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Hybrid adaptive control strategy was developed and tested for the degradation of propylene glycol, a major component in de-icing waste, in an anaerobic fluidized bed bioreactor (AFBR). A linearized model with time-varying parameters was first employed to describe the dynamic behavior of the AFBR using a recursive off-line system identification method. A hybrid adaptive control strategy was then tested using a recursive off-line system identification routine followed by an on-line adaptive optimal control algorithm. The objective of the controller was to achieve the desired set point value of the propionate concentration (stand-alone control output variable) by manipulating the dilution rate (control input variable). To do so, the optimal control law was developed by minimizing a cost function with constraint equations. This novel idea was successfully applied to the underlying system for 200 h. The set point (700 mg HPrl(-1)) was achieved even in the case where the feed concentration suddenly increased by 50% (9000 mg HPrl(-1) to 13500 mg HPrl(-1)).

This work examines the transient response of an anaerobic fluidized bed bioreactor to an overload of propylene glycol (PG), the primary component in aircraft de-icing waste. Under favorable operating conditions, PG was converted to n-propanol (n-PrOH) and propionate (HPr), and subsequently n-PrOH was completely converted into HPr. HPr was then fully degraded to methane and carbon dioxide via acetate. Under an overload condition, n-PrOH conversion to propionate was completely blocked but propionate degradation continued, contrary to free-energy computations in which n-PrOH should rapidly degrade and HPr should accumulate. When the imposed overload condition was relieved, the accumulated n-PrOH was rapidly converted into propionate. n-PrOH, then, could act as a temporal sink for reducing equivalents (XH(2)) and could regulate the overall PG methanazation process. n-PrOH should be monitored along with typical VFAs such as HPr to avoid sudden VFA accumulation and thus to optimize process performance for PG methanization., (Copyright 2002 Elsevier Science Ltd.)

An integrated mathematical model for propylene glycol (PG) degradation in an anaerobic fluidized bed bioreactor is described. Special attention is put on biomass distribution, bed expansion, and bed segregation, associated with the biofilm accumulation process. In order to interpret the segregation of the bed during the initial development of the biofilm, the model postulates various mixing intensities along the bed height and thereby different exchange rates of microbial cells between the biofilm and the bulk liquid. The model incorporates stoichiometry of PG methanization, hydrodynamics, and reaction kinetics for elucidating microscopic interaction among microbial trophic groups inside the biofilm as well as the macroscopic behavior of the reactor such as bed expansion. A biofilm diffusion mechanism is also taken into account focusing on the spatial distribution of multiple species of micro-organisms. Employing moving boundaries makes the model flexible in computation, which permits simplifying the implementation of the biofilm accumulation and the bed expansion phenomena. [ABSTRACT FROM AUTHOR]

This paper examines the influence of bed segregation on the performance of an anaerobic fluidized bed bioreactor (AFBR) using both an integrated mathematical model previously described in Part I of this study, and experimental data obtained from a laboratory-scale AFBR continuous flow system and batch serum vial tests. Local hydrodynamics within the bed are shown to determine mixing intensities and patterns of bioparticles thereby controlling biofilm thickness and composition along the bed height. Results of the model simulations and the experimental data indicate that shallow biofilms that allow full substrate penetration are dominantly populated with faster growing micro-organisms. The internal mass transfer resistance in thicker biofilm significantly influences population distribution resulting in the increase of population of slower growing micro-organisms in a deeper layer of the biofilm. The serum bottle tests also confirm that microbial distribution inside a multispecies biofilm is determined by the hydrodynamic condition of the reactor. This study illustrates the importance of hydrodynamic regimes in the AFBR, and demonstrates the impact of bed segregation on bioparticle properties and total system performance. [ABSTRACT FROM AUTHOR]