Transformations of organic matter in domestic wastewater under changing aerobic/anaerobic conditions were studied in laboratory scale reactors and in a real pressure sewer emphasizing measurements of oxygen utilization rate (OUR). During 3-10 hours of anaerobic conditions, the aerobic, heterotrophic biomass was maintained and a net production of readily biodegradable substrate originating from hydrolyzable substrate was observed. A process model description which includes aerobic and anaerobic processes in the bulk water phase as well as in the biofilm was presented and model parameters were determined from OUR measurements. Based on experimental findings, it was possible to simulate wastewater quality changes in terms of organic matter under transient aerobic/anaerobic conditions in sewer systems.

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