Expansion set new challenges

The Minnesota Pollution Control Agency (MPCA) regulations created some unique problems for the Southern Minnesota Beet Sugar Co-operative in Renville, USA. An expansion project begun in 1998 increased the volume of wastewater produced from beet processing, outstripping the capacity of the on-site lagoons and spray irrigation system that the company had traditionally used for wastewater treatment and disposal.


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Beets and the resulting thick juice and molasses are processed in three distinct

periods during the year. The ‘Slice Campaign’ – when the beets are sliced, the

juice produced and the sugar granulated – lasts from September to March. Sugar

beets are approximately 75% water; the most wastewater is generated during this

six-month period. The ‘Juice Campaign’ from March to July, when the stored thick

juice from the beets is converted to sugar, requires less water. The July to

September ‘Molasses Campaign’ produces only a limited amount of wastewater.

Odour complaints

The expansion of the processing facility would require treatment of additional

quantities of wastewater. Expansion of the wastewater storage pond system was

explored and the option dismissed, since it did not address the odour concerns.

The Co-op turned to Applied Technologies, an engineering firm which specialises

in wastewater projects. Applied Technologies reviewed the plant requirements

and decided the solution should be designed to accommodate the varying amounts

and types of wastewater output.

Applied Technologies recommended a wastewater treatment system that would permit

continuous direct discharge of treated wastewater into a drainage ditch that

empties into the Minnesota River system. ‘For the plant to continue its expansion,

we needed a treatment plant that was efficient, flexible and cost effective,’

said John Kouba of Applied Technologies.

The wastewater from the beet washing process was to be piped to a flume clarifier.

Most of the clarified water would be returned to the processing facility for

washing beets.

Any excess wastewater that could not be reused in the plant, primarily in the

fall and winter, was to be briefly stored in a 124ft dia by 24ft high equalisation

tank. The wastewater would then be processed through an anaerobic reactor, then

two 38ft x 26ft high anoxic tanks, which were centered inside two 126ft x 26ft

high aeration tanks. The activated sludge was sent to concrete clarification

tanks for settling.

After thickening, the settled sludge would go to a 124′ diameter by 24′ high

sludge storage tank. Here the sludge would be stored before land application.

Roof decks were specified for the equalisation and sludge storage tanks to control

odours.

Meanwhile, the mixed liquid from the aeration tanks flows to concrete secondary

clarifier tanks. The clarifier overflow goes to tertiary treatment sand filters

and then to post-aeration in an inground concrete tank to drainage discharge.

For up to four months of the year, treated wastewater is stored in a large pond

on site.

Once the project was approved, another set of challenges arose. The Minnesota

climate, with its short summer and unpredictable weather, made construction

of the new storage tank components uncertain. ‘We’d designed a system that would

solve the problem and produce “just in time” delivery for the plant,’

Kouba said. ‘Next we had to find a “just in time” construction solution.’

The critical new components were the equalisation and sludge storage tanks,

and the combination anoxic and aeration tanks.

‘The scheduling issues dictated the selection of factory-coated, bolted steel

tanks,’ said Kouba. ‘We knew from experience that bolted tanks could meet the

deadlines and that they would also cost less than concrete or welded tanks.’

Bolted tank solution

After all proposals were reviewed, Columbian Steel Tank Company (now Columbian

TecTank) was selected. Columbian had committed to having the bolted tanks shipped

to the site by truck, assembled and commissioned within the critical ten week

period. ‘It was a pretty standard arrangement for us,’ Mark Eklund, Columbia’s

sales engineer, said. ‘Weather has a lot less effect on bolted tank construction.

We knew that barring some highly unseasonable circumstances, we’d be able to

meet the deadline.’

The coating systems for steel tanks are often a critical consideration. The

interior and exterior coatings were selected for the application. Bolted tanks

are factory-coated in a controlled environment. The surface is prepared and

the coating system applied and baked in accordance with exact specifications.

The finished parts were shipped in protective pallets.

The aerobic portion of the wastewater treatment system began operation in February

2000. ”The results were immediate,’ said Glenn Augustine, Factory Manager of

Southern Minnesota Beet Sugar Co-operative. ‘Some of the toughest NPDES permit

limits in the State of Minnesota were met.’ Direct discharge began in March

2000. The new system enabled the company to increase its production capacity.

The water system is more efficient and SMBSC no longer receives complaints about

odours.


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