Process of treating wastewater with oxygen

In a process for treating wastewater with oxygen, wherein oxygen of a higher concentration than atmospheric oxygen is contacted at least intermittently with the water surface and introduced into the wastewater through an agitation of said water surface, the improvement comprising the treatment of at least part of the total volume of a circulating horizontal course of the wastewater by carrying out alternately at least two of the following three modes of operation:

• (a) surface aerating the wastewater using atmospheric air,
• (b) oxygenating the wastewater in at least one section of said course under a gas-filled compartment defined by a cover over said one section while maintaining atmospheric pressure above the wastewater under said compartment, and
• (c) oxygenating the wastewater in said one section of said course under said compartment while maintaining positive pressure above the wastewater under said compartment, with the operating modes alternating with each other in response to variations in the oxygen content of the wastewater and/or the oxygen concentration in said gas-filled compartment with the wastewater beng exposed to the introduction of gas only in said one section of said course and being exposed to the atmosphere on the remainder of said course and with said one section being the only part of said course covered by a cover and the wastewater is retained in said course for a period sufficiently long to allow the volume of said wastewater to pass beneath said cover several times, and wherein in any of the operating modes carried out the introduction of oxygen and air into the water is controlled by control means to maintain the oxygen content of the wastewater within a range defined by a preselected upper limit and a preselected lower limit.