SLUDGE AND SCUM-SUCKING SUBMERSIBLE SEWAGE PUMP STATION (A UNIQUE APPROACH TO THE SELF-CLEANING WET WELL DESIGN)

The City of Harrisonville, Missouri, a municipality in the Kansas City metropolitan area, experienced frequent basement backups and overflows in their sanitary sewer collection system. A study of their collection system, completed in 1999, recommended a plan of action to address the capacity problems. This plan included the replacement of a major pump station that conveyed flow to the wastewater treatment plant.This paper presents the design approach taken to replace the Pump Station No. 2 with a new pump station employing several unique features. The paper concludes with a discussion of lessons learned during the project and points to be considered when undertaking the design of a trench-type self-cleaning wet well.In 1990's, several nationally-renowned experts in pump station design and the Hydraulics Institute (HI) began to promote the concept of trench-type “self-cleaning” wet wells for sewage pump stations. Benefits of this wet well configuration include reduction of deposited and floating solids in wet wells, smaller foot prints and lower costs than traditional pump stations, and reductions in wet well flow currents that cause pump intake problems.Based on these benefits, the decision was made to employ this wet well design for the new Pump Station No. 2. However, several hurdles stood in the way of moving forward with the design. First, this design had not been used in the Kansas City metropolitan area. This lack of experience was resolved through two avenues. Discussions were held with Arnold Sdano, of Fairbanks Morse Pump. Mr. Sdano is one of the working committee members for HI that produced the American National Standard for Pump Intake Design (ANSI/HI 9.8-1998). Mr. Sdano's provided input into the design.Additional input came from Mr. Robert Sanks, the principal author on several papers regarding the self-cleaning wet well design and a member of the HI working committee. After listening to a paper presented by Mr. Sanks at the 2002 WEFTEC, an opportunity occurred in which to discuss the proposed design with Mr. Sanks. Several ideas were presented and incorporated.Another hurdle revolved around pumping a wide range of flows, from 2.3 MGD to 18.4 MGD. Through discussions with the City, the decision was made to reuse the existing 12-inch force main for dry weather flows and install another 20-inch force main for conveying wet weather flows. The hydraulic design included two (2) dry weather pumps that pumped through the 12-inch force main directly to the headworks at the City's 3.0 MGD wastewater treatment plant.Three (3) wet weather pumps were designed to pump the remainder of the flow through the 20-inch force main directly to a flow equalization basin on the WWTP site.Modifications of the self-cleaning wet well design were made to enable the station to accept the wide range of flows. A second, parallel wet weather trench was added to allow for segregation of the two types of pumps. To prevent flow from stagnating in the second trench, a weir wall between the trenches was designed to be automatically topped if the dry weather pumps can not pump the flow. A sump pump was also added to dewater the second trench following the end of the wet weather event.The paper describes modifications of the trench-type “self-cleaning” wet well for submersible pumps. This includes addition of a suction tube with internal guide vanes connected to the bottom of each submersible pump and an anti-rotation baffle at the end of the dry weather trench.The paper discusses the lessons learned during the project from preliminary planning to start-up. As an example, the paper relates the problems encountered with the fabrication and installation of the anti-rotation baffle provided with the pumps and will present an approach to addressing this issue on future projects.The paper concludes with recommendations to be considered when pursuing the use of the trench-type self-cleaning wet well design including modifications to the designs for submersible pumps and planning level cost estimates for trench-type “self-cleaning” pump stations versus traditional designs.