FILTER UPGRADES: NEW UNDERDRAIN OPTION FOR TRAVELING BRIDGE FILTERS

Traveling Bridge Filters have been used extensively for tertiary treatment of wastewater, wastewater reuse applications and drinking water treatment over the past 50 years. There are over 1500 traveling bridge filter installations throughout the world. These filters use various different types of underdrains and materials of construction. Many units currently in service would benefit from an upgraded underdrain that would substantially extend their useful life and eliminate the need for the extensive capital investment associated with replacing the entire filter.Traditional plate underdrain systems use single piece cell sheets that divide the filter bed into channels and utilize hold down angles to secure the porous plates that support the media. These underdrains typically use aluminum oxide (approximately 8” by 12”) or polyethylene porous plates (approximately 8” by 40”) that require extensive sealing arrangements to prevent sand media loss caused by migration to the effluent channels of the underdrain system. Historically the installation of these underdrain systems has required considerable care and time to ensure adequate sealing and to prevent media loss from the filter bed.Infilco Degremont, Inc. (IDI) has recently developed a new underdrain system that uses a polyethylene porous plate (QuickPlate™) that is 6-9 times larger than previous polyethylene designs. In conjunction with a two piece cell divider system this plate minimizes the media loss from the filter bed. This unique design has increased strength due to the two-piece design and continuously supported porous plates. The QuickPlate underdrain eliminates at least 75% of the adhesive sealant (also referred to as caulk) required for traditional underdrains and uses isobutyl rubber strips to provide a strong seal. The two-piece cell divider system and the large porous plates enable significantly easier installation of the underdrain system and virtually eliminate the problems associated with media loss.This paper describes in detail the design of the new underdrain system including the unique approach to ensuring the integrity of the installation and the features that allow QuickPlate to be the first porous plate wider than a single filter cell. It describes the features and the benefits that greatly improve the performance of traveling bridge filters in treating water and wastewater and details the significant installation labor and material savings achieved at the first two QuickPlate installations in Harris and Dickinson County, TX.