One way to avoid the wash-out limitation in continuous biological systems and toincrease productivity, is to retain the cells inside the reactor via immobilisation onto a supportmaterial. From a physical-chemical point of view, adhesion of cells to surfaces is determinedby the interplay of electrostatic and hydrophobic interactions. Thus, the nature of the surfaceof the microorganisms and supports are determining factors. The two characteristics ofrelevance are electrokinetic potential and hydrophobicity. The adhesion of Alcaligenesdenitrificans to several polymeric materials is being studied. The supports are: high densitypolystyrene (HDPS), high density polyethylene (HDPE), polypropylene (PP),Polyvinylchloride (PVC) and polymethyl-methacrylate (PMMA). The electrokinetic potentialof the cells and the supports was determined by measurements of electrophoretic mobility.Cell hydrophobicity was assessed by measuring adhesion to hexadecane and thehydrophobicity of supports was determined by water contact angle measurements. Underphysiological conditions, the bacterial cells and most solid surfaces are negatively chargedcausing a potential energy barrier which difficults the adhesion process. Nevertheless,Alcaligenes denitrificans for pH values above 5.0 is positively charged. Under theseconditions, all the above mentioned materials to be used as carriers were found to benegatively charged. In this way, adhesion seems to be favourable and as cells and supports arehydrophobic, it might be dominated by hydrophobic interactions. So, it is necessary todetermine if in the adhesion process prevails the electrostatic interaction or hydrophobicity.Studies are being performed to determine which type of material promotes a strongeradhesion and the development of the most stable biofilm, to be used as biomass carrier indenitrifying inverse fluidised bed reactors.
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