Disclosed is an installation for treating sewage sludge, which comprises a sewage reception station (1) and in which a device (6) for separating sludge and filtrate is arranged. A membrane unit for treating waste water is provided with a membrane module (106) and a flushing box (105) that is located below said membrane module (106) and encompasses an air inlet via which purging air can be blown into the membrane module (106). The membrane module (106) comprises a waste water inlet and a sludge and permeate outlet. A sieve (108,110) is disposed between the flushing box (105) and the membrane module (106) in order to distribute the purging air to the entire membrane module (106) and/or at the waste water inlet into the membrane module (106) so as to retain large size suspended matter. The invention further relates to an installation for treating waste water by means of a membrane unit (103) and a method for operating such an installation. According to said method, operation of the permeate pump (123) is controlled in accordance with the water level in the receptacle, the permeate pump (123) being turned on or off and being operated at variable conveying quantities, and/or an additional permeate pump being activated.