Cyclic aeration system for submerged membrane modules

An apparatus is described for cleaning or inhibiting the fouling of a plurality of membrane modules 20 immersed in tank water 18 in a tank 12. Air is supplied through aerators 38 to produce a flow of bubbles 36 effective to clean or inhibit fouling of the membrane modules. The flow of air is alternated between a higher flow rate and a lower flow rate, the lower flow rate being less than one half of the higher flow rate, in repeated cycles of not more than 120 seconds in duration while the permeate is withdrawn through the membrane modules 20. This produces transient flow conditions in the tank water 18, the tank water 18 being accelerated or decelerated for much of each cycle so that it is rarely in a steady state during permeation. This gives rise to efficient cleaning of the membranes without overloading the aerator system. The flow rates are controlled by a set of valves 254 and a controller 256. <IMAGE>