Garco Construction Inc. is the apparent low bidder for an $800,000 piping system modification project at the city of Spokane's Riverside Park Water Reclamation Facility sewage-treatment plant, in northwest Spokane.

The Spokane office of the Denver-based engineering firm CH2M Hill Inc. designed the system improvement, which is part of a $100 million second phase of improvements that started about a year ago.

The improvement is designed to provide operational flexibility and enhanced safety as the plant manages foam buildup in two large egg-shaped digesters, says Lars Hendron, the city's principal wastewater management engineer. The plant's digesters use a bacterial process to convert sludge in wastewater into fertilizer.

"The sludge will produce a lot of foam," Hendron says. "Our concern with the foaming is we want to make sure it doesn't overwhelm the digester gas systems. It would cause pressure problems within the digesters."

Hendron says that if too much foam accumulates, the piping modification will allow the plant to "pump sludge between the two digesters and therefore reduce the level of foaming within a digester if it's having a foaming problem."

A gas transfer pipe will be installed above the two digesters where the piping is least likely to be affected by foam or to become plugged, and a piping modification will be placed at the liquid level that ties into both digesters to allow draining if pressure in one of the two gets too high.

"We often run these digesters with different levels," Hendron says. "If one is getting higher, we can quickly transfer to the other one, so it's an equalizing pipe essentially. If we drop the liquid level that the foam sits on top of (in one digester), then the foam can build up without pressure."