A water treatment module (100; 300; 500; 600) comprising: at least one elongated gas enclosure (110; 210; 310; 510; 610) comprising a gas inlet (112; 512) and two vertical walls (114; 116; 214; 216; 314; 316), at least one vertical wall comprising a water-impermeable and gas-permeable membrane having a water-facing side (114a; 116a) and a gas-facing side (114b; 116b), the two vertical walls having a top end and a bottom end, the two vertical walls being longitudinally sealed at the top end and at the bottom end, and separating between water external to said enclosure and gas within said enclosure, the gas enclosure (110; 210; 310; 510; 610) being in a rolled or folded configuration to thereby define a convoluted horizontal path and one or more water-treatment spaces formed between opposite water facing sides of the enclosure; and a diffuser arrangement (126; 226; 326; 402; 526; 626) comprising gas diffusers configured for introducing a stream of gas into the one or more water treatment spaces to cause mixing of water therein, the gas diffusers comprising gas release openings (134; 232; 334) positioned below the one or more water treatment spaces; said water treatment module being characterized in that the module comprises a reinforcing structure that includes a top support element (140; 540; 640), and a bottom support element (142; 542; 642) that are connected to a top portion and to a bottom portion of said enclosure and configured to maintain the enclosure in its intended convoluted horizontal path configuration when submerged in water, said bottom support element (142; 542; 642) being structured as a mesh to enable air passage from said diffuser arrangement to the entire one or more water treatment spaces as well as water passage into the one or more water treatment spaces. at least one elongated gas enclosure (110; 210; 310; 510; 610) comprising a gas inlet (112; 512) and two vertical walls (114; 116; 214; 216; 314; 316), at least one vertical wall comprising a water-impermeable and gas-permeable membrane having a water-facing side (114a; 116a) and a gas-facing side (114b; 116b), the two vertical walls having a top end and a bottom end, the two vertical walls being longitudinally sealed at the top end and at the bottom end, and separating between water external to said enclosure and gas within said enclosure, the gas enclosure (110; 210; 310; 510; 610) being in a rolled or folded configuration to thereby define a convoluted horizontal path and one or more water-treatment spaces formed between opposite water facing sides of the enclosure; and a diffuser arrangement (126; 226; 326; 402; 526; 626) comprising gas diffusers configured for introducing a stream of gas into the one or more water treatment spaces to cause mixing of water therein, the gas diffusers comprising gas release openings (134; 232; 334) positioned below the one or more water treatment spaces; said water treatment module being characterized in that the module comprises a reinforcing structure that includes a top support element (140; 540; 640), and a bottom support element (142; 542; 642) that are connected to a top portion and to a bottom portion of said enclosure and configured to maintain the enclosure in its intended convoluted horizontal path configuration when submerged in water, said bottom support element (142; 542; 642) being structured as a mesh to enable air passage from said diffuser arrangement to the entire one or more water treatment spaces as well as water passage into the one or more water treatment spaces.