Client: City of Willmar

Location: Willmar, Minnesota

Donohue & Associates was retained by the City of Willmar to provide planning, design, and program management services for a new 7.5 mgd average-day wastewater treatment facility and associated collection system improvements. The service area covers approximately 5,500 acres and serves a resident population of nearly 20,000. The new plant was built 5.5 miles to the west of the existing plant.

The prior collection system included 93 miles of pipe, 1,700 manholes, and 25 lift stations. Major portions of the sewers and numerous lift stations were approaching capacity limits. Donohue’ conducted a collection system analysis and developed a Comprehensive Sanitary Sewer Master Plan to address these issues cost-effectively.

The analysis identified future growth areas and problem areas within the existing collection system and evaluated the condition and capacity of the collection system. A new collection system map was prepared showing coordinate locations for sewer structures and data on all pipes and manholes.

The Plan’s major objectives include routing a new interceptor system to convey wastewater flows to the new plant, replacing or relieving lift stations and sewers with insufficient capacity, and addressing future growth.

Other tasks included flow monitoring oversight; critical flow level inspection; identification of infiltration sources; lift station flow evaluation, inspection, and assessment; capital improvement and maintenance budget preparation; and project identification with risk analysis and schedule.

The chosen alternative was a 6-mile primary interceptor of 48-inch and 54-inch diameter, with a secondary 24 inch diameter interceptor to serve the western part of the City and a new industrial park. This alternative eliminated two existing lift stations and two planned future lift stations. By rerouting flow, the collection system will have sufficient capacity to serve the community for the next 50 years.