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Innovative Capacity Expansion

Client: City of Hutchinson

Location: Hutchinson, Minnesota

The City of Hutchinson, Minnesota retained Donohue & Associates to explore means of increasing capacity at the City’s wastewater treatment facility. Prior to the upgrade, the facility was operating at or above capacity for biological oxygen demand (BOD), total suspended solids (TSS), and ammonia nitrogen; additionally, future new phosphorus and mercury limits necessitated upgrades.

The existing treatment train included two oxidation ditches and clarifiers. Conventional methods of increasing capacity were evaluated, including adding a third oxidation ditch and final clarifier, but were eventually rejected due to large footprints and considerable operator control. Donohue also explored converting the entire plant to a Membrane Bioreactor (MBR) process, but the hefty price tag associated with such a conversion proved to be prohibitive. The final solution, an MBR system that runs in parallel to the existing treatment train, combines modern and conventional technologies to maximize effluent quality and minimize costs.

An additional benefit of the MBR process is its potential to eliminate a separate disinfection process. Donohue conducted a study to state regulators to demonstrate that a separate disinfection step for MBR effluent is not necessary. The results were exceptionable: the count for fecal coliform and E. coli was 0; the permitted limit for fecal coliform is 200 per 100 mL. Based on this data, the City received approval to not separately disinfect future MBR effluent.

In order to be eligible for low interest funding, the facilities were designed in less than 10 weeks. This project received a Grand Award in the 2009 ACEC Minnesota Engineering Excellence competition. The project was also an ACEC National Finalist.