PRELIMINARY COMPARISON OF
CONSOLIDATED WASTEWATER AND CSO TREATMENT OPTIONS
FOR THE
BOSTON HARBOR SDEIS
Prepared for
U.S. Environmental Protection Agency, Region I
Environmental Evaluation Section
JFK Building, Room 2103
Boston, Massachusetts 02203
DRAFT
April 30, 1984
CE MAGUIRE, INC.
Architects • Engineers • Planners
60 First Avenue. Waltham. Massachusetts 02254
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INTRODUCTION
As requested by the Environmental Protection Agency (EPA), a
preliminary investigation has been made into the feasibility of
a scheme developed by the Massachusetts Division of Environmental
Quality Engineering (DEQE) which would develop conveyance and
treatment facilities for Combined Sewer Overflows (CSO) at Deer
Island. These CSO facilities would be combined with the option for
use of Long Island as a consolidated treatment plant site.
The feasibility of this new proposal from the State may be compared
with the existing option of no treatment at Deer Island and
consolidated treatment facilities at Long Island. To this existing
option have been added existing plans of the MDC for CSO abatement
facilities in Boston Harbor.
DEER ISLAND CSO TREATMENT FACILITY SCHEME
In summary, the major new facilities required under this new CSO
option, as shown on Figures No. 1 and No. 2, consist of the following:
1. Construction of a new 111611 diameter 21,500 foot long deep
rock tunnel to convey sewage from the Columbus Park
Headworks to Long Island.
In essence this scheme would convert the existing Deer Island
Treatment Facility into a CSO treatment facility. The existing Boston
Main Drainage Tunnel (BMDT) which currently conveys sewage from the
Columbus Park Headworks to Deer Island for treatment would be
converted to transport newly consolidated CSO’s to Deer Island.
Sewage flows from the Columbus Park Headworks would be rerouted via a
new tunnel to a wastewater treatment facility on Long Island. Sewage
flows from the separate Ward Street and Chelsea Creek Headworks would
be conveyed from Deer Island via a new tunnel to Long Island for
treatment.
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2. Construction of a new 12’ diameter 11,000 foot long deep
rock tunnel to convey sewage from Deer Island to Long
Island.
3. Construction of a new 438 mgd influent pumping station at
Long Island.
4. Rehabilitation of Deer Island pumping station to provide for
North Metropolitan System and CSO flows.
5. Construction of a new headworks facility next to the
existing Columbus Park Headworks for CSO discharges into the
existing ll’6” diameter Boston Main Drainage Tunnel to Deer
I si and.
6. Construction to convert the existing Deer Island Treatment
Facility into a CSO treatment facility.
7. Construction of approximately 9,000 feet of CSO
consolidation sewer in the South Boston Old Harbor area.
8. The complete facility requirements for wastewater treatment
under this scheme, relating to consolidated treatment
facilities at Long Island and headworks at Nut Island are
not factored into the discussion nor are their costs
reflected in the tables. These costs would be the same
under either of the schemes analyzed. Therefore, this
discusion focuses only on the comparable differences between
the two schemes.
Under this new CSO scheme, the various flow consolidation, headworks,
conveyance, pumping and treatment facilities are based on components
derived from plans of the South Boston Area Old Harbor CSO mitigation
program in the Metropolitan District Commission’s (MDC) Report on
Combined Sewer Overflows in Dorchester Bay Area (October 1981). Flows
from the proposed 9,040 foot long consolidation conduit recommended in
the above report would discharge through a new CSO headworks into the
existing Boston Main Drainage Tunnel (BMDT). The existing Columbus
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Park Headworks would remain and be connected to the new 11 ‘6” diameter
deep rock tunnel to Long Island. The proposed 3.0 million gallon
capacity storage/containment facility included as part of that report
would not be required.
Other proposed CSO mitigation plans, such as for the Inner Harbor area
and other parts of Dorchester Bay area (see References attached) as
addressed in MDC’S recent planning reports do not appear feasible for
consolidation into the CSO Treatment Facility being proposed at Deer
Island.
