Modelling aerobic granular SBR at variable COD/N ratios including accurate description of total solids concentration

The operation of a sequencing batch reactor (SBR) with aerobic granular biomass was successfully simulated using a one-dimensional biofilm model. The biological processes considered were described based on the activated sludge model (ASM) platform with two main modifications: (i) simultaneous growth and storage of organic substrates by heterotrophic bacteria; and (ii) inclusion of nitrite as intermediate compound in the nitrification and denitrification processes. Three different operational conditions were evaluated, characterized by different chemical oxygen demand to nitrogen (COD/N) ratios in the influent of: 0, 1.25 and 5.5 kg kg^-1, representing a purely autotrophic media and two heterotrophic media, respectively. An accurate description of the experimental concentrations of COD, ammonium, nitrite, nitrate, dissolved oxygen (DO) and alkalinity along the cycles was obtained. Total solids concentration inside the reactor (5.0, 2.0 and 1.0 kg VSS m^-3 for (COD/N) ratio of 5.5, 1.25 and 0 kg kg^-1, respectively) and biofilm density (23 kg m_granule^{- 3}) were correctly described with the model. To obtain an accurate description of both solids concentration and biofilm density different densities were defined for the particulate compounds and a porosity profile along the granule was imposed. Oxygen penetration depths obtained with the model were 0.35 × 10^-3, 0.30 × 10^-3 and 0.12 × 10^-3 m for (COD/N) ratio of 5.5, 1.25 and 0 kg kg^-1, respectively. The values were in agreement with those used in the description of the porosity profiles.