Ammonia is one of the major pollutants existing in sewage. The major removal process for ammonia is biological nitrification which is a two-step process including oxidation of ammonia to nitrite and further oxidation of nitrite to nitrate. It is widely used together with denitrification, which reduces nitrate/nitrite to nitrogen gas, in wastewater treatment plants (WWTPs) to achieve nitrogen removal target for environmental protection.

For a long time, a group of bacteria, called ammonia-oxidizing Bacteria (AOB) have been thought to be mainly responsible for the oxidation of ammonia in WWTPs and natural environments until it was found a few years ago that ammonia-oxidizing Archaea (AOA) is another major player and may contribute even more to oxidation of ammonia to nitrite than AOB. AOA is diverse and abundant in various natural environments, such as sediment, soil, estuary and seawater.

So far, most studies on AOA have been focused on their distribution and quantity in various natural environments. Only a few pioneer works on the presence of AOA in WWTPs have been done.

In this study, we have found that the AOA existing in Hong Kong WTTPs were closely similar to those from sediment and soil, but distinct from those from activated sludge in other places, by applying DNA-based molecular techniques (for further details, please refer to our paper: Occurrence of ammonia-oxidizing Archaea in activated sludges of a laboratory scale reactor and two wastewater treatment plants, Journal of Applied Microbiology 107(3):970-7, 2009).

　

　

　

Dr T Zhang’s Environmental Biotechnology research group

Additionally, we also observed that the dominant AOA enriched in a laboratory reactor belonged to Crenarchaeotal Group I.1a in phylum Crenarchaeota , while that the dominant AOB in the reactor fell to the genus Nitrosomonas. The analyses also suggested that AOB community were more sensitive than that of AOA to operative conditions, such as ammonia loading and dissolved oxygen (please refer to our paper: Characterization and quantification of ammonia-oxidizing Archaea (AOA) and Bacteria (AOB) in a nitrogen-removing reactor using T-RFLP and qPCR, Applied Microbiology and Biotechnology 87: 1167-1176, 2010).

Furthermore, we developed a reactor operated under limited dissolved oxygen (DO) level to conduct the partial nitrification process for energy saving nitrogen removal. The findings showed that nitrogen in wastewater could be removed through the Complete Autotrophic Nitrogen Removal Over Nitrite (CANON) process for saline wastewater in a continuous reactor at DO concentrations of 0.5 mg L^-1 (please refer to our paper, Autotrophic biological nitrogen removal from saline wastewater under low DO, Journal of Chemical Technology and Biotechnology 85(10): 1340-1345, 2010).

The results of this project has provided new scientific knowledge about the roles of AOA in partial nitrification (ammonia oxidization to nitrite) in wastewater and valuable information
that will be needed to further develop the partial nitrification processes to save energy and reduce carbon footprint, including single reactor system for high activity ammonium removal over nitrite, complete autotrophic nitrogen removal over nitrite and oxygen limited autotrophic nitrification and denitrification.

Dr Tong ZHANG
Environmental Biotechnology Lab
Department of Civil Engineering
The University of Hong Kong
zhangt@hkucc.hku.hk