Red Oak, IA

Wastewater Treatment Improvements

  • Client City of Red Oak
  • Type Wastewater
  • Services Evaluation, Design, Construction
  • Construction Cost $6.8 million
  • Project Manager Steve Van Dyke, P.E., BCEE
  • Project Engineer Jennifer Ruddy, P.E.

Challenge

The city had operated a trickling filter wastewater treatment plant since the early 1960s. The plant was expanded and rehabbed in the late 1980s. Since that time, the facility reliably met its effluent limits and provided Red Oak with more than 20 years of operation with relatively few improvements.

However, the treatment facility was facing several challenges. Recent changes in Iowa’s Water Quality Standards required disinfection of the plant’s effluent. Several structures were not adequately protected against flooding, which occurs at the plant site due to its location near the East Nishnabotna River. Much of the treatment equipment and several structures required repair and/or replacement due to age and general deterioration, and some did not meet current regulations for redundancy and fire protection, among others.

Pump Station

Pump Station

Solution

In response to these concerns, the city hired FOX Engineering to evaluate the treatment plant and provide design, bidding, and construction services for improvements recommended in the facility plan. Bidding was purposely delayed several months to accommodate potential funding requirements. This proved to be very beneficial, as FOX helped the city secure a $2.0 million I-JOBS grant for the project. The remainder of the project was funded with an SRF loan and local funds.

Improvements took place throughout the plant, starting with a new raw wastewater pump station at the front end of the plant and ending with a new ultraviolet light disinfection system at the tail end. The new raw wastewater pump station is equipped with five pumps; the two smaller pumps deliver normal flows, up to 4 million gallons per day (MGD), to the treatment plant.

When peak flows occur due to inflow and infiltration in the collection system, two of the larger pumps are dedicated to deliver flows above 4 MGD to the flow equalization basin. With all four pumps in operation, the peak capacity of the pump station is 12 MGD.  The fifth pump serves as common standby, capable of pumping to either the plant or equalization basin.

The pump station includes a trench-type, self-cleaning wet well with ramped entrance. This design was used to minimize the volume needed and to facilitate a simple and easy cleaning procedure. The pump station includes a mechanical bar screen and screenings washing press.

Grit removal equipment was also replaced; the old, heavily corroded steel grit chamber was replaced with an aluminum grit chamber, and the grit pump and piping were replaced. Grit washing equipment was added, providing grit that can be land-filled.

Pumps

Pumps

The plant included a trickling filter and two biotowers; all three of these units were maintained in use in the upgraded plant. Pumps for the trickling filter and biotowers were replaced, and improvements were made to the pump piping to improve accessibility for maintenance. One of these pumps had been in nearly continuous operation for 25 years before finally failing during the construction project. The rotary distributors for the two biotowers were replaced due to age and corrosion issues.

The existing plant had included two final clarifiers, and a third clarifier was added to meet current reliability requirements. Fiberglass launder covers were installed at all three clarifiers to prevent problems with algae, and the mechanisms for all of the existing clarifiers were sandblasted and painted.

A new ultraviolet (UV) light system was installed to provide disinfection. UV disinfection is not always well-suited to trickling filter plant effluent, so FOX coordinated with the city to perform extensive testing during design. Testing revealed that Red Oak’s effluent was actually a better candidate than most activated sludge plants for UV disinfection.

 UV Disinfection

UV Disinfection

All of the equipment for the two anaerobic digesters was replaced as part of the project. Two new steel floating digester covers were installed, including draft tube mixers. The boiler was replaced with a new unit that can use either gas produced in the digesters or natural gas as fuel. The boiler provides heat to the building and digester tanks. The gas safety equipment and gas flare were also replaced as part of the project.  A new floating mixer was added to the sludge storage tank.

Change orders for the project were less than 0.65% of the original bid amount.

Outcome

The project was bid in June 2010.  In July 2010, the construction contract was awarded in the amount of $6,797,000, well under FOX Engineering’s estimate of $7,000,000. Work began in September 2010, and the project was substantially complete by March 2012.

As is typical for wastewater plant rehabilitation projects, some surprises were found during construction. For example, when the old digester covers were removed, it was found that the old concrete corbels that support the covers were mostly rotted away and needed to be replaced.

Even so, change orders for the project were less than 0.65% of the original bid amount. The upgraded facility has performed very well since construction was complete.

Clarifiers

Clarifiers

Packed tower

Packed tower