Equipment prevents potential raw sewage bypass from reaching a scenic Minnesota river.
by Steve London

Workers place and secure the volute before the remainder of the pump is received.

Emergency Replacements
Johnson turned to a representative of a factory-authorized brand of pumps that the utility had used exclusively in the other 13 stations along the collection system. A nationwide search located two 80-horsepower, self-cleaning, high-efficiency submersible pumps in Atlanta that were immediately available but would require modest modifications for use in the dry pit versus a fully-submersible installation.

To expedite the process, the city responded by declaring an emergency procurement upon recommendation of its consulting engineering firm. This allowed direct procurement without the time for a customary bid procedure and brought the pumps into service within three months. Large horsepower pumps are rarely held in inventory due to their cost, but the two found in Atlanta  reduced the lead time to manufacture units, according to a spokesman. Only modest piping modifications were necessary for the pumps to mount vertically in Station No. 9’s dry pit.

The pumps feature highly efficient, clog-resistant hydraulics. The semi-open impeller and a relief groove in the volute reduce the risk of clogging and allow for the passage of solids to maintain consistent pumping efficiency, even under high-solids conditions. This feature is important in handling the effluent flow from the local turkey and other meat processing plants. The relief groove in the volute complements the self-cleaning impeller to maintain a consistent flow path through the pump. The concept eliminates most blockages and stringy material that can entangle on a conventional impeller and induce energy-sapping drag.

The expedited delivery and installation resolved the station’s immediate vulnerability and prevented a potential nightmarish bypass once the first replacement pump became operational in June 2012. Because of the recurring blockages, a second factory-manufactured submersible pump was added to the station in mid-October 2012.