Abstract: Process for the purification of water that is polluted with diffuse substances originating from ablation, such as fertilizers, mineral and soil particles, oils, wastes, detergents, and primarily inorganic substances, which might accelerate undesirable entrophication of the receiving waters such as lakes, comprising the steps of introducing the polluted water to be purified into a defined area (e.g. a basin); spreading the water cross-wise to increase its surface area; and extracting the diffuse pollutants by applying a biological treatment by the expedient of at least one group of suitable aquatic plants. An apparatus for performing the process comprises means that provide a flow path for the water; at least one dividing structure that performs the spreading; and at least one aquatic plant region, with at least one plant zone, wherein the plants contribute to the above-mentioned biological treatment. Several optional procedural and structural details are also disclosed.