Nitrification by artificially immobilized cells : model and practical system

The research described in this dissertation started after extended studies on the fundamental aspects of the growth of immobilized nitrifying cells. In these previous studies dynamic models were developed and experimentally validated, which resulted in knowledge about which steps determine the rate of the nitrification process and to which parameters the immobilized-cell process is sensitive. Furthermore, it showed that this system may be interesting as a practical system as well.The aim of the work described in this thesis was to obtain more insight in the complex interactions in the nitrification process with artificially immobilized cells in air-lift loop reactors. The knowledge obtained may form the basis for a rational design of compact systems for practical applications, e.g. nitrification of domestic wastewater. This knowledge was built up by developing dynamic models, by studying the applicability and mechanical stability of the immobilization support and by doing experiments in domestic wastewater.In Chapter 1 an overview of the previous studies and the approach, followed in our group for the development of a process with immobilized cells, are given. The subjects in this Chapter provide the starting points of the following chapters, which are arranged in three themes: dynamics, supports and application.The first theme, the dynamics of immobilized nitrifying cells entrapped in carrageenan, is described in Chapter 2 and 3. The results in Chapter 2 clearly show the reduced temperature sensitivity of immobilized nitrifying cells caused by diffusion limitation. In Chapter 3 is described that, for example at low substrate concentrations and low hydraulic retention times, the death rate of cells is an important parameter. If cells die as a result of e.g. a low substrate loading, the process reacts slower to changes in substrate concentrations than if all cells remain viable.The second theme (Chapter 4 and 5) deals with the selection and characterization of support materials for cell immobilization. In order to establish which support materials are the most suitable, characteristics of several natural and synthetic materials have been determined and compared, Natural gel materials, like alginate and carrageenan, have a mild immobilization procedure such that few cells die and they grow well in these supports. These supports, however, appeare to be soluble, biodegradable and sensitive to abrasion.Synthetic gels, on the contrary, have better mechanical properties, but the immobilization conditions are less mild resulting in low biomass retention. For application of entrapped nitrifying cells synthetic gels, like polyethylene glycol, polyvinyl alcohol and polycarbamoyl sulphonate, are, however, promising (Chapter 4). In Chapter 5 the relevance of rheological properties of gel beads for their mechanical stability is described. It is concluded that tests based on rupture of 'virgin' beads does not provide relevant information on the mechanical stability and sensitivity to abrasion. It is likely that abrasion of gel beads is related to 'fatigue' of the gel materials.Application of artificially immobilized nitrifying cells is the topic of the last theme, in which the two formerly discussed themes are combined. In the work described in Chapter 6 the applicability of promising support materials was studied in more detail and a suitable support was used to immobilize nitrifying cells. High nitrification capacities could be reached in pretreated domestic wastewater, demonstrating that it is indeed possible to apply this system. The possibilities, drawbacks and prospects of immobilized cells for environmental applications are discussed further in Chapter 7. It is demonstrated there on the basis of the gathered knowledge that the immobilized-cell process has promising characteristics to meet the desired demands for future wastewater-treatment plants i.e. high biomass concentration, small volume of aeration tank, low liquid retention times, reduced sludge production and a high operational flexibility. Furthermore, it was calculated that the costs for production of immobilized cells are more than compensated for by the reduced costs for construction of a smaller reactor.