a rainfall event to be
diverted to the main STP;
32
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7. total number of times that sewers are mechanically flushed;
8. pounds of suspended solids and BOD removed during dry
weather flushes; and
9. pounds of suspended solids and BOD in the sewer deposits
resuspended by stormwater runoff and contributing to the
combined sewage. ;
The above information identifies the principal sources of pollu-
tant contribution. Pollution abatement strategy can be determined on
a quantitative basis.
34
-------
SECTION IX
MODEL INPUT DATA
Input data to STORM includes (a) meteorological data for con-
tinuous long-term rainfall-runoff simulation, (b) watershed charac-
teristics which include parameters for surface runoff and pollutant
loading computation, land use, dry-weather flow, evaporation rate,
solids and associated pollutants deposition rates, and sewer flushing
and street cleaning practices.
LONG-TERM METEOROLOGICAL DATA
An historical record, spanning 1960-1975, was used to develop
rainfall-runoff characteristics of the study area for the period of
May through November. Such records are available at the Logan
Airport weather station and Blue Hill Observatory. Statistical
analysis of rainfall records at these two stations were presented in
the PRI Report. (5) The average antecedent dry days and the average
rainfall intensity are about the same at each station and are,
respectively, four days and 0.08 inches/hour. The average rainfall
duration at the Blue Hill Station is 7.0 hours or 1.3 hours longer,
and average rainfall per storm is 0.52 inches or 0.09 inches greater
than those recorded at the Logan Airport Station. Because the Blue
Hill station is closer to the .study area and is about as far inland
as the study area, its records were used. The ongoing CSO Facility
Plans also use the Blue Hill Station records.
The amount of rainfall from May through November, averaged over
the 16-year period used, is 28.26 inches. Table 8 shows the amount
of annual precipitation from May through November. The period
includes two record wet years (1972 and 1975) when rainfall was about
145 percent of average; two record drought years (1964 and 1965) when
rainfalls were about 55 percent of average; one year (1962) when
rainfall was about 122 percent of average; one year (1971) when
rainfall was about 82 percent of average; and ten years when rainfall
was within plus or minus 10 percent of average. Because rainfall,
runoff and combined sewage overflow characteristics are affected by
three extremely variable inputs, namely, the antecedent dry period,
rainfall intensity and duration, the period selected for simulation
should exhibit as many combinations of these random variables as
possible. The period of 1960 through 1975 appears to fit this
criterion well.
35
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TABLE 8. PRECIPITATION, MAY THROUGH NOVEMBER
Year
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
Precipitation
Inches
24.90
28.82
32.18
25.68
14.82
13.98
23.92
27.66
24.84
25.91
27.24
21.57
38.04
27.49
25.02
38.40
Percent of
Average
94.
109.
122.4
97.7
56.4
53.2
91.0
105
94
98.6
.3
,1
104.
82.
144.7
104.6
95.2
146.1
Average
26.28 Inches
Figure 7 shows the cumulative probability distributions of rain-
fall durations and antecedent dry periods as obtained from the PRI
reports. These distributions were derived from the observed hourly
rainfall data at the Blue Hill Station for a period of 1958 to 1972
from May through November. The same type plots for rainfall intensi-
ty and total rainfall per storm ;event are given in Figure 8. Using
these figures, the one-year storm (Figure 2) used to design the EMMA
Alternative 1 can be related to the observed rainfall data as fol-
lows :
].. The rainfall amount of 1.78 inches is exceeded about 4 per-
cent of the time.
2. The rainfall duration of six hours is exceeded 40 percent
of the time.
3. The seven antecedent dry days are exceeded about 18 percent
of the time.
4. The maximum hourly intensity of 0.98 inches is exceeded
less than 2 percent of the time.
A single precipitation event cannot determine the pollution
control effectiveness, of an alternative. Hence a facility designed
using a hypothetical storm should be evaluated using a continuous
36
-------
10'
o
I I
c
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c
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,
8
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Blue Hill
Observatory Data
1958 - 1972
May through November ;
Reproduced from PRI Report :(5)
Rainfall Intensity
10 T
15 20 30 40 50 60 TO 60 85 9O 3o 98%
Cumulative Probability
Figure 8. Cumulative probability plots of rainfall amount and
intensity.
38
-------
simulation model and real .rainfall events. Based on such an evalu-
ation, this design storm appears very conservative for evaluating
pollution control benefits.
WATERSHED CHARACTERISTICS
STORM can now analyze only one watershed at a time. It cannot
automatically transfer computed outflow quantity and quality data
from an upstream to downstream watershed. For this reason, the en-
tire study area has been considered as one watershed. Data required
for STORM, therefore, should represent the gross values over the en-
tire study area.
Data required for the STORM program, in addition to that pre-
viously described, includes:
1. Land Use and Pollutant Loadings
COM furnished land use data. The area and percent distri-
bution of land uses and percent imperviousness are summarized in
Table 9 for Overflow Numbers 100 and 103, which receive drainage
from the study area. The values for Overflow Number 1'03 were
used throughout the study. These numbers are shown as PRCNT on
Table 6. For comparative analyses, these numbers, if within a
reasonable range, should not affect the validity of the conclu-
sions reached. The percent imperviousness for single family,
multiple family, commercial, industrial and open space are as-
sumed to be 50, 70, 80, 80 and 20, respectively, and are shown
in Table 6 under FIMP. These values appear reasonable and about
65 percent of the watershed area is impervious. In computing
surface runoff by STORM only the average value is significant.
Input data required for computing pollutants generated in
the watershed during dry days include density of curb length
(STLEN), dust and dirt accumulation rate (DD), conversion rates
to compute pollutant components from DD, street sweeping inter-
val (NCLEAN), and street sweeping efficiency (REFF). None of
the above for the Dorchester area was available at the time of
this study, therefore it is necessary to make reasonable assump-
tions and evaluate their effects on the end result.
The density of curb length is assumed as 300 feet per acre
for all land uses. This is the average value found in the on-
. going CSO facilities plan study in Elizabeth, New Jersey, with a
population density about the same as that in Dorchester. Com-
pared to the national average (13), it is low for residential and
commercial areas and high for industrial areas and open spaces.
The dust and dirt accumulation rates and conversion factor's to
calculate SS and BOD in dust and dirt are also borrowed from the
Elizabeth study(10). These values were obtained from a calibra-
tion of SWMM parameters using around ten sets of rainfall-runoff
quantity and quality data sampled at 7.5 minute intervals. For
39
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40
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Elizabeth, the SS and BOD accumulation rates are about 10 and
0.3 Ibs/acre/day. The BOD rates are comparable to those re-
ported in Milwaukee, Washington and Seattle(14), all comparable
in population density. The conversion factors for settleable
solids, total nitrogen, phosphate and total coliform were not
calibrated in the Elizabeth study. The values shown are the
default values internally assumed in both SWMM and STORM. This
study considers SS and BOD in the evaluation of pollutant re-
moval. If is assumed, based on other tests, all discharges
during the recreation period will require disinfection. Other
pollutants were not included.
Street sweeping interval was. assumed as ten dry days. Ac-
cording to Figure 7, only about ten percent of the dry periods
equals or exceeds ten days. This implies that the street sweep-
ing activity is not intensive, which is typical in most old
urbanized areas. The sweeping efficiency is assumed to be 50
percent which has been shown to be high considering the parked
car problem and contribution of pollutants from areas other than
the streets.
2. Depression Storage, Recovery Rates and Runoff Coefficients
The depression storage for pervious area (DEPRS) and that
for impervious area (DETIMP) is assumed to be 1/4 and 1/16 inch
respectively. Unlike the original STORM program, which requires
a single depression storage capacity averaged over the entire
drainage area, the modified program requires both DEPRS and
DETIMP and computes the average storage capacity internally.
STORM allows depression storage to recover to its maximum
capacity at a constant rate to account for evaporation. The
recovery rates for each month were computed using the Meyer
formula which expresses the evaporation rates as an empirical
function of air vapor pressure and wind speed, all monthly
averaged. Table 10 shows the data required for calculating
recovery rates.
Runoff coefficients for the pervious area (CPERV) and for
the impervious area (GIMP) are assumed to be the same as those
internally assumed in STORM and are 0.15 and 0.90, respectively.
The surface runoff is computed by applying these coefficients to
the effective rainfall and represents all losses other than de-
pression storage.
