Bethel Park, Pennsylvania installed two 4MGD traveling bridge filters in 1980. According to Joe Witkowski, who manages the facility, "We had serious media leakage with our old filters. We had continuous backwashing and mechanical problems - they just couldn't handle high flows. Plus, they had serious media leakage problems."
The problem recently was solved by retrofitting the traveling bridge filters with Gravisand technology from USFilter-Davco Products. This technology incorporates proprietary tubular non-segmented underdrain components which eliminate media loss and are not subject to irreversible biological fouling. Another significant advantage unique to the Gravisand approach is a "No-Wear" backwash shoe.
In addition, this technology uses a unique method to clean the filter media. Rather than using a continuously moving bridge - which is known to displace the filtration media - the Gravisand approach employs an indexing system that isolates each filter cell to ensure maximum media cleaning without the troublesome media migration problems that have haunted old-style, two-pump TBFs. The indexing system also eliminates the frequent filter-bed maintenance common with old-style TBFs.
Older TBF design employed a strip liner-contact seal which caused a side load on the bridge and, in addition to seal wear, resulted in bridge alignment problems. The Gravisand approach, however, does not use a strip liner-contact seal but utilizes a non-load "expansion shoe" that seals directly to the basin wall automatically upon pump energization at the backwash point.
"We retrofitted with Gravisand components and I will tell you I am well pleased. The backwash is beautiful and there's no way they can leak media now. It's simple, we don't have to shovel sand anymore," explains Mr. Witkowski.
Gravisand components use a simple "bolt-in" design and are particularly beneficial to installations that have older, existing, two-pump filter systems in need of an upgrade.