Studies of the factors influencing the flocculation and sedimentation of microbial cells in the treatment of Kraft mill effluents. Appendix II.

The treatment of paper mill effluents involves a succession of unit processes to remove the impurities that may vary in size by about six orders of magnitude. process of primary importance in this study The unit was the biological treatment stage, where an understanding of the flocculation and sedimentation of micro-organisms is needed in order to operate it efficiently.

A bench-scale chemostat was constructed to operate as a model treatment plant, and the physico-chemical properties of the effluent were determined. Similarly the properties of pilot-plants and paper mill treatment plants were determined. The results were compared with treatment data available from other International Paper Company mills.

The electrokinetic properties of the predominantly microbial particles in the treatment systems were measured using microelectrophoresis. DetaiI s of the natural variation in the electrophoretic mobility were obtained, and the effects of calcium chloride and aluminium sulphate on these systems were measured.

The variation in the mobility of a single population was found to be small for such a heterogeneous population and the implications are that a common material is adsorbed on the particle surfaces.

Estimates for the particle concentration, volume fraction and Debye-Huckel parameter of the biological effluents were obtained.

Using a multi-equation computer programme, based on DLVO theory and modified to include adsorbed layers, the magni tude of the long-range (>3nm) van der Waal sand electrostatic forces were calculated for interacting biological colloids of 0.5-2.0 ~m diameter. The effect of adsorbed layers and changes in the particle composition on the stability of these systems appears to be minor. The validity of applying DLVO theory to such heterodisperse and poorly defined dispersions is discussed. Consideration was also given to the role that these forces play in the flocculation of biological effluents.

The rate of sedimentation was measured at constant temperature in precision bore glass columns. Using an interactive computer programme, based on the work of Carstensen ~ Su (1970a,b), the data was analysed. The validity of using this model for studying effluent dispersions was tested and discussed.

The effect of aluminium sulphate, calcium chloride and sodium chloride on the sedimentation of the effluents was examined with reference to the electrophoretic mobility data for the same systems.

This study has given an idea of the complexity of paper mill effluents and the need for model studies on the individual components of the system.