DRY-WEATHER FLOW AND POLLUTANT LOADING
As shown in Table 3, the average dry-weather flow is 12.96 cfs,
based on an average per capita contribution of 150 gallons per day
obtained from .field monitoring in 1974 ^\ Both the quantity and
quality of dry-weather flow generally vary with the hour of the day
and the day of the week. For this study, it was assumed that hourly
41
-------
TABLE 10. MONTHLY EVAPORATION RATES
Month
January
February
March
April
May
June
July
August
September
October
November
December
Mean Air
Temperature
(Degrees F)
27.3
29.2
36.8
47.0
56.9
67.8
73.3
71.9
64.4
54.7
43.7
33.6
Relative
Humidity
(Percent)
55
62
65
70
76
81
82
83
84
79
82
68
Wind
Speed
(mph)
14.
14.
14.
13.9
12.8
11.7
11.7
11.6
11.4
12.4
12.9
14.0
,5
,3
.3
Evaporation
:Rate
(in/day)
.075
.074
.108
.163
.216
.272
.331
.289.
.203
; .172
' .085
.082
variations are the same for any day of the week as shown in Table
11.
The hourly variations of Idry-weather flow rates were, derived
from field data furnished by CDM. The field data were obtained in
September and October of 1978 at three sampling points in Dorchester
and three in South Boston, all using Manning level recorders. The
sampling stations in Dorchester include Victory Road near the
Dorchester Interceptor; Geneva Avenue near Tonawanda Street; and Bay
Street near Maryland Street. ' The three stations in South Boston
include Mt. Vernon near Southeast Expressway; Sidney Street near
Dorchester Interceptor; and K Street near Marine Street.
The average daily concentrations of pollutants in the waste-
waters adopted for long-term simulation are shown in Table 12 and are
consistent with the lower sewage strength used in this report. Only
suspended solids and BOD were used for evaluation of alternatives.
These pollutant concentrations are classified as "medium" according
to an EPA study report (-"-5). They are more representative of sewage
strength reaching the sewage treatment plant than the sewage strength
at the source of origin. Because of deposition of solids in sewers
and dilution resulting from infiltration/inflow, the sewage strength
may be reduced as sewage travels downstream. Recent field sampling
at four upstream sewers in Dorchester ' ' indicates the mean SS con-
centration can be as high as 1800 mg/1 and mean BOD about 1000 mg/1.
The average per capita suspended solids contribution of four sampling
stations is 0.56 Ibs/day. Using the SS strength of 250 mg/1 and 150
gallons per capita per day of waste flow, the daily SS rate is 0.313
Ibs/capita/day. The actual sewage solid concentration could be
within these ranges. Using the SS rate of 0.313 Ibs/capita/day, the
daily solid deposition the study area becomes 1134 Ibs/day, with 824
42
-------
Table 11. DIURNAL VARIATION OF DRY-WEATHER FLOW
Hour
Flow/Average*
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
.900
.865
.850
.845
.855
.885
.930
1.030
1.070
1,
1,
1,
1,
1,
1,
1.
1.
1.
1.
,080
080
070
040
030
020
020
030
050
070
1.080
1.080
1.060
1.010
.950
11.65
11.20
11.00
10.94
11.07
11.46
12.04
13.34
13.85
13.98
13.98
13.85
13.47
13.34
13.21
13.21
13.34
13.60
13.85
13.98
13.98
13.73
13.08
12.30
*Average flow rate = 12.96 cfs
TABLE 12. AVERAGE DAILY POLLUTANT CONCENTRATION OF
DOMESTIC WASTEWATER
Pollutant
Suspended solids
BOD
Settleable solids
Total nitrogen
Total Phosphate
Total coliform
Concentration
250 mg/1
200 mg/1
6 ml/I
30 mg/1
10 mg/1
5.37 x 107 MPN/100 ml
43
-------
Ibs/day in separate sanitary sewers and 310 in combined sewers. The
effect of using the deposition rates based on 0.56 and 0.313 Ibs/
capita/day are evaluated.
With a sewage .strength of 250 mg/1 for SS and 200 mg/1, for BOD,
the average daily SS load is 17,458 Ibs and BOD load is 13,967 Ibs.
These are part of input data to STORM.
Interceptor should be designed with capacity at least equal to
the peak dry-weather flow of the drainage area served. For the
evaluation of alternatives, interceptor capacity is assumed as equal
to the peak flow. Harmon's ratio ^ 'is used to determine the peak
flow rate:
14
where:
4 +
M = the instantaneous peak flow
Q s the average daily domestic flow
P - the tributary population in thousands.
For an estimated population of 55,844, the ratio of peak to
average flow is 2.2 and the peak dry-weather flow rate equal to 18.58
mgd.
44
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SECTION X
DEVELOPMENT AND EVALUATION OF ALTERNATIVES
Alternatives for CSO pollution abatement can include: (1) sewer
flushing; (2) storage by flow routing in pipes or in off-line basins;
(3) treatment; or (4) a combination of several. The cost and
pollutant removal effectiveness of the alternatives developed are
compared with that of EMMA Alternative 1.
DEVELOPMENT OF SEWER FLUSHING ALTERNATIVE
ter
From the PRI Report and recent field study report in Dorches-
the following is observed:
1. Seventy-five percent of sewage solids deposits in Dorches-
ter occur in 17 percent of the sewer length.
2. Sewer flushing for pollution control purposes is effective.
Beyond 1000 feet SS tend to resettle and the removal rate
is greatly reduced; however, organics and nutrients are
conveyed much further. Thirty-three, 50, and 60 percent of
the BOD, Total Kjeldahl Nitrogen, Total Phosphorus, res-
pectively are estimated to remain in suspension 3000 feet
from the point of flush.
3. Because a significant portion (33-45 percent) of. the BOD
would remain in suspension after flushing, BOD removal rate
is greater than SS removal rate.
4. Smaller lateral sewers may contain more deposits than the
trunk sewers but have a disproportionatly greater length,
thus trunk sewers have greater deposition rates per unit
length .
5. Available experimental data applies to flushing 12-inch and
15-inch sewers. In these instances, flushing volumes of
approximately 50 cubic feet discharged at the rate of
approximately 0.5 cfs would effectively flush the sewers.
Shear stress computations indicate larger flow volumes may
be required to flush larger sewers. Field demonstrations
in larger size sewers appear desirable.
In the Dorchester area sewer flushing, to be effective, should
concentrate on moderate to heavy deposition areas . For a long later-
al sewer, sequential flushing may be required to transport solids and
its associated pollutants to reach trunk sewers.
45
-------
In developing alternatives, flushing locations have been selec-
ted to the extent possible on the lateral sewers. Locations, how-
ever, have also been selected in some smaller sized combined sewers.
Pending demonstration to define the criteria for flushing such sew-
ers, the estimates of the benefits of flushing might be overstated.
Because of the feasibility of flushing large size sewers demonstrated
in Detroit ^^ the amount of overstatement should not significantly
effect the conclusions in this report.
Tables 13 and 14, respectively, rank sanitary and combined sewer
segments with deposition rates greater than or equal to 3.0 Ibs/day.
Figure 9 shows both these heavy deposition segments and strategic
flushing locations. There are 28 flushing locations shown; 11 in
combined sewer segments and 17; in sanitary sewer segments. Flushing
at 11 locations would resuspend a part or all pollutants in 14
deposition segments or at least 137.8 Ibs out of 198.1 Ibs of solids
deposited daily in the combined sewers, or about 70 percent.
Flushing at 17 sanitary sewer segments would affect 20 deposition
segments or at least 172 Ibs out of 526.5 Ibs deposited daily, or
about 35 percent of the total deposited in sanitary sewers. These
deposition rates were computed based on the SS generation rate of 0.2
Ibs/capita/day. At higher SS generation rates, which have been
observed, sewer flushing could be proportionately more effective.
Table 15 summarizes, for both combined and sanitary sewers, the
number of deposition segments, cumulative deposition rates of these
segments and percent of total solids deposition segments with deposi-
tion rate equal to or greater than 2.0, 1.5 and 1.0 Ibs/day. The
results are plotted in Figure 10. The benefit of sewer flushing
beyond 28 flushing stations becomes increasingly more marginal as the
solids flushed per station reduces rapidly. For this study, the 28
flushing locations shown on Figure 9 have been used to evaluate the
effects of alternatives involving sewer flushing for comparison with
the EMMA Alternative 1 and storage. Flushing alternatives, with the
number of flushing segments both greater and less than 28S were also
considered to evaluate the cost-effective number of flushing stations
for the Dorchester area.
DEVELOPMENT OF STORAGE ALTERNATIVE
STORM can consider only one storage facility at a time. STORM
assumes this storage to be downstream near the interceptor system.
