An enclosure system for use with a membrane module in membrane supported biofilm reactors, the membrane module of the type having an upper and lower headspace (104,106) separated by an array of gas-permeable hollow fibre membrane cassettes secured in parallel in the module, wherein the cassettes are a linear arrangement of potted hollow fibre membranes, comprising an upper and lower manifold (98,99) into which are potted a large number of hollow fibre membranes or a number of bunches of hollow fibre membranes, characterised in that the enclosure system comprises: (i) a plurality of panels (3a,3b,3c,3d) configured to seal the membrane module to form an enclosed membrane module which is open at the top and bottom but which is encased within the enclosure system; (ii) a modular collar configured to attach vertically to the enclosed membrane module, increasing the height of the upper headspace; and (iii) a low-pressure airlift mixing system, which is integrated into at least one panel of said plurality of panels (3a,3b,3c,3d), which is configured to transport liquid either from inside the membrane module to outside of the membrane module, or vice versa, so that liquid is pumped in an upward or downward direction through the enclosed membrane module, to encourage good liquid flow through the enclosed membrane module when it is installed in a bioreactor tank, wherein the airlift mixing system comprises: an airlift channel and a first downcomer, which are in fluid communication with each other at their bottom, forming two sides of a substantially U-shaped tube which is open at both ends; a vertical return channel having its top in fluid communication with the top of the channel of the substantially U-shaped tube proximal the vertical return channel; and an air injection port configured to accept and deliver air into the base of the airlift channel to induce upward flow of liquid above the air injection port in the airlift channel, causing a corresponding downward flow of liquid in the first downcomer; and wherein the open end of the U-shaped tube distal the vertical return channel forms a port with access to inside the enclosed membrane module, the open end of the U-shaped tube proximal the vertical return channel forms a port with access to outside of the enclosed membrane module and the lower end of the vertical return channel allows a fluid communication between the vertical return channel and the bottom of said bioreactor or tank. (i) a plurality of panels (3a,3b,3c,3d) configured to seal the membrane module to form an enclosed membrane module which is open at the top and bottom but which is encased within the enclosure system; (ii) a modular collar configured to attach vertically to the enclosed membrane module, increasing the height of the upper headspace; and (iii) a low-pressure airlift mixing system, which is integrated into at least one panel of said plurality of panels (3a,3b,3c,3d), which is configured to transport liquid either from inside the membrane module to outside of the membrane module, or vice versa, so that liquid is pumped in an upward or downward direction through the enclosed membrane module, to encourage good liquid flow through the enclosed membrane module when it is installed in a bioreactor tank, wherein the airlift mixing system comprises: an airlift channel and a first downcomer, which are in fluid communication with each other at their bottom, forming two sides of a substantially U-shaped tube which is open at both ends; a vertical return channel having its top in fluid communication with the top of the channel of the substantially U-shaped tube proximal the vertical return channel; and an air injection port configured to accept and deliver air into the base of the airlift channel to induce upward flow of liquid above the air injection port in the airlift channel, causing a corresponding downward flow of liquid in the first downcomer; and wherein the open end of the U-shaped tube distal the vertical return channel forms a port with access to inside the enclosed membrane module, the open end of the U-shaped tube proximal the vertical return channel forms a port with access to outside of the enclosed membrane module and the lower end of the vertical return channel allows a fluid communication between the vertical return channel and the bottom of said bioreactor or tank. an airlift channel and a first downcomer, which are in fluid communication with each other at their bottom, forming two sides of a substantially U-shaped tube which is open at both ends; a vertical return channel having its top in fluid communication with the top of the channel of the substantially U-shaped tube proximal the vertical return channel; and an air injection port configured to accept and deliver air into the base of the airlift channel to induce upward flow of liquid above the air injection port in the airlift channel, causing a corresponding downward flow of liquid in the first downcomer; and