Modeling wastewater treatment by mobile biofilm and activated sludge processes configured for biological nutrient removal at 10 and 20 °C

ABSTRACT Hybrid biological wastewater treatment (WWT) processes contain biofilms and suspended biomass. Biofilms and carriers are retained by physical selectors, while suspended biomass is managed by its controlled withdrawal and recirculation. Compared with activated sludge (AS) processes, hybrid biological WWT processes require a smaller physical footprint and, in the case of mobile organic biofilms (MOBs), accumulate biomass with exceptional settling characteristics. This article presents a model-based comparison of MOB and AS processes for BOD5, NH4-N, and PO4-P removal. Process, numerical 1D-biofilm, geometric, migrating-carrier, and hybrid-bioreactor submodels were applied simultaneously. At 10 °C, only the MOB process achieved treatment criteria because slow-growing nitrifiers and phosphorus-storing heterotrophic bacteria were retained in biofilms. The hybrid MOB process increased suspended-growth solids retention time due to biofilm detachment and prevented secondary clarifier overload through superior biomass settleability. Biofilm carriers caused the total suspended solids concentrations in the MOB process to be greater than those in AS, but MOB had less total active biomass. For the modeled biofilm, layers near the biofilm's surface were dominated by phosphorus-storing and ordinary heterotrophic bacteria and protein extracellular polymeric substances (EPS) and carbohydrate-EPS; layers near the substratum were dominated by polyphosphates and nonbiodegradable endogenous decay products.