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This laboratory study presents a detailed evaluation of the effects of dissolved oxygen concentration and accumulation of storage polymers on sludge settleability in activated sludge systems with an aerobic selector. The oxygen and substrate availability regime were simulated in laboratory sequencing batch reactor systems. The experiments showed that low dissolved oxygen concentration (< or =1.1 mg O2 l(-1)) had a strong negative effect on sludge settleability, leading to the proliferation of filamentous bacteria (Thiothrix spp., Type 021N and Type 1851). This negative effect was stronger at high chemical oxygen demand loading rate. This indicates that a compartmentalised (plug flow) aerobic contact tank, designed at short hydraulic residence time to guarantee a strong substrate gradient, with low dissolved oxygen concentration, might be worse for sludge settleability than an "overdesigned" completely mixed contact tank. Contrary to the general hypothesis, the maximum specific acetate uptake rate, poly-beta-hydroxybutyrate production rate, and resistance to short starvation periods are similar in both poor- and well-settling sludge. The results of this study support our previous hypothesis on the importance of substrate gradients for the development of filamentous structures in biological flocs, from soluble organic substrate gradients to dissolved oxygen gradients in sludge flocs.

An individual-based model, originally developed for a biofilm system, was adapted to simulate three-dimensional formation of activated sludge flocs. The model was extended to two different bacterial morphotypes (floc-forming and filamentous bacteria), allowing spatial development of the floc according to the bacterial morphology, diffusion, reaction, and growth processes. The model needed also extension with a process for attachment of individual cells. Despite being in an early stage of development, the model is already a tool that enables us to obtain useful information about the microfloc environment. The model indicates that filamentous bacterial morphology and substrate microgradients are important aspects in the formation of bacterial structures. In mass transport-limited regimes filamentous bacterial structures prevail, whereas in growth-limited regimes irregular shaped flocs with fingerlike structures are dominant. These modeling results suggest that activated sludge flocs and biofilms might be different manifestations of the same phenomena. The model results support the hypothesis that floc-macrogradients can be the most important parameter for development of bulking sludge. The model suggests that attachment has a very strong effect on floc structure, leading to enhancement of the effect of substrate microgradients.