Council Bluffs, IA

Screening and Grit Removal Improvements

  • Client City of Council Bluffs
  • Type Wastewater
  • Services Evaluation, Design, Construction
  • Project Manager Keith Hobson, P.E., BCEE
  • Project Engineer Laurie Twitchell, P.E., BCEE

Challenge

The water pollution control plant has been operating at its current location since the mid 1970s. The challenge was to retrofit the aerated grit basins that had deteriorated due to age with a grit removal technology to remove finer “sugar sand” grit particles and keep grit particles out of downstream clarifiers and digesters. By removing grit particles earlier in the treatment process, equipment lasts longer, due to less abrasion, and basins are kept cleaner to maintain their full capacity. Various grit removal technologies were investigated, and the decision was made to use new headcell technology for finer grit removal.

Screening

Screening

Solution

At the inception of the project there were only two installations (one in Oregon and one in Indiana) of the headcell technology, both on relatively small treatment facilities. The Council Bluffs project was the first implementation of the larger diameter (12 feet) units for grit removal; it was the largest installation in the world.

Several challenges were encountered on the project including arranging the headcell units in the existing aerated grit basins, providing a building opening to allow installation of the units, and providing channels to distribute the flow to the various headcells under different hydraulic loading conditions.

Four headcell units were designed and installed with each unit containing seven 12-foot diameter grit removal trays.  The headcells are designed to remove grit down to a size of 75 micron; a size much smaller than the old system removed.  Stainless steel channels were constructed to deliver the flow to the four headcell units.  The grit is pumped from the headcells with five new grit pumps and transported to grit slurry cups where the organic matter is removed from the grit.  A grit “Snail” was also installed to dewater the cleaned grit prior to disposal.

The project included replacement of two existing bar screens with two new fine screens made of stainless steel material to prevent corrosion. Screening spacing was reduced to 3/8 inch to retain more screenings and remove them from the flow stream. Two screening washer presses were installed with the screens to wash the screens of organic material and then compress the screenings to reduce water content and volume for disposal.

Slurry Cups

Slurry Cups

Outcome

Performance of the new headcell units has been outstanding. Much less grit passes through to the downstream treatment process. Completion of this project has also resulted in cleaner grit and screenings that can be disposed of in a more environmentally friendly manner. Collection of additional grit with the new headcell system and slurry cups will reduce wear on downstream equipment and reduce basin cleaning costs.

Implementation of the headcell technology also eliminated the need for additional wastewater pumps at the plant.  Other technologies would require additional hydraulic headloss and pumping of the entire wastewater flow through the units. By eliminating the pump requirements, additional cost savings were realized in both initial capital costs and long term operational costs.

Headcell System Overlay from Eutek

Headcell System Overlay from Eutek