SEATTLE'S APPROACH TO EVALUATING COSTS AND
BENEFITS OF -COMBINED SEWKR OVERFLOW CONTROL PER PGM-
Preparec For
U.S. EPA Technology Transfer Program
Seminar on Combined Sewer Overflow Assessment
and Control Procedures
May 18, 1978
Jack Warburton, P.E.
Brown and Caldwell, Consulting Engineers
100 West Harrison Street, Seattle, WA 98119
-------
-1-
The City of Seattle is located along the southeasterly shorelines
of Puget Sound about 90 miles inland from the Pacific Ocean, in the
shadow of the Olympic mountains. The city is bounded on the west by
the marine waters of Puget Sound and the east by the fresh waters of
Lake Washington. Additional water bodies within the city include
Lake Union, a canal.that connects Lake Washington with Puget Sound,
the Duwamish River and numerous smaller creeks and seasonal water
courses. Shorelines adjacent to these water bodies have provided
excellent sites for homes and parks for recreational pursuits and
for industrial and commercial development.
Sewerage Service
Initial sewerage provisions for the area were construction of a
combined sewer system with discharge to the nearest water course.
Subsequently, interceptors were built to transfer raw sewage discharge
from the inland fresh waters to the marine waters of Puget Sound. In
1958, to address the serious water pollution problems of the area, a
regional agency, the Municipality of Metropolitan Seattle (Metro), was
created with responsibilities for interception, wastewater treatment
and disposal for the entire Seattle metropolitan area. Responsibilities
for local collection remained the responsibility of local agencies.
By 1970, Metro's construction program had eliminated discharge of un-
treated dry weather flows. The City of Seattle combined sewered areas
are served by two Metro primary treatment plants: West Point and Alki,
both plants discharge to Puget Sound, see Figure 1. During storms
combined sewage and.storm flows surcharged both the local city and
Metro interceptor systems, resulting in overflows ,from both Seattle
and Metro sewers to all the city's major water bodies.
Substantial "progress to date has been made to reduce combined sewer
overflows. The City of Seattle, through a partial sewer separation
program, has eliminated problems of localized backups and reduced the
incidence of overflows. Metro has maximized the existing in-line storage
potential of the existing large diameter sewers, inherited from the City
of Seattle, by construction oi: regulator stations and further optimized
the transport system by installation of a centralized computer control
network designated the CATAD system (Computer Augmented Treatment and
Disposal).
The impact of Seattle's partial sewer separation and Metro's in-
line storage/CATAD progeam has cut annual overflow volumes by approx-
imately half. Currently, out of the 50,000 acre tributary to West Point
15,000 acres are combined, 19,000 acres partially separated and 16,000
sanitary only. The tributary area of the Alki plant is 4,000 acres,
all of which are partially separated.
Combined Sewer Overflows - Current Status
Statistically speaking, there are 107 overflow locations in the
Seattle area, of which 30 are listed under Metro NPDES permits and the
remainder are the responsibility of the City of Seattle. Frequency of
-------
-------
-3-
overflow at present, averages about 40 occurrences per year of which
approximately six take place in the summer recreation season. Geo-
graphically, the overflows are scattered all over the area, with dis-
charges of varying volume to all the major water bodies around the
City"of Seattle, see Figure 2. Annual volume of overflows exceeds
1 billion gallons.
Public Law 9 2-50 0
To meet Section 201 of the law Metro is required to prepare a
Facility Plan, addressing upgrading its existing primary treatment
plants and a program of combined sewer overflow control. The regula-
tory aaencies have not prescribed levels of control for combined sewer
overflows. Guidance is provided in PGM-61 (PRM 75-34) namely:
1. Alternative combined sewer overflow controls must; be con-
sidered and evaluated in terms of costs and benefits over
a range of levels of pollution control.
2. Marainal costs and benefits must be determined and displayed
such that incremental dollar costs can be compared with
quantified pollutant reductions and subsequent increased
beneficial ui-es.
This pragmatic approach is in contrast to treatment plant evaluation
where the required degree of treatment is given leaving the planner
the task of optimizing facilities for a specific level of treatment.
Even thouqh operated by separate agencies, the interdependence of
the Metro and city collection system elements dictated that the devel-
opment of combined sewer overflow controls consider the collection
system as one coinmon unit.
CSO Control Planning Approach
To address the requirements of an incremental cost/incremental
benefit analysis consistent with PG-61 ir-olves both quantifiable
and judgmental factors. Tm<-- ^ost of cont.cOs and subsequent reduction
in discharge of pollutants can be quantified, however, to relate
the reduction in pollutants to benefits, even with substantial moni-
toring data, takes us into the realm of judgment. The key to address-
ing the funding limitations of PG-61 is development of sufficient data,
circumstantial and otherwise, to present a reasonable case to the regu-
latory agencies.
