Abstract: In water treatment using alum and yielding a sludge containing aluminum hydroxide, alum is recovered by acidulating the sludge with sulphuric acid, conditioning the sludge with an inert additive for facilitating filtering, and supplying the conditioned sludge to a filter press. Alum in the filtrate is reused and make-up alum added as required. Periodically the cake in the filter press is neutralized with lime and disposed of. When the recovered alum becomes unsuitable for further use, the system is convertible to a sludge dewatering system in which acid is discontinued and lime added with the inert additive to condition the sludge for the filter press, the filtrate is returned to the water treatment system, and make-up alum is discontinued until all the stored recovered alum has been used up. New alum is then introduced and the sludge dewatering continued until the recovered alum has passed through the system and its coagulant product eliminated, the cake in the filter press being dumped as required.