Research was undertaken to develop a model (Aquifas Unified Model) that can be applied to activated sludge, Integrated Fixed Film Activated Sludge (IFAS) and Moving Bed Biofilm Reactor (MBBR) systems. The model has been developed to operate with up to 12 reactors (cells) in series with
biofilm media incorporated in one or more reactors. The process configuration can be any combination of anaerobic, anoxic, aerobic, post anoxic with or without supplemental carbon and reaeration; it can also be any combination of step feed and recycles including recycles for mixed liquor,
RAS, nitrates and membrane bioreactors. This paper presents the structure of the Aquifas model and the results of its application to a facility. This model embeds a biofilm model into a multi-cell activated sludge model. The advantage of such a model is that it eliminates the need to run
separate computations for a plant being retrofitted from activated sludge to IFAS or MBBR. The biofilm flux rates for organics, nutrients and biomass can be computed by two methods - a diffusion model that is computationally intensive, or a semi-empirical model of the biofilm that is relatively
simpler. The values of the kinetic parameters for the model were measured in pilot scale units of activated sludge, IFAS and MBBR systems. For the semi-empirical version, a series of Monod equations were developed for COD, ammonium-N and oxidized-N uptake by the biofilm. These equations
were used to develop equations for fluxes of COD, ammonium-N, DO, oxidized-N and VSS into/out of the biofilm. For the biofilm diffusion model (Aquifas 4), the biofilm is divided into 12 layers and a stagnant liquid layer. The diffusion and substrate utilization are calculated for each
layer and the equations solved simultaneously using a finite difference technique. The biofilm flux model is then linked to the activated sludge model. Both versions of the model are largely open source code that can be downloaded from www.aquifas.com. They can be used to quantify the amount of media and surface area required to achieve nitrification, identify the locations for the media, and optimize the DO levels and nitrate recycle rates. Some of the advanced features include
the ability to compute the biofilm thickness and impact of biofilm thickness on performance; apply different media types and fill fractions in different reactors operating in series; design the aerobic zone in a manner such that the media nitrifies and denitrifies within the aerobic zone.
The model has been able to predict the improvement in nitrification, or the deficiencies in the design which limited nitrification in IFAS systems at several full scale facilities.
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