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.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.
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