However, the location of storage within reasonable restraints, i.e.,
receiving runoff from perhaps 70 percent of the watershed, is not
critical.
i
In the PRI Report, 11 potential upstream and two potential down-
stream storage locations were identified. These locations are shown
in Figure 11. The potential upstream storage capacity totals about
50 million gallons and the downstream storage about 17.3 million gal-
lons. Fifty million gallons of storage is equivalent to 1.15 inches
of water depth over the entire watershed and 17.3 million gallons
46
-------
TABEL 13. SANITARY SEWER SEGMENTS RANK BY DEPOSITION'RATE
Deposition
Rank
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Branch
Area
Centre St.
Centre St.
Centre St.
. Centre St.
Geneva Ave.
Centre St.
Geneva Ave.
St. Marks Rd.
Centre St.
Geneva Ave.
Centre St.
Centre St.
Deer St.
Geneva Ave.
St. Marks Rd.
Geneva Ave.
Centre St.
Centre St .
Geneva Ave.
Co f fey St.
Street
Name
Norfolk
Talbot
Talbot
Southern
Park
Stanton
Park
St. Marks
Wainright
Geneva
Centre
Wainright
Dorchester
Josephine
Roseland
Easement
Torrey
Southern
Washington
Co f fey
Rate
(Lbs/Day)
28.3
28.2
18.2
16.9
8.9
8.1
7.4
6.7
6.4
6.4
4.5
4.0
4.0
3.8
3.8
3.6
3.3
3.2
3.2
3.1
Sewer
Slope
.0006
.0003
.0007
.0004
.0019
.0007
.0012
.0010
.0008
.0016
.0005
.0008
.0013
.0016
.0019
.0019
.0019
.0016
.0025
.0017
Sewer Lateral
Size
(inches)
30 x 36 oval
36 x 48 egg
36 x 48 egg
12 circ.
18 circ.
12 circ.
27 x 35 egg
40 x 60 egg
30 x 36 oval
27 x 35 egg
26 x 48 egg
30 x 36 oval
15 circ.
12 circ.
48 circ.
24 circ.
15 circ.
15 circ.
15 circ.
15 circ.
or
Trunk
• T
L
L
L
L
L
T
T
L
T
T
L
T
L
L
L
L
L
L
T
TABLE 14. COMBINED SEWER SEGMENTS RANK BY DEPOSITION RATE
Deposition
Rank
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Branch
Area
Hancock St.
Centre St.
Geneva Ave.
Centre St.
Romsey St.
Kimball St.
Commercial Pt.
Centre St.
Centre St.
Neponset Av. (N)
Centre St.
Hancock St.
Centre St.
Kimball St.
Street
Name
Hancock
Centre
Geneva
Washington
Sagamore
Adams
Freeport
Adams
Washington
Boutwell
Duribar
East
Centre
Leedsville
Rate
(Lbs/Day)
59.
11.
11.
10.
8.
5.
5.
4.
4.
3.
3.
3.
3.
3.
5
9
6
1
3
9
4
5
0
6
5
4
1
0
Sewer
Slope
.0004
.0004
.0010
.0006
.0010
.0041
.0014
.0010
.0011
.0020
.0014
.0018
,0006
.0005
Sewer
Size
Lateral
(inches)
24
36
18
20
20
16
15
32
20
12
12
12
36
12
x 31
x 48
circ.
x 26
x 26
x 20
circ.
x 42
x 26
circ.
circ.
circ.
x 48
circ.
oval
egg
oval
oval
egg
egg
oval
egg
or
Trunk
L
T
T
L
T
T
T
T
L
T
L
L
T
L
47
-------
HEAVY DEPOSITION SEGMENT
FLUSHING SEGMENT
Figure 9. Locations of heavy deposition segments
and flushing stations.
48
-------
TABLE 15. TOTAL DEPOSITION OF SEGMENTS RANKED BY DEPOSITION
RATES
Total
Deposition Rate Deposition
equal to or greater Sewer*. No. of Rate % of**
than (Ibs/day) Type Segments (Ibs/day) Total
3.0 C 14 138 70
2.0 C 22 156 79
1.5 C 30 169 85
1.0 C 56 198 100
3.0 S 20 172 33
2.0 S 33 203 39
1.5 S 48 229 43
1.0 S 89 278 53
C = combined sewer
S = sanitary sewer
Total deposition in combined sewers
Total deposition in sanitary sewers
**
198.1 Ibs/day
526.5 Ibs/day
equivalent to about 0.4 inches. Generally, a storage capacity of
about 0.15-0.3 inches over the entire drainage area can be effective
in abating combined sewage overflow pollution^- '. Consequently,
storage alternatives will be considered within this range.
EVALUATION OF ALTERNATIVES
Table 16 compares the computed performance of various alterna-
tives for a 16-year period (1960-1975). Column (1) identifies the
alternative. For all cases, the maximum flow to the existing STP is
equal to 18.6 mgd, the peak dry-weather flow rate. Column (2) shows
the maximum combined sewage intercepting rate to the CSTP proposed in
the EMMA Alternative 1 (See Figure 5). The amount of off-line stor-
age (see Figure 5) is indicated on Column (3). In Cases 8 through
11, it equals the sum of the volume in the proposed interceptor and
the CSTP., The sewer flushing interval is shown in Column (4). The
30-day flushing interval represents no sewer flushing since no dry
period lasted longer than 30 days during the 16-year simulation per-
iod. Column (5) 'shows the average number of times from May through
November the sewers would have been flushed at each of 28 flushing
locations. Column (6) indicates the average number of times from May
through November the combined sewage flow rate exceeded the inter-
ceptor capacity and untreated overflow to the bay occurred. Column
(9) indicates both untreated overflow (Column 7) and that discharged
from the CSTP after receiving primary treatment (Column 8). The
amount of pollutant in the overflow also includes that untreated,
49
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-------
STORAGE LOCATION
"^ ,f
Figure 11. Potential storage locations
51
-------
(Columns 10 and 14) and that treated by the CSTP (Columns 11 and 15).
The total pounds of SS and BOD in the overflow are shown in Columns
(12) and (16). The percent removal (Columns 13 and 17) refers to
Case 1 (the existing condition) as a base. Column (18) is the volume
of combined sewage and Column (19) is the amount of surface runoff
included in the combined sewage. The difference represents the
volume of sanitary sewage during wet-weather period. Considering the
high peak flow rates of combined sewage the amount of storm water
runoff routed to the STP is relatively small. The amount of wet-
weather flow pollutants diverted to the main STP is shown on Columns
(20) and (21). The pollutants shown in Table 16 assume a street
sweeping interval of 10 dry days and sewer solids deposition rates
computed with a per capita suspended solids contribution of 0.56
Ibs/day. Table 17 presents information similar to that in Columns
(10) through (17) of Table 16, except that solids deposition in sew-
ers is computed assuming a per capita solids contribution of 0.313
Ibs/day. In both tables, the deposition rate used for BOD is 40
percent of the SS deposition rate. Cases 2, 3 and 4 are not included
in Table 17 since sewer flushing alone would not provide pollution
control comparable to the EMMA Alternative 1. Cases 9 and 11 are not
included since sewer flushing with the EMMA Alternative 1 is not
pertinent to the conclusions of this report.
Case No. 1 represents a system in which the maximum combined
sewage flow rate diverted to the main STP equals the peak dry-weather
flow. Based on past studies, providing effective treatment capacity
for greater flow rates is not likely to be cost-effective. Under
this condition, 66 percent of combined sewage volume, 77 percent of
SS and 62 percent of BOD contained in the sewage would be discharged
directly to the receiving waters. On average, overflows could be
expected 53 times from May through November or about once every five
days. If the sewers are flushed daily at 28 locations (Case No. 2),
the SS discharged would be reduced by 7 percent and the BOD by 17.6
percent. The BOD removal rate is higher than the SS removal rate
since flushing efficiency is higher for BOD than that for SS . There
would be an average of 165 times that sewer flushing would be per-
formed from May through November. To achieve this condition, some
form of automatic installation appears essential. Its operation
would require triggering after 24 dry hours. In addition, the flush-
ing operation at all points might have to be almost simultaneous.
