The technique is used for the physico-chemical and microbiological purification of water polluted by organic matter of faecal origin. It can also be applied to the treatment of various agricultural wastewaters and industrial wastewaters. The depollution method uses 2 techniques of hydroponic production: the nutrient film technique (NFT) which involves removing by gravitational flow, along channels containing plants with bare roots, a film (a few mm thick) of nutritive liquid; nutrition by continuous flowing, (NCF), a variant of the NFT, where the layer of liquid in the circuit may be several cm thick. The characteristics of both these techniques are to aerate the liquid which can be purified by a single passage through several channels placed end to end to form a channel for example 20 m long. The treatment can also be performed by pumping and recirculating the liquid along a shorter channel which requires less space. The plants which are selected to ensure optimum depollution must be specially grown and may require fixation by mesh. The levels of purification vary. As an example, wastewater from a water treatment plant treated in a channel measuring 50 m gave the following results: suspended matter, 85%; biological oxygen demand (BOD) 5, 75%; DCO, 30%; total nitrogen, 40%; total phosphorus, 25%. For microorganisms, the removal rate is generally around 99% which is the recommended level for water for bathing. Applying this technique to industrial waters also showed that heavy metals were stopped by the passage of liquid in the channels. By their uptake of nutritive mineral elements, the plants also contribute to reducing eutrophizing matter such as nitrates and phosphates. The treatment of sludge requires dilution prior to purification and the water used can later be employed in fertilizing by irrigation. The remaining sludge and roots can be used for composting and top part of the plants can be employed in forrage production.