A system for treating wastewater comprising: at least one water-treatment pathway having at least one wastewater inlet, and at least one treated wastewater outlet and arranged for at least aerobic treatment of said wastewater as it flows from said at least one wastewater inlet to said at least one treated wastewater outlet; at least one wastewater supply conduit, supplying said wastewater to said at least one wastewater inlet of said water-treatment pathway; and at least one treated wastewater conduit, supplying treated wastewater from said at least one treated wastewater outlet of said at least one water-treatment pathway; said system being characterized in that: the at least one water-treatment pathway has oxygen-permeable, water-impermeable vertically extending walls sealed at top and bottom edges (127,128) thereof by heat pressing, ultrasonic welding or similar means applied onto several folds of the wall material over itself or onto an additional film of compatible material over the edges of the wall material and defining a spiral wound horizontal water-impermeable, wall-enclosed water-treatment pathway, the vertically extending walls separating an interior of said at least one horizontal water impermeable wall-enclosed water-treatment pathway from outside air, the at least one water treatment pathway is arranged to define a vertical airflow passageway having a spiral cross-section between the windings thereof; the vertically extending walls of said at least one horizontal water-treatment pathway being arranged to support a biofilm on an interior surface. at least one water-treatment pathway having at least one wastewater inlet, and at least one treated wastewater outlet and arranged for at least aerobic treatment of said wastewater as it flows from said at least one wastewater inlet to said at least one treated wastewater outlet; at least one wastewater supply conduit, supplying said wastewater to said at least one wastewater inlet of said water-treatment pathway; and at least one treated wastewater conduit, supplying treated wastewater from said at least one treated wastewater outlet of said at least one water-treatment pathway; said system being characterized in that: the at least one water-treatment pathway has oxygen-permeable, water-impermeable vertically extending walls sealed at top and bottom edges (127,128) thereof by heat pressing, ultrasonic welding or similar means applied onto several folds of the wall material over itself or onto an additional film of compatible material over the edges of the wall material and defining a spiral wound horizontal water-impermeable, wall-enclosed water-treatment pathway, the vertically extending walls separating an interior of said at least one horizontal water impermeable wall-enclosed water-treatment pathway from outside air, the at least one water treatment pathway is arranged to define a vertical airflow passageway having a spiral cross-section between the windings thereof; the vertically extending walls of said at least one horizontal water-treatment pathway being arranged to support a biofilm on an interior surface. the at least one water-treatment pathway has oxygen-permeable, water-impermeable vertically extending walls sealed at top and bottom edges (127,128) thereof by heat pressing, ultrasonic welding or similar means applied onto several folds of the wall material over itself or onto an additional film of compatible material over the edges of the wall material and defining a spiral wound horizontal water-impermeable, wall-enclosed water-treatment pathway, the vertically extending walls separating an interior of said at least one horizontal water impermeable wall-enclosed water-treatment pathway from outside air, the at least one water treatment pathway is arranged to define a vertical airflow passageway having a spiral cross-section between the windings thereof; the vertically extending walls of said at least one horizontal water-treatment pathway being arranged to support a biofilm on an interior surface.