Case No. 3 represents flushing at a 48-hour interval. This is pro-
bably a more reasonable alternative. The flushing operation could
start after 24 dry hours and would not have to be done nearly as
simultaneously. The average number of flushes would be reduced from
165 to 70. Approximately 5.2 percent of the SS and 14.1 percent of
the BOD would be removed. Neither Case 2 or 3 approaches the
effectiveness of the EMMA Alternative 1 (Case 8). If the flushing
interval is extended to 7 dry days (Case 4), only about 1.2 percent
of the SS and 4.0 percent of the BOD would be removed. The number of
flushing events would be reduced to 7. It does not appear that sewer
flushing by itself can be considered as an effective CSO pollutant
abatement technique.
52
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Case No. 8 is the EMMA Alternative 1. Combined sewage flow at a
rate of 497.4 mgd is pumped to the CSTP while 18.6 mgd is conveyed by
the interceptor sewer to the main STP. The peak runoff rate of the
design storm for the area tributary to the Overflow No. 50 (Figure 1)
was provided by M&E as 346 mgd. (21)The peak flow rate was moderated
to 152 mgd by using the storage:provided in the 4000-feet long, 126-
inch diameter sewer connecting outfall Nos. 49 and 50. The conduit
storage behaves as off-line storage in the STORM simulation. Because
of large treatment capacity and off-line storage, untreated overflow
would have occurred only about ten times in 16 years. Thus the
amount of unchlorinated discharges would be very small. The chlori-
nated overflow from the CSTP is, however, significant. Of 2,706,600
Ibs of SS and 485,000 Ibs of BOD entered the CSTP, 1,265,900 Ibs of
SS and 248,400 Ibs of BOD are discharged to the receiving waters for
a removal rate of 53.2 percent for SS and 48.8 percent for BOD.
These removal rates are somewhat higher than the reported overall
removal rates at the Cottage Farm facilities which remove 45 percent
of SS and 42 percent of BOD probably due to the larger volume at the
proposed EMMA Alternative 1 facility. Including, untreated discharged
pollutants (Columns 10 and 14 of Table 16), the percent removal with
respect to the Case No. 1 is 52.4 percent for SS and 47.8 percent for
BOD. This appears low considering the cost of this alternative
(Table 2). The major advantage of this proposal, the elimination of
practically all unchlorinated discharges, may be achieved by less
costly means.
Supplementing the EMMA Alternative 1 proposal by daily flushing,
Case No. 9, SS and BOD removals increase to 55.2 and 56.2 percent
respectively. Sewer flushing, when integrated with other structural
alternatives, such as storage, may be cost-effective. Alternatives
5, 5A, 6, 6A, 7 and 7A were developed to explore cost-eff ectivenes's
of combinations of storage and sewer flushing.
Case Nos. 5, 6, and 7 are alternatives without sewer flushing
but with offline storage capacities equal to 5.1, 6.8 and 13.1 mil-
lion gallons respectively. As the amount of storage increases, the
number of overflow events decrease, as do the pollutants discharged
to the receiving waters. In contrast to Case 8, Cases 5, 6 and 7
contemplate gravity flow to storage and pumping out of storage to
deliver flows to the dry weather interceptor at much lower rates
after the storm flow subsides.
Using Case 1 (no control/treatment) as the reference, Cases No.
5, 6 and 7 would remove 32.2, 40.3 and 63 percent respectively of the
SS that would have been discharged under the reference case. The BOD
removal rates are higher and are 38.3, 46.2 and 67.1 percent, re-
spectively. The SS and BOD removal rates of the EMMA Alternative 1
(Case No. 8) are somewhat more ;than that for Case 6 and substantially
less than that for Case 7. The storage provided in Case 8 is also
somewhat more than that provided in Case 6 and substantially less
than that provided in Case 7. The primary benefits with respect to
56
-------
tion abatement in Case 8 are those associated with storage rather
than treatment, except for chlorination.
Case Nos. 5A, 6A and 7A supplement the offline storage alterna-
tives with daily sewer flushing. As a result of flushing, the SS
removal rates are increased to 36.2, 43.7 and 65.0 percent, respec-
tively for Cases No. 5A, 6A and 7A, or a marginal improvement of 3.9,
3.4 and 2.0 percent over those obtained for Cases No. 5, 6 and 7.
With no storage, the net improvement attributable to sewer flushing
is maximum, or 7 percent. Sewer flushing is more effective in re-
ducing BOD in the overflows. With flushing, the percent BOD removal
rates are 48.2, 54.6 and 72.1 for Case Nos. 5A, 6A and 7A, respec-
tively. The net improvements over the no-sewer-flushing alternatives
are 9.9, 8.4 and 5.0 percent for storage capacities equal to 5.1, 6.8
and 13.1 million gallons respectively. With no storage, the net im-
provement is 17.6 percent. The net improvement decreases with in-
creased storage capacity. The provision of flushing may permit a
reduction in other pollution abatement techniques but is not likely
to result in their elimination.
BOD may be a more appropriate measure of pollution control ef-
ficiency because (1) field sampling, especially in sewers, is diffi-
cult due to inaccuracies inherent in the methodology and (2) the in-
accuracies most greatly affect solids with little effect on BOD, and
(3) solids deposition is seldom or never discrete and non-cohesive as
assumed in the models. Figure 12 compares percent SS and BOD removal
versus offline .storage for the no sewer flushing alternatives (Cases
No. 5, 6 and 7) and daily sewer flushing alternatives (Cases No. 5A,
6A and 7A). As shown in Figure 12, an offline storage capacity of
about 7.1 million gallons without sewer flushing, or a storage
capacity of 5.1 million gallons with daily sewer flushing at 28
heavily deposited sewer segments, would result in the same reduction
in BOD discharged between May through November to receiving waters as
the EMMA Alternative 1. The ineffectiveness of EMMA Alternative 1 is
graphically demonstrated in this comparison. The high pumping rate
through the CSO treatment plant actually reduces efficiency as
compared to simple storage and bypassing flows in excess of the
storage capacity. This is not surprising since, in a major storm in
areas with a high degree of imperviousness, the pollutant strength in
the combined sewage decreases very greatly as the storm proceeds.
Case No. 9 is EMMA Alternative 1 supplemented by daily sewer
flushing at 28 locations in the study area. The BOD removal rate is
improved from 47.8 percent to 56.6 percent or by about 18 percent.
The SS removal rate is improved only from 52.4 percent to 55.2 per-
cent or by about 5 percent. Referring to Figure 12, Case 9 ,is equi-
valent to providing a storage of about 9.6 Mgal for equal BOD
removal.
Case No. 10 is similar to EMMA Alternative 1 except that the
conduit diameter is reduced to 5 feet instead of 10.5 feet and pump
capacity to the CSTP to 100 mgd instead of 497.4 mgd. The conduit
57
-------
§
Q
<
CO
CO
cd
M
I
00
C
O
00
70
60
50
40
30
20
IO
INTERCEPTING CAPACITY TO THE MAIN S.T.P,
= 18,6 MGD (PEAK DWF)
DEPOSITION RATE BASED ON SOLID RATE
OF 0,56 LBS/CAPITA/DAY
DAILY SEWER FLUSHING
EMMA ALT, 1
SS
EMMA ALT, 1
BOD REMOVAL = 47.8%
3 2 4 6 8 10
OFFLINE STORAGE (MILLION GALLONS)
Figure 12. Effect of off-line storage on pollutant removal,
high deposition rate
58
-------
storage in 4000 feet of 5-foot diameter'pipe is about 0.6 million
gallons. This alternative would overflow untreated discharge more
often (16 times during May through November versus 0.6 times for the
EMMA Alternative 1), but the overall pollutants discharged to the
receiving waters would be about the same. The long-term averaged SS
removal rate is 51.1 percent as compared to 52.4 percent for Case 8
and the BOD removal rate, 47.3 percent as compared to 47.8 percent.
Case No. 10 has obvious cost advantage as compared to Case 8. While
Case No. 10 discharges more pollutant at the overflow, Case 8 dis-
charges about the same amount more at the treatment plant. The
greater pumping rate in Case 8 also reduces the effectiveness of the
treatment provided. This implies that for a given storage basin
capacity in a treatment plant, there is a pumping capacity and con-
duit size that provides the cost-effective pollution abatement. Five
alternatives are compared in Table 18. Cases B, C, 10 and D are com-
pared with Case 8. Case D would provide the greatest reduction in
pollutants discharged and Case 10 the least.
Case D compares to Case 8 except the pumping capacity is reduced
to 1/5 that of Case 8. It provides better pollution control and
would cost less than Case 8. It appears that the cost of EMMA Alter-
native 1 may be substantially reduced while providing the same over-
all CSO pollution abatement in the Boston area. To provide a more
reasonable comparison of the basic merits of storage/sewer flushing
alternatives, Case No. 10 of Table 16 is cost estimated in the next
chapter.
