UNIFIED COMPUTATIONAL MODEL FOR ACTIVATED SLUDGE, IFAS AND MBBR SYSTEMS

Research was undertaken to measure kinetic parameters for an Unified computational model that can be applied to activated sludge, IFAS and MBBR systems. The model is for COD removal, nitrification and denitrification. There are significant advantages of such a model: 1) it eliminates the need to run separate computational models for each of the three systems; and (2) a single model can be used to evaluate all three configurations for plant design and retrofit.Pilot scale continuous flow units were operated in the UCT/VIP configuration to measure the kinetic parameters for the model. The IFAS and MBBR systems (units) were operated in parallel with an activated sludge system. The systems were operated under identical wastewater loads, tank and nitrate recycle configuration, except for the biofilm support media in the aerobic zone of the IFAS and MBBR systems. The flow rate was 208 L/day and the nominal HRT was 12 hours. The operating temperature of 12 C was low enough to stress the nitrifiers in the mixed liquor suspended solids as the mixed liquor MCRT was lowered. Specific phases of the study included:Activated sludge system operated at aerobic zone mixed liquor MCRTs of 3.1 days, 2.4 days, 1.7 days and 1.0 days;Integrated Fixed Film Activated Sludge (IFAS) system operated with biofilm support media in the aerobic zone. The IFAS unit was operated in parallel and at the same mixed liquor MCRTs as the activated sludge system;Moving Bed Biofilm Reactor (MBBR) system operated without return activated sludge (RAS) recycle with biofilm support media in the aerobic zone. The MBBR unit was operated in parallel to a control system without biofilm media and RAS.The aerobic zone of each of the three systems was divided into three cells with equal volume to determine how the biofilm would develop, and COD uptake and nitrification rates would vary with the soluble biodegradable COD and ammonium-N along the aerobic zone.The most important equations identified as part of this research effort are the rates of COD removal and nitrification per unit surface area of biofilm, and their dependence on the soluble biodegradable COD and ammonium-N concentrations. The structure of these equations is similar to the Monod equations used for removal (uptake and oxidation) of COD and ammonium-N in the mixed liquor suspended solids. By adding the removal rates of COD and ammonium-N in the biofilm to that in the mixed liquor suspended solids, the model is able to compute the total removal in each reactor.The COD removal, ammonium-N removal and nitrification and denitrification, the concentration profiles and the effluent values computed by the model compared satisfactorily to the concentrations measured in the pilot unit. Additionally, the model was able to determine the amount of biofilm surface area that would be required and the best locations for the media to maximize the increase in nitrification. Based on this, the user is able to compute how much media will be required to retrofit a plant that is not nitrifying or is partially nitrifying.