Modeling aerobic granules in continuously flowing wastewater-treatment processes.

Continuously flowing wastewater-treatment processes can be configured for biological and physical selection to form and retain large biological aggregates (LBAs), along with suspended biomass that contains ordinary biological flocs and biomass that has detached from the LBAs. Suspended biomass and LBAs have different solids residence times (SRTs) and mass-transport resistances. Here, mathematical sub-models that describe metabolic processes, a 1-D biofilm, and spherical carriers that can migrate throughout a wastewater-treatment process were combined to simulate a full-scale demonstration train having anaerobic, anoxic, and oxic zones, as well as side-stream enhanced biological phosphorus removal. Hydrocyclones were utilized for physical selection. Simulation results and experimental observations agreed for soluble chemical oxygen demand, nitrogen, and phosphorus removals, as well as mixed liquor concentration and characteristics. The model outputs demonstrated that suspended biomass was responsible for most of the transformations in the bioreactor, but LBAs contributed importantly to P accumulation as polyphosphate. The simulated LBAs accumulated a higher density of phosphorus-storing bacteria, polyphosphate, and total- and protein-extracellular polymeric substances (EPS), particularly near their core. Protein-EPS accumulated near the substratum because protein-EPS hydrolyzed more slowly than carbohydrate-EPS, while the SRT in each layer increased from the surface layer to the layer adjacent to the LBA core. PRACTITIONER POINTS: Combined models well represented observed solids components in a full-scale demonstration train as A2O with S2EBPR. Simulations described aerobic-granule structure and function consistent with what is known about aerobic granules in BNR processes. Suspended biomass dominated most of the simulated transformation rates, but the LBAs accumulated ~2000 mg P/L as polyphosphosphate. The simulated aerobic granules did not intensify WWT overall but should have improved the net solids-settling characteristics. Aerobic granules had more EPS than the suspended biomass and protein-EPS accumulated inside the LBA by slower hydrolysis kinetics.