an assembly of modules on the order of a cassette or larger are aerated simultaneously in a series of air - on or high flow rate periods , each in the range of 0 . 5 to 20 seconds long , separated by longer air - off or low flow rate periods , each in the range of 5 to 40 seconds long . a plurality of aerators located in association with a plurality of modules are provided with a burst of air simultaneously from a shared air accumulator or pulsator connected to all of the aerators . the air accumulator or pulsator may be fed with a continuous supply of air . the plurality of modules may be connected together in a cassette or rack . the accumulator or pulsator may be located outside of a tank of water containing the membranes .