Filamentous bacteria grow in filaments or strands, rather than floc, and are a major source of problems in activated sludge systems including paper mills, refineries, chemical plants, food processing, etc. When present in small or moderate numbers, the filamentous bacteria are beneficial because they form a backbone upon which the floc formers grow and thus improve clarification and settling, resulting in lower effluent total suspended solids (TSS). However, if levels of filamentous bacteria become excessive, then settling is hampered.
Filaments become prevalent when their greater surface area gives them a competitive advantage over the floc formers. Some of the conditions leading to filamentous bulking are low dissolved oxygen, low nutrients, low food to mass ratio and septicity or the presence of organic acids. There are approximately twenty species of filamentous bacteria, each of which is associated with one or more of these four causes.
Haliscomenobacter Hydrossis, or more commonly called H. hydrossis, is a typical filament in pulp and paper systems and other activated sludge systems. H. Hydrossis is a small, sheathed, filamentous bacteria without a cell septa. H. Hydrossis can be straight or bent and range from 0.3 to 0.5 micrometers in diameter, and 10 to 100 micrometers in length. It is the shortest and narrowest filament, and can often be overlooked using a non-phase contrast microscope or at magnifications less than 400X. They usually grow outward from the floc surface, and their needle or pin like appearance enables them to be identified fairly easily. Their appearance is similar to pins in a pincushion. However in severe cases these filamentous bacteria can grow freely in the bulk water.
Although one of the most commonly observed filaments in pulp and paper mill activated sludge systems, H. Hydrossis is responsible for only a modest number of filamentous bulking episodes. The presence of H. Hydrossis is associated with several causes: Low dissolved oxygen concentrations, low food to mass (F: M) ratio, and nutrient deficiency. Because of these multiple causes, H. Hydrossis is not a good indicator organism, but the combination of low dissolved oxygen and low food to mass conditions appears to be particularly selective for this organism.