Abstract: There is described a system for the treatment of