A water treatment unit has a support housing vertically extending between a support housing upper end and a support housing lower end. The support housing includes a pair of opposing primary housing walls and a pair of opposing secondary housing walls. Each of the secondary housing walls is formed separately from, and mechanically joined to, each of the primary housing walls. Each of the primary housing walls is perforated to allow airflow in a first direction laterally through a treatment cavity defined by the housing walls. A plurality of hangers extend in the first direction between the primary housing walls and are supported by the support housing. The hangers are mutually spaced between the secondary housing walls across the treatment cavity (23). A plurality of biotreatment membranes each carry a biomass culture and are draped over, and supported by, one of the hangers to define two opposing membrane walls each extending downwardly from the hanger.