After passing through the bar screen and the grit chamber, the wastewater reaches the comminutor, also known as the grinding pump.  In the comminutor, water is passed through a rotating cutting screen.  This cutting screen shreds any large chunks of organic matter in the wastewater into smaller pieces.  This makes it easier for the the microorganisms to use the organic matter as food and prevents the large chunks from harming the internal workings of the treatment plant.