In recent years, there has been a growing awareness of the ill-defined nature of environmental processes. To provide a frame of reference for discussions regarding ill-defined systems, a taxonomy and terminology of the modelling and simulation of systems is presented. Due to the complexity of ill-defined systems, it is not only necessary to describe the nature of models, but also to describe modelling procedures. This enables the modeller to obtain the model which best fits his goals (optimal model). For meaningful description of models, different model formalisms will be presented. Furthermore, modelling procedures will be described at a generic level and different model formalisms will be presented. Throughout this paper, Waste-Water Treatment Plants and processes occurring within these plants will illustrate the definitions given.
The use of an automatic on-line titration unit for monitoring the effluent quality of wastewater plants is presented. Buffer capacity curves of different effluent types were studied and validation results are presented for both domestic and industrial full-scale wastewater treatment plants. Ammonium and ortho-phosphate monitoring of the effluent were established by using a simple titration device, connected to a data-interpretation unit. The use of this sensor as the activator of an effluent quality proportional sampler is discussed.
Spare nitrification capacity is usually needed for a nitrifying activated sludge plant to counter nitrogen shock loads and/or toxicity incidents. The traditional way to provide this capacity is to apply a sludge retention time which is much longer than what needed to obtain a stable nitrification, resulting in over designed plants. Another approach, which is investigated in this paper, is to store the spare biomass in a separate sludge storage tank and return it to the main stream process when a shock nitrogen load or a toxicity incident occurs. Model based analysis reveals the unique feature of the scheme: different particulate components in the sludge have a different retention time, and more specifically, active biomass stays longer in the plant than inert solids. This results in that a plant with a storage tank can have the same amount of active biomass as a traditional plant but less sludge, opening the potential of reducing the volume of the plant. Analysis shows that a plant with a sludge storage tank can be about twenty percent smaller than a traditional plant that has the same treatment capability. Analysis is verified by simulation studies.
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