HomeMy Public PortalAboutExhibit MSD 37B2 - 2013 Update (LTCP) - Supplement 2Supplement No. 2
September 2013
Exhibit MSD 37B2
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Metropolitan St. Louis Sewer District
CSO LTCP Supplement No. 2
1 September 2013
Introduction and Purpose
The Metropolitan St. Louis Sewer District (MSD) prepared an update to its Combined Sewer
Overflow Long-Term Control Plan, dated February 2011, that described the development and
selection of MSD’s plan for controlling combined sewer overflows to area waterways. The plan
was approved by the Missouri Department of Natural Resources on June 1, 2011. The CSO
Control Measures defined in the plan—including descriptions, design criteria, performance
criteria, and critical milestone dates—were subsequently incorporated into a Consent Decree
between the United States EPA, the Missouri Coalition for the Environment Foundation, and
MSD. The Consent Decree was entered on April 27, 2012.
On March 26, 2013, MSD proposed a modification to two of the CSO Control Measures for
Maline Creek described in the Long-Term Control Plan and Consent Decree: CSO Treatment
Unit at Bissell Point Outfall 051 and Bissell Point Outfall 052 Storage Tank. The CSO Control
Measures described in the Long-Term Control Plan are depicted in Figure 1 below. The
treatment unit at Outfall 051 was to be an Enhanced High Rate Clarification facility with a
capacity of 94 MGD, providing the equivalent of primary treatment and seasonal disinfection to
CSO prior to discharge to Maline Creek. The storage tank at Outfall 052 was to temporarily store
up to 1 million gallons of CSO until secondary treatment capacity became available at the Bissell
Point WWTP.
Figure 1: CSO Control Measures for Maline Creek as Described in LTCP
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CSO LTCP Supplement No. 2
2 September 2013
MSD proposed that the two above-named CSO Control Measures be replaced with a single
12.5-million-gallon CSO storage facility to accommodate overflows from both outfalls until
secondary treatment capacity becomes available at the Bissell Point WWTP; see Figure 2. In
essence, the flow that would have received primary treatment and seasonal disinfection in the
CSO treatment unit at Outfall 051 and been discharged to Maline Creek, will now be stored and
transported to the Bissell Point WWTP for secondary treatment and seasonal disinfection before
being discharged to the Mississippi River.
This Supplement No. 2 to the Long-Term Control Plan addresses modifications to the plan to
incorporate these two modified CSO Control Measures. This Supplement addresses each of the
minimum elements of the Long-Term Control Plan, as defined in the CSO Control Policy
(59 FR 18688):
Characterization, Monitoring and Modeling of the Combined Sewer System
Public Participation
Consideration of Sensitive Areas
Evaluation of Alternatives
Cost/Performance Considerations
Operational Plan
Maximizing Treatment at the Existing POTW Treatment Plant
Figure 2: Proposed New CSO Control Measures for Maline Creek
Metropolitan St. Louis Sewer District
CSO LTCP Supplement No. 2
3 September 2013
Implementation Schedule
Post-Construction Monitoring Program
Characterization, Monitoring and Modeling of the Combined Sewer System
Additional flow meter data acquired since the completion of the LTCP’s planning-level model
was used for building and calibrating a design-level model. The calibrated design-level model
predicts higher CSO volumes in the typical year and higher peak flow rates compared to the
LTCP’s planning-level model, as shown in the table below.
Model CSO Volume
(MG)
Peak Flow Rate
(MGD)
LTCP’s planning-level 147 179
New, design-level 250 282
The new CSO volumes and peak flow rates exceeded the hydraulic capacities of the LTCP’s
selected CSO Control Measures. This fact necessitated a re-sizing of the LTCP’s CSO Control
Measures. Due to the significant change in design parameters and costs, MSD sought a re-
evaluation of the selection of CSO controls, as explained below under “Evaluation of
Alternatives.”
Public Participation
The modification to the CSO Control Measures for Maline Creek will not result in a change to
the level of CSO control which was presented to the public during the development of the LTCP
and included in the approved Plan. The original LTCP considered a local storage option as one
of its alternatives. No adverse public comments were received regarding either alternative.
Additionally, since these modifications gave rise to the need to modify the above mentioned
Consent Decree there was a public comment process included in that effort. The Consent Decree
modification process did afford the public an opportunity to comment on the proposed
modification to the CSO control measures. As part of the Consent Decree modification process
The United States published notice of these proposed amendments in the Federal Register on
July 11, 2013 (78 Fed. Reg. 41803), and invited the public to submit comments on the Decree.
The public comment period closed on August 26, 2013 and no comments were received.
Consideration of Sensitive Areas
The modification to the CSO Control Measures for Maline Creek does not result in CSO
discharges to any receiving waters other than those previously described and characterized in the
Long-Term Control Plan. Therefore, MSD’s evaluation of Sensitive Areas as presented in the
Long-Term Control Plan, and approved by MDNR, remains unchanged as a result of the
modification.
Evaluation of Alternatives
As noted above, the design-level model indicates higher CSO volumes and peak flow rates in the
typical year compared to the planning-level model. Costs associated with the CSO Control
Measures to meet the Performance Criteria of the Consent Decree also increased significantly as
a result of the CSO flow and volume increases. Consequently, MSD sought a new evaluation of
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4 September 2013
CSO controls for the Maline Creek to determine if the LTCP’s solution was still the best choice
economically and environmentally.
