Nitrification by immobilized cells

Nitrosomonas europaea and Nitrobacteragilis are nitrifying bacteria. Subsequently they oxidize ammonia to nitrite and nitrite to nitrate. Nitrification is a key process for the removal of nitrogen compounds from wastewater.Ammonia is removed from wastewater by supply of oxygen to a reactor containing nitrifying bacteria. The hydraulic retention time in a reactor with suspended nitrifying bacteria needs to be sufficient to prevent wash-out of the bacteria. By immobilization, i.e., attach or entrap the organisms in a support material and retain the support material in the reactor, it is possible to run a reactor under wash-out conditions and consequently execute the nitrification process in compact systems. In practice, biofilm reactors are most commonly used for this.In this research, artificial immobilized nitrifying bacteria are studied. The bacteria were suspended in a gel (carrageenan) and solid spheres were produced from this gel. The artificial immobilized cells were used as a model system for biofilm processes. By using the model system we were able to perform fundamental studies of growth within the support and substrate conversion by the immobilized cells.Growth of immobilized cells has been described qualitatively in chapter 2. Immobilized cells form small colonies as the result of growth. Initially, such expanding micro-colonies were observed over the entire gel bead, but after some time growth just under the surface of the spheres was faster because of diffusion limitation over the support.The qualitative observations have been quantified by modelling the significant transport processes (radial diffusion through the support and external mass transfer across the stagnant liquid layer surrounding the spheres), substrate consumption and growth (chapter 3). The model that we developed calculates substrate consumption and growth over time and shows for example that initially growth will take place all over the bead and after some time especially just under the surface of the beads. This is in agreement with our previous qualitative observations. The model has been validated experimentally by immobilized Nitrobacter agilis cells by determination of macroscopic substrate consumption rates, substrate profiles and biomass profiles (chapter 4 and 5). Experimental results were in close agreement with the model results.The model consisted of general equations and the parameters were obtained from separate experiments or literature. For this reason the model should be general applicable. To demonstrate the general applicability the model was also used for Nitrosomonas europaea (chapter 6). The calculated macroscopic consumption rates, however, were much larger and the biomass concentration much lower than we found experimentally. We could show that in this experiment not only diffusion limitation over gel beads but also diffusion limitation over the micro-colonies was important. The size of micro-colonies of Nitrosomonas europaea was much larger than of Nitrobacter agilis in the previous experiment. This has been the reason to implement growth of biomass by expansion of colonies and taking diffusion limitation over those colonies into account in our model. Validation of this colony-expansion model showed that experimental and simulated results agreed much better. Application of the colonyexpansion model to our previous experiment with Nitrobacter agilis also gave good results.The size of the micro-colonies formed is dependent to a higher degree on the amount of organisms that is immobilized than on the type of organism. At low initial biomass concentrations, a few but large colonies will be formed. At high initial biomass concentrations much more and much smaller colonies will be formed. Those smaller colonies experience less diffusion limitation and consequently the substrate-utilization capacity of such beads will be higher. This effect could be demonstrated, both experimentally and by simulations with immobilized Nitrosomonas europaea (chapter 7).The models that are presented in this thesis give insight in growth and substrateconversion capacity of immobilized cells. Our approach was general and as a consequence the insight that is obtained is not only important for nitrification, but can be used for other immobilized-cell processes as well.