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COLLECTION SYSTEM O&M CONSIDERATIONS: Reduced pumping costs, enhanced force main flushing, and equalized flow to the treatment facility with system configuration and SCADA control

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The planning and design of an all-new 13.4 million wastewater collection and treatment system for the Community of Dallesport, Washington included an emphasis on

operations and maintenance (O&M) considerations. Key issues addressed during project development have included minimizing labor requirements, minimizing power usage, optimizing flushing capabilities, and reducing capital costs by partially equalizing flows to the treatment facility. Addressing these issues has been done in two major steps. The first step – planning – included careful consideration of what type of collection system was most appropriate for the project. The second step – system design – included consideration of innovative features to lessen operation and maintenance requirements, optimize system performance, and minimize capital costs. This paper outlines the basis for the selection of collection system components and describes some of the ideas considered during design such as:

Equalizing flows to the wastewater treatment facility by rethinking the proposed system layout and by telemetry control.

Selectively using grinder systems for individual connections and a neighborhood connection to eliminate the need for an additional lift station.

Using telemetry control of multiple lift stations to minimize pumping costs.

Using telemetry control of multiple lift stations for automated flushing

The Dallesport Wastewater System addresses a long-term need on the Dallesport peninsula in Klickitat County, Washington. Failing septic systems has been a recurring problem in this community of 1,000 over the years. The failing septic systems and the need for sewer infrastructure at an Industrial Park owned by the Port of Klickitat provided an impetus for this project. Shallow soil, a predominance of basalt rock at or near the surface, a relatively large service area (two residential areas of three square miles and two square miles, respectively) with relatively low population density, and strong population growth projections provided challenges to the Owner and the Engineer.

One of the solutions developed uses telemetry control of multiple lift stations to minimize pumping costs. Two lift stations pump into a common 10″ force main. A third lift station will eventually pump into the force main as well. While the system is capable of operating with both lift stations operating simultaneously, it was desired to limit the frequency of these events. Spread spectrum radio telemetry not only offers the advantages of remote monitoring and⊘or operation, but also allows operating strategies to be developed to offset lift station startup sequences in most situations.

The peak hydraulic flow requirements of the new wastewater treatment facility (WWTF) was minimized by a combination of activities including the telemetry control system described previously and by rethinking the originally planned layout of the system. Original planning had the force main discharging directly to the treatment facility. The revised layout included placement of about 3,000 ft. of gravity pipe between the treatment facility and the force main discharge. This allowed for some attenuation of peak flows, eliminating a lift station in the industrial park, and eliminating the need to either slightly increase the hydraulic capacity of the treatment facility headworks or place variable speed drives on the lift station pumps.

The three parallel lift stations – including the planned future station – each provide adequate scouring velocities in their respective force mains. The combined two-mile force main, however, is a larger diameter and one lift station operating independently provides only the bare minimum in scouring velocities. Through telemetry control, the system can periodically scour the system at higher velocities. This could be done by running all lift stations simultaneously – in contrast to the normal operating scenario. This can be done automatically on a weekly or monthly basis or manually as part of a collection system cleaning operation. The duration of the cleaning cycle is dependent upon the amount of flush water available at the lift stations.
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Document Type: Research Article

Publication date: 2001-01-01

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