Design Considerations
In order to determine the approximate magnitude of CSO that could be
readily diverted into the Boston Main Drainage Tunnel (BMDT) various
CSO reports on Boston Harbor were utilized as noted above. In
addition, the hydraulic capacity of the interceptor sewers including
the new Boston Main Interceptor, tributary to the Columbus Park
Headworks were reviewed from plans of the Boston Water and Sewer
Commission. Based on the information contained in these plans and
reports, the following approximate flows during storm conditions would
be transported to the Columbus Park Headworks:
1. Dorchester Interceptor 60 mgd
2. New Boston Main Interceptor 166 ingd
3. Old Boston Main Interceptor 20 mgd
Total 246 mgd
Since the design capacity of th Columbus Park Headworks is 182 mgd, a
CSO of approximately 64 rngd may be expected. This amount of CSO could
readily be conveyed to a new facility at Deer Island.
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In addition to the above flows, the proposed South Boston Old Harbor
CSO, estimated at 149 mgd in the Metropolitan District Commission
October 1981 Report on Combined Sewer Overflows in the Dorchester Bay
Area , could also be diverted into the BMDT. Therefor, a total CSO
flow of about 213 mgd (l49mgd+64mgd) could be available for transport
to treatment facilities at Deer Island.
Based on the preceding, it can be readily seen that the 438 mgd
capacity of the BMDT would not be fully utilized for projected CSO
flows under this scheme. It should be realized that under this scheme
the BMDT will be subjected to intermittent flow conditions which must
be considered. Between storm flows when there is virtually no flows in
the BMDT, and during CSO conditions when the BMDT would be operating
at half its capacity, sedimentation of suspended solids and possible
septic conditions may develop similar to that being experienced in the
present Calf Pasture — Moon Island Tunnel. The problems of odors and
systems operation plus any furtehr consideration of possible added CSO
flows to the BMDT under these conditions would require additional
consideration and would result in significantly higher construction
and O&M costs to develop the CSO portion of this scheme.
Although beyond the scope of this study to evaluate in detail,
possible solutions to these problems could be developed. They
include:
1. Periodic flushing of the BMDT with ocean water; however,
this would introduce salt water into the pumping and
treatment facilities at Deer Island.
2. Dewatering the BMDT between storms at a considerable added
cost.
3. Additional CSO consolidation to increase flows into the
BMDT. Possible options include rehabilitation of the Old
Boston Main Interceptor (BMI) slated for abandonment as well
as further consolidation of CSO’s not readily conveyed to
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Deer Island. However, the required interceptor/sewer
construction throughout the City of Boston under such an
approach would significantly increase both the costs and
environmental impacts of CSO mitigation under this scheme.
Construction and Equipment
The following briefly outlines the construction and equipment required
for wastewater treatment and CSO elements under this scheme.
New CSO Headworks and Consolidation Conduit
As shown on Figure No. 2, a new CSO headworks is required in
the vicinity of the existing Columbus Park Headworks. This
new facility will provide screening and degriting of
overflows from the Dorchester Intercepter, New Boston Main
Interceptor, Old Boston Main Interceptor and from the
previously mentioned South Boston Old Harbor Consolidation
Conduit. The facility should be designed for up to the
maximum future CSO anticipated, but not in excess of the 438
mgd capacity of the BMDT. Flows from the headworks will be
discharged via a new shaft into the BMDT.
At the location of a new shaft the BMDT must be plugged to
prevent flows from the Columbus Park Headworks. The
significant and major environmetnal consequences of such
work are discussed in a later section of this memo.
Deep Rock Tunnels
Two deep rock tunnels are required under this scheme as
shown on Figure No. 1. First, a 21,500 foot ll 6” diameter
tunnel (New BMDT) is needed to convey sewage from the
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Columbus Park Headworks to treatment at Long Island.
Second, a 11,000 foot 12’ diameter tunnel is needed to
convey sewage from Deer Island to treatment at Long Island.
The new 21 ,500 BMDT from the Columbus Park Headworks would
have the same 438 mgd capacity as the existing BMDT. New
shafts approximately 9’ diameter at Columbus Park and 11’
diameter at Long Island would be required. Flows would
discharge into a new influent pumping station at Long
Island. The new pumping station was assumed in this
analysis to be similar to that existing at Deer Island with
a capacity of 438 mgd.