Case 11 supplements Case 10 with sewer flushing. The marginal
benefit of pollution control over Case No. 10 is about the same as
that of Case No. 9 over Case No. 8.
Data in Table 16 were developed for solids deposition rates in
sewers calculated using a per capita solid generation rate of 0.56
Ibs/day. This generation rate is based on field measurements. The
calculated solids generation rate, based on the assumed 150 gpcd flow
and a concentration of 250 mg/1 of SS, is 0.313 Ibs/capita/day.
Table 17 was prepared using the deposition rate calculated with a
generation rate 0.313 Ibs/capita/day. Figure 13 presents percent BOD
removal versus off-line storage for this lower solids generation
rate. The estimated BOD removal of EMMA Alternative 1 may be
achieved by providing either storage of 7.3 million gallon capacity,
or storage of 6.0 million gallons supplemented by daily flushing.
These alternatives are compared in Table 19, together with Case No.
10 which provides about the same-degree of pollution control with
less cost as the original EMMA Alternative 1.
All alternatives reduce the BOD discharged to the receiving
waters about the same amounts. Case 12 assumes the wet-weather flow
enters the storage basin by gravity and the storage is drained by
pumps with capacity to supplement the interceptor flow so that it
equals the peak dry-weather flow or 18.6 mgd. The required pump
capacity has been assumed as 15 mgd to allow for a minimum
59
-------
TABLE 18. OPTIMAL COMBINATION OF PUMPING AND STORAGE CAPACITIES
FOR POLLUTANT REMOVAL
Case
No.
8
B
Pumping Conduit
Capacity Diameter
(mgd) (ft)
497.4
125
C 115
10 100
D 100
10.5
7
6
5
10.5
Conduit
capacity
(Mgal)
; 2.60
1.15
0.85
0.60
2.60
% Removal
ss
52.4
54.6
53.4
51.1
55.9
BOD
47.8
49.0
48.4
47.3
50.0
interceptor flow of 3.6 mgd. To allow filling by gravity, the flow
line of the storage basin was assumed at least ten feet below the
ground level. This is in contrast to the EMMA Alternative 1 which
pumps into the basin at a maximum capacity of 497.4 mgd and drains
the basin by gravity. The storage envisioned for Case 12 could be
located near Overflow Nos. 49 and 50 (Figure 1). The large sewers
near the two outfalls could also be used by employing flow routing
techniques to store most of the frequently occurring rainfall-
runoff.
Case 13 is a daily sewer flushing/storage alternative with pump-
ing capacity equal to 15 mgd |as explained earlier. Case 10 is a less
costly version of the EMMA Alternative^!, and is included to provide
a more reasonable comparison with other alternatives. Cases 14 and
15 are equivalent to Cases 12 and 13, respectively, but assume the
lower solid generation rate previously discussed. Case 16 utilizies
storage in larger sewers near two outfalls. As much as 7.5 million
gallons are potentially available.
Column 7 of Table 19 presents the volume of combined sewage to
be chlorinated, excluding that disinfected at the main treatment
plant. The volume includes combined sewage which bypasses either the
storage basin or flows through the CSTP and should be chlorinated
before discharge. For Cases 12, 13, 14 and 15, chlorination facili-
ties can be integrated with the storage facility. For Cases 8 and
10, chlorination facilities are included at the CSTP. The capital
and operating cost of the chlorination facilities would be about the
same for all alternatives since the volume to be chlorinated is about
the same. Consequently, for comparison of costs, the cost of chlori-
nation can be dropped from further consideration.
Table 20 shows alternatives which reduce the amount of SS dis-
charged to receiving waters about equally. Case No. 17, which is a
60
-------
so
INTERCEPTING CAPACITY TO MAIN S.T.P.
= 18.6 MGD (PEAK DWF)
70
§
co
CO
H
1
1
60
50
40
30
eo
- 10
DEPOSITION .RATE BASED ON SOLID RATE OF
0,31 LBS/ CAPITA/ DAY
DAILY SEWER FLUSHING
EMMA ALT. 1
_S£ REMOVAL^ =52.0%
EMMA ALT. 1
BOD REMOVAL =47,5
4 6 8 10
OFFLINE STORAGE (MILLION GALLONS)
Figure 13. Effect of off-line storage on pollutant removal low
depositions rate
61
-------
TABLE 19. EQUIVALENT BOD ABATEMENT ALTERNATIVES
Daily
Case Sewer
No . Flushing
(1) (2)
12
13
8
10
14
15
16
*
**
No
Yes
No
No
No
Yes
No
Pumping Conduit
Storage* Capacity Size
(Mgal) CSTP (mgd) (ft)
(3) (4) (5) (6)
7.1 No
5.1 No,
7.7 Yes
5.7 Yes
7.3 No
6.0 No
7.5** No
Includes off-line storage,
Volume of large sewers near
TABLE
Daily
Case Sewer
No . Flushing
(1) (2)
17
18
8
10
19
20
21
No
Yes
No
No
No
Yes
No
15
15
497.4 10.5
100 5.0
15
15
Small
Chlorination
(Mgal)
(7)
321
358
341
340
319
337
340
conduit storage and storage in CSTP
outfall Nos. 50 and 49 (Figure 1)
20. EQUIVALENT SS ABATEMENT ALTERNATIVES
Off-line*
Storage
(Mgal) CSTP
(3) (4)
10.0 No
9.1 No
Yes
Yes
10.0 No
9.5 No
10.0** No
Pumping Conduit
Capacity Size
(mgd) (ft)
(5) (6)
15
15
497.4 10.5
100 5.0
15
15
15
Chlorination
(Mgal)***
(7)
295
302
341
340
295
298
295
* Not including conduit storage and 5.1 Mgal storage at the CSTP
** Includes 7.5 Mgal of pipe storage near two outfalls and 2.5 Mgal
of off-line storage
*** Excluding volume disinfected at the main STP
'. 62
-------
storage alternative (as is Case No. 12), requires 10 million gallons
(or greater storage than that required for the latter) to remove the
same amount of SS as Case No. 8. This is because the storage in the
CSTP of the EMMA Alternative 1 removes a greater proportion of SS
than BOD (Figure 6). With daily sewer flushing, storage required for
equivalent SS removal is reduced by 0.9 million gallons, compared to
two million gallons for equivalent BOD removal (Cases No. 12 and 13).
Cases No. 19 and 20, which used the deposition rate calculated with a
SS generation rate of 0.313 pounds per capita per day, also show less
benefit from daily sewer flushing on the storage required to achieve
equal SS removal efficiency. Case 21 is equivalent to Case 16 in
Table 19 except that 2.5 million gallons off-line storage is provided
to supplement in-pipe storage in large sewers near two outfalls so
that the total storage becomes about 10 million gallons.
63
-------
SECTION XI
COST ESTIMATE AND ALTERNATIVE COMPARISONS
The cost of the alternatives includes those for sewer flushing,
off-line storage, pumping facilities and conveying conduit connecting
the two outfalls. The capital, operating arid maintenance (O&M) costs
are considered.
When possible, the unit costs reported in the EMMA study were
used after updating the ENR Index from 2200 to the present value of
2800. The EMMA construction costs for conveyance systems are shown
in Table 21 and for pumping stations in Figure 14. THE EMMA plan
(Case 8) and the modified EMMA Plan (Case 10) also include submarine
outfalls, conveying conduit connecting the two outfalls, and CSTP,
none of which are required for the other alternatives identified in
Tables 19 and 20. The EMMA O&M costs for interceptor, pumping
station and treatment plant facilities were also discussed. The
estimates of individual components were not provided. The total
annual O&M cost for the 10 proposed decentralized facilities was
estimated as $3.9 million (based on ENR 2200). No breakdown of this
cost for each individual facility was given in the report.
This study determined O&M costs based on data found in EPA
published reports. These cost ^data are given in curves with respect
to sizes of facilities such as storage volume and pump capacity. MDC
has provided, after alternatives had been cost estimated, a breakdown
of O&M costs incurred in 1978 to operate the Cottage Farm Facility.
For comparison, the O&M cost for the Cottage Farm Facility was esti-
mated based on the EPA report data and was estimated at about one
half of the MDC reported cost incurred. The major difference in the
two cost bases is in the estimated versus actual manpower require-
ments. The O&M costs, if revised, would not change the conclusions
of this study.
