Improvements in and relating to waste water treatment

A process for removing nitrogen from organic waste water containing nitrogenous compounds, which includes the steps of

• a. feeding waste water to a first stage,
• b. passing active sludge recycled from a subsequent stage of the process, to the first stage and inducing turbulence in the first stage for mixing the waste water and the active sludge to provide a mixed liquor, said first stage being a biological denitrification stage for allowing heterotrophic bacteria in suspension in the active sludge to convert nitrates to nitrogenous gases under anoxic conditions,
• c. passing mixed liquor from the first stage to a second stage,
• d. aerating the second stage to maintain aerobic conditions, to obtain biological nitrification of nitrogenous compounds to nitrates by nitrifying bacteria contained in the active sludge in the mixed liquor,
• e. passing mixed liquor from the second stage to a third stage and inducing turbulence in the third stage while allowing a sufficient residence time of the mixed liquor in the third stage for endogenous respiration of bacteria in the active sludge, to reduce any oxygen in the mixed liquor and then reduce nitrates to nitrogen gas to remove at least 60% by weight of the nitrates entering the third stage with the mixed liquor from the second stage,
• f. recycling mixed liquor containing nitrates formed in the second stage, from the second stage to the first stage at a sufficient rate to maintain the level of nitrates passing from the second stage to the third stage with the mixed liquor, below the level where at least 60% by weight of the nitrates entering the third stage can be removed by the endogenous respiration of the bacteria in the third stage,
• g. providing a sufficient residence time of mixed liquor in the first stage in relation to the respiration rate of bacteria in the active sludge in the first stage, to obtain denitrification of a major proportion of the nitrates recycled from the second stage to the first stage with the recycled mixed liquor,
• h. passing mixed liquor from the third stage to a solids separation stage to provide a clarified effluent and an active sludge having a concentrated suspension of bacteria,
• i. withdrawing clarified effluent from the solids separation stage,
• j. recycling at least a portion of the active sludge from the solids separation stage to the first stage to provide the recycled active sludge in the first stage, the active sludge being recycled at a sufficient rate to maintain an effective bacterial population level for nitrification and denitrification in the first, second and third stages,
• k. wasting sludge from the process at a sufficiently low level to provide a sufficient residence time of nitrifying bacteria in the second stage, in relation to their growth rate, to prevent washing out of the nitrifying bacteria at a rate faster than their growth rate.