The activated sludge from the aeration basin of the Su-yeong municipal wastewater treatment plant which has operated by a standard activated sludge process in Busan, Korea was investigated during April 2004 and January 2005 with several bio-indicators. The number of bacteria and fungi per gram of dry weight of MLSS were estimated to be 3.1×106~1.5×108 and 1.1×103~1.1×105 colony forming units, respectively, by the plate agar method. By cultivation-independent methods, such as 4’,6-diamidino-2-phenylindole stain and fluorescence in situ hybridization, the ratio of eubacteria to the entire biomass was evaluated by more than 80% (v/v). The ratio of ammonia-oxidizing bacteria and nitrite-oxidizing bacteria to the total eubacteria was determined to be 7.0~9.8% and 3.3~6.2% without heavy variation in spite of a period of relatively low temperature in the basin. It would be expected that the nitrification would occur or at least co-exist throughout the year in the sludge of many municipal WWTP with influents that contain the sufficient nitrogen sources although the WWTP does not have any specialized processes for the removal of nitrogen.

• Sun-Ja Cho(Department of Microbiology, Pusan National University)
• Yong-Ju Jung(Department of Microbiology, Pusan National University)
• Tae-Joo Park(Department of Environmental Engineering, Pusan National University)
• Sang-Joon Lee(Department of Microbiology, Pusan National University) Corresponding Author