The influence of aeration on nitrification and the nitrifier distribution in an upflow biological aerated filter for tertiary treatment of municipal sewage

A series of two pilot-scale aerobic and anaerobic upflow biofilters were constructed and operated for tertiary treatment of municipal sewage. The effects of aeration on the nitrification performance and nitrifier distribution between attached and suspended biomass of the aerobic upflow biofilter (UBAF) were studied. Process operating results revealed that, for the secondary effluent with volumetric loads of 0.31-0.65 kgCOD/m(3) d and 0.14-0.34 kgNH(4)-N/m(3) d, the UBAF showed an unstable ammonia removal rate of 60.6-72.2% with an average bulk DO concentration of 2.18 mg/L at an aeration rate of 0.6 m(3)/h. Partial nitrification was observed at the first 1.2 m of the filter (DO < 1.42 mg/L), and more than 38.4% nitrite accumulation ratio was detected. When the aeration rate was increased to 1.0 m(3)/h, the nitrification efficiency of UBAF was stabilized at the range of 90.1-93.5% with an average bulk DO concentration of 3.82 mg/L. No significant nitrite accumulation took place in the whole filter. Specific activity tests of attached and suspended biomass of the filter demonstrated that the buildup of nitrification efficiency at an aeration rate of 1.0 m(3)/h mainly arose from the improvement of the nitrifying activity of the attached biofilm. The increase of nitrifying activity of attached biofilm increased the stability of nitrification performance of the UBAF. Most probable number (MPN) emuneration showed that about 60.8% of ammonia oxidizing bacteria (AOB) and 90.8% of nitrite oxidizing bacteria (NOB) were present in attached biofilm at aeration rate of 0.6 m(3)/h. The increase of aeration rate gave rise to 38.9% and 66.7% increase of AOB and NOB of attached biofilm, but only 10.6% and 23.1% increase of AOB and NOB of suspended biosolid. The increase of aeration rate showed little effect on the growth of AOB and NOB of suspended biosolid. NOB was more inclined to grow in attached biofilm than AOB at two aeration rates.