Anaerobic Digesters Receive More Than a Facelift!

5 WGG - CB Digester Complex Upgrade.pdf

The Council Bluffs, Iowa, wastewater plant was built in the early 1970s and, in spite of conscientious maintenance, the anaerobic digester complex was showing signs of age.

FOX designed improvements to the NW primary digester cover after assisting with the SW primary cover repairs several years earlier. Evaluations of the structural stability of support beams and gas collection dome were conducted to identify needed repair from corrosion.

The project included removal of existing roofing, reuse of the precast concrete roofing panels, and repair of the structural portions of the cover. The cover was then reinsulated and new membrane roofing was installed. The digester gas compressor and controls for digester mixing were replaced. In addition, the original gas safety equipment and the hoses to remove methane gas and biosolids were replaced.

In 2009, FOX Engineering was hired to evaluate the capacity of the anaerobic digesters, as well as the electrical and structural integrity of two secondary digesters and the associated equipment in the entire digester complex. The study included assessment of the existing floating gasholder covers, biogas safety equipment, sludge pumping and heating system, and the digester gas use and waste gas burner.

FOX looked at alternative methods of biogas holding and utilization. It was determined that the existing, almost 40 year old, floating covers were in need of extensive structural repair, but still had many years of service left. Replacement steel covers or membrane covers were also evaluated. The sludge heating systems (combination heat exchanger/boilers) were deemed at the end of their useful life and required replacement with new, more efficient equipment. In addition, new grinders were installed upstream of the sludge recycle pumps and a new motor control center was recommended. All of the original gas safety equipment needed to be replaced and a new waste gas burner was installed.

For this project, each gasholder digester cover was removed and placed onto the ground with a large crane so that the contractor could complete the repairs to the covers and tanks, sandblast and repaint, and then replace it in the digester. The contractor decided to lift the covers with the ballast in place – the total weight of each digester cover was 72 tons when it was removed. By the time the repairs were completed, approximately 8 tons of additional steel had been added to each cover. The 500 ton crane required seven semis to deliver it to the site!

The contractor for the project, SBI General & Mechanical, was able to manufacture new rollers and guides in their machine shop. Ed Jankowski, a public works engineer (now retired) with the city, had sketched up a design for a sliding/rolling  walkway onto the floating covers that would also serve as a support for the sludge and biogas hoses. FOX was able to fine-tune this idea and incorporate it into the design of the digester repairs.
 
The Council Bluffs WPCP staff take a great deal of pride in the maintenance of their facility; choosing repair and reconditioning over replacement of the almost 40 year-old covers, the city was able to save significantly and provide reliable digestion and gas collection for the future. 

Contact:
FOX Engineering Associates, Inc.
Winnie G. Gleason, P.E.
800.433.3469
www.foxeng.com