Against the background of drastically rising global water demand and increasing pollution and overexploitation of regional water resources, the demand-driven water supply of households and industry is of central importance. Water reuse and desalination are seen as key technologies to overcome potential regional and local water shortage. In the joint projects funded by the German Federal Ministry of Education and Research (BMBF) ‘Future-oriented Technologies and Concepts to Increase Water Availability by Water Reuse and Desalination (WavE)’, evaluation approaches for analysing innovative technologies and concepts are being developed and assessed. All evaluation methods and criteria used were selected based on the decision situation at hand and the decision-makeŕs preferences. Based on the analysis of six multi-criteria evaluation concepts used in selected WavE projects, this paper presents a general approach for comparative multi-criteria evaluation of water reuse systems consisting of prerequisites, minimum requirements, evaluation criteria (qualitative, semi-quantitative or quantitative) and a final aggregation of results. Exemplary sets of criteria for the application in a more industrial, municipal and/or international context are presented as an aid for the application of holistic evaluation approaches for (process) concept and technology selection in the context of water reuse and desalination.

The performance of two systems of semi-industrial up-flow biological aerated filters (BAF) with pre-denitrification followed by nitrification was studied and compared under various operating and loading conditions. The first system consisted of two separate reactors for the denitrification and the nitrification step, whereas in the second system the aerobic nitrification zone was packed on top of the anoxic denitrification zone in one reactor. The second system potentially offers substantial savings in investment costs and space requirements for a large scale treatment plant. Regarding the elimination of carbonaceous pollution and denitrification the systems did not show significant differences. However, nitrification in the combined system suffered from the mixing of different biocenosis by daily backwashing and was reduced to 50-70% of the separated system's performance. Factors such as oxygen concentration, raw water composition and loading rates affected both systems' nitrification rates in similar ways. Since it is impossible to optimise the nitrification and denitrification processes separately, the combined system should only be considered for large scale applications if space is very scarce and if a stable raw water composition can be expected. If strict limit values for nitrate have to be met in the effluent, a combination of pre- and post-denitrification is advantageous and advisable.

The incorporation of enhanced biological phosphorus elimination in the single sludge process is a contemporary challenge in the design of biological wastewater treatment plants. The simultaneous biological removal of nitrogen and phosphorus is a process of considerable technical and biological complexity owing to the mixed substrates and the diversity of microorganism populations. In addition, an interaction takes place between the phosphorus-accumulating organisms and the nitrogen-eliminating organisms owing to the physiological demands. When designing wastewater treatment plant, it is necessary to establish anaerobic zones for producing the short fatty acids necessary for phosphorus removal, an xic zones for denitrification, and aerated zones for nitrification and phosphorus removal. We have developed a mathematical model based on IA WPRC-model No I for this process scenario. In addition to the degradation kinetics of carbonaceous components and nitrogen, the model describes the microkinetics of enhanced biological phosphorus elimination. The assumptions of the model are discussed in relation to the recent literature. The range of values found in the literature and the used default values of the model parameters are listed. The simulation program is presented, and first results of simulation are shown.

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