For Seattle's case the following distinct analysis steps were
taken:
Development of an analysis tool to allow optimization of
of control.
x* i4 «- — „
success levels of control.
2. Development of an analysis tool to quantify overflow reduc-
tions for specific control levels.
3. Determine combined sewer overflow quality parameters.
-------
r. , i r - „ r 1
V * ^ ' ¦ .V *•
y
V'" " ' , '
fv
11/2 0 I
scale in miles
; A
;VP''
¦ •, I"
' /S,v
M '"..V'.' \ ,'• . 10,' ••
' " r/r>. " .M-
*¦•'/• v: \:r-f--?¦•, i:?r '; ' \
, ^ J '•- . . . .-.-. J1
«/r ,
( MILLION GALLONS PFR YEAR)
• <
Q
o
o
i - 10
10 - 40
40-100
> 100
... fy V ¦ "u':; * -v - ¦ i
~.-v.
4vfV^;v
J.: "--¦< s-iWrtSi. .;&!».#,. .<"
*'C ? , . / * • I '• ''V' • 'tf^ ^ •|;1;-,rt-^ v ^ " . !• • "i\ V-" .t
..K Ai' .'l-, » vv-. '.'.iDir ;ri'. sfc .' . ' . J- i-. •.¦¦ j
• ' i***i vV *• '"' ¦*' ->A '•> ir ~••• • ¦ *V*U-«»" • •• •'•.*¦*;» • \.i V ''»
;#?$; '-X
-~-'X. ;n4v J: ! r.i.-;.rr::i;'r.,»r'. ! ¦ , 1 Is^.r' f
M i; v—^
•
r"t^Ss
*¦- rfe'
,nV .¦
r!SS!i|:u ¦ Vv&BS?;!teSf
: • \irn^hp$
. :. ...
„,' .-¦*•,*:- • • i • i-i- ^ f
4Np'i3L,J^i"C-
,JA: > > V'' (^Xa ¦>
4 +-!:; I 'S'ift cSJS;
/c"~ f- j- * \ tr ';
f-r* v'f'r,-'-' «Hvi-y. •>•.' •' *..' '-V'1 -tl. vh vv..v
••-: -A
"V "
v; ¦¦j.Tjrv.u.Hf-;
Vr|'r>,- " -
-------
-5-
4. Measure impacts on receiving waters.
5. Identify beneficial uses/sensitivity of cso's on receiving waters.
6. Relate CSO impacts to beneficial uses.
Collection System Analysis
Both West Point and the Alki tributary collection systems arP
extensive and extremely hydraulically complex. To enable analvqis
of variable control levels, to optimize alternative control facili
ties and differing arcal emphasis, dictated the use of computer
mathematical modeling. omputer based
Several hundred storm and sewer networks simulation models are
available in the current literature, however, none could handTp tho
complexities inherent in the West Point system (CATAD, reaulator
stations, inverted siphons for example) without extensive"modification*
Those models that had the basic sophistication to handle the flood
routing aspects were too detailed and thus too time consuraina for-
Planning effort. Thus at the start of the planning it was decided tn
custom build a model that would not only address the needs of the stnrtv
out would also, with minimum refinement, be suitable for subsequent
detailed design and be a useful tool for future Metro planning
The adopted model consists of two parts, a "runoff model" based
°n the unit hydrograph technique which provided the input to the
transport model which simulates the flow of the runoff model throuah
the system. 9
Recognizing the spatial rainfall patterns, design storms required
°r input to the runoff model were developed for five regions within
Alki/West Point areas utilizing rainfall data from 18 gaging sta-
tions. Rainfall intensity-duration relationships were developed and
trom these, storm hyetographs.
The West Point system was divided into 10 subareas which were then
Droken down into a further 114 subcatchments. For each of these 114
subcatchments, runoff, and thus inflow to the Metro system, based on
"e unit hydrograph, method developed from storm hyetograph input and
®ubcatchment characteristics were developed. The Metro system itself
was broken into 190 elements representing pipes, pumping stations,
"verted siphons, overflow structures and regulator stations. The
Jjodel was calibrated by comparing its output with metered discharge
jjydrographs from subcatchments or groups of linked subcatchments where
°th rainfall and discharge histories are known. Special use was
ade of CATAD-ctored data, plus specific flow monitoring.
Once the model was built and calibrated we had the tool to enable
Yaluation of alternatives to optimize control alternatives, and deter-
ge CSO volume reductions for specific control levels.
