Abstract: A method for the secondary treatment of sewage (and apparatus therefor) having a step in which sewage is circulated in a system having a riser and a downcomer communicating with each other at their upper and lower ends and in which an oxygen-containing gas is supplied to the sewage as it passes through the downcomer. The method is intended for use in the aeration and/or digestion steps of the secondary treatment of sewage. The riser and downcomer are preferably located in a shaft extending into the ground for at least 40 meters below a basin containing the sewage.

Abstract: Process and apparatus for treating waste water by circulating it, together with oxygen-containing gas, through communicating downflow and upflow pipes enclosed in a treatment chamber, the pipes being open at their bottom ends, and controlling the flow of waste water/gas mixture from the downflow pipe to cause gas bubbles to escape downwardly, or part downwardly and part upwardly, in the treatment chamber.

Abstract: Method for treating sewage, wherein sewage is circulated around a system comprising a downcomer and a riser communicating with each other at their upper and lower ends, the downcomer having an upper portion of enlarged cross-section and the riser having a lower portion of enlarged cross-section.