SITUATION

The existing perforated plate screens in the headworks were not performing well and required significant maintenance. The project objective was to replace the underperforming headworks screenings removal and handling equipment with more reliable equipment to increase screenings capture and to reduce the operations and maintenance (O&M) effort. LEE + RO, as design engineer of record, teamed with Stanek Constructors, as the prime contractor, for IEUA’s $1.9 million RP-4 Influent Screen Replacement design-build project.

Challenge

Replacing the headworks screenings removal and handling equipment posed a few unique challenges on top of the typical challenges associated with a headworks rehabilitation project in an active wastewater treatment plant.

"Cutting-edge techniques like 3-D scanning were utilized to confirm wall plumbness of the deep channels, ensuring that the new screens fit in the existing channels without concrete modifications."

Solutions

1.Plumbness of the deep channel walls was in question.

The influent channel walls are 18-feet deep, which raised concerns about their plumbness. This could, in turn, complicate the bar screen installation and warrant structural modifications. To determine the extent of this potential issue, a 3-D scan of the influent channel was performed.

"Several industry standards and guidelines were consulted to estimate the anticipated screenings volume in order to size the screenings handling equipment in the absence of reliable historical screenings off-haul data"

2.Lack of reliable historical screenings off-haul data.

The plant was equipped with screenings removal equipment for decades, so it would only be logical to use the plant screenings off-haul data to size the new screenings handling equipment. However, the screenings off-haul data was deemed unreliable since the plant staff firmly believed that the existing screens were not performing up to par because of screenings bypassing the existing screens. This was confirmed when the existing screening data was analyzed to determine the average screenings removal rate and compared to industry standards and guidelines. To provide the IEUA RP-4 plant a greater degree of operational flexibility to handle the large seasonal screenings load variations, a conservative design guideline was used to size the new screenings handling equipment.

"To avoid potential controls interface issues resulting from mixing and matching screening removal and handling equipment from different manufacturers, the washer compactor offered by the manufacturer of the preselected bar screens was thoroughly researched and studied to confirm its suitability for this application."

3.Controls interface between the screenings washer compactor and the bar screens.

The industry commonly installs screens and screenings washer compactors from the same manufacturer to avoid complicated controls interface issues. It was not so straight forward on this project as the screens were pre-selected by IEUA and the design team and plant staff were not familiar with that manufacturer’s screenings washer compactor. To avoid potential controls interface issues, the pre-selected screen manufacturer’s screening washer compactor was thoroughly researched and compared to the screenings washer compactor equipment commonly used in the industry to confirm that it would be a good choice based on economic and non-economic factors.