Project Profile: St. Charles, Missouri

In 2008, HDR, Inc. recommended the replacement of brush aerators and internal “boat clarifiers” in three parallel operated oxidation ditches.  The brush aerators were experiencing problems with icing and shaft breakage, and many of the aeration “stars” were corroded and had broken off.  The boat clarifiers—originally designed as a pre-clarification step to lower the Solids Loading Rate on the existing final clarifiers—were inefficient and interfered with channel velocity, causing mixed liquor suspended solids (MLSS) to settle in the basins.  The ditches  were  retrofitted with Evoqua with 12 VLR mounted disc aerators to achieve a  7.54 MGD design flow plant, which allowed for the treatment of higher organic loading within the existing footprint. The disc aerators resulted in lower noise, provided better mixing, minimized dead zones and resulted in fewer odors resulting from good oxygenation within the basins.

The existing oxidation ditches flow pattern were changed from parallel to series to create independent environmental zones to increased the nutrient removal capacity of the plant. This allowed denitrification of recycled nitrates, as well as simultaneous nitrification/ denitrification.  This process allows some of the influent ammonia to be converted from nitrite directly to nitrogen gas, reducing energy consumption and total nitrogen discharges.  Controlling the oxidation-reduction potential of the first tank in the series also enhances the biological phosphorus removal capacity of the system. The series flow pattern increases the efficiency of treatment without increasing tank volume.  Denitrifying bacteria use the chemically combined oxygen to stabilize influent biological oxygen demand, rather than absorbing mechanically injected dissolved oxygen.  Reducing mechanical oxygen injection reduces purchased power.  The design model indicates the new treatment system is 20–25% more energy efficient than the parallel brush aerator installation.

The disc aerators provide access platforms to the drives and outboard bearings, making them easier to maintain.  Because the discs have no deterioration or corrosion, the expected lifetime of the system is 20+ years.

Fort Dodge, Iowa

Expansion of the existing VLR system to increase primary treatment capacity to 12.0 MGD and secondary treatment to 18.6 MGD. The schedule for the project was very tight. McClure Engineering designed the plant with a start-up in mid-2014. The contractor was Gridor Construction of Buffalo, Minn. The plant is operated for the city by US Water.