Abstract
Sponge carrier media can be effectively used to upgrade existing activated sludge plants to augment nitrification. With an internal anoxic zone and ambient oxic conditions, it can perform simultaneous nitrification and denitrification (SND). In order to accurately predict SND, it is essential to construct a proper mass transfer model. A simple model for the simulation of bio-clogging in sponge carrier media using multiple biofilm reactor compartments was constructed. The relation between pore diffusivity and porosity was developed which was based on a critical porosity for the transition from advection to diffusion in the pores of the sponges. By inclusion of heterotrophic organisms in the model, the pore diffusivity was calibrated with experiments of denitrifying sponges. In order to simulate SND, the model was extended using slow-growing ammonia-oxidizing organisms (AOO) and nitrite-oxidizing organisms (NOO). At the effluent, a clarifier was modelled with a defined sludge wastage rate. At low sludge recycle fractions, the heterotrophs were dominant inside the sponge and outcompeted AOO and NOO. Higher sludge recycle fractions resulted in the presence of AOO and NOO inside the sponge which made it possible for SND via nitrate or short cut SND via nitrite.
