Manufacturing sugar involves the heavy use of pure water and the discharge of a significant amount of category III waste water, which, in its physicochemical indices, is characterized by high biochemical (BPK5) and chemical (KhPK) consumption of oxygen, a putrid odor, a lack of dissolved oxygen, and an acid medium. This is caused by the presence of a large amount of organic substances. The harmfulness of sugar industry waste water is worsened by their content of saponin, which is poisonous for fish, even in small quantities. An important measure for sugar plants which have filtration fields with poorly filtering soils is a method of intensifying the natural processes of purifying waste water by the aid of mixtures of Protococcaceae chlorella and cenedesmus. The method of purifying waste water by the aid of algae is a combination of natural purification processes and artificially conducted measures having the goal of intensifying purification. The measures include liming and algolization of waste water in the charts in filtration fields with subsequent release of the purified water into common use reservoirs.