Modeling the activated sludge floc microenvironment

Integrated modeling of activated sludge systems will only be complete when settling characteristics can be determined within the model. These characteristics are fundamentally related to the size, shape and porosity of both the sludge flocs and their component microcolonies. Microcolony porosity is of particular interest since it may both influence, and be influenced by, the substrate and electron acceptor gradients that establish competitive microenvironments. A model was developed to explore the relationship between biomass growth, death, and hydrolysis; soluble component diffusion and microcolony porosity. A key element of the model is the assumed distribution of daughter cells following replication. Preliminary results indicate rapid progression toward minimum porosity with relatively slow expansion of the microcolony surface. Although little is documented on porosity variation within flocs, these results contrast with observations for biofilms and may emphasize the difference between the mechanisms involved in the structural development of microcolonies and larger scale aggregates. (C) 1998 IAWQ. Published by Elsevier Science Ltd.