Abstract
A method for measuring the density of activated sludge to a precision of 0.005g/ml was devised. In the method, which was a modified step density-gradient centrifugation, colloidal silica sol containing a proper amount of sodium chloride was used as the medium. Amount of NaCl addition was determined so as to simulate the actual condition the activated sludge existed by means of electric conductivity. This method revealed that the density of activated sludges collected from 16 different waste treatment plants ranged from 1.00 to 1.07g/ml, depending on the content of ash and capsule in the activated sludge. It was shown also that the settlability of activated sludges was influenced strongly by the density besides by the content of filamentous organisms.
