Filamentous bacteria are long strands of bacteria growing end to end, resembling strands of hair or spaghetti, which interlock with each other to form a mesh. This “mesh” will help to hold the sludge floc structure together to form larger pieces for better settling. Filaments are most prevalent in activated sludge systems, but are typically found in low concentrations in aerated stabilization basins as well.
The types of filamentous bacteria present can indicate certain things about a wastewater system, like nutrient availability or dissolved oxygen concentration. Excessive amounts of filaments can lead to poor settling or bulking sludge, especially in activated sludge systems. In normal amounts, they aid floc formation and help to catch small particles during settling yielding a less turbid effluent.
Furthermore, due to their spaghetti like structure, filamentous bacteria have a high surface area to volume ratio, thus making them excellent BOD degraders.
Influent composition and plant operating parameters determine the type of microbial population that will be present in the activated sludge plant. Theoretically, filamentous bacteria predominate under long sludge ages, low dissolved oxygen, low organic matter (food) to microorganism ratio and take up nutrients under nutrient deficiency conditions.
Microscopic analyses to identify filamentous bacteria in the sludge samples can be achieved by assessing the morphological characteristics using staining reactions based on key factors followed by Jenkins et al (2004) and Eikelboom (2000). Wet mount, Gram staining and Neisser staining reactions are done to evaluate the activated sludge floc shape, size, morphology, filament abundance and the effect of filaments on floc quality in connection to activated sludge separability.
The filamentous bacterial population in mixed liquor are quantified using a subjective scoring method that assesses both the abundance of the dominant filamentous population, and the diversity of secondary filaments present. The scoring method is based on a seven point scale (0 -6) termed the filament index (FI). Secondary filaments are usually ranked below 3 on the subjective scoring scale, while dominant filaments being the most abundant in a system are ranked between 4- 6 on the subjective scoring scale. There may be up to 3 dominating filaments occurring simultaneously in a bulking sludge, thus causing filamentous bulking.
The activated sludge plants in South Africa were initially established in 1921. A survey conducted by Blackbeard et al (1987) in 33 biological nutrient removal plants in South Africa indicated that filamentous bulking were common in South African wastewater treatment plants and were found in approximately 75% of all plants. The most common, dominating filamentous organisms in bulking AS WWTPs in descending order were found to be Type 0092, Type 0675, Type 0041, M.parvicella, Type 0914 and Type 1851. Type 0092 and M.Parvicella were frequently observed in low F/M conditions (long sludge age).
In South African plants the low F/M, conditions are the most likely cause as most activated sludge plants are operated at long sludge ages. The systems with low F/M and nutrient deficiency showing incidence of certain types of filamentous bacteria (viz., H. hydrossis Gordonia spp., Thiothrix spp., Type 0041,Type 0675,Type 0092,Type 1851) clearly indicate that reduction or elimination of the above mentioned filamentous organisms, is likely to make a significant impact to reducing the problems of bulking in nutrient removal WWTPs.
Type 0092, M. parvicella and Gordonia amarea were most frequently in foaming samples than in the mixed liquor of the nutrient removal plants.