The ability of various additives to improve the dewaterability of activated sludge was determined and the surface properties of additives characterized in order to arrive at a mechanism for ash conditioning of activated sludge. The primary additives investigated were fly ash and 3 sludge incinerator ashes. Since the presence of salts and a solid surface complicates the interpretation of experimental results, a series of experiments was devised where either salts or solids alone were added to the activated sludge for settleability measurements. The solids consisted of the washed ashes themselves, as well as two silicas, two carbon samples, and magnetite, which contained high surface areas and exhibited different surface properties. The salts consisted of the washings obtained from the ashes and a number of inorganic salts and ionic polymeric surfactants.