Treatment of a waste water stream comprising soluble biological contaminants issuing from a primary facility uses selective active yeast and comprises:(a) filtering the waste water stream into a permeate stream comprising water and a concentrate stream comprising a concentrated mixture including the soluble biological contaminants and water; (b) heating the concentrate stream to a temperature sufficient to break up the soluble biological components and sterilise the concentrate stream; (c) cooling the concentrate stream to a temperature sufficient to promote yeast growth; (d) introducing an enzyme into the concentrate stream to convert the biological contaminants into glucose; (e) introducing the concentrate stream into a fermentation vessel and introducing and active yeast into this vessel; (f) introducing pressurised air into the fermentation vessel; and (g) continuously withdrawing a yeast mass from the fermentation vessel and filtering the yeast mass to form a yeast mass pro duct stream and a water stream. The process converts a waste water stream issuing from a primary facility into a relatively clean water stream and a useful yeast product.    USE -   The process is used for high strength organic waste water streams issuing from primary facilities including food processing plants (e.g. potato, rice, grain processing), brewery plants, dairy processing, pharmaceutical plants, etc. The yeast by-product has direct commercial use as animal feed and as a protein and flavouring supplement in human foodstuffs.    ADVANTAGE -   The effluent discharge into municipal sewage systems and water ways is environmentally safe and free from harmful biological contaminants. The amount of water in the biological process is minimised, thus the predictability and controllability of the process are increased and the amount of additives required is reduced. The fermentation parameters are carefully controlled to permit optimal yeast growth while avoiding bacteria strains and pollutant by-products. A valuable single cell protein product is produced in the process.