An aerator for immersed filtering membranes has an aerator shell with
openings for discharging bubbles from its upper surface and a shape
capable of temporarily containing a volume of air in fluid communication
with the openings. The shell is open to tank water below it and located
so that discharged bubbles will rise through an assembly of the filtering
membranes. The shell may be wholly or partially made of parts of the
assemblies of filtering membranes. A supply of air is provided to the air
space in the aerators alternating between a high flow rate and a low flow
rate in short cycles of between about 10 seconds and 100 seconds. A
filtration system has an inlet for adding substrate and an outlet for
retentate are located so as to create a horizontal flow of substrate
through the tank. Membrane assemblies are located within the horizontal
flow of substrate. Aerators as described above are provided and operated
as described above.
