Use of real-time PCR to examine the relationship between ammonia oxidizing bacterial populations and nitrogen removal efficiency in a small decentralized treatment system ‘Johkasou’

The aim of this study was to examine the relationship between ammonia oxidizing bacterial populations and biological nitrogen removal in a small on-site domestic wastewater treatment system “Johkasou”. The population dynamics of ammonia oxidizing bacteria (AOB) in six full-scale advanced Johkasous was surveyed using real-time PCR assay over a period of one year. These Johkasous were selected to compare the AOB populations in different treatment performance. When the effluent NH₄-N concentration was higher than 2 mg L⁻¹, it was difficult to meet the effluent standard of advanced Johkasous (T-N 10 mg L⁻¹). In contrast, the nitrogen removal efficiency was hardly affected by nitrite oxidation and denitrification in these systems. In other words, ammonia oxidation was a rate-limiting step. Furthermore, we focused on the relationship between NH₄-N loading per AOB cell and nitrogen removal. Real time PCR monitoring results demonstrated that it is important to regulate NH₄-N loading per AOB cell below 210 pg cell⁻¹ day⁻¹ to meet the effluent standard of advanced Johkasou. It is considered that NH₄-N loading per AOB cell is a useful parameter for determining suitable nitrogen loading and small decentralized system design.