-------
-6-
CSO control alternatives evaluated included:
1. Full separation in partially separated areas.
2. Full and -partial separation in combined areas.
3. Roof-top storage.
4. In-line storage of existing system (CATAD).
5. In-line storage with new pipe/tunnels.
6. Off-line storage.
7. Localized storage/transfer and centralized storage.
8. Local off-shore discharge.
9. Local treatment.
10. Transfer and centralized treatment.
Controls were optimized for the 114 sub-basins considering overall
cost economics based on controls from existing of 40 per year to 10
per year, 1 per year, and 1 in 10 years.
In basins tributary to the fresh inland waters of Lake Washington
and the ship canal downstream to the outlet of Lake Union, control
alternatives were limited to storage, transport, and source control.
In other drainage basins, additional alternatives of localized treat-
ment or upgraded outfalls were evaluated. Once the range of alterna-
tives was established, the analysis was conducted for each drainage
basin by detailing the size of physical facilities required and
establishing their cost.
Systemwide cost optimization was accomplished by matching flows
at drainage boundaries and apportioning the cost for downstream facil-
ities based on their proportion of total facility required. Selected
facilities were based on flexibility for areal emphasis and stageable
controls within specific areas.
The facility arrangements which yielded most economical control
at three selected storm frequency control levels, 10 per year, 1 per
year, and 1 in 10 years were identified for both Metro and City of
Seattle control elements.
In general, localized and/or centralized holding was found to be
the most cost effective in the remote areas from the treatment plants.
A combination of holding, transport and increased treatment capacity
was found to be the most economical for controlling overflows closer
to the West Point and Alki plants.
-------
-7-
From the above tho least cost for controlling any combination
of overflows and incremental control level and corresponding
reduction in overflows can be determined.
CSO Characteristics
Of the 110 overflow points, five were selected from representa-
tive runoff areas and subjected to detailed analysis. Investigation
included analysis of the overflows, dye studies, coliforics dye
off, and benthic studies. Sampling at each point through at least
five storms was conducted, grab samples at each phase of the storm
and overflow composits were analyzed. A large range of values were
obtained from the analyses, however, some general conclusions could
be made; the tributary land use did not significantly affect con-
ventional pollution characteristics, the phenomena of the first
flush was not substantiated, the season was not significant, and
the size of the storm was not significant. Thus the use of average
pollutant concentrations could be used for determining pollutant
loadings, see Table 1.
Dye studies indicated that the areal effect of overflows was
up to one-half mile for specific wind and localized current con-
ditions. Coliform levels exceeded local public health water contact
quality standards for up to 3 days. Benthic analysis at the over-
flow indicated significant "dead areas" overlain by sludge deposits.
Identify Beneficial Uses/Sensitivity of CSO on Receiving Water
The impact of overflows will differ depending on the sensi-
tivity of the receiving water and their attendant uses. A geo-
graphical inventory of x^ater use areas, aquatic life habitats and
ranking of relative risk to pollutant loadings based on physical
characteristics (e.g., w.ii circul.. tio: dilution factors, flush-
ing, etc.) was performed to assist in ranking overall sensitivity
of the various water bodies to degradation from pollution loads.
Individual environmental risk maps, depicting recreational use,
biotic life zones, and water quality sensitivity were summed utilizing
the overlay technique as developed by Mcliarg, from which three levels
of risk were identified. This prioritization does not constitute a
cost-effective analysis for abatement techniques, but simply groups
the overflows relative to the degree of environmental risk they
impact. It is the first step in grouping overflows with specific
beneficial uses. The more localized the analysis, the easier it
will bo to identify the relationships between beneficial use and
CSO impact.
The next step was evaluation of comrainality of collection sub-
system, CSO impact overlaps,water body physical characteristics and
dominant beneficial uses. This step resulted in defining nine
separate areas, that were prioritized utilizing the initial risk
analysis concept.
-------
-8-
Table 1. CSO Pollutant Levels
CSO
POLLUTANT
LEVELS
PARAMETER •
units
MINIMUM
MAXIMUM
AVERAGE
BOD
nig/I
15
82
60
COD
mg/l
100
330
236
ss
rng/l
141
296
217
nh4-n
nig/i
0.5
1.5
0.9
p
mg/l
1.2
1.7
1.4
Cu
mg/l
0.1
0.3
0.2
Pb
mgfl
0.5
0.9
0.6
Hg
mg/l
o; oi
0.01
0.01
Cr
rng/l
0.02
0.20
- 0.10
Cd
mg/l
0.01
0.02
0.01
Zn
mg/l
0.2
0.5
0.4
TOTAL
COLIFORMS
J/100 ML
CO
X
1—»
7000 x 103
200 x 133
FECAL
COLI FORMS
f/100 .ML
3.0 xlO3
780 x 103
250 x 103
Table 2. Beneficial Uscs/CSO Impact
CSO USE IMPACT
USE
CSO IMPACT
RESIDENTIAL
COL 1 FOIiMS /FLOATABLE S
SWIMMING
COLIFOIWS-TLOATAISIES
SHELLFISH
COLIFORMS/VIRUS
FISH SPAWNING'REARING
TOXICITY/SUSPENDED SOLIDS
JUVENI1F FISH MIGRATION
TOXICITY
RECREATIONAL BOATING
FLOATACI.ES
SHORELINE PARKS
FLOATABIES
COMMERCE
MINIMAL
INDUSTRY
NEGLIGAP-LE
-------
-9-
Cot Con t. ro I Re 1 a t ion .o] i ip
Ff>f each of." the nine groups of: overflows, utilizing data de-
veloped in tho control level optimization a plot: of cost versus
control level was made, see typical example? (Figure 3). in all
cases a pronounced "knee" indicating a dramatic increase in control
cos ts was indicated in the 1 per year to 1 per 10 year control level.