The 12’ diameter tunnel from Deer Island to Long Island, as
shown on Figure No. 1, is required to transport wastewater
flows from the existing North Metropolitan Relief Tunnel and
Winthrop Terminal Facilities at Deer Island to the proposed
Long Island Treatment Facility. Capacity of the new tunnel
is 475 mgd based on 350 mgd from the North Metropolitan
Relief Tunnel and 125 mgd from the Winthrop Terminal
Facility.
Rehabilitation of Deer Island Pumping Stations
The main influent pumping station at Deer Island will
require new pumping equipment to handle both the existing
sewage flows from the North Metropolitan Relief Tunnel
(NMRT) and proposed CSO flows from the existing BMDT. The
10 existing pumps would be replaced with 8 new pumps. Four
pumps would lift the 350 mgd flow from the NMRT and
discharge into the new 12’ diameter tunnel to Long Island;
an additional pump would be stand-by. Two pumps would lift
the estimated 213 mgd CSO flows from the BMDT into the
proposed Deer Island CSO Treatment Facility; an additional
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pump would be stand-by. Use of the existing pumping station
for both sewage flows and CSO flows is made possible through
the utilization of any combination of the 8 pumps. In other
words, no single pump would be dedicated to a single system.
Each of the new pumps should have a rated capacity of 88 mgd
against a Total Dynamic Head of about 120 feet. It is
assumed herein that 2,200 horse power variable speed
electric motors will be used to drive the pumps.
tinder normal operation, the two pumping systems, sewage and
CSO, would be separated. However, should the necessity
arise, the CSO portion of the pumping station could also be
used to pump sewage flows to Long Island.
Some reconstruction of the discharge piping would be
required as part of the rehabilitation process.
The Winthrop Terminal Facility would be upgraded from 60 mgd
capacity to 125 rngd capacity as proposed by the MDC. This
pumping station would also discharge into the 12’ diameter
tunnel to Long Island. A total of four pumps including one
stand-by pump is suggested. Each pump would have a rated
capacity of 42 mgd against a Total Dynamic Head of about 54
feet. It is assumed that 500 horse power variable speed
electric motors will be used.
It is important to note that the new pumps will provide
sufficient hydraulic head on the new 12’ diameter tunnel to
Long Island to preclude the need for influent pumping of
Deer Island flows at the proposed Long Island Treatment
Plant.
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Long Island Influent Pumping Station
A new 438 mgd pumping station will be required at Long
Island to lift flows from the new BMDT into the treatment
facilities. The new station would be similar to the
existing Deer Island Pumping Station in depth but smaller in
overall size.
A total of six pumps including one stand-by pump is
suggested. Each pump would have a rated capacity of 88 mgd
against a Total Dynamic Head (TDH) of 105 ft. The TDH is
the same as now required at Deer Island. Variable speed
2,000 horse power electric motors are assumed herein to
drive the pumps.
It may prove feasible to consolidate the flows of both the
New BMDT and Long Island Headworks into a single influent
pumping station. In such case, the equipment requirements
and costs would not change; however, some savings in
construction costs would be realized. These potential cost
savings have been factored into the cost comparison to
reflect additional potential benefits under this scheme.
Conversion of Deer Island into CSO Treatment Facility
The present Deer Island Sewage Treatment Plant, (STP) can be
readily converted into a CSO facility. The only
modifications necessary relate to the intermittent use of
the facility when operating as a CSO facility.
In the CSO mode the primary settling tanks would be drained
back to the sewer after each storm event. The sludge and
scum would also be returned to the sewer for treatment at
the plant.
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The essential work necessary to effect conversion to a CSO
facility would include:
1. Provide effective tank drain and wash down system.
2. Divert present sludge and scum pump discharge to the
sewer (either Winthrop facilities or Chelsea - Long
Island System).
3. Revis present chlorine facilities (tank truck delivery
not cost effective for intermittent use).
4. Revise pump discharge piping so that any pump can be
utilized for either CSO system or Chelsea - Long Island
system use.
5. Demolition of facilities no longer needed such as
digesters, sludge thickners, etc.
The present main pumping equipment or an equivalent replacement
could serve the CSO needs. rn fact, with a peak requirement of
less than the present 438 mgd, the pump head requirement would be
less. However, with the utilization of the existing pumping
station for both the CSO flows and the Chelsea—long Island flows,
it would seem prudent to have all pumping units suitable for
either service. The attached pump curve illustrates (Figure No.
