Facility Plan Amendment
Introduction
1. Background
2. Supplement
3. Conveyances

4. RTF
5. Schedule
Appendices

Midland Avenue RTF and Conveyances: Facility Plan Amendment, June 2003
by Environmental Engineering Associates, LLP
for Onondaga County Department of Water Environment Protection

4. Midland Avenue RTF

Quick reference
— 4.1 General
— 4.2 Description of facilities and operation
— 4.3 Design flow rates
— 4.4 Preliminary basis of design

4.1 General

This section provides a description of the facilities and operation, design flow rates, and preliminary basis of design for the Midland Avenue RTF.

4.2 Description of facilities and operation

The purpose of the Midland Avenue RTF is to provide high-rate treatment, including floatables removal, settleable solids removal, and disinfection of CSOs up to the one-year design storm event. As shown on Figure 2-3, the RTF will be located in the vicinity of Oxford and Blaine Streets on the east side of Onondaga Creek adjacent to the existing CNYRTA bus garage.

As indicated in the CSO volumetric capture table for existing conditions (Table 3-2), approximately 200 MG of untreated combined sewage is discharged from the Midland Avenue CSO service area to Onondaga Creek on an average annual basis. Upon implementation of the Midland Avenue RTF and Conveyances Project, there will be no untreated combined sewage discharged from the Midland Avenue CSO service area up to the one-year design storm event.

It is anticipated that the Midland Avenue RTF will receive influent flow from the Midland Avenue CSO service area approximately 50 times per year. The majority of these events and associated flows will be stored in the conveyances, underground storage tank and RTF. Long-term simulation sewer system modeling for the service area sewer indicates that on an average annual basis there will be approximately nine treated discharges per year from the RTF, totaling approximately 77 MG of treated, disinfected and dechlorinated effluent.

When flows exceed the storage capacity of the conveyances, 2.5 MG storage tank and influent wet well, flows will discharge to the RTF, which will include the following treatment processes:

Influent stilling well/grit sump
Coarse screening/manually cleaned trash racks
Influent pumping
Vortex solids separators (USEPA swirl concentrator) to remove floatables and settleable solids
Pumped conveyance of vortex solids separator underflow and floatables to the Main Interceptor Sewer (MIS)
Pumped dewatering flows from vortex separators, disinfection tank and influent wet well to the MIS following a storm event
Disinfection by chlorination/dechlorination
Outfall to Onondaga Creek for treated overflows
Emergency controlled diversion for RTF facilities/equipment protection during unexpected operating conditions
Odor control facilities.

All treatment components will be designed in accordance with the latest edition of the Recommended Standards for Wastewater Facilities (Ten States Standards).

Figure 4-1 is a prelim plan for influent pumping, vortex separators, odor control, and chemical feed/storage facilities.
Figure 4-2 is a preliminary section these same items.
Figure 4-3 shows a preliminary plan for the RTF underground disinfection tank.
Figure 4-4 shows a preliminary section of the disinfection tank.
Figure 4-5 presents a preliminary hydraulic profile for the RTF.

As described in Section 3.5, combined sewer flow will enter the RTF influent stilling well/grit sump from the Midland Avenue Conveyances/Off-Line Storage Facility. With the lead pump "on" activation elevation set above the crown of the conveyances at the RTF, continued flow into the RTF will backup into the conveyances and upstream, underground storage tank. Flow storage will continue in this manner until the maximum upstream storage of 6.7 MG is achieved. When this maximum storage is achieved, the lead influent pump will activate and pump RTF influent flows through the downstream unit processes. Vortex separator treated flows will be discharged to the disinfection tank and will be chlorinated/dechlorinated prior to discharge.

For smaller storm events, it is anticipated that, on occasion, there will be flows that enter the RTF disinfection tank that will not be discharged to Onondaga Creek. These stored flows will be returned to the MIS and conveyed to Metro for subsequent treatment, following the storm event.

All stored CSO within the RTF, conveyances and underground storage tank (8.45 MG total storage capacity) will be pumped back to the MIS for subsequent treatment at Metro by dewatering pumps located in the RTF. Sluice gates will be used to control the conveyance of stored flows from the conveyances, underground storage tank and underground disinfection tank to the RTF influent stilling well/grit sump, with subsequent conveyance into the RTF dewatering wet well. The dewatering pumps will be sized to dewater the maximum stored volume of 8.45 MG in a maximum 48 hour time period pursuant to NYSDEC stated requirements for CSO storage facilities and accepted operating procedures.

Following a CSO event at the RTF, County certified collection system maintenance staff will perform maintenance and cleaning of the RTF and prepare the RTF for the next CSO event. Flushing gates will be installed in the upstream underground storage tank and tipping buckets will be installed in the underground disinfection tank to facilitate the washdown of these large structures. Manual washdown facilities will also be provided throughout the RTF to accomplish effective facilities washdown following a CSO event. Standard Operating Procedures for the RTF will be developed by County staff during the detailed facilities design and construction.

4.3 Design flow rates

The proposed Midland Avenue RTF will be designed to convey and provide treatment up to the one-year design storm event. Based on the intended closure of select existing CSOs (see Section 2.4) and the Midland RTF Size Evaluations (see Appendix 6), the required influent pumping rate of the RTF is 330 cfs. This influent pumping rate allows for maximizing the storage in the Conveyances/Off-Line Storage Facility and prevents overflows from the remaining CSOs in the Midland Avenue service area (i.e., CSOs 039, 042, and 044; RTF emergency controlled diversion) up to the one-year design storm event.

Although designed to process a peak RTF influent flow rate of 330 cfs, in accordance with Ten States Standards the RTF will be designed to include a standby influent pump that is equivalent in capacity to each of the two equal capacity standard operating pumps. To achieve supplemental CSO treatment for the Midland Avenue CSO service area, the RTF will be hydraulically designed to process the full influent pumping capacity of the RTF, i.e., 495 cfs.

Although there is anticipated to be a reduction in the quality of the treated effluent from the vortex separators into the disinfection tank at this higher influent flow rate, downstream disinfection/dechlorination of this additional flow prior to its discharge to Onondaga Creek will achieve additional environmental benefits. At this higher influent pumping rate (i.e., 495 cfs), sewer system modeling performed by Brown and Caldwell indicates that the RTF emergency controlled diversion will activate at the 2-year design storm event and above.

4.4 Preliminary basis of design

Appendix 9 presents a preliminary basis of design for the key components of the Midland Avenue RTF.