INFLUENCE OF SUBSTRATE C N RATIO ON THE STRUCTURE OF MULTISPECIES BIOFILMS CONSISTING OF NITRIFIERS AND HETEROTROPHS/

A bench-scale biological aerated filter (BAF) was operated under a ran ge of substrate loadings and C/N influent ratios. Start-up with a subs trate loading of 4.8 COD kg m(-3) d(-1) and an influent C/N ratio of 1 .5 gave no nitrification, due to the high biofilm detachment rate caus ed by the daily backwashing. Subsequent runs with lower substrate load ings and backwashing rates allowed stable nitrification. A final run w ith the initial substrate loading and backwash rate gave stable nitrif ication. Oligonucleotide probes targeted to the 16S rRNA of the nitrif iers, as well as to other types of microorganisms, were used to charac terize how the changes in loading and detachment affected the microbia l structure in the biofilms. The proportion of nitrifiers decreased wi th increasing C/N ratio. The biofilm remaining a;ter backwashing also contained a higher fraction of nitrifiers than did the backwash solids stripped from the cuter layers of biofilm, which consisted mainly of active heterotrophs. Thus, stable nitrification was possible only when the nitrifiers became established deeper inside the biofilm.