The success of biological wastewater treatment is to a large extent governed by the ability of bacteria to induce floc formation, thereby facilitating the separation of particles from the treated water. We performed studies on the dynamics of floc stability, the desorption of cells from the flocs, and the reflocculation of detached material. The floc stability was affected by the ionic strength of the medium in a way that strongly suggests that the interactions between the floc components can be explained by the theory of Derjaugin, Landau, Verwey, and Overbeek (DLVO theory). At increasing concentrations of electrolytes, the stability of the flocs increased. However, above an ionic strength of about 0.1 the floc stability decreased, and it seems that at this high electrolyte concentration the DLVO theory cannot be applied. The reversibility of the electrostatic double-layer effects was experimentally shown by treating the sludge repeatedly with a low-ionic-strength solution until parts of the flocs detached. When salt was added at this point, flocs reform , resulting in a dramatic decrease in the turbidity of the supernatant liquid. Both reflocculation and detachment of floc material were seen with calcium as well as with potassium. This finding clearly indicates that the reflocculation and destabilization of flocs were due to changes in double-layer thickness rather than bridging effects of multivalent ions such as calcium. The results indicate that the ionic strength may well be an important factor for the floc stability in wastewater in situ.

We have shown that increased cell surface hydrophobicities of both well-characterized Escherichia coli strains and isolates from wastewater correlate well with increased adhesion to sludge flocs (Spearman rank correlation coefficient, 0.773; P < 0.01).

Wastewater is often treated using the activated sludge process. Flocculation and subsequent sedimentation of flocs are vital steps in this process 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 understand the mechanisms of bacterial adhesion to flocs. The green fluorescent protein (GFP) was used as a cellular marker to study bacterial adhesion to activated sludge flocs in sifu in sludge liquor. Cell surface hydrophobicity (CSH) was shown to be an important factor that determined the relative bacterial adhesion potential. High CSH correlated with high numbers of attached cells. However, the absolute adhesion of two test bacteria to different sludge flocs varied and could not be explained by the f loc characteristics. Confocal laser scanning microscopy of GFP-marked cells showed their position in the floc matrix in situ. Hydrophobic cells attached not only on the surface but also within the floc, whereas hydrophilic cells did not. This indicates that cells may penetrate the flocs through channels and pores and increase the effective surface, which in turn makes the clarification of the wastewater effluent more efficient. The addition of polymers is common practice in wastewater treatment and was shown to increase bacterial adhesion to the flocs. A decrease in surface tension caused by addition of DMSO decreased adhesion, indicating the detrimental effect of surfactants on flocculation. An understanding of basic bacterial adhesion and aggregation mechanisms is important for the managment and control of biotechnological wastewater treatment.

Bacterial cell surface hydrophobicity is one of the most important factors that influence bacterial adhesion. A new method microsphere adhesion to cells, for measuring bacterial cell surface hydrophobicity was developed. Microsphere adhesion to cells is based on microscopic enumeration of hydrophobic, fluorescent microspheres attaching to the bacterial surface. Cell surface hydrophobicity estimated by microsphere adhesion to cells correlates well with adhesion of bacteria to hydrocarbons or hydrophobic interaction chromatography for a set of hydrophilic and hydrophobic bacteria (linear correlation coefficients, R2, were 0.845 and 0.981 respectively). We also used microsphere adhesion to cells to investigate the in situ properties of individual free-living bacteria directly in activated sludge. Results showed that the majority of the bacteria were hydrophilic, indicating the importance of cell surface hydrophobicity for bacterial adhesion in sludge, and for the overall success of the wastewater treatment process.