Modelling nitrification, heterotrophic growth and predation in activated sludge

A mathematical model describing the interaction between nitrifiers, heterotrophs and predators in wastewater treatment has been developed. The inclusion of a predation mechanism is a new addition to the existing activated sludge models. The developed model considered multi-substrate consumption and multi-species growth, maintenance and decay in a culture where nitrifiers, heterotrophs and predators (protozoa and metazoa) are coexisting. Two laboratory-scale sequenced batch reactors (SBRs) operated at different sludge retention time (SRT) of 30 and 100 days for a period of 4 years were used to calibrate and validate the model. Moreover, to assess the predator activity, a simple procedure was developed, based on measuring the respiration rate with and without the presence of the predators. The model successfully described the performance of two SBRs systems. The fraction of active biomass (ammonia oxidisers, nitrite oxidisers and heterotrophs) predicted by the proposed model was only 33% and 14% at SRT of 30 and 100 days, respectively. The high fraction of inert biomass predicted by the model was in accordance with the microscopic investigations of biomass viability in both reactors. The presented model was used to investigate the effect of increasing sludge age and the role of predators on the biomass composition of the tested SBR system.