(143h) Comprehensive Distributed Parameter Model of an Upflow Anaerobic Sludge Bed (Uasb) Reactor

Upflow anaerobic sludge bed (UASB) reactors are often used in anaerobic digestion for treating high strength wastewaters. Typically, an UASB reactor has a sludge bed thickness of 2-5 m and is operating at a liquid upflow velocity of 1 m h-1 or below and a retention time of 8 h or above. Under these operating conditions the existence of significant substrate and biomass gradients in UASB-type reactors might be expected and has been experimentally demonstrated in a number of studies [1, 2]. Nevertheless, existing UASB reactor models most often use the assumption of ideal mixing, e.g. as in the International Water Association (IWA) Anaerobic Digestion Model No 1 (ADM1) developed by the IWA task group for mathematical modeling of anaerobic digestion processes [3]. The ADM1 is a structured model, which accounts for steps of disintegration, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Also, it uses seven populations of microorganisms to describe a multi-step transformation of organic matter to methane. Because of the comprehensiveness of the bioconversion processes in ADM1, the model is applicable for simulating a wide range of anaerobic digestion processes. The CSTR assumption however, limits the model applicability to systems with intensive mixing.

In the present study, the ADM1 model is used as a basis for developing a comprehensive distributed parameter model of the UASB reactor. First, hydraulics of the UASB reactor is studied using on-line measurements of a fluorescent tracer. Tracer concentrations are simultaneously measured at different reactor heights permitting an evaluation of dispersion coefficient dependence on reactor height. A significantly better agreement between the model outputs and the observed tracer distribution is obtained when the dispersion coefficient is proportional to a constant raised to the power of the reactor height [4]. Furthermore, measurements of chemical oxygen demand (COD) and volatile fatty acids (VFAs) are also carried out at different reactor heights. Significant COD and VFA gradients are observed at a flow rate to external recirculation ratio of 4 and below. To model these results, material balances of ADM1 are transformed to a set of partial differential equations (PDEs) describing transport and biotransformation phenomena in the UASB reactor. The orthogonal collocation method is applied to solve the distributed PDEs model. Parameter estimation of the model is carried out using a zero-order minimization algorithm of Nelder and Mead yielding a good agreement between model outputs and the measurements. In comparison to CSTR model, the distributed parameter model provides better fitting of the experimental measurements. More importantly, the distributed model makes it possible to study the influence of upflow velocity on reactor dynamics. Conversely, a CSTR model is unable to do so because of the assumption of ideal mixing. Overall, the study suggests that the distributed parameter model provides better accuracy in describing the UASB reactors than the CSTR model. The distributed parameter model can be used to develop new control strategies for UASB reactors, i.e. using upflow velocity to reduce the impact of organic overload on reactor removal efficiency. Furthermore, application of the distributed model enables optimization of the design and operation of UASB reactors by investigating the effect of biomass and substrate distribution on reactor performance.

References [1] S. V. Kalyuzhnyi, V. I. Sklyar,M. A. Davlyatshina, S. N. Parshina, M. V. Simankova, N. A. Kostrikina & A. N. Nozhevnikova, Organic removal and microbiological features of uasb-reactor under various organic loading rates, Bioresource Technology 55 (1996) 47-54 [2] S. V. Kalyuzhnyi, V. Fedorovich and P. Lens, Novel dispersed plug flow model for UASB reactors focusing on sludge dynamics, the Proceedings of the 9th World Congress ?Anaerobic Digestion 2001?, Antwerpen, Belgium [3] D.J. Batstone, J. Keller, R.I. Angelidaki, S.V. Kalyuzhnyi, S.G. Pavlostathis, A. Rozzi, W.T.M. Sanders, H. Siegrist and V.A. Vavilin, Anaerobic Digestion Model No.1, ISBN: 1900222787, IWA publishing, London, UK. 2002. [4] Y. Zeng, S. J. Mu, S. J. Lou, B. Tartakovsky, S. R. Guiot, P. Wu, Hydraulic Modeling and Axial Dispersion Analysis of UASB Reactor, Biochemical Engineering Journal, In press, 2005