4) how this can be effected.
LONG ISLAND CONSOLIDATEO TREATMENT OPTION AND SEPARATE CSO MITIGATION
Under this option, no wastewater or CSO treatment would be provided at
Deer Island. All present inflows to the Deer Island Wastewater
Treatment Facility would be conveyed via a new 16’ diameter deep rock
tunnel to a consolidated wasterwater treatment facility on Long
Island. This option incorporates, for comparison purposes only,
existing CSO mitigation plans consistent with those developed under
the new CSO option described in a previous section.
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As shown on Figure No. 3 the major new facilities required under this
option consist of the following:
1. Construction of a new 16’ diameter 11,000 foot long deep
rock tunnel from Deer Island to Long Island.
2. Rehabilitation of the existing Deer Island pumping stations
to pump the flows to Long Island.
3. As noted in the previous section, facility costs of the
complete treatment plans which would be the same for both
pptions are not factored into this analysis.
Design Considerations
The sewage flows to be conveyed from Deer Island to Long Island
consist of 438 mgd from the Boston Main Drainage Tunnel (BMDT), 350
mgd from the North Metropolitan Relief Tunnel (NMRT) and 125 mgd from
the Winthrop Terminal Facilities for a total flow of 930 mgd. All of
these flows currently are conveyed to Deer Island and would, under the
MDC’S upgrade plans, continued to be conveyed there.
As part of the Deer Island pumping station rehabilitation design
criteria the new pumping equipment will provide sufficient hydraulic
head on the new 16’ diameter tunnel to preclude the need for influent
pumping at the Long Island Treatment Facility.
Construction and Equipment
The following outlines the construction and equipment requirements
under this option.
Deep Rock Tunnel
The new 11,000 foot 16’ diameter deep rock tunnel shown on
Figure No. 3 from Deer Island to Long Island will have a
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total capacity of 930 mgd. Two new shafts will be required,
one at Deer Island and the second at Long Island.
Rehabilitation of Deer Island Pumping Stations
The main influent pumping station at Deer Island will
require new pumping equipment to provide the additional
hydraulic head to meet the influent hydraulic gradient at
the new Long Island Treatment Facility. The existing 10
pumps would be replaced. Normal operation at the design
flow of 805 mgd (930 mgd minus 125 mgd for the Winthrop
Terminal Facility) would consist of 9 pumps with one pump as
stand-by. Each of the new pumps should have a rated
capacity of 89 mgd against a Total Dynamic Head of about 120
feet. For this analysis 2,200 horsepower variable speed
electric motors were assumed to drive the pumps.
The Winthrop Terminal Facility would be upgraded from 60 mgd
capacity to 125 mgd capacity as proposed by the MDC plans.
This pumping station would also discharge into the new 16’
diameter tunnel to Long Island. A total of four pumps
including one stand—by pump was used for this analysis.
Each pump would have a rated capacity of 42 mgd against a
Total Dynamic Head of about 54 feet. It is assumed that 500
‘horsepower variable speed electric motors will be used.
Some reconstruction of the discharge piping would be
required as part of the rehabilitation process.
COST COMPARISON
The comparative costs estimated for the two schemes examined show a
differential of about $82 million. Table No. 1 outlines the separate
construction cost elements developed for each scheme. It is important
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to note that these costs are not the total construction costs for each
option. They reflect only those costs which would be alternates under
each scheme’s different approach to developing wastewater and CSO
treatment facilities. The costs for common elements under both
schemes such as facilities at Nut Island, would be the same and are
omitted in this comparison.
The pertinent elements of construction under Scheme “A” would cost
$123.3 million. This total includes 11,000 feet of tunnel to convey
consolidated North System flows from Deer Island to Long Island; it
requires rehabilitation of the existing Deer Island pump station; and
it adds $39 million for the portions of existing CSO plans readily
adapted to this alternative.
Scheme “B” would cost $214.4 million for the comparable volume of
wastewater and CSO treatment. This total would include two separate
tunnels to convey existing wastewater flows to Long Island; a new pump
station on Long Island; rehabilitation of the existing pump station at
Deer Island; a new headworks facility for CSO at Columbus Park;
construction of collection interceptor lines for CSO to Columbus Park;
and conversion of the Deer Island SIP into a CSO treatment facility.
