Membrane Filtration of Fresh Water

Abstract

MEMBRANE filtration is widely used as a standard procedure in treating water samples before they are subjected to chemical analysis. Acceptance of this procedure involves the implicit assumption that filtration separates the raw water into well defined fractions; this belief is, however, only partly correct. In fact, membrane filtration separates the solid phase (particulate matter: detritus, plankton) from the liquid phase (water with the solutes), with distribution of colloidal matter between these major fractions. An undefined part of the colloids is retained with the solids on the filters, chiefly as a result of electrostatic forces; the remaining part of the colloids accompanies the liquid phase¹. Colloid retention by the filter is evidenced by the colouring of the filters even when no plankton is present, and by the observation that filtration of a specified volume of water is often difficult to perform, even in the case of samples containing hardly any planktonic organisms or detritus².