Abstract: This invention provides a method for treating wastewater by circulating wastewater around a system comprising a downcomer and a riser which communicate directly at their lower ends, the level of the wastewater in the downcomer being maintained above the level of the wastewater in the riser to provide a hydrostatic pressure head which causes circulation of the wastewater around the system, supplying a gas containing oxygen to the wastewater as it passes through the downcomer, pumping the wastewater from the top of the riser back into the top of the downcomer and removing treated wastewater from the riser at the same rate as wastewater is introduced into the system characterized in that the height of the hydrostatic pressure head is controlled by adjusting the height at which treated wastewater is removed from the riser and the level of the wastewater in the riser. The invention also provides an apparatus for putting the method into practice.

Abstract: A method and apparatus for the treatment of wastewater in which the wastewater is circulated around a system comprising a downcomer and a riser communicating with each other at their lower ends, the level of the wasterwater in the downcomer being maintained above the level of the wastewater in the riser and providing a hydrostatic pressure head which causes circulation of the wastewater around the system at a pre-selected rate, a gas containing free oxygen is supplied to the wastewater as it passes through the downcomer, and the wastewater is pumped from, or form near, the top of the riser back into the downcomer (or vessel communicating therewith) at a position(s) above the highest level to which the wastewater in the downcomer can fluctuate during normal operation of the system when operating at maximum load.