As a further assumption, it was concluded that under Scheme “B”
additional cost savings of approximately $9 million could be realized
if the pump station at Long Island were built to consolidate flows
from Nut Island and Columbus Park. This savings would therefore reduce
the comparative costs of this scheme to $205.4 million.
The cost savings between these two options can now be compared. Table
No. 2 graphically portrays these savings. Due to the considerable
additional construction necessary to implement wastewater treatment
under Scheme “B”. This option would cost an extra $82.1 million to
develop. The volume of sewage to be treated under both schemes is 930
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mgd, while a total of 213 mgd CSO can readily be consolidated and
treated in each. As described in the following section on
Environmental Consequences, there are additional costs, not readily
quantified, which further limit the feasibility of the proposed new
CSO scheme.
POTENTIAL ENVIRONMENTAL CONSEQUENCES
As a further element of comparison between the two schemes, the
potential environmental consequences of each were examined. Under
both schemes, there would be significant environmental impacts
resulting from the construction and operational activities necessary
to develop these major metropolitan area wastewater treatment
facilities. By contrast, the potential impacts of the proposed scheme
which introduces new CSO treatment at Deer Island would be beyond
those associated with the siting of major treatment facilities as
proposed in the other options. Such impacts involve broader
construction activities in the City of Boston and significant water
quality impacts from temporary raw sewage discharges to Boston Harbor.
As described in the screening review, the potential impacts of the
option which consolidated all treatment at Long Island and implements
CSO abatement plans of the MDC (Scheme “A’ t ) are associated with the
construction and operation of major treatment facilities and would be
comparable in many cases to those for other siting options. While
the impacts associated with consolidation of treatment at Long hand
would be significant, particularly including construction activities
at Deer Island and Long Island plus traffic through Wintrhop and
Quincy, there would also be introduced the potential for benefits at
both Deer Island and Nut Island and in the surrounding neighborhoods
from the reduction of treatment facilities.
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The scheme which proposed new CSO facilities at Deer Island (Scheme
11811) would introduce additional adverse impacts, beyond those noted
above, associated with the conveyance of sewage from the Columbus Park
Headworks to Long island via a New BMDT. It would also eliminate any
possible benefits to the neighborhoods closets to the Deer Island
site.
Specifically, this scheme would require a connection of a proposed New
BMDT to be constructed from the existing Columbus Park Headworks which
now receives up to 438 mgd of sewage daily. In order to make the
final tap-in of a new deep rock tunnel at the headworks that volume of
sewage would have to be diverted while the headworks were dewatered
and the final connection was made. Due to the complexity of the
anticipated construction work involving undergruond pressurized
sewers, it appears likely that the time needed to complete such final
connections of sewer lines to headworks could take between 4 to 8
weeks. The existins sewage volume of up to 438 mgd would have to be
diverted and most like’y would be discharged directly to the Harbor
during this period of time.
It should be pointed out that the substantial environmental impact of
raw sewage overflows associated with the diversion of the
Ward-Columbus system to Long Island could be alleviated, although not
eliminated, by utilization of the old BMI. The old BMI could be
utilized to convey flows through the proposed CSO headworks and
through a temporary connection to the new shaft in the tunnel to Long
Island.
In order to effect this temporary diversion, the following would be
necessary:
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1. l1’-6” slide gate in new Long Island tunnel shaft suitable
for 125 psi seating pressure.
2. Temporary construction from new CSO headworks to this shaft.
3. Temporary unrestricted connections from all sewer systems to
new CSO facility.
Under this scheme, all sewage flows would first be diverted through
the new CSO headworks and the new tunnel to Long Island SIP. After
this was done, the new shaft into the existing Columbus-Deer Island
tunnel could be completed, the tunnel dewatered, and the connection
at the Columbus Park shaft and the connection to the new tunnel
completed.
As stated above, this possible method of construction would not avoid
sewage overflows because of the hydraulic restrictions in the total
system, but could substantially alleviate that serious problem during
construction. Determination of the amount of overflows under this
scheme would require extensive hydraulic analysis, and the potential
improvements from such a method of construction of the connections at
Columbus Park will need detailed engineering analysis before its
suitability is judged. Such an approach is presented here to reflect
the assumuption that this scheme does have possible benefits which
warrant consideration of such methods.
