CONTROL OF HETEROTROPHIC LAYER FORMATION ON NITRIFYING BIOFILMS IN A BIOFILM AIRLIFT SUSPENSION REACTOR

A Biofilm Airlift Suspension (BAS) reactor was operated with nitrifyin g biofilm growth and heterotrophic suspended growth, simultaneously co nverting ammonium and acetate. Growth of heterotrophs in suspension de creases the diffusion limitation for the nitrifiers, and enlarges the nitrifying capacity of a biofilm reactor. Neither nitrifiers nor heter otrophs suffer from additional oxygen diffusion limitation when the he terotrophs grow in suspension. Control of the location of heterotrophi c growth, either in suspension or in biofilms over the nitrifying biof ilms, was possible by manipulation of the hydraulic retention time. A time delay for formation and disappearance of the heterotrophic biofil ms of 10 to 15 days was observed. Surprisingly, it was found that in t he presence of the heterotrophic layers the maximum specific activity on ammonia of the nitrifying biofilms increased. The reason for the in crease in activity is unknown. The effect of heterotrophic biofilm for mation on oxygen diffusion limitation for the nitrifiers is discussed. Some phenomena compensating the increased mass transfer resistance du e to the growth of a heterotrophic layer are also presented. (C) 1997 John Wiley & Sons, Inc.