The IWA Anaerobic Digestion Modelling Task Group was established in 1997 at the 8th World Congress on Anaerobic Digestion (Sendai, Japan) with the goal of developing a generalised anaerobic digestion model. The structured model includes multiple steps describing biochemical as well as physicochemical processes. The biochemical steps include disintegration from homogeneous particulates to carbohydrates, proteins and lipids; extracellular hydrolysis of these particulate substrates to sugars, amino acids, and long chain fatty acids (LCFA), respectively; acidogenesis from sugars and amino acids to volatile fatty acids (VFAs) and hydrogen; acetogenesis of LCFA and VFAs to acetate; and separate methanogenesis steps from acetate and hydrogen/CO 2 . The physico-chemical equations describe ion association and dissociation, and gas-liquid transfer. Implemented as a differential and algebraic equation (DAE)

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This work reviews the existing methodologies for assessing microbial activity and inhibition under anaerobic conditions. The anaerobic digestion process consists of several metabolic steps: the Anaerobic Digestion Model No. 1 (ADM1) has attempted to describe these steps in the form of a mathematical model with the intention of providing a reference base for all further efforts in the modelling of anaerobic processes. The existence of a reference point for modelling has highlighted the fact that there is a lack of coherence between the many different methodologies for experimentally assessing anaerobic activity and inhibition.A working group of the International Water Association was recently founded to harmonise the existing methodologies with the ultimate intention of developing a unified reference procedure: a primary objective of the group will be the establishment of a standard terminology in the field of anaerobic digestion, activity and inhibition assessment. Secondly, it will compare the existing methodologies and develop standard protocols for assessing the kinetic parameters (e.g. maximum uptake rate, half-saturation constant) of anaerobic processes that may be entered directly into ADM1 and its successors. This paper revises and enlarges a contribution presented by the authors at the workshop ''Harmonisation of anaerobic biodegradation, activity and inhibition assays'' (Ligthart & Nieman 2002, Proc. workshop held in Orta (Italy) June 7-8, 2002) and aims to promote a clear understanding of the currently established methodology.Numerous methods have been developed over the past 30 years, since Van den Berg et al. (1974, Biotechnol Bioeng 16(11): 1459-1469) measured methanogenic activity, by using a manometric device equipped with a photoelectric sensor to quantify the gas production. Methanogenesis is often the rate limiting step of the entire process and since the quantification of gas flowrate is relatively easy to perform, most of the methods reported in literature monitor the production of biogas. These methods can be termed volumetric or manometric methods, as the volume of biogas produced or the pressure increase due to gas production inside a close vessel are assessed, respectively. However, this same concept can be employed to assess activity or inhibition of individual metabolic steps preceding the methanogenic one, providing that they are rate limiting for the whole process. The reliability of activity assessment through gas measurement has been proven to be strongly dependent on the equilibrium between liquid and gas phase in a closed vessel. This can be influenced by many factors, e.g. the amount and characteristics of the test substrate; the concentration of the biomass; the gas-to-liquid ratio: all these aspects will need to be addressed in the standard procedure. Other direct or indirect methods, targeting physico-chemical or microbiological parameter exist and have been investigated by many authors. Besides the interest for research purposes, the definition of reference methods to assess ...

The reduction of the duration of start-up and the improvement of process control are important factors in order to increase the competitiveness of anaerobic high-rate reactor systems. This paper discusses and reviews the specific similarities and differences of UASB, filter and expanded/fluidized bed reactors with respect to start-up, operation, parameter monitoring and process control. The influence of microbial, biochemical and physical parameters upon reactor start-up and process performance is evaluated and methods for process monitoring and control are described. The different role of stability indicators, which give an early warning signal of oncoming unstable process conditions, and control variables, which must be kept constant during operation, is discussed with respect to process control and reactor start-up. The merits and weak points of each reactor system are presented and all systems are qualitatively compared.