While no cost estimates have been made 0 f the environmental
consequences under either construction scenario, it appears that such
potentially harmful inipacts as those associated with Scheme “B” could
in themselves, render this scheme unacceptable.
A further contrasting element between these two schemes is also
important to note. Unlike Scheme “A”, this scheme would eliminate any
potential benefit at Deer Island from the reduction of present
wastewater treatment facilities there. The conversion of the Deer
Island SIP, as proposed in this scheme to a CSO facility, would
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maintain the existing tanks, support buildings and garages in addition
to the necessary pumping facility. The size and operational elements
of the Deer Island site as a CSO facility would be seen as similar to
its current usage. Impacts from traffic during construction and
operation, potential odor problems, and visual impacts would remain
and would require further mitigation measures to address.
Another environmental consequence associated with this new CSO scheme
involves the problem, noted in a previous section, of the
under-utilization of the od BMDT for consolidated CSO flows to Deer
Island. The CSO readily conveyed to the old BMDT under this scheme
would only divert 213 mgd to the BMDT conduit which has a capacity of
438 mgd. The odor and operational problems of such usage were
described previously. In order to increase the CSO flows up the
capacity of the BMDT additional flows from the Inner Harbor area, the
old BMI conduit, or other parts of the Dorchester Bay area would have
to be consolidated and conveyed to the new Columbus Park Headworks.
Such additional consolidation and conveyance of CSO up to 225 mgd
appears to be beyond what is readily accomplished. It would involve
further disruption, higher costs, and greater environmental impacts
beyond those described above. By contrast, the alternatives provided
by existing CSO plans for additional treatment would appear to
accomplish the same CSO abatement goals at a more acceptable level of
environmental consequence and with comparable costs.
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TABLE 1
CONSTRUCTION COSTS
Descri ption
SCHEME “A”: CONSOLIDATED TREATMENT AT
LONG ISLAND WITH SEPARATE CSO MITIGATION
1. 11,00 L.F. of 16’ Diameter Tunnel
2. Rehabilitate Pump Station at Deer Island
3. Demolition at Deer Island
4. Collection of CSO in Old Harbor (from
CDM Report)
II SCHEME “B”: CONSOLIDATED TREATMENT AT LONG
ISLAND WITH DEER ISLAND CSO MITIGATION
1. 21,500 L.F. 11’06” Diameter Tunnel
from Columbus Park to Long Island
2. 11,00 L.F. 12’ Diameter Tunnel
from Deer Island to Long Island
3. New 438 MGD Pump Station at Long Island
4. Rehabilitate Existing Pump Station
at Deer Island
5. New Headworks at Columbus Park
6. Demolish and Convert Deer Island into
CSO Treatment Facility
7. Construction of 9000 L.F. of (Collection
only) CSO Interceptor (CDM Report)
Million $
66.3
17.0
1.0
39.0
*Jf Nut Island and Columbus Park flows are combined into one pump
station at Long Island, an additional $9 million savings is estimated.
Total 123.3
*
89.5
45.7
40.0
14.0
11.6
3.0
10.6
214.
- 9.0
205.4
Total
Total
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TABLE 2
COST COMPARISON
OF
DEER ISLAND SUBOPTIONS
SCHEME A
( ?\
$3 9m
SCHEME B
$32.2m
Sewage Treatment
TOTAL
$123.3m $
205.4m
Di
fference
+
$82.lm
CSO Pollution Abatements
$84 3m
$173.2m*
*TIlis figure assumes a $9 million savings for consolidated pumping at Long Island
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REFERENCES
1. MDC, Report on Combined Sewer Overflows in the Dorchester Bay Area .
Camp, Dresser & McKee, Inc. - October 1980.
2. MDC, Discharges from Moon Island .
Camp, Dresser & McKee, Inc. - Draft Report
March 28, 1984.
3. MDC, EMMA Study - Volume No. 9.
Metcalf & Eddy, Inc. - October 1975
4. MDC, Combined Sewer Overflow Report .
MDC, Inner Harbor Area Facilities Plan .
O’Brien & Gere Engineers, Inc. - January 1981
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