The aim of this paper is to provide a conceptual basis for the systematic treatment of uncertainty in model-based decision support activities such as policy analysis, integrated assessment and risk assessment. It focuses on the uncertainty perceived from the point of view of those providing information to support policy decisions (i.e., the modellers' view on uncertainty) -uncertainty regarding the analytical outcomes and conclusions of the decision support exercise. Within the regulatory and management sciences, there is neither commonly shared terminology nor full agreement on a typology of uncertainties. Our aim is to synthesise a wide variety of contributions on uncertainty in model-based decision support in order to provide an interdisciplinary theoretical framework for systematic uncertainty analysis. To that end we adopt a general definition of uncertainty as being any deviation from the unachievable ideal of completely deterministic knowledge of the relevant system. We further propose to discriminate among three dimensions of uncertainty: location, level and nature of uncertainty, and we harmonise existing typologies to further detail the concepts behind these three dimensions of uncertainty. We propose an uncertainty matrix as a heuristic tool to classify and report the various dimensions of uncertainty, thereby providing a conceptual framework for better communication among analysts as well as between them and policymakers and stakeholders. Understanding the various dimensions of uncertainty helps in identifying, articulating, and prioritising critical uncertainties, which is a crucial step to more adequate acknowledgement and treatment of uncertainty in decision support endeavours and more focused research on complex, inherently uncertain, policy issues.

A review of anaerobic treatment of wastewater in fixed film reactors is presented. The biochemistry and microbiology is discussed, with emphasis laid on kinetic parameters like growth constants, substrate removal rates and growth yields. The influence of temperature, toxic substances, nutrients and pH upon process performance is evaluated. The kinetics of anaerobic biofilms is discussed. The effect of diffusional limitation and methane generation within the biofilm are important factors not studied yet. The various reactor types and process configurations are commented upon. Most studies of anaerobic processes until now have been with fixed bed reactors, and with soluble substrates. The design of reactors should be based upon biofilm area or biomass in the reactors. At present biofilm area can not be used for design due to lack of investigations on anaerobic biofilm kinetics. Anaerobic fixed film processes can be used for almost all types of industrial wastes containing organic matter at reasonable concentrations. Treatment of municipal wastewater is at present not regarded feasible, but the development of new effective types of reactors is believed to change this in the near future.

[1] A general framework for regional analysis and modeling of extreme rainfall characteristics is presented. The model is based on the partial duration series (PDS) method that includes in the analysis all events above a threshold level. In the PDS model the average annual number of exceedances, the mean value of the exceedance magnitudes, and the coefficient of L variation (LCV) are considered as regional variables. A generalized least squares (GLS) regression model that explicitly accounts for intersite correlation and sampling uncertainties is applied for evaluating the regional heterogenity of the PDS parameters. For the parameters that show a significant regional variability the GLS model is subsequently adopted for describing the variability from physiographic and climatic characteristics. For determination of a proper regional parent distribution L moment analysis is applied for discriminating between the exponential distribution and various two-parameter distributions in the PDS model. The resulting model can be used for estimation of rainfall intensity-duration-frequency curves at an arbitrary location in a region. For illustration, the regional model is applied to rainfall data from a rain gauge network in Denmark.

Carbon cycling and the availability of organic carbon for nutrient removal processes are in most wastewater treatment systems restricted by the rate of hydrolysis of slowly biodegradable (particulate) organic matter. To date, the mechanisms of hydrolysis are not well understood for complex substrates and mixed populations. Most mathematical models use a simple one-step process to describe hydrolysis. In this article, mechanisms of hydrolysis and mathematical models to describe these processes in wastewater treatment processes are reviewed. Experimental techniques to determine mechanisms of hydrolysis and rate constants are discussed.

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