Relationship between population dynamics of nitrifiers in biofilms and reactor performance at various C:N ratios

The relationship between time dependent population dynamics of nitrifiers and heterotrophs in undefined mixed-population biofilms and their nitrification efficiency was experimentally investigated at various C:N ratios of feed solutions. Five types of biofilms were cultured in partially submerged rotating biological contactors (RBCʹs) at different C:N ratios and were used as test materials. The results indicated that initial microbial composition in the biofilms and substrate composition (e.g. C:N ratio) strongly influenced the later population dynamics and the nitrification efficiency. Higher influent C:N ratio retarded accumulation of nitrifying bacteria, especially NO2-oxidizers, resulting in a considerably long start-up period for complete and stable nitrification due to competition for dissolved oxygen and space in the biofilm. Furthermore, a start-up inoculum was very important to keep start-up time of nitrification to a minimum. Time-dependent population dynamics in the biofilms reflected well the bulk water quality and microbial community structure in the bulk liquid. These results suggest that the structure of microbial community in the biofilm can be predicted from monitoring the water quality and microbiology of the bulk liquid. Physiologically inactive cells in the biofilm were determined by an INT dehydrogenease assay. These cells gradually accumulated up to about 30% of the total bacterial population within the biofilms. The results of this study will provide a rational basis for developing and controlling desired biofilm population dynamics to maximize nitrification efficiency of wastewater biofilms