EVALUATION OF MULTIPLE-SPECIES BIOFILM AND FLOC PROCESSES USING A SIMPLIFIED AGGREGATE MODEL

A simplified model of microbial aggregates is developed to evaluate ho w the key features of aggregation affect the stability of nitrificatio n in multiple-species biofilms and flocs. The foundation of the model is a layered system in which each layer contains a different type of b iomass, includes mass-transport resistance for all substrates (i.e., C OD, NH4+, and O-2), and included formation and consumption of soluble microbial products. The model describes how the outer biofilm layer of heterotrophs protects the inner layers of nitrifiers and inert biomas s from detachment, so that an inner layer can have a low specific deta chment-loss rate. This protecting function is not important for suspen ded flocs, because the entire flee is wasted. Comparison of biofilm an d suspended flocs shows that the protecting function of the biofilms p rovides greater nitrification stability. The model also demonstrates t hat extensive heterotrophic growth induced by high organic loading can cause a deterioration of nitrification when the dissolved oxygen is d epleted in the inner layer of the aggregate. Although it is based on a simplified spatial distribution, the aggregate model identifies key d ifferences among biofilm, floc, and dispersed-growth processes.