Process design for nitrogen removal using nitrifying biofilm and denitrifying suspended growth in a biofilm airlift suspension reactor

Operation of a Biofilm Airlift Suspension (BAS) reactor was possible with nitrifying biofilm growth and heterotrophic suspended growth, simultaneously converting ammonium and acetate. Control of the location of heterotrophic growth, either in suspension or in layers over the nitrifying biofilms, was established by manipulation of the hydraulic retention time. Two process configurations (based on the BAS reactor) are identified, for nitrogen removal from wastewater by integration of nitrification in biofilms and denitrification in suspension. The first set-up consists of a conventional nitrifying BAS reactor, coupled with a suspended biomass reactor (without biomass retention). The second process configuration is a single BAS reactor which is sequentially aerated and non-aerated, by cyclically recirculating the offgas. Process design schemes for these configurations are presented and the design aspects are evaluated. Three limiting processes are identified: gas-liquid oxygen transfer, the hydraulic capacity of the settler and the minimal time needed for oxygen depletion of the gas phase during offgas recirculation. It was found that the two-reactor configuration is mainly suitable for pre-treatment of concentrated ammonia containing wastewaters. The one-reactor configuration can be applied to obtain low nitrogen concentrations in the effluent.