2. Evaluation of Midland Avenue RTF and conveyances flows, Onondaga Lake Improvement

Midland Ave Regional Treatment Facility & Conveyances—Facility Plan, Jan 2003
by Environmental Engineering Associates, LLP
for Onondaga County Department of Water Environment Protection

2. Evaluation of Midland Avenue RTF and conveyances flows

The following sections summarize evaluations conducted to develop design flow requirements for the Midland Avenue RTF and Conveyances project.

2.1 Sewer separation evaluation

Pursuant to the ACJ, the County was required to conduct sewer separation evaluations to identify where sewer separation may be a more cost-effective alternative to consolidation of CSO flows for treatment at the Midland Avenue RTF. Specifically, the ACJ states:

A cost evaluation of sewer separation projects proposed by the city, or by any party to this agreement and order, shall be implemented by the county. When doing this evaluation, the cost of separation shall be compared with the savings realized from the sum total of:
a.) reduced CSO abatement project costs;
b.) reduced operating costs at the Metro sewage treatment plant;
and any other savings as applicable; and achieves comparable water quality benefits.

The County is obligated to construct only those sewer separation projects for which the anticipated savings exceed expected costs.

To fulfill this requirement, a detailed evaluation of potential opportunities for sewer separation of the nine CSOs tributary to the proposed Midland Avenue RTF and Conveyances not scheduled for sewer separation or previously permanently closed (i.e., CSOs 039, 042, 043, 044, 052, 060 [includes former Overflow 077], 061, and 076) was conducted. Appendix A presents a Sewer Separation Evaluation memorandum (dated October 15, 1998) detailing this evaluation.

This analysis concluded that with the exception of the minor Glenwood area sewer separation project described in Appendix A, in comparing the costs of conveying CSO flows for storage and/or treatment at the Midland Avenue RTF, there are no cost benefits to separating additional CSOs within the Midland Avenue service area.

Subsequent to the preparation of the Sewer Separation Evaluation memorandum presented in Appendix A, the County had a separate analysis of up-to-date sewer separation costs completed by its ACJ Oversight Program Manager, CDM/C&S Joint Venture. This analysis, presented in a CDM/C&S memorandum to the County dated April 30, 2001 (Issued September 10, 2001), established recommended unit costs for sewer separation for both residential areas and commercial areas in the metropolitan Syracuse area. See Appendix B.

These recommended unit costs for sewer separation were identified to be 70% (residential areas) to 215% (commercial areas) higher than the unit cost for sewer separation (dollars per linear foot) used for cost comparisons in the October 1998 Sewer Separation Evaluation memorandum. This information further supports the conclusion of the October 1998 Sewer Separation Evaluation that in comparing the costs of conveying CSO flows for storage and/or treatment at the Midland Avenue RTF, there are no cost benefits to separating additional CSOs within the Midland Avenue service area.

2.2 RTF hydraulic evaluations

As presented in the "Combined Sewer Overflow Facilities Plan" (Moffa and Associates and Blasland and Bouck Engineers, PC, February 28, 1991), the Midland Avenue RTF was preliminarily sized to pump, treat, and convey up to 650 cubic feet per second (cfs) of tributary area CSO. This flow rate was based on flow projection models and was selected as the flow rate which resulted in no overflows in the Midland Avenue tributary area up to the one-year design storm event. Due to the large size of the Midland Avenue facilities, a review of the previous flow projections was conducted for the purpose of updating previous flow projection models and potentially minimizing facility costs. To complete this hydraulic evaluation, previous sewer system models were verified and updated to incorporate the best available information regarding recent sewer rehabilitation and replacements, additional CSO storage requirements, field observations of the existing sewer system, and other conditions that may affect the flow projections for the project. Appendix C presents a summary memorandum detailing the verification and updating of the previous flow projection models as described above.

Based upon these updated flow projection models, the Midland Avenue RTF peak influent pumping rate was determined to be 500 cfs, rather than the previously estimated peak influent pumping rate of 650 cfs. This decrease in the influent pumping rate was primarily attributed to a more accurate predicting flow projection model following completion of the model upgrade noted above.

Following completion of the Sewer Separation Evaluation described in Section 2.1, the County elected to undertake an additional evaluation of the potential for the closure of CSOs tributary to the Midland Avenue RTF and Conveyances project not currently scheduled for sewer separation or permanently closed (i.e., CSOs 039, 042, 043, 044, 052, 060 (includes former Overflow 077), 061, and 076). Specifically, this evaluation was conducted to determine the facility modifications that would result if one or more of the remaining nine CSOs tributary to the proposed Midland Avenue RTF and Conveyances were closed to prevent future discharges from these CSOs. Appendix D presents the CSO Closure Evaluation memorandum detailing the results of this evaluation.

Based on review of the results of the CSO Closure Evaluation, the County has determined that it is feasible to permanently close the following CSOs as part of this project: 043, 052, 060 (includes former Overflow 077), 061, and 076. The RTF hydraulic evaluation discussed below includes the impacts of facility design flows necessitated by the permanent closure of these existing CSOs.

To evaluate the effects of closing the CSOs identified for closure as described above on the RTF influent pumping rates, a separate analysis of the RTF influent pumping rate was conducted using the updated flow projection models. Appendix E presents a memorandum summarizing the results of this hydraulic evaluation.

