Strip diffusers are disclosed for use in wastewater treatment plants. Such a diffuser has an elongate diffuser body (280) with a gas supply channel and a support member (285) on which a flexible membrane (298) is mounted. Each longitudinal edge of the membrane is held in a groove (295) in the diffuser body by means of one or more locking or jamming members (300). This holds the membrane in place, and with the support member, at least when the diffuser is operating, defines a gas chamber from which gas passes through the membrane. The locking or jamming members may be held in place by a flange extending over the groove, or by engagement of the locking or jamming member with the membrane and compressing the membrane against the groove sidewalls (297).