Sphaerotilus natans, S. natans for short, is a type of filamentous bacteria found in many activated sludge processes and in aerated stabilization basins (ASBs). It is commonly found in systems where there is a low dissolved oxygen concentration, nutrient deficiency, especially phosphorus, and/or in the presence of soluble, readily-metabolizeable substrates, like organic acids or simple sugars. It is also common in complete mix systems where the wastewater and biomass are completely mixed throughout the aeration basin.
S. natans should be identified by filamentous bacteria identification by trained personnel. By using activated sludge microscopic examination techniques, microbiologists will look for the following characteristics of this filament: straight or smoothly-curved trichomes that extend from the floc and resemble spaghetti, sausage-shaped rods or round-ended rods, clear cell septa, and a tight sheath. The filament length ranges from 100 to greater than 500 microns in length and around 1.5 microns in diameter. Because of the long length of these filaments, they can cause filamentous bridging of the floc or open floc structure. The filament is not motile and usually does not have attached growth unless is it not growing. If attached growth is present, it is not substantial. S. natans has Gram and Neisser negative staining reactions. It presents itself with false branching, where the filaments appear to be branched, but are actually just lying very close together. In this case, there is no continuous flow of cytoplasm between the two filaments. While many filament s can metabolize sulfates and incorporate them as elemental sulfur in the cells, this filament does not contain sulfur granules.
S. natans can cause filamentous bulking sludge in industrial wastewater treatment plants that contain easily degradable materials, like sugars or organic acids. It can be temporarily controlled by chlorination of the filaments and concurrent wasting. However, the root cause for the bulking has to be corrected to prevent the filament from returning. During chlorination, empty sheaths or empty cells are observed within the filaments. Bulking episodes are identified by microscopic examination and settling analyses.