A technique was developed to measure the equivalent MLSS in the fixed film Captor® media-activated sludge process. The technique accounts for the wear in the media as well as unequal wear in different media. The microbial population variations encountered during three years of full scale operation of the Captor® process, at the Moundsville WWTP were recorded. It has been found that fixed film media seem to support predominantly rotifers, followed by free swimming ciliates, and single stalk ciliates. In contrast, the suspended growth activated sludge population contained lower level organisms, mostly free swimming ciliates, followed by stalk ciliates, flagellates, and a few rotifers.