COST OF STORAGE
Table 22 is a summary of off-line storage costs compiled as of
1978. Most of the costs shown include pumping, chlorination and
sludge removal facilities. Based on these costs, it would appear
that a cost $1.0 per gallon would be reasonable for the tank volume
above the flow line in Cases 12, 13, 14, 15 (Table 19), and 17, 18,
19 and 20 (Table 20). This volume was determined assuming a tank
freeboard of 10 feet. This freeboard was assumed to insure gravity
flow into the tanks. The unit cost for the tank volume ab.ove the
flow line is lower than that below because it includes only a wall
64
-------
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to 4i9**v.^ .-Of
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41 v 41 * 4c
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a.
lu
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f-i
uj
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•*oo^O'>oirtr-miA>»j-**** ^
5» irtir>Minmr-ii^-a>Oo<****
• .»«»».«••••.••«.«•«•«**** rHirsiir>(M(Ma>mo-H^-o-*O'-«r-»»** §
^4 t--*4i**$ a
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to
o
o
o
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4-1
en
a
o
o
a
o
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a
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a.
-------
TABLE 22. SUMMARY OF OFFLINE STORAGE COSTSa*
Location
Akron, Ohio [21]
Milwaukee,
Wisconsin [13]
Humboldt Avenue
Boston,
Massachusetts
Cottage Farm
Detention and
Chi ori nation.
Station [17]°
Charles River
Marginal Conduit
Project [19]
New York City,
New York [22, 23, 25]
Spring Creek
Auxiliary Water
Pollution
Control Plant
Storage
Sewer
Chippewa Falls,
Wisconsin [18]
Storage
Treatment
Chicago, Illinois
[2, 11, 26]
Tunnels and pumping
Reservoirs
Total storage
Treatment
Sandusky, Ohio [16]
Washington, D.C.
[2, 15]
Columbus, Ohio
[2, 3, 12]
Whittier Street
Cambridge,
Maryland [14]
Storage
capacity,
Mgal
1.1
3.9
1.3
1.2
12.39
13.00
Z5TT9
2.82
ot
2,998
41 315
44 313
44 313
0.36
0.20
3.75
0.25
Drainage
area,
acres
188.5
570
15 600
3 000
3 260
3 260
90
M
240 000
240 000
240 000
14.86
30.0
29 250C
.20
Capital cost,
$
455 700
1 774 000
6 495 000
9 488 000
11 936 000
11 936 000
744 000
189 000
933 000
870 000 000
682 000 000
1 552 000 000
1 001 000 000
2 553 000 000
520 000
883 000
6 144 000
320 000
Storage
cost,
$/gal
0.41
0.45
5.00
7.91
0.96
Q74T
0.26
0~72t
0.29
0.02
0.04
ot
1.44
4.41
1.64
1 28
Cost per
acre,
$/acre
2 420
3 110
416
3 160
3 660
3~660
8 270
2 100
10 370
3 630
2 840
6 470
4 170
10 640
35 000
29 430
210
16 000
Annual operation
and maintenance
cost, $/yr
2 900
51 100
80 000
97 600
100 200
100 200
2 700
8 000
10 700
8 700 000
6 200
3 340
14 400
a. ENR 2000.
b. Estimated values; facilities under design and construction.
c. Estimated area.
*Reproduced from EPA Report 600/8-77-017
$/acre x 2.47 = $/ha
$/gal x 0.264 = $/L
Mgal x 3785 = m3
67
-------
'extension and does not include 'foundation slab or appurtenant works
such as pumping and sludge removal facilities. This freeboard was
assumed to insure gravity flow into the tanks. For the total storage
cost, cost of effective tank storage obtained from Figure 15 was
added to the above cost. Figure 15 was derived from the EMMA's
estimates shown in Table 2. This estimated cost may be quite high
and, if anything, should favor the EMMA Alternative.
COST OF SEWER FLUSHING
Verifiable costs for sewer flushing facilities, based on long-
term operating experience, are not available. Pisano, et al/- ' , has
presented cost estimates based on a demonstration program. The cost's
so developed appear low for a permanent arrangement, particularly
where larger sewers are involved. The preferred type of installation
would probably include a new manhole structure to house a
hydraulic-pneumatic control gate and a sidewalk vault to house
duplicate air compressors, a compressed air tank, electrical service
for heating and operation, a control system to permit operation of
the gate at stated intervals, and appropriate heating and ventilating
equipment. Similar installations in Cleveland to provide storage for
combined sewage in 1974 were estimated to cost about $100,000. Their
current cost might be $125,000. The sewers in which automatic flush-
ing equipment is proposed would generally be smaller than those used
for storage and as shown in Table 23, the construction cost per in-
stallation appears to be about $52,000 for small sewers (up to 27
inches) and increases to $63,000 for larger sewers (30 inches to 48
inches). Annual maintenance supplies and power are assumed at three
percent of the equipment costs. A three-man crew should assure that
the equipment is fully operational and they are provided with a truck
fully equipped with safety equipment and maintenance tools and sup-
plies. The estimated present worth, including O&M costs (20 years at
6-5/8 percent) is between $90;,000 and $113,000. For purposes of
estimating, a present worth value of $100,000 per module will be
used.
OPERATION AND MAINTENANCE COST
Data reported in the literature has been used to estimate O&M
costs. The EPA Report entitled "Cost Estimating Manual - Combined
Sewer Overflow Storage and Treatment"^**) summarized the data in
curves which are reproduced here.
Figure 16 shows the labor required to clean the storage reser-
voirs after a storm event using a spray system. The labor require-
ment depends upon how often the storage is used, as does the energy
consumption shown in Figure 18. Miscellaneous supply costs shown in
Figure 17 are arbitrarily established. The costs include repair
parts, truck time, tools, insurance, janitorial supplies, gas, oil
and other miscellaneous consumable products. Figures 19, 20 and 21,
respectively, show the labor requirements, miscellaneous supply costs
and energy requirements of a pumping facility. Labor requirements
68
-------
01
60
B)
5-1
O
4J
CO
M-l
O
J-l
CO
8
cd
4.1
•rl
oo
•H
1SOO IVlldVO
69
-------
TABLE 23. ESTIMATED COST OF AUTOMATIC SEWER FLUSHING (ENR 2800)
Sewer Size
Capital Cost
New Manhole
Hydraulic Slide Gate
Cnamber
2 Air Compressors
Electrical Control System
Sump Pump
Contingencies
Total
Small to 27"
$ 3,000
12,000
10,000
4,000
10,000
3,000
10,000
$ 52,000
30" to 48"
$ 6,000
20,000
10,000
4,000
10,000
3,000
10,000
$ 63,000
Annual O&M Cost
Maintenance $ 1,500 $ 1,900
3 men @ 18,000 yr/module 1,900 1,9.00
Truck & Equipment - $25,000/28 900 900
Total/Module $ 4,300 $ 4,700
Present Worth/Module $ 46,000 $ 50,000
(n=20 Year, i=6-5/8%)
Total Present Worth/Module
$ 9ti,000
$113,000
70
-------
10,000
o
<
1,000
100
•JUMBER OF STORAGE
EVENTS PER YEAR
3 4 56789 2 3 4 5 6 789
10 JOO
VOLUME - MILLION GALLONS
2 3456 789
1,000
Figure 16. Storage reservoir man-hour requirements
10,000
<
~t
6
5
4
2
2
1.001
Q
7
6
5
4
3
2
.-..,***'
r^*
-"
.^
^•**
^^-
2 34 56789 2 34 56789 2
1 I0 100
VOLUME - MILLION GALLONS
3
4
5 6 789
1.000
Figure 17. Storage reservoirs - miscellaneous supply cost
(ENR 2200)
71
-------
|
o
Of
iu
100
0
6
5
4
3
2
10
9
81
6
S
4
3
2
1
S
6
4
3
2
0.1
NUMBER
OF OPE
PER YE/
1C
so
4Q
2C
OFD
UTIC
iR .
