Type 1701 is a type of filamentous bacteria found in many activated sludge processes and in aerated stabilization basins. It is commonly found in systems where there is a low dissolved oxygen concentration. It is also common in complete mix systems where the wastewater and biomass are completely mixed throughout the aeration basin. Type 1701 readily grows on simple carbohydrates and organic acids using ammonia as a nitrogen source. It is a strict aerobe; however, it has the ability for growth at quite low oxygen concentrations (low half-saturation values for DO on the order of 0.01-0.03 mg/L DO), less than those supporting the growth of floc-forming bacteria examined. It is important to note that these filamentous organisms require oxygen for growth and do not proliferate under anaerobic conditions; they become a problem in activated sludge only when DO is present but at low concentration. There is some evidence that type 1701 may occur when DO deficiency is severe. In addition type 1701 is observed over a wide range of F/M values extending downward to approximately 0.2-0.3.
Type 1701 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, smoothly-curved, or bent trichomes that are found within the floc or extending from the floc, sausage-shaped rods or round-ended rods, clear cell septa, and a tight sheath. The filament length ranges from 10 to greater than 150 microns in length and around 1 micron in diameter. The filament is not mobile. Attached growth is usually present. Type 1701 has Gram and Neisser negative staining reactions. It rarely presents itself with false branching, where the filaments appear to be branched but are actually just lying very close together. This filament does not contain sulfur granules. Cells almost always contain PHB granules, which appear as dark, spherical intracellular granules. A distinguishing feature of type 1701 is the generally profuse attached growth of epiphytic bacteria which makes an observation (and identification) of individual cells difficult. Occasionally type 1701 is observed radiating outward from the floc surface into the bulk solution and without attached growth. In this case, type 1701 is growing fast enough to not yet become “colonized” by attaching bacteria. Observation of the sheath can be difficult but is essential to the proper identification. Look for missing cells in the filament, or spaces where the filaments bend.
Type 1701 is equally found as a cause of bulking in plants treating domestic, industrial, and mixed domestic-industrial wastes. Industrial wastes promoting type 1701 generally contain readily degradable carbon compounds as found in wastes from fruit and food processing, brewing and malting, pulp and paper processing, and dairy and meat processing. 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 analysis.