Start off on the Right Foot with Fine Screen Selection
- Colin Beck, PE; Simon Lobdell, PE; Patricia Drummey Stiegel, PE
Screening involves the physical separation of objects from wastewater in order to ensure the operability and effectiveness of all downstream processes. Target materials can range from large debris such as logs, to medium sized rags, down to fine objects such as human hair. In general, screens are categorized by the size of the openings or apertures and the technique of solids removal.
An easy trap is to assume that the goal should be to capture as much of the solids from the flow stream as possible. The actual purpose in most wastewater applications is to selectively remove only those materials that may negatively impact downstream processes. The larger debris material coarse screens remove may block flow paths, damage pump impellers, accumulate in digesters or hang on the myriad pieces of rotating equipment in the typical wastewater plant. However, the selection criteria for fine screen technology is a bit more complicated.
Important fine screen selection criteria include performance, capacity and maintenance. Performance is gauged by identifying the type of materials requiring screening and the owner’s tolerance for pass through. Capacity involves the relationship between aperture size, aperture type and gross flow area. Smaller screen apertures increase screenings capture, but also increase headloss as a result of the decreased flow area. Different types of screen openings and materials can target more specific screenings such as fibers as opposed to granular materials. Finally, if operations and maintenance expectations are not met, the system may simply be abandoned in frustration. Examples of O&M concerns include screenings washing, screenings conveyance, bypassing, mechanical replacement and off-site disposal to name a few.
Hazen and Sawyer worked together with the City of Durham and successfully completed a fine screen selection process for preliminary treatment upgrades at both the North and South Durham Water Reclamation Facilities. Hazen and Sawyer and the City of Durham are confident that the selected centerflow band screens will result in the best treatment process start possible.
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