Purificn. of sewage from one or more properties, comprises the addn. by means of a pump (1) regulated by a control unit (5) of a suitable dose of pptn. chemicals to the sewage being led into the mains drain (6), thereby pptg. out phosphorus in the subsequent sludge sedimentation chamber (7). The flocked sediments remain on the chamber floor whilst the purified sewage is led away to the outflow.