In aerobic wastewater treatment process, low dissolved oxygen (DO) concentration was generally thought to be harmful to aerobic microorganisms, especially to nitrifying microbes. In fact, ammonium could be well converted under low DO and low DO condition could obviously lead to energy saving. In recent years, some new phenomena were found in nitrogen removal reactors operated at low DO concentration, such as sludge granulation;short-cut nitrification -denitrification; simultaneous nitrification-denitrification and denitrification by autotrophic nitrifier. These phenomena were valuable because they could give new method or some convenience to nitrogen removal process. Fig 1, Tab 1, Ref
The biological aerated filter （BAF） was used to treat the oil-field produced water. The removal efficiency for oil, COD, BOD and suspended solids （SS） was 76.3%-80.3%, 31.6%-57.9%, 8.6.3%-96.3% and76.4%--82.7%, respectively when the hydraulic loading rates varied from 016m·h^-1 to 1.4m·h^-1. The greatest partof removal, for example more than 80% of COD removal, occurred on the top 100cm of the media in BAF. The kinetic .performance of BAF indicated that the relationship of BOD removal efficiency with the hydraulic loadingrates, in biological aerated filters could be described by c1/c1=l-exp（-2.44/L^0.59）. This equation could be used topredict the B OD.removal efficiency at different hydraulic loading rates.