^^
^
AYS
IN,
X
-------
1
I
2 3 456789
1,000
FIRM PUMPING CAPACITY - MILLION GALLONS PER DAY
Figure 19. Raw wastewater pumping - man-hour requirements
i
6
5
4
2
S 2
_j
_i
o
o
' 1,000
1- 9
o »
8 7
< ?
z
z 4
3
2
100
X
x
'
/
^ _
/
/
/
/
~ ~ -yr—
'
Z 34S6789 2 3456789 2
3 4
5 6 789
100 1,000
FIRM PUMPING CAPACITY - MILLION GALLONS PER DAY
Figure 20. Raw wastewater pumping - miscellaneous supply cost
(ENR 2200)
73
-------
ce
2
i
B
1,000
g
8
7
6
5
4
3
2
100
9
7
6
5
4
3
2
10
i
6
5
4
3
2
1
1
80^
60-^
XO-*
20-5
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__,
/
NUMBER 0
OF OPERA
PER YEAR
T~
/
/
S
1
'-tit
not
/
^
/
ors
i
/
^ /
/ /
/
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y
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' / /
' / /
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2 34 56789 234 56789 2 34 56783
10 100 1,000
FIRM PUMPIKG CAPACITY -MILLION GALLONS PER DAY
Figure 21. Raw wastewater pumping - energy requirements
74
-------
are assumed to be proportional to the operating time of a pump
station, with a constant requirement of eight man-hours to wash the
wet well after each storm event and 24 hours per year to check and
test equipment and controls between storm events. In this study, the
cost of energy is assumed to be 4 cents per KWH.
COST OF ALTERNATIVES
Table 24 presents a cost summary of alternatives with the same
amount of BOD removal as the EMMA Alternative 1. Costs were calcu-
lated based on the physical description of the alternatives in Table
19 and unit cost of conduit (Table 21), pumping facilities (Figure
14), storage (Figure 15) and sewer flushing (Table 23). All costs
are expressed in terms of present ENR Index of 2800 applicable to
Boston.
It is clear that the EMMA Alternative 1 is too expensive. Its
modified version, with conveyance conduit and pump capacity reduced,
by 80 percent is, however, quite comparable to other off-line storage
and sewer flushing alternatives. Case 13, which is a daily flushing
and off-line storage alternative, has the lowest total cost if
storage available in the system is neglected. Most of the costs of
Cases 12, 13, 14 and 15 are attributable to the storage cost. As
mentioned earlier, these costs are perhaps too high and, therefore,
the more detailed development of the costs may indicate further cost
advantage compared to the EMMA Alternatives. It should be noted
that the BOD removal efficiency of Case 10 is slightly lower than the
other alternatives. Consequently, the actual cost of Case 10 would
be somewhat higher if it is to remove the same amount of BOD dis-
charged to receiving water as the other alternatives. Cases B and C
of Table 18 would prevent more pollutants from discharging to receiv-
ing water than Case 10 but they would cost more.
The difference in costs between Case 14 and Case 15 is not as
much as that between Case 12 and Case 13 since the merit of sewer
flushing is reduced as the strength of wastes entering the sewers is
reduced.
A storage capacity of about 7 million gallons, or a uniform run-
off depth over the entire drainage basin equal to 0.16 inches, would
result in the same BOD removal efficiency as the EMMA Alternative 1.
A properly designed sewer system would have at least this amount of
storage in trunk sewers. Such storage in trunk sewers can be uti-
lized by installing flow regulating devices to effectively provide
combined sewage pollution control. Such pipe storage is available in
the study area.
It is estimated that about 3.5 million gallons or more of pipe
storage is available in sewers and outfall pipes near the Outfall No.
50 (Figure 1) and about 4.0 million gallons or more near Outfall No.
49. It appears that these sewers,.some of which are as large as 168-
in x 138-in (horseshoe) and 144-in x 144-in (horseshoe), are above
75
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the elevation of the Dorchester Interceptor, therefore, a.pumping
facility may not be required. If required, it would be of small
capacity, due to limitations imposed for discharge to the Dorchester
Interceptor and of low head. The cost of using this pipe storage
includes flow regulating devices such as an inflatable dam and rou-
tine flushing of sewers to maintain effective storage capacity. The
cost could be ten percent of that of the other alternatives shown in
Table 24. This pipe storage alternative of about 7.5 million gallons
is designated as Case 16. The potential of this pipe storage should
be explored. Such storage could replace all or most of storage re-
quired in Cases 12, 13, 14 and 15. The costs of these alternatives
would then be far lower than Case 10.
Table 25 presents a cost summary of alternatives with the same
amount of SS removal as the EMMA Alternative 1. The EMMA Alternative
1 is still far too expensive while its reduced scale (Case 10) be-
comes the lowest cost alternative when compared with off-line storage
alternatives. However, as explained earlier, the cost estimates of
storage for Cases 17, 18, 19 and 20 are perhaps much top
conservative. Because sewer flushing is less efficient in removing
SS than BOD, the advantage of daily sewer flushing/storage
alternatives over only storage alternatives vanishes.
Case 21 is the alternative which supplements 7.5 million gallons
of storage available in large sewers near the two outfalls with about
2,5 million gallons of off-line storage. The total storage capacity
of this alternative is 10 million gallons which is comparable to Case
17. It should result in about the same amount of SS being discharged
to receiving waters as other alternatives in Table 25. The cost of
Case 21 is about ha.lf of the second lowest cost alternative, Case 10.
The economy of Cases 16 and 21 indicates that there are definite cost
incentives to explore pipe storage potential in the study area as
well as other combined sewer areas in Boston.
OPTIMAL NUMBER OF FLUSHING STATIONS
The flushing alternatives described in the comparative analysis
assumed 28 flushing stations. These stations would affect those
sewer segments in which solids deposits equal or exceed 3.0 Ibs/day.
As indicated in Figure 10, 28 flushing stations are probably the
maximum .number that should be considered for the Dorchester area. To
estimate the cost-effective number of flushing stations, alternatives
using 5, 12 and 104 stations were developed. The 104 flushing sta^-
tion alternative would affect all of the combined sewer deposits and
43 percent of sanitary sewer deposits. The 12 flushing station al->
ternative would affect 51,2 percent and 22.1 percent of the solids
deposits in combined and sanitary sewers respectively while the 5
flushing station alternative would affect 30 and 17.4 percent. Table
26 compares costs of 8 equivalent SS and BOD abatement alternatives
with daily sewer flushing assuming high sewage strength. The amounts
of off-line storage required to supplement daily sewer flushing for
77
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equal amounts of SS and BOD removal as the EMMA Alternative 1 are
shown.
Increasing the number of flushing stations from 28 to 104 re-
sults in small savings in storage costs while sewer flushing costs
increase substantially using either BOD or SS removals as a criteria*
If 12 stations are used instead of 28, the total cost is reduced by
about 4 percent if SS is used as the criteria and negligibly if BOD
is used. The 5 station alternative appears as the least cost under
either criteria. Should the cost of a sewer flushing station be
$50,000 instead of $100,000 per station, the break-even number of
flushing stations might be between 5 and 12.
SOLIDS HANDLING CONSIDERATION
Assuming a high strength ^sewage, the dry-Weather flow in the
study area would contain about 31,230 Ibs/day of solids, of which
about 2000 Ibs. would settle in collection sewers and the remaining
29,230 Ibs. would reach the Deer Island treatment facilities* If
sewers are flushed every dry day at 28 flushing stations, about 1800
Ibs., on the average, would be; resuspended and eventually reach the
treatment plant, assuming that the Dorchester Interceptor has ade-^
quate transporting capacity. Consequently, on a dry day, the solids
transported to the treatment plant from this tributary area would be
increased by about 6 percent. ;The increase in the plant O&M cost as
the result of this 6 percent increase of solids loading should be
small.
The interceptor carrying capacity was assumed as equal to the
peak dry-weather flow of 18.6 mgd. Further, all pollutants reaching
the interceptor were assumed to be transported to the Deer Island
treatment plant. Field surveys conducted during the PRI study indi-
cated substantial sea water intrusion into the Dorchester Interceptor
as well as sediment deposits for its entire length. The deposits
blocked as much as 30 percent of the flow area. The MDC has recently
cleaned the Dorchester Interceptor. Unless causes of sedimentation
in the Interceptor are found and corrective measures taken, it may be
blocked again. Correction of problems in the Interceptor is beyond
the scope of this study.
80
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SECTION XII
FLUSHING - POSSIBLE LIMITATIONS AND ADVANTAGES
Since sewer flushing could be a valuable adjunct to reduce CSO
pollution, a theoretical investigation was made to determine the re-
lationship of wall sheaf stress, flow, pipe size, arid pipe slope.
