SOUTHERN TIER INTERCEPTOR SYSTEM
Des Moines, IA
In 1988 Veenstra & Kimm, Inc. was retained by the City of Des Moines on behalf of the regional wastewater authority to design the Southern Tier Interceptor. The Southern Tier Interceptor project is located in portions of the Des Moines River basin near its junction with the North River basin as well as in both the North River and Middle Creek basin. The Southern Tier Interceptor consists of two segments of gravity sewer, a pump station and a force main. The Southern Tier Interceptor was designed to allow for the abandonment of five treatment facilities.

The Southern Tier Interceptor trunk sewer consists of approximately 61,000 feet of trunk sewer ranging in size from 18-inch diameter to 66-inch diameter. The peak design flow for the Southern Tier Interceptor is approximately 27 mgd.

In addition to the trunk sewer component of the Southern Tier Interceptor the project included the design of a pump station and force main. The pump station is used to lift the flow from the North River basin to the Des Moines River basin. The vertical lift from over the ridge between the North River and Des Moines River basin is approximately 120 feet. Taking into account friction loss in the 9,000-foot force main, the total design head ranges from approximately 170 feet to 190 feet.

The design of the Southern Tier Pump Station took into account several design challenges, including peak flow rates from existing subsystems with relatively high flows and the projected increase in flow as development occurred within the basin.

The design challenges for the Southern Tier Interceptor were addressed through the use of a unique two-stage pumping station. The first stage of the pump station is an Archimedes' screw pump station. The screw pumps are used to lift the flow from the trunk sewer to ground elevation. The flow from the screw pumps can be conveyed to the wet well for the second stage pumps or to a diurnal equalization basin. The diurnal equalization basin is used to maintain a relatively constant flow to the second stage pumps. The diurnal equalization basin can be used to assist in solids removal prior to second stage pumping.

The second stage of the pump station consists of four high head centrifugal wastewater pumps. The pumps are designed for a flow rate of approximately 5,000 gpm at a TDH of 195 feet.

The pump station discharges to a single force main which conveys the flow to the lower reaches of the Southern Tier Interceptor. Under low flow conditions the wastewater flow does not maintain minimum scouring velocity. The pump station includes use of the diurnal equalization basin to store flow and allow for periodic and programmed enhanced pumping rates to maintain scouring velocity on a periodic basis. This self-scouring protocol has maintained a self-cleaning velocity within the force main even with relatively low flow.

The total cost for the Southern Tier Interceptor system was approximately $17,500,000. Design and construction of the project was completed over an 18 month period utilizing three sewer construction contracts and a separate lift station contract.