EFFECT OF SHORT-TERM AND LONG-TERM CHANGES IN HYDRAULIC CONDITIONS ON NITRIFYING BIOFILM

Hydraulic conditions affect the biofilms used in the treatment of very low concentration of pollutants. Effect of hydraulic conditions on nitrifying biofilm grown under a NH4-N concentration of about 1 mg/l (substrate-limiting condition) was investigated ina cylindrical reactor (phi-40 cm x 46 cm (working height)). Velocity and turbulence at 4 mm from the wall of the reactor mixed by paddles were measured by a hotfilm anemometer. Nitrifying biofilm was grown on the wall of the reactor at a base hydraulic condition with average turbulent intensity of 6.6 cm/s. Turbulent intensity was reduced for a period of three to four times the hydraulic retention time and the resulting flux was measured at the end of this period (short-term). This short-term effect of turbulent intensity on specific flux (NH4-N flux / NH4-N concentration in bulk water) varied with turbulent intensity according to a power law of turbulent intensity in agreement with the theoretical expression derived for filamentous biofilms under turbulent conditions. When turbulent intensity was changed and kept at the changed value for several days (long-term), filamentous type biofilm with higher substrate flux was observed at high turbulent intensities but colony-type biofilms under low turbulent intensities showed reduced mass transfer.