Garco Construction Inc., of Spokane, has submitted an apparent low bid of $34.1 million to build and install two new digesters at the city of Spokanes Riverside State Park sewage-treatment plant.


The 130-foot-high digesters, which process sewage sludge during the waste-water treatment process, will be shaped like eggs. They will replace three digesters, including one that was destroyed in a May 2004 explosion that killed one worker and injured others.


The digesters use a newer technology thats being applied in the waste-water plants of larger cities such as Los Angeles, Boston, and Washington, D.C., says Tim Pelton, administrative superintendent at the Riverside State Park facility, at 4401 N. Aubrey L. White Parkway. The new technology, focused primarily around the smaller diameter and increased height of the tank, reduces costs and maintenance, he says.


By separating the scum at the top and the gravel on the bottom, and by pumping out the new tanks periodically, we wont have to put workers inside to shovel and hose out the interior, and will have less down time, Pelton says. At our current projections, these digesters will serve the citys sewer needs of about a 5-million-gallon capacity until 2015.


The new digesters each will have a capacity of 2.8 million gallons of biological solids, compared with the 2.25-million-gallon capacity of each of the two older remaining digesters and the one that was destroyed, Pelton says. Still, the usable capacity of those old digesters is limited by the fact that one has to be removed from service for maintenance purposes every three years, for up to two months, he says.


Current plans are for work to begin in June on the new digesters, with the steel tanks to be on line between the fall of 2007 and early 2008, says Pelton.


Because they are smaller in diameter than the old tanks (which are only 40 feet tall), they will be easier to clean, and we wont have to drain them so often, Pelton says.


He also says the new digesters will be constructed of steel, instead of concrete like the current ones, making them safer because they will be able to withstand more pressure than the old tanks.


Other projects under way at the treatment plant are the relocation of a cogeneration building so that its not too close to the new digesters, and a changeover from a gas to a liquid chemical to treat waste at the facility, Pelton says. He says within the next five years the waste-water plant will be using an ultraviolet disinfection process to treat waste.


Contact Rocky Wilson at (509) 344-1264 or via e-mail at rockyw@spokanejournal.com.