August 2009, Vol. 21, No.8

Plant Profile

Hutchinson Wastewater Treatment Plant

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Location: Hutchinson, Minn.
Startup date: July 2008
Service population: 14,000
Number of employees: 8
Design flow: 3.67 mgd (13,900 m³/d)
Average daily flow: 2.29 mgd (8670 m³/d)
Peak flow: 9.62 mgd (36,400 m³/d)
Annual operating cost: $4.4 million

It’s fair to say that oxidation ditches and membrane bioreactors (MBRs) are at the opposite ends of the technology spectrum, but since June 2008 at the Hutchinson (Minn.) Wastewater Treatment Plant, those two processes have worked hand in hand. The plant is the first in the Midwest and one of the first in the country to have an MBR process paralleling an oxidation ditch–final clarifier process to meet more-stringent effluent limits.

The City of Hutchinson negotiated with the Minnesota Pollution Control Agency to receive variable effluent limits for the 5-day carbonaceous biochemical oxygen demand (CBOD) parameter for the plant’s effluent. The plant’s new CBOD limit is 5 mg/L but allows CBOD to increase to 10 mg/L when the dilution ratio is at least 10 parts river flow to one part effluent. Downstream dissolved-oxygen and un-ionized ammonia conditions also must be met.

                    

Dueling Treatment Trains
Influent from Hutchinson’s 14,000 users enters the preliminary treatment building and splits. Two-thirds flow to the oxidation ditch–final clarifier process and one-third flows to the new MBR process. The water headed to the oxidation ditch passes through four static screens and three vortex grit-removal units operating in parallel before entering the oxidation ditch. On the MBR side, two rotary fine screens operate in parallel to remove screenings and grit. The fine screens have openings of only 1.0 mm to protect the membranes from debris.

                   

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The Hutchinson (Minn.) Wastewater Treatment Plant is the first plant in the Midwest to operate oxidation ditch and membrane bioreactor processes side by side. (Photo credit: Donohue & Associates [Sheboygan, Wis.])

After preliminary treatment, two oxidation ditches and two final clarifiers biologically treat two-thirds of the wastewater. The oxidation ditches, an extended aeration process, include nitrification. The final clarifiers remove the solids from the wastewater.

The MBR process includes two aeration tanks, each with an anoxic and aeration zone. The anoxic zone was added to denitrify, recover alkalinity, and recover oxygen. The MBR mixed liquor averages 8000 to 10,000 mg/L, which reduces the size of the aeration process by one-half to one-third off what is normally required with an extended aeration process. The membrane portion of the system includes two trains of six cassettes of membranes. The membranes filter out basically all bacteria and most of the viruses due to the size of the pores of the membrane material.

Effluent from both processes recombines for ultraviolet disinfection and extra aeration. The final effluent is discharged to the South Fork Crow River.

However, the plant has hopes soon to skip disinfection for one-third of its effluent. The city is conducting a study to confirm to its regulator that the MBR effluent has such low counts of fecal coliform that disinfecting the MBR effluent is unnecessary. If the study confirms this hypothesis, the MBR discharge will bypass the ultraviolet disinfection facilities and be discharged directly to the receiving stream.

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The membrane bioreactors at the Hutchinson (Minn.) Wastewater Treatment Plant handle about one-third of the plant’s flow. The mixed liquor in the reactors averages 8000 to 10,000 mg/L, which reduces the size of the aeration process by as much as one-half, compared to an extended aeration process. (Photo credit: Donohue & Associates [Sheboygan, Wis.])

                     

Solids Solutions
The sludge handling at the Hutchinson plant includes two gravity belt thickener–presses, thickened sludge storage, a heat dryer, and a pelletizing process. The gravity portion of the two gravity belt thickener–belt filter presses thicken waste activated sludge from the final clarifiers and the MBR process to reduce storage requirements. The thickened sludge is then dewatered using the belt filter press function of the same equipment.

Depending on the season, dewatered sludge is land-applied as a Class B cake or heat-dried to produce a Class A biosolids product. The dried sludge is then pelletized for easier handling during disposal.

The cake is land-applied on agricultural land within 10 mi (16 km) of the city. When agricultural land is not available for land application, the dryer and pelletizer are used to produce biosolids pellets. A local golf course takes most of the pellets, which are clean and odorless, and spreads them on the golf course as a soil conditioner.

While pilot-testing the dryer project a few years ago, city staff found that the copper levels in the biosolids exceeded the exceptional quality Class A limits. An investigation revealed the copper’s source as the city’s copper drinking-water-distribution piping. Due to the slightly acidic pH of the water supply, the water was leaching the copper from the pipes. To prevent the leaching and reduce the copper                     

concentration in the biosolids, the city installed a water-softening process, which includes biofiltration, reverse osmosis, and antiscalent chemicals to raise the pH of the finished water.

Interim Aeration
Finding simple and effective solutions is nothing new to the staff at the Hutchinson plant. In 2005 and 2006, as biochemical oxygen demand and total Kjeldahl nitrogen loadings to the plant continued to rise, the plant staff grew concerned about continuously meeting permitted limits. The MBR upgrade was coming soon, but they needed an interim solution to maintain compliance.

To bide their time, the plant operators installed surface aerators on the sides of the existing oxidation ditches to provide more air to the extended aeration process for BOD removal and nitrification. The plan worked and kept the plant in compliance with its BOD and ammonia effluent limits.

Interim Aeration
Finding simple and effective solutions is nothing new to the staff at the Hutchinson plant. In 2005 and 2006, as biochemical oxygen demand and total Kjeldahl nitrogen loadings to the plant continued to rise, the plant staff grew concerned about continuously meeting permitted limits. The MBR upgrade was coming soon, but they needed an interim solution to maintain compliance.

To bide their time, the plant operators installed surface aerators on the sides of the existing oxidation ditches to provide more air to the extended aeration process for BOD removal and nitrification. The plan worked and kept the plant in compliance with its BOD and ammonia effluent limits.

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The effluent from the Hutchinson (Minn.) Wastewater Treatment Plant membrane bioreactor has such low fecal coliform counts that the plant is conducting a study to confirm to its regulator that disinfection of the membrane effluent is unnecessary. (Photo credit: Donohue & Associates [Sheboygan, Wis.])