| Filamentous bulking, the most common solids separation problem in activated sludge systems, is caused by the excessive growth of filamentous bacteria over floc-forming bacteria, resulting in decreased sludge settling ability. The competition between the two types of organisms has been historically described using kinetic selection. However, it has been suggested that other factors, such as the presence of a filamentous "backbone", differences in decay rates, bacterial storage abilities, and substrate diffusion limitation may also affect the microbial selection. In this research, various hypotheses that integrated several of those factors were tested using modeling, reactor and molecular studies, and a new conceptual qualitative model combining kinetics and diffusion explaining bulking was developed.; As a first step, a bacterial competition model integrating kinetic selection theory, filamentous backbone theory, decay rates and storage abilities of filaments and floc formers was set up to predict and explain coexistence in a completely mixed reactor. Sensitivity analysis showed that the kinetic parameters mu max and Ks, storage rate constants and backbone coefficient had the greatest effect on the simulation results. It was found that non-bulking sludge had higher maximum substrate uptake rates than bulking sludge, consistent with results from respirometry. Quantitative fluorescence in situ hybridization (FISH) showed that the filaments Eikelboom Type 1851, Type 021N and Thiothrix nivea were dominant in bulking sludge, comprising 42.0% of mixed liquor volatile suspended solids (MLVSS), with 61.6% of the total filament length extending from flocs into bulk solution. Only low levels of Type 1851 filament length (4.9% of MLVSS) occurred in non-bulking sludge, 83.0% of which grew inside the flocs. This result seemingly supported the kinetic selection theory, but contradicted our previous experimental data that showed that bulking and non-bulking sludge have similar levels of total filaments length, and thus supporting the diffusion limitation hypothesis. To resolve this contradiction, a new conceptual qualitative framework was developed in this study. To test this framework, sequencing batch reactors (SBRs) were operated with various influent substrate concentrations, and sludge settleability was measured at various floc sizes induced by different mixing strength. A model integrating both mechanisms was developed to simulate the substrate concentrations at different floc sizes. The modeling results showed the occurrence of diffusion limitation inside the flocs at a certain range of activated sludge floc sizes, and the experimental data supported this framework in the bulking and transitional regions. (Abstract shortened by UMI.)... |
