Growth of filamentous bacteria in an enhanced biological phosphorus removal system

Activated sludge bulking caused by filamentous bacteria Eikelboom type 021N and Thiothrix spp. is reported for the first time in a multistage pilot-scale wastewater treatment plant for enhanced biological phosphorus removal (EBPR). Both types of microorganisms grew gradually reaching their maximum cumulative length within a period of 2 months. For identification of the strains, in addition to physiological observations under the microscope, Gram staining, Neisser staining and sulfur storage test took place. Results confirmed that biological nutrient removal systems may also suffer from sludge bulking. Plant configuration, wastewater composition, removal efficiency and filaments’ growth stages were examined in relation to the occurrence of the certain strains in an EBPR system.

Surveys on filamentous microorganisms [2,9] report that only Microthrix parvicela has been reported in EBPR plants. However, present work shows that other filamentous microorganisms can grow in such units as well. Experimental results clearly showed that a period of 2 months is necessary for filamentous bacteria Thiothrix spp and Eikelboom type 021N to achieve cumulative length and cause severe sludge bulking. Thus, early detection of either Thiothrix spp or Eikelboom type 021N before reaching their cumulative length could save time and costs for their suppression. Synthetic medium with high organic load containing peptone, glycine, yeast extract, sucrose, acetic acid, urea and KH2PO4, as well as macronutrients, favored the growth of sulfur filamentous microorganisms, such as Thiothrix spp and Eikelboom type 021N, in a continuously operating pilot-scale EBPR plant. Present findings clearly show that high floc-former population is necessary for sufficient nitrogen removal, whereas removal of organic pollution and phosphorus is achieved even at lower biomass concentrations (sludge bulking). Evidently synthetic media totally failed to replace real wastewater. Easily biodegradable (metabolizable) substances favored over-proliferation of filamentous microorganisms, which led to sludge bulking. Influent wastewater composition in relation to filamentous bacteria growth should be further investigated, whereas continuously operating units using real wastewater should be motivated. This kind of information will provide valuable data to researchers, who are also challenged to control sludge bulking. Routine microscopic investigation is a useful tool for optimum operation and control of a wastewater treatment plant. This is also supported by the observation that FI is a more sensitive indicator for detection and evaluation of filamentous bacteria proliferation than SVI. Efficient biomass settling results in a stable plant operation, satisfactory performance and well-clarified effluent.