Treatment of aluminous or ferrous sludge from water-treatment plants comprises dehydrating, opt. treating with a sodium silicate-based fluidiser, and/or adding fly ash from thermal power stations. Dehydration pref. comprises centrifuging, or makes use of a filter press (opt. with a membrane), a band filter operating by electroosmosis, or a rotary press with 1-4 pipes. The prod. obtd. after dehydration and treatment with fluidiser is dried by heating. Treatment with a fluidiser involves addn. of an aq. sodium silicate soln. contg. more than 50% water, pref. 53% water, 31% silica and 16% Na2O, opt. with addn. of 1-100% of fly ash. The process may also include addn. of a cationic flocculant in the form of an emulsion or powder, together with a filter aid (sawdust) or calcium hydroxide or iron perchloride.
