What are the observational units?

There are several research questions we will ask in this investigation. The first one is: On average, is there a significant difference in the E. coli counts between the water that has just been filtered and water that is sitting in the bottom of a filter after it was filtered the previous day? The data set we will use contains results from 14 water filters each giving E. coli counts (per 100 mL) on the first day and the second day after the water was filtered.