The analysis assumed steady flow arid that the Manning formula ap-
plied. The errors introduced by these assumptions could not be eval-
uated within the scope of this work. Figures 22, 23 and 24 present
the results of analyses for sewers rangirig in diameter from 12 inches
to 7 feet and slopes from 0*0005 to 0.01 for flows of 0.5, 1.0 and
1.5 cfs, respectively. Pisano, et al.^4' have reported success in
flushing sewers 12 to 15 inches in diameter by maintaining flows of
0.5 cfs for about two minutes to create a Wave of celerity. This
would indicate a shear stress equal to 0.04 pounds per square foot
(psf) could be sufficient for effective flushing. For flushing of
lighter organic particles a shear stress less than 0.04 psf may be
satisfactory. This relatively small flow might not be successful in
flushing larger sized pipe, unless their slope equalled 0.005 or
more. At a flushing flow of 1.0 cfs^ it appears that all size pipes
up to 7 feet diameter with a slope of 0.003 might be flushed success-
fully. Further, at a flushing flow of 1.5 cfs, all pipe sizes up to
7 feet diameter and a slope of 0.002 or more, appear to be suitable
candidates for flushing. For a given slope and flow, the shear
stress is relatively constant. Hence, relatively large pipes may be
successfully flushed With relatively small quantities of water. This,
if proven, could offer significant aid in cleaning sewers of deposit
after wet weather flows have been stored to permit routing combined
sewage to treatment. This potential, plus possible savings in CSO
pollution abatement facilities, urge strongly the continuation of
investigations into the effectiveness of sewer flushing in large
sewers.
81
-------
FLOW = 0.5 CFS
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345
PIPE DIAMETER (FEET)
Figure 22. Wall shear stress in circular pipes, flow =0.5 cfs
82
-------
FLOW= 1.0 CFS
.005
3 4 5
PIPE DIAMETER (FEET)
Figure 23. Wall shear stress in circular pipes, flow = 1.0 cfs
83
-------
FLOW = 1,5 CFS
.005
34 5
PIPE DIAMETER (FEET)
Figure 24. Wall shear stress in circular pipes, flow = 1.5 cfs
84
-------
SECTION XIII
REFERENCES
1.
2.
3.
4.
5.
6.
8.
9.
10.
Metcalf & Eddy, Itic*, "Wastewater Engineering and Management
Plan for Boston Harbor - Eastern Massachusetts Metropolitan Area
- EMMA study," Main Report and Technical Data Volumes 1 through
16, prepared for the Metropolitan District Commission, 1976.
FMC Corporation, "A Flushing System for Combined Sewer Clean-
ing," USEPA Report No. 11020DN003/72, NTIS PB 210 858, 1972*
Pisano, W.C*^ and C.S* QueiroZj "Procedures for Estimating Dry
Weather Pollutant Deposition in Sewerage Systems," USEPA Report
No* EPA-600/2^77-120s NTIS PB 270 695» July 1977.
Pisano, W.C., et al*, "Dry-Weather Deposition and Flushing for
Combined Sewer Overflow Pollution Control", USEPA Report No.
EPA-600/2-79-133, November 1979.
Process Research, Inc., "A Study of Pollution Control Alterna-
tives for Dorchester Bay" prepared for the metropolitan District
Commission, Boston, Massachusetts •, December 1975.
Camp Dresser & McKee, tnc*j "Water Quality Improvement of Tenean
and Malibu Beachesj" prepared for the Metropolitan District Com-
mission, Boston, Massachusetts, November 1972.
. 'I
Brandstetter, A., "Assessment of Mathematical Models for Storm
and Combined Sewer Management," USEPA Report No. EPA-600/2-*76i-
175 a, NTIS PB 259 597, August 1976*
Hydrologic Engineering Center, U.S. Army Corps of Engineers *
"Storage, Treatment, Overflow, Runoff Model (STORM): Generalized
Computer Program 723-s8-L7520," July 1976.
Huber, W.C., et al., "Interim Documentation, November 1977 Re^
lease of EPA SWMM^" USEPA Report, Project No. R-802411, to be
published.
Clinton Bogert Associates, ongoing CSO Facilities Plan study for
the City of Elizabeth, New Jersey, EPA Grant C-34-^447.
85
-------
11. Fair, G.M., J.C. Geyer and D.A. Okun, Water and Wastewater En-
gineering , John Wiley and Sons, Inc., 1968.
12. Smith, G.F., "Adaptation of the EPA Storm Water Management Model
for Use in Preliminary Planning for Control of Urban Runoff,"
Master Thesis, University of Florida, 1975.
13. Heaney, N.P., et al., "Stormwater Management Model: Level I -
Preliminary Screening Procedures," USEPA Report No. EPA-600/2-
76-275, NTIS PB 259 916, October 1976.
14. Heaney, J.P., et al., "Nationwide Evaluation of Combined Sewer
Overflows and Urban Stormwater Discharges: Volume II, Cost
Assessment and Impacts," USEPA Report No. EPA-600/2-77-0646,
NTIS PB 266 005, 1977.
15. U.S. Environmental Protection Agency, "Water Quality Studies,"
Water Program Operations Training Program, NTIS-PB 237 586, May
1974. \
16. Harmon, W.G., "Forecasting Sewage System Discharge at Toledo,"
Engineering News-Record, 1918.
17. Lager, J.A., et al., "Urban Stormwater Management and Techno-
logy: Update and Users' Guide," USEPA Report No. EPA-600/8-77-
014, NTIS PB 275 654 September 1977.
18. Benjes, H., Jr., "Cost Estimating Manual-Combined Sewer Overflow
Storage and Treatment," USEPA Report No. EPA-600/2-76-286, NTIS
PB 266 359, December 1976.
19. Watt, T.R. et al., "Sewerage System Monitoring and Remote Con-
trol," USEPA Report No. EPA-670/2-75-020, NTIS PB 242 126, May
1975.
20. Kaufman, H.L. and Fu-hsiung Lai, "Conventional and Advanced Sew-
er Design Concepts for Dual Purpose Flood and Pollution Control
- A Preliminary Case Study, Elizabeth, New Jersey" USEPA Report
No. EPA-600/2-78-090, NTIS PB 285 663, May 1978.
21. Letter dated December 7, 1978 from Richard A. Moore of M&E to
Dennis F. Lai of Clinton Bogert Associates.
22. American Society of Civil 'Engineers, "Wastewater Treatment Plant
Design", Manuals and Reports on Engineering Practice -No. 36,
1977.
86
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-80-118
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
REVIEW OF ALTERNATIVES FOR EVALUATION OF SEWER
FLUSHING
Dorchester Area - Boston
5. REPORT DATE
August 1980 (Issuing Date)
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Herbert L. Kaufman and Fu-hsiung Lai
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Clinton Bogert Associates
2125 Center Avenue
Fort Lee, New Jersey 07024
10. PROGRAM ELEMENT NO.
A35B1C
11. CONTRACT
68-01-4617
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency, Region I
Boston, Massachusetts 02203
Municipal Environmental Research Laboratory—Cin.
Cii cinnati, Ohio 45268
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
OH
EPA/600/14
15. SUPPLEMENTARY NOTES
Project Officer: Daniel K.
Technical Advisor: Richard
O'Brien, Phone: (617) 223-7213
P. Traver, Phone: (201) 321-6677
16. ABSTRACT
i i \r-t\j i
Alternatives employing sewer flushing were developed for the Dorchester area
of Boston and their cost effectiveness compared with the decentralized combined
sewer overflow (CSO) storage/treatment and disinfection facilities proposed as
Eastern Massachusetts Metropolitan Area (EMMA) Alternative 1. Thirty-three
alternatives were evaluated. These alternatives included sewer flushing, off-
line storage, in-pipe storage, storage/treatment facilities, and a combination
of the above. A study objective was to determine if additional expenditures to
develop sewer flushing techniques and devices were indeed appropriate. The
feasibility and efficiency of sewer flushing was based on literature review
including a report containing sewer flushing data for four small sewer segments
in the Dorchester area. Continuous simulation runs using 16 years (1960-1975)
of hourly rainfall data from May through November were made to determine the
level of CSO pollution control obtained. The STORM program was modified to
include continuous simulation of solids and organic material deposited in sewers
during dry days, the removal of those deposits by dry day sewer flushing and
wet-weather flow, and the storage and treatment effects of a CSO storage/treat-
ment facility on the wet-weather discharge.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
c. COS AT I Field/Group
*Rainfall, *Runoff, *Storm sewers, Com-
bined sewers, *Sanitary sewers, *0verflows,
*Sewage treatment, *Cost effectiveness,
*Mathematical models, Computer programs.
Combined sewer overflows,
Combined sewage treat-
ment plant, Offline
storage, In-pipe storage,
Sewer flushing.
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
99
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (Rev. 4-77)
87
U.S. GOVERNMENT PRINTING OFFICE: 1980-657-165/0138
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