Tetrazolium Reduction-Malachite Green Method for Assessing the Viability of Filamentous Bacteria in Activated Sludge

A method was developed to assess the activity of filamentous bacteria in activated sludge. It involves the incubation of activated sludge with 2(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride followed by staining with malachite green. Both cells and 2(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride-formazan crystals can be observed in prepared specimens by using bright-field microscopy. This procedure allowed us to distinguish between inactive and actively metabolizing filaments after chlorine application to control the bulking of activated sludge.