The following alternatives were originally evaluated in the LTCP:
1. Local storage for both Outfall 051 and Outfall 052
2. Local treatment for Outfall 051 and local storage for Outfall 052.
The LTCP determined that the alternatives provided similar benefits, differing in only a few
ways. The storage alternative provided the higher benefit to Maline Creek and greater public
acceptance, but these benefits were outweighed by its significantly greater expense compared to
the treatment alternative. Therefore, the LTCP selected the less expensive treatment alternative.
MSD re-evaluated the two alternatives based on the new CSO volumes and peak flow rates at a
level of control equivalent to the Performance Criteria in the Consent Decree, four overflow
events in the typical year. MSD selected the local storage alternative for both outfalls for the
following reasons:
The estimated project cost to store the flow from both outfalls in a below-grade storage
facility is approximately $72 million; the cost for a treatment system for Outfall 051 and
a storage system for Outfall 052 is approximately $97 million.
The local storage solution will provide lower pollutant loadings to Maline Creek. A local
storage solution will result in all captured flow receiving full secondary treatment and
seasonal disinfection prior to discharge to Mississippi River. The treatment alternative
provides the equivalent of primary treatment and seasonal disinfection prior to discharge
to Maline Creek.
Loadings to Maline Creek in the Typical Year
Parameter Local storage for both outfalls
Local treatment for Outfall
051 and local storage for
Outfall 052
BOD (tons) 9.4 24.9
TSS (tons) 118 206
Ammonia N (tons) 0.45 1.46
Organic N (tons) 0.93 2.46
E. Coli (106
counts) 5.32 E+08 5.06 E+08
A below grade storage system will be better for the largely residential community.
A below-grade storage system will eliminate the need for electrical and mechanical
equipment like the large remote pumping and treatment facilities of the treatment
alternative which require O&M to maintain proper performance during wet weather.
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CSO LTCP Supplement No. 2
5 September 2013
Cost/Performance Considerations
The LTCP indicated that the appropriate level of control, based on a cost-benefit analysis, was at
four overflow events in the typical year. MSD has estimated the capital cost of the selected local
storage solution at various levels of control to verify that the “knee-of-the-curve” still occurs at
the same level of control as indicated in the LTCP. Figure 3 below provides the cost-performance
curve for the local storage solution.
As indicated in the curve, expenditures for a level of control beyond four overflow events result
in diminishing benefits. At this level of control, the maximum untreated overflow volume in the
typical year increases to 70 million gallons due to the increased peak flow rates and volumes
predicted by the design-level hydraulic model.
The proposed modification to the CSO control measures for Maline Creek will involve the
construction of a 12.5 MG storage facility to control overflows from Bissell Point Outfalls 051
and 052 to four overflow events in the typical year. The total capital cost is estimated at
$72 million.
Figure 3: Cost-Performance Curve for Storage Solution
Operational Plan
Operation of the CSO Control Measures will change as noted in the Evaluation of Alternatives
section. It is expected that the new solution will reduce O&M demands due to the passive nature
of below-grade storage systems. Specific O&M manuals will be developed for the CSO storage
$0
$50
$100
$150
$200
$250
02468101214Total Capital Cost (Millions)Number of Untreated Overflows in Typical Year
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6 September 2013
facility and associated diversions and de-watering components as the project is designed and
constructed.
Maximizing Treatment at the Existing POTW Treatment Plant
The use of storage tanks for CSO control will increase flows to the Bissell Point WWTP
compared to the LTCP’s treatment unit. Stored flows will be drained to the Bissell Point
Interceptor Tunnel to receive full secondary treatment at the Bissell Point WWTP. Peak
instantaneous flows to the treatment plant will not change, but sustained flow may increase
modestly during tank drainage. The existing collection system and treatment plant have the
hydraulic capacity to convey and treat, respectively, the small increase in post-event flows.
The net benefit is an additional 180 MG receiving full secondary treatment in the typical year.
Implementation Schedule
MSD has determined that the implementation schedule for the below-grade storage system, and
the Critical Milestone Dates defined in the Consent Decree and originating from that schedule,
do not change, based on timely approval so that MSD can progress to the next step in the design,
construction, and delivery process.
Post-Construction Monitoring Program
Minor changes to the post-construction compliance monitoring program as defined in the
February 2011 Long-Term Control Plan and further developed in the CSO Post-Construction
Monitoring Program plan (PCMP), dated April 27, 2013 and approved by EPA on June 20,
2013, will be needed as a result of the proposed modification. Monitoring elements associated
with the treatment unit at Outfall 051 will no longer be needed and MSD anticipates that the
monitoring program for the proposed storage facility will be similar to that identified for the
Maline Creek Storage Tank, corresponding to CSO Outfall 052, in the EPA-approved PCMP.
Many elements of the monitoring program will not change. Both CSO outfalls will still be
continuously monitored for outfall activation. The monitoring program will still utilize MSD’s
network of long-term rain gages for record of rainfall depth, duration, intensity, and event
distribution. The monitoring program will also record flow measurements and additional
monitoring to ascertain event storage volume at the locations identified for the Maline Creek
Storage Tank Monitoring Program for a period of one year or as sufficient to properly update the
hydraulic model.
The final changes will be documented in the detailed monitoring plan that will be submitted one
year prior to the Achievement of Full Operation date, December 31, 2020. The plan will show
final monitoring site selection based on final design configurations.