The activated sludge process is one of the most commonly used methods of municipal and industrial wastewater treatments and the microbes are playing very important roles on the removal of organic pollutants in wastewater. Therefore, it might be interesting to identify the predominant microbial population structure in the activated sludge of municipal wastewater treatment plant. The purpose of this study was to test the applicability of the API 20E and API Rapid NFT systems for the identification of some predominant bacteria isolated from activated sludge treatment systems.

The API 20E and API Rapid NFT systems have been widely used for the identification of microorganisms. Since both API identification systems are more convenient and easier to test, they can also be used to identify the microorganisms isolated from activated sludge. However, the microbial population structure is very complex in the activated sludge, by using both API systems it is always difficult to obtain the identification of most microbes in the sludge at the species level or even at the genus level. Many other additional traditional tests, such as morphological, biochemical, and physiological tests, are also required to obtain the exact identification of each microorganism.

Sequencing batch reactor (SBR) and continuous-flow stirred tank reactor (CFSTR) are popular types of biological systems in wastewater treatment plants. In this study, both SBR and CFSTR systems were setup. After both reactors had reached equilibrium, many pure cultures isolated from the activated sludge in both systems were obtained and the API identification systems and many other traditional tests were conducted to identify each pure culture. The diversity of predominant microorganisms in the activated sludge of each reactor was also observed.

In SBR the substrate is filled into the system during a short period of time, and the microbes must suffer high organic loading in the beginning of each cycle. At the end of each cycle the microbes have to survive under a low substrate condition. Hence, the system is never really at equilibrium. It is thought that the microbes unable to tolerate such conditions might disappear gradually and the diversity of microbial population should be low in the SBR system. On the contrary, the substrate is filled into the CFSTR continuously and its concentration is always relatively constant. Therefore the dominating microbial population in the CFSTR system should remain more stable and the predominant microbes isolated from the system should show more diversity than from the SBR. The results from this study show the microbial population structures in the SBR and CFSTR systems, and the conclusions display the characteristics mentioned above as well.

According to the results of this study, it seems that the API identification systems could not provide a very good cross-section of bacteria present in the activated sludge. Therefore, it is recommended that the new more advanced technologies such as 16S rRNA can be applied in the future to obtain a comprehensive view of the total microflora.