2022/09/07 Council Agenda Packet

N� EXHIBIT A G� R K ENGINEERING SERVICES THE CITY OF EVERETT WATER POLLUTION CONTROL FACILITY COMBINED SEWAGE CONVEYANCE IMPROVEMENTS ALTERNATIVES PROJECT INTRODUCTION The purpose of this project is to determine the preferred approach for bypassing combined sewage (CS) flows at the Water Pollution Control Facility (WPCF) to the Oxidation Pond. Combined sewage (CS) flows that exceed the plant headworks and the primary clarifier capacities are bypassed to the head of Aerated CeII 1. These flows pass through Aerated CeII 1 to Aerated Cell 2 and then into the Oxidation Pond recirculation channel. High CS bypass flows wash solids out of the aerated cells into the recirculation channel. The Oxidation Pond is not designed to handle these solids causing operation and maintenance issues The Combined Sewage Conveyance Improvements Project will route the CS bypass flows directly to the Oxidation Pond recirculation channel, preventing solids washout from the aerated cells to the Oxidation Pond. A study was performed in 2021 to identify changes needed to the CS bypass conveyance system to route flows directly to the Oxidation Pond recirculation channel. The study evaluated plant hydraulics, identified maximum bypass flows, provided a concept for CS bypass conveyance changes needed, evaluated alternative conveyance conduits, and set forth a recommended improvement project. The study is documented in the Engineering Report, Everett WPCF Combined Sewage Conveyance Improvements, January 17, 2022, Stantec (CS Conveyance Study). The CS Conveyance Study recommended the CS bypass flows be redirected immediately after Diversion Structure 1 to the Oxidation Pond recirculation channel. The proposed project constructs a diversion structure downstream of Diversion Structure 1 to intercept the bypassed CS flows and route them through two buried 78-inch diameter pipes constructed using open -cut methods through the adjacent wetland and Oxidation Pond embankment to the recirculation channel. An alternative to the CS Conveyance Study recommendation is to convey CS bypass flows to the location where flow passes from Aerated Cell 2 to the Oxidation Pond recirculation channel Currently five pipes convey flow from Aerated Cell 2 to the Oxidation Pond recirculation channel. These pipes, and the culverts that connect wetlands on each side of the pipe crossing, are aged and scheduled to be replaced. This alternative project would combine the CS Conveyance Improvement project with replacement of the conveyance system between Aerated Cell 2 and the Oxidation Pond recirculation channel and the wetland culverts in one project. Under this scope of work, the Consultant will compare the alternative CS bypass project to that recommended in the CS Conveyance Study and facilitate selection of the preferred alternative by the City. Following selection of the preferred alternative, the Consultant will prepare predesign documents, prepare applications federal and state resource permits authorizing the construction, and complete detailed design and pv:\NA\Everettl_Proposals\Combined Sewage Conveyance Improvements\AdmrnVA PCF CS Bypass Project.doc Page 1 of 5