SPATIAL MICROBIAL DISTRIBUTIONS OF NITRIFIERS AND HETEROTROPHS IN MIXED-POPULATION BIOFILMS

Spatial microbial distributions of nitrifiers and heterotrophs in unde fined mixed-population biofilms were experimentally investigated using a microslicer technique and correlated with nitrification efficiency of the biofilm system. The general stratification of different bacteri al groups in the biofilm was simulated using a one-dimensional (1-D) m athematical biofilm accumulation model (BAM) and compared with the exp erimental results. Biofilms were cultured at three C:N ratios of feed solutions In a partially submerged rotating biological contactor (RBC) . It was shown that the biofilms were vertically stratified (from biof ilm surface to substratum). At C:N = 0, heterotrophs and nitrifiers co existed in the outermost biofilm and heterotrophs dominated in the inn ermost biofilm. At C:N = 1.5, heterotrophs outcompeted nitrifiers for dissolved oxygen and space; thus, heterotrophs dominated in the outerm ost biofilm and nitrifiers were present only in the deeper biofilm. Ni trifiers and heterotrophs coexisted in the innermost biofilm. An incre ase in the influent C:N ratio resulted in stronger stratification of m icrobial species, as well as inhibition of nitrification. In batch exp eriments, NH4-N utilization rate (R(NH4-N)) was almost the same at eac h substrate C: N ratio even though NH, oxidizers were predominantly pr esent in the deeper biofilm. The biofilm performance could not be suff iciently explained by the obtained microbial spatial distribution, sug gesting that one-dimensional description of microbial distribution was not good enough and three-dimensional measurements of microbial spati al distribution is necessary. Total bacterial densities increased by a factor of 3-17 with biofilm depth. The metabolically active cell frac tion decreased from 35 +/- 13% in the outermost biofilm to 15 +/- 4% i n the innermost biofilm, presumably due to substrate limitation. The m odel predicted more pronounced stratification of nitrifiers and hetero trophs than the observed results. This discrepancy could be attributed to the real biofilms that were structurally heterogeneous (e.g., wate r channels), which could not be described by the one-dimensional model . The results of this study clearly indicate the limitation of 1-D bio film models to describe the extent of stratification of nitrifiers and heterotrophs and suggest a 3-D model is necessary. (C) 1996 John Wile y & Sons, Inc.