The conclusion of this evaluation was that, by maximizing storage in the conveyances, the minimum RTF pumping/treatment flow rate that would be required without overflows occurring at the remaining three tributary area CSOs (i.e., 039, 042, and 044) or the RTF emergency controlled diversion up to the one-year design storm event, or that would not result in elevated hydraulic grade lines upstream of the existing CSOs scheduled to be closed (i.e., 043, 052, 060 [includes former Overflow 077], 061, and 076), was 667 cfs.

The 667 cfs RTF influent pumping rate was also demonstrated to achieve compliance with the ACJ-required elimination or capture for treatment of not less than 85 percent by volume of combined sewage collected in the overall combined sewer system during precipitation events on a system-wide average annual basis. The RTF Hydraulic Evaluation memorandum in Appendix E summarizes and details this analysis.

Subsequent to the preliminary design development for the aforementioned 667 cfs Midland Avenue RTF, a series of mediation meetings were held with representatives from the County, the City of Syracuse, NYSDEC, the Partnership for Onondaga Creek, Atlantic States Legal Foundation, and the Onondaga Nation to discuss alternatives to the 667 cfs RTF. Non-County mediation representatives expressed concern regarding the size of the proposed above-ground structure for the 667 cfs RTF and the issue of chlorinated/dechlorinated discharges from the RTF. Complete underground storage of the Midland Avenue CSOs with subsequent conveyance and treatment at Metro was identified as the preferred CSO Abatement approach for the non-County mediation participants.

Through multiple meetings and discussions with the mediation representatives, various reduced size RTF and large underground storage alternatives were evaluated. The results of these evaluations demonstrated that the three most viable alternatives for the Midland Avenue CSOs were as follows:

Capital, operation and maintenance (O&M) and 25-year present worth costs were developed for the above three alternatives. These costs are presented in Table 2-1.

Table 2-1. MOST VIABLE ALTERNATIVES ESTIMATED COSTS
Alternative	Estimated project cost	Estimated annual O&M cost	Estimated 25-year present worth
1. 667 cfs RTF (plus 4.2 MG upstream storage)	$54,780,000	$839,000	$71,300,000
2. 330 cfs RTF (plus 6.7 MG upstream storage)	$54,420,000	$712,000	$68,000,000
3. 12 MG underground storage tank (plus additional 4.2 MG upstream storage)	$79,220,000	$394,000	$87,400,000
Notes: 1. Costs based on Dec '02 dollars (ENRCCI=6563). 2. 25-year present worth based on 5% discount rate; rounded to nearest $100,000.

Based upon the cost data presented in Table 2-1, the County has determined that Alternative 2 - 330 cfs RTF plus 6.7 MG of upstream storage (total system storage equals 8.45 MG) is the preferred approach to abating the Midland Avenue CSOs. In addition to having the lowest estimated project cost and 25-year present worth, this alternative provides a smaller above grade structure for the RTF due to its reduced size and maximizes the cost-effective use of upstream, underground storage, while decreasing the number of treated discharges.

2.3 Overflow elimination plan summary

Based upon the above series of evaluations, the following presents the resultant disposition of the existing CSOs in the Midland Avenue tributary service area upon completion of the Midland Avenue RTF and Conveyances Project (Project).

CSOs 039, 042, 044 - Active Overflow Diversions; to activate only above one-year design storm event upon completion of Project (three total).

CSOs 043, 052, 060 (includes former Overflow 077), 061, 076 - Active Overflow Diversions; to be closed upon completion of Project. (Note: as part of these facilities planning efforts, the County completed a separate evaluation of eliminating the Newell Street (CSO 067) RTF and conveying the Newell Street CSO service area flows to the Midland Avenue CSO transmission pipeline and RTF. A memorandum summarizing the results of this evaluation is presented in Appendix G. Based upon the results of this evaluation, it has been determined to be cost-effective to convey Newell Street CSO (067) to the Midland Avenue CSO transmission pipeline and subsequently the Midland Avenue RTF for treatment. Therefore , CSO 067 will be added as an Active Overflow Diversion; to be closed upon completion of this project, as part of the Midland Avenue Regional Treatment Facilities and Conveyances Project) (six total).

CSOs 038, 040, 041, 045, 046A, 046B, 047, 048, 050, 051, 053, 054 - Active Overflow Diversions; to be converted to storm only discharge or closed by sewer separation upon completion of Project (12 total).

2.4 Sequence of operations

In order to meet the minimum 85 percent volumetric capture of combined sewage on a system-wide average annual basis pursuant to the ACJ, it is necessary for the Midland Avenue RTF and Conveyances to be operated in a manner that allows for filling and maximizing storage in the Conveyances, the underground storage tank, and the influent wet well of the RTF prior to initiating the RTF influent pumps. This would be achieved through the setting of the RTF influent pump "on" elevation above the crown of the Conveyances at the RTF.

Additionally, to maximize storage in the remote, upper end of the Midland Avenue CSO transmission pipeline, a weir structure will be required to fully optimize available storage. Preliminarily, this flow control structure has been modeled as a bottom outlet orifice device to limit flow through the structure to 100 cfs until water levels upstream of the structure reach the crown of the Conveyance pipeline. After the flows reach the crown of the Conveyances, the flow control structure would automatically increase flow capacity to the downstream Conveyances to relieve upstream surcharge.

In order to maximize storage in the Conveyance pipelines, the RTF influent pumps would operate until such time as the influent flows recede below the crown of the Conveyances at the RTF. Subsequent to this RTF influent pump shutdown, CSO volume stored in the RTF and Conveyances would be pumped to the MIS for transport to and treatment at Metro.