FLOC STABILITY AND ADHESION OF GREEN-FLUORESCENT-PROTEIN-MARKED BACTERIA TO FLOCS IN ACTIVATED-SLUDGE

Wastewater is often treated using the activated sludge process. Floccu lation and subsequent sedimentation of flocs are vital steps in this p rocess that have direct influence on the quality of the effluent water from wastewater treatment plants. Since cells that remain free-living will decrease the quality of the effluent water it is important to un derstand the mechanisms of bacterial adhesion to flocs. The green fluo rescent protein (GFP) was used as a cellular marker to study bacterial adhesion to activated sludge flocs in situ in sludge liquor. Cell sur face hydrophobicity (CSH) was shown to be an important factor that det ermined the relative bacterial adhesion potential. High CSH correlated with high numbers of attached cells. However. the absolute adhesion o f two test bacteria to different sludge flocs varied and could not be explained by the flee characteristics. Confocal laser scanning microsc opy of GFP-marked cells showed their position in the flee matrix in si tu. Hydrophobic cells attached not only on the surface but also within the floc, whereas hydrophilic cells did not. This indicates that cell s may penetrate the flocs through channels and pores and increase the effective surface, which in turn makes the clarification of the wastew ater effluent more efficient. The addition of polymers is common pract ice in wastewater treatment and was shown to increase bacterial adhesi on to the flocs. A decrease in surface tension caused by addition of D MSO decreased adhesion, indicating the detrimental effect of surfactan ts on flocculation. An understanding of basic bacterial adhesion and a ggregation mechanisms is important for the managment and control of bi otechnological wastewater treatment.