The "knee" of the curve is only significant if control levels
beyond it are required, to address a specific beneficial use. We
are now at the point where we have the cost versus reduction in
pollutant relationships. The next step is to relate the reduction
in pollutant to increase in benefit.
Body Beneficial Use
A list of all existing beneficial uses and beneficial uses
lost because of the existing CSO's was prepared for each of the
nine CSO groups. Use .information was based on field observations,
state environmental and wildlife departments, local universities
and colleges, county health department, city parks department,
~ ocal community groups, and through the public hearing process.
These beneficial uses were then listed in. order of importance
based on a combination of factors including public risk, biota
sensitivity, and city zoning/planning policies for the area.
A list of identified beneficial uses is shown in Table 2.
CSO_Contro1 Levels - Beneficial Use
For each of the nine groups of CSO's and for each beneficial
use within the group, the incremental level of CSO control bene-
ficial use relationship was evaluated. This was accomplished by
evaluating existing conditions and asking the question what benefit
would accrue by increased reduction?; in overfjow events.
For illustrative purposes, priority 2 area, )l,ake Washington
south is shown. The prioritized beneficial uses are:
1. Swimming
2. Fish Rearing
3. Fish Spawning
4. Recreational Boating
5. Shoreline Parks
Swimming. Up to twenty overflows per year discharge to the
near'^hore, "resulting in up to three days of health standard coli-
form count'violations for each occurrence. Up to five overflows
occur during the summer recreation season. _ CSO's have not pre-
cluded swimming activity, except that p
-------
-10-
substaritiation for this approach has recently been provided for
in a recent court case involving the State of Illinois versus the
City of Milwaukee. The judge stated that "exposure to a hazard
is itself actionable, whether or not that exposure results in the
actual contraction of a disease".
Other factors that were taken into consideration were prior
community commitment to CSO control (i.e., sewer separation), local
political policy to reduce overflows to one per year, 60 percent of
the shoreline is public park access, and the city operates a number
of swimming areas within the CSO's areas of influence. For swim-
ming use, funding of facilities to control overflow s to one event
per year was agreed, this is equivalent to one summer overflow every
two years. The resultant publicity of the CSO question has prompted
the County Health Department to develop beach closure procedures
for the coming recreation season.
Fish Rearing/Spawning. Combined sewer overflows are potentially
toxic to fish rearing and spawning. It was the general concern of
fishery experts and the regulatory agencies that overflows do stress
these activities. Available information was lacking on the degree
of CSO stress, i.e., did it increase from one to two percent, or
was it one to seventy percent. Until such time as a closer defini-
tion of stress can be determined, the funding agencies will not.
participate in CSO controls to protect fish rearing/spawning.
Recreational Boating/Shoreline Parks. Control levels beyond
that developed for swimming would not be justified by considering
recreational boating shoreline park use.
Similar analyses were conducted for each water body and in each
case it was only the human use beneficial uses that could meet the
PGM-61 unit benefit-requirement, namely, residential areas subjected
to CSO's, swimming, and shellfish harvesting. The case for CSO
control to protect fishery related uses could not be made sufficiently
strong to meet the rigors of PGM-61.
Conclusions of Seattle Case
Reduction of potential health risks in high public use areas
meets PG-61 requirements. Fish stress, interference with commercial
and industrial activity did not. Further studies at identifying
relative fish stress are currently being developed by Metro. It
was shown that the EPA will consider favorably funding CSO controls,
where reduction of CSO is one element in an overall comprehensive
201/208 solution approach,to addressing a water quality problem.
-------
-11-
25 r
4 2
0 25 50
ANNUAL OVERFLOW VOLUME REDUCTION
4.10.5 MILLION GALLONS
75 100 PERCENT
PRIORITY 5 LAKE UNION ( SOUTH AND EAST SHORES ) AND PORTAGE DAY
CSO CONTROL
Figure 3. Example Cost-Overflow Control Curve
------- |