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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, GAC progressively occupies a narrower size range.