In September 1999, the Michigan Department of Environmental Quality issued the City an Administration Consent Order (ACO) to address wet weather sanitary sewer overflows. The ACO required the City to eliminate all emergency overflows at the City's wastewater treatment plant (WWTP) associated with weather events up to those generated by 25 year-24 hour storm event.

A 43.5 MG open earthen storage basin along with a 52 MGD pump station was designed, and is currently near completion, to address the ACO requirements. The new basin was constructed on a site adjacent to the WWTP with known low levels of soil contamination, which led to additional constraints during design and construction as follows:

• Earth balancing was critical because only the first 6" of soil was permitted to be excavated, yet the interior of the Basin had to be graded to allow complete drainage. Fill also had to be controlled to maximize the volume available for storage.
• A Health and Safety Plan was prepared to protect both construction personnel and the public while work was performed at the site.
• Dust had to be controlled and minimized.
• Allowances were included in the contract for handling and disposal of any contaminated materials (Type I or II), none of which were encountered during construction.
• Approximately 185,000 cubic yards of suitable clay were required to construct the Basin embankments and liner system. The City's landfill site was used as a borrow source after testing confirmed the materials there met the requirements for the clay liner. The clay was required to be capable of compaction to achieve a permeability of no greater than 1 x 10-7 cm/sec.

The following are benefits resulting from implementation of the 43.5 MG Storage Basin:

• Allowed the City to comply with their ACO issued by the MDEQ.
• Emergency bypasses to the Tittabawassee River of partially-treated sewage generated by the City's collection system will no longer occur during wet weather events up to the 25-year storm. This will be a significant benefit to the environment by reducing the pollutant load to the River.
• The GCL liner system also substantially reduces the infiltration of rainwater into the soil below the basin, reducing the impact of any existing contamination on the groundwater.
• Using the City's landfill as a borrow area for clay has begun the initial development of a landfill cell for Type II refuse.
• The Storage Basin was built on property which was not suitable for other construction, thereby saving prime developable City property from being used for the large basin.