NITRIFICATION BY IMMOBILIZED CELLS

Nitrification has been studied extensively for its significance in the nitrogen cycle and within the treatment of wastewater. Often nitrogen removal from wastewater is problematic because of the low growth rare of the bacteria involved. Immobilization is an efficient method to re tain slow-growing organisms in continuous-flow reactors. Immobilized c ells can be divided into ''naturally'' attached cells (biofilms) and ' 'artificially'' immobilized cells. Biofilm systems are widely used in practice. Immobilized-cell reactor cells entrapped in gel beads form a n alternative to these attached-cell systems. An overview is given of studies with immobilized nitrifying cells, with special attention to m odel development and evaluation. Dynamic models, including growth, dif fusion limitation in the support material and in the microcolonies, an d external mass transfer, are discussed. Experimental evaluation of th e models is essential for their control if the models provide insight in the complex processes and for their use as a tool in reactor design . Several techniques for evaluation are given, including macroscopic s ubstrate consumption rates and local analyses in the support material.