EFFECTS OF IONIC-STRENGTH ON BACTERIAL ADHESION AND STABILITY OF FLOCS IN A WASTE-WATER ACTIVATED-SLUDGE SYSTEM

The success of biological wastewater treatment is to a large extent go verned by the ability of bacteria to induce floc formation, thereby fa cilitating the separation of particles from the treated water. We perf ormed studies on the dynamics of floc stability, the desorption of cel ls from the flocs, and the reflocculation of detached material. The fl oc stability was affected by the ionic strength of the medium in a way that strongly suggests that the interactions between the floc compone nts can be explained by the theory of Derjaugin, Landau, Verwey, and O verbeek (DLVO theory). At increasing concentrations of electrolytes, t he stability of the flocs increased. However, above an ionic strength of about 0.1 the floc stability decreased, and it seems that at this h igh electrolyte concentration the DLVO theory cannot be applied. The r eversibility of the electrostatic double-layer effects was experimenta lly shown by treating the sludge repeatedly with a low-ionic-strength solution until parts of the flocs detached. When salt was added at thi s point, flocs re-form, resulting in a dramatic decrease in the turbid ity of the supernatant liquid. Both reflocculation and detachment of f loc material were seen with calcium as well as with potassium. This fi nding clearly indicates that the reflocculation and destabilization of flocs were due to changes in double-layer thickness rather than bridg ing effects of multivalent ions such as calcium. The results indicate that the ionic strength may well be an important factor for the floc s tability in wastewater in situ.