OARS Deep Sewer Tunnel
A crucial requirement of the consent order between the City of Columbus and the State of Ohio is to reduce the environmental impacts caused by Combined Sewer Overflows (CSOs). The OSIS (Olentangy Scioto Interceptor Sewer) Augmentation Relief Sewer, known as OARS, is the key component in meeting this goal. This sewer tunnel will intercept wet weather overflows that currently empty into the Scioto River and carry the flows instead to the city’s Jackson Pike and Southerly wastewater treatment plants.
The construction of Phase 1 began in September 2010 and involved digging a 20-foot diameter tunnel with a total length of about 23,300 feet. The tunnel will provide relief to the existing OSIS trunk sewer from just north of the Arena District to the Jackson Pike facility (see an aerial view of the alignment).
The overall length of the OARS tunnel is just under four and a half miles, and will include three relief structures that will divert wet weather combined sewer flow from the OSIS to the OARS tunnel. The OARS project will control all of the regulators (relief structures where the sewers currently overflow to the river multiple times per year) in the downtown riverfront area to a level in which they might overflow once every ten years on average.
The tunnel will be approximately 170 feet below ground, reducing risks associated with construction at more shallow levels. This will require special drop structures (or vertical shafts) to direct flow from existing shallow sewers to the new deep tunnel. In addition, the drop structures will dissipate energy and minimize air entry into the tunnel. The tunnel shafts are designed to handle hydraulic surges which will prevent impacts on upstream sewers. A screening system will be installed upstream of the Jackson Pike facility. A pump station will empty the tunnel directly to the treatment plants after significant rain events thereby utilizing as much treatment capacity as possible.
The as-bid construction cost for Phase 1 of the project is approximately $264.5 million and for Phase 2, approximately $77 million. Phase 2 of the project involves construction of three shafts, pump station, pump electrical building, river overflow structure and other infrastructure. Construction of Phase 2 began in August of 2011 and is expected to be completed by the end of 2014. Overall, the entire project is expected to be completed in 2017.
To finance the construction costs of Phase 1, the City of Columbus obtained $264.5 million from the Ohio Water Pollution Control Loan Fund (WPCLF) at a 3.25% interest rate over a 20-year construction loan. The city has requested similar funding for the OARS Phase 2 construction. Financing this project through the Ohio EPA’s below-market interest rate loan program will save millions of dollars in interest payments over the life of the loan, compared to financing through higher interest rate bonds. The savings will be passed on to the city’s sewer utility rate payers, thereby minimizing the slight increase expected in sewer rates over the next several years due to the cost of this project.