Redesign & Refine

Sept. 11, 2008

About the author: Caitlin Cunningham is associate editor for Water & Wastes Digest. Cunningham can be reached at 847.391.1025 or by e-mail at [email protected].

Located approximately 60 miles west of Minneapolis, the city of Hutchinson, Minn., is home to more than 13,600 residents and a fast-growing industrial base. In light of recent population and business growth, the city’s wastewater treatment plant (WWTP) had been operating at or above capacity for several parameters, including biological oxygen demand (BOD), total suspended solids (TSS) and total nitrogen. Furthermore, the Minnesota Pollution Control Agency (MPCA) concurrently set forth new, more stringent effluent limits with which Hutchinson’s WWTP needed to achieve compliance. City staff quickly recognized the need for a facility expansion.

Constructed in the late 1980s, the existing Hutchinson WWTP was designed to handle 3.21 million gal per day (mgd) at an average daily flow. The city’s main wastewater treatment process involves flow equalization, static fine screens, vortex grit units, oxidation ditches, final clarifiers and post-aeration. Sludge handling at the WWTP is achieved via gravity belt thickener/presses, sludge storage and a sludge pelletizing process. Hutchinson’s wastewater treatment components are well maintained and operate smoothly, according to city staff, and the decision to move forward with a complementary expansion project was finalized in March 2006.

Funding & Infrastructure

In order to achieve the facility’s new 3.67-mgd capacity and meet regulatory requirements, city representatives and private consultant Don Roecker assumed an active role in pursuing state and federal financial assistance in the form of grants and low-interest loans.

Meanwhile, engineering and consulting firm Donohue & Associates worked with city staff—namely Water and Wastewater Manager Randy DeVries, Wastewater Superintendent Brian Mehr and City Engineer Kent Exner—for two months to prepare the Year 2006 Facilities Plan Amendment, which was submitted to the MPCA in April 2006. Overcoming spatial design challenges posed by onsite power lines and a water-filled burrow pit, Donohue & Associates turned in its completed design documents that June, meeting the deadline for securing a low-interest state loan.

The existing WWTP utilized two oxidation ditches and two secondary clarifiers for secondary treatment. Several options were considered during the planning phase of the upgrade, including taking no action, introducing a third oxidation ditch and secondary clarifier and installing a membrane bioreactor (MBR) train system parallel to the oxidation ditches/final clarifier process. The MBR alternative, with a comparable price tag and several perceived benefits (e.g., smaller footprint, superior effluent quality and facilities for meeting potentially stricter effluent limits), was ultimately implemented.

At the Hutchinson WWTP’s preliminary treatment building, influent flow is now split two-thirds to the existing oxidation ditch/final clarifier process and one-third to the new MBR system. When flows exceed designed maximum daily flow conditions, flow is limited to the MBR section and excess flow is sent to the oxidation ditches or flow-equalization ponds.

New rotary fine screens with 1-mm openings and a vortex grit unit have been added to remove screenings and grit from wastewater before it reaches the MBR process’ two aeration tanks, which feature anoxic and aeration zones. The anoxic zone is intended to denitrify in addition to recovering alkalinity and oxygen. Mixed liquor averages of 8,000 to 10,000 mg/L reduce the aeration process size to roughly one-third or half of what is normally required.

The membrane portion of the system includes two trains of six membrane cassettes and replaces clarifiers by separating solids from incoming wastewater. The membranes, according to staff, filter out nearly all bacteria and most viruses due to the material’s pore size.

Finally, a new ultraviolet disinfection system sized to handle the flow from the oxidation ditch/final clarifier process was employed to replace the WWTP’s chlorination/dechlorination system. The city is currently conducting a study to confirm to the MPCA, however, that its MBR effluent contains so little E. coli that disinfection is not needed.

Performance & Perception

Construction crews completed improvements to the Hutchinson WWTP in July 2008, and new components began operation in June 2008. The most recent performance research indicates that facility improvements are meeting effluent limits: The total plant discharge of BOD measures 3 mg/L (new permitted limit is 5 mg/L), and the total plant discharge of TSS comes in at 6 mg/L (new permitted limit is 30 mg/L).

Overall public response to the expansion project has been positive, according to city staff. DeVries, Mehr and Exner highlighted three immediate benefits Hutchinson has seen as a result of the work: a virtually eliminated threat of National Pollutant Discharge Elimination System violations, higher-quality effluent reaching the Crow River and sufficient WWTP capacity for continued city growth. City staff continues to pursue state and federal grants to help fund the upgrade.

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About the Author

Caitlin Cunningham

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