Abstract: Phosphate containing basic aluminum solutions are provided by reaction aluminum sulphate solution with phosphoric acid, adding chloride, basifying by precipitation of calcium sulphate from solution and cooling the resultant solution in a controlled manner. Preferably addition of chloride and basification occurs at a temperature of 80.degree.-90.degree. C and the solution is cooled to below 60.degree. C soon after the basifying salts have been introduced. The solutions, preferably diluted to an aluminum content of 1 to 2% Al.sub.2 O.sub.3 are useful for clarifying water and dewatering sewage sludge.