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Water softening by precipitation with lime is widely practiced throughout the Midwestern United States. This slide show illustrates some of the treatment practices and classical equipment used by several municipal utilities.

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Algal blooms, followed by low lake water temperatures in the early winter of 2004, led to taste-and-odor producing compounds in the influent to the drinking water treatment plant at Bloomington, Illinois. The operational procedures adopted to mitigate the taste-and-odor along with extensive data on the presence and removal of geosmin and 2-methyl isoborneol (MIB) are presented in this slide show.

 

A major finding was that water recovered from the lime sludge storage lagoons contributed significantly to the plant influent geosmin/MIB concentrations.

 

Since most geosmin/MIB removal occurred primarily during filtration through Bloomington's GAC-capped filters, laboratory studies were undertaken to determine how rapidly geosmin adsorption capacity might change with GAC service age. These results indicated that adsorption on virgin carbon was most effective in geosmin removal. Despite increased colonization with microorganisms, GAC became progressively less effective after one and two years in service.

 

It was decided that virgin carbon should best be installed in the autumn in preparation for winter taste-and-odor challenges.

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On a rotating basis, over a three-year period, the granular activated carbon (GAC) caps on Bloomington, Illinois' 18 sand filters are replaced with virgin GAC. Microscopic examination of both the carbon and sand shows that extensive microbial colonization has occurred on the media.

 

Moreover, the GAC has changed markedly in size distribution. As 'fines' are lost and larger particles are reduced in size owing to abrasion, GAC progressively occupies a narrower size range.