The basis of the sludge activity concept is described together with experimental applications. As a result of these it has been concluded that activated sludge plants normally operate under oxygen limiting conditions and that increases in the rate of aeration which enable the plants to deal with greater loadings require equal increases in the sludge and dissolved oxygen concentration. The increases necessary are not proportional to the increased loadings but to the square roots of them. Increase in turbulence of the mixed liquor does not appear to be a factor of importance down to retention periods of the order of two hours when treating an average strength sewage, but may have had an effect at lower aeration periods. Calculations using data obtained from a hybrid mechanical aeration plant at Manchester indicate that the percentage oxygenation of the sludge in a low intensity carbonaceous oxidation plant may be as low as 15%. Even in a high intensity plant treating sewage in two hours the calculations gave an estimate of 33% aerobicity. It appears that the sludge activity approach can be applied to nitrifying organisms and that they also tend to a maximum as the sludge concentration is increased. In full scale practice there will be an optimum dissolved oxygen level which gives the highest oxygen deficiency, and thus power economy, consistent with attaining the required nitrification.

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