AEROBIC GRANULATION DURING THE START UP PERIOD OF A PERIODIC BIOFILTER

The paper reports the results of an experimental investigation specifically aimed at assessing the mechanism of biomass granulation during the start up of a period biofilter under aerobic conditions. The role of the hydrodynamic shear forces, calculated by a methodology developed by authors, was assessed, and a mechanism for granulation was proposed: (1) formation of a thin biofilm that completely covers the carrier; (2) increase of biofilm thickness; (3) break-up of the attached biofilm with release of biofilm particles; (4) rearrangement of biofilm particles in smooth granules. The hydrodynamic shear forces trend during the start-up period provides a key for explaining the process of biomass granulation. In fact, during the first two steps, the biofilter is characterised by rather weak shear force values (lower than 1 dyne/cm2). Under these weak forces, the biofilm grows, increasing its thickness through a porous structure characterized by weak adhesion strengths. The continual increase of biofilm thickness produces a corresponding increase of the shear forces with negative effects on biomass stability that cause the detachment of biofilm particles. In turn, such detachment causes a further sharp increase of the shear forces that promotes the rearrangement of the detached biofilm particles into smooth granules.