<p>A sewage purification system used an oval basin with a central portion in which >=2 of the following modes of operation alternate, depending on the oxygen absorption by the fluid: (a) surface aeration by ambient air; (b) oxygen aeration at atmospheric pressure above fluid level; (c) oxygen aeration at excess pressure above fluid level. Combines an economical utilisation of the oxygen with an optimum purification efficiency and good flexibility for adaptation to fluctuating working conditions.</p>
