The City of Stevens Point and Donohue & Associates received a 2023 Engineering Excellence Best of State Award from the American Council of Engineering Companies (ACEC) of Wisconsin for their Liquid to Dried Biosolids Conversion project.

The City of Stevens Point’s wastewater treatment plant was facing annual hardships to recycle biosolids as a liquid fertilizer on agricultural land due to factors outside their control. The City collaborated with Donohue for guidance in planning, design, and construction of what became a major overhaul to not only the solids handling process, but also the water department’s main garage and office space.

The resulting solution identified relocating the non-wastewater department vehicles from a structure that was properly positioned to serve as a new solids processing building for wastewater treatment, enabling both departments to solve multiple system needs. The creation of a new, larger Water Garage enabled all the common departments (collection systems, water, stormwater, and wastewater) to have a common vehicle, equipment, and parts storage building. Bringing the inventory under one roof also allows for shared office space to facilitate inter-department discussions and work sharing. The new facility is serviced with geothermal heating and cooling using clean wastewater effluent powered by a rooftop solar array, producing 150% of the power consumption and offsetting nearly all the heat load.

The existing garage space could then be converted to a process space. The selected paddle dryer was one of few devices that could fit in the limited ceiling height and provide the necessary evaporative capacity. Being the first dryer of its type in the State, the team forged new ground, adapting it to run on the City’s over-abundance of anaerobic digester biogas (methane) saving 90% of the natural gas cost, equivalent to $300,000 per year.

Upon converting the liquid biosolids to a dried product, the City no longer had a need for the three million gallons of liquid storage tanks. Thus, to repurpose these salvageable tanks, the team identified multiple uses: 1] as additional anaerobic digestion to expand capacity for outside wastes and generate more biogas, 2] as fail-safe storage for unexpected equipment failures, and 3] equalization of nutrient rich filtrate return flows. Each of these features resulted in reduced operations and capital expenditures, aside from helping the system run more consistently.

The combination of the above series of events is proof that planning departments must look beyond their fence lines and budget silos to develop solutions that serve the needs of the greater community at the lowest cost. Secondly, the project’s technologically advanced dryer with biogas reuse proves that sustainable drying is obtainable at medium sized facilities all throughout Wisconsin.  

Engineering Excellence awards recognize quality engineering achievements. Entries are judged on the basis of excellence in design, the degree to which the client’s needs are met, as well as benefit to the public welfare and to engineering practices. Learn more at acecwi.org/news/liquid-to-dried-biosolids-conversion.