Abstract: The present invention is concerned with a process for the purification of highly polluted water containing paper mill waste products and algae, which comprises: (a) treating said highly polluted water with hydrogen peroxide and alum; (b) allowing flocculation and then sedimentation to take place, while pre-chlorinating the treated water by subjecting it to the action of chlorine during flocculation or after sedimentation, or both; (c) filtrating the resulting water to remove therefrom solid particles still in suspension; and (d) post-chlorinating the filtrated water by treating it with a liquid composition obtained by mixing chlorine, an alkali metal chlorate and a non-toxic mineral acid. The water treatment process according to the invention enables one to obtain a drinking water which is colorless, odorless and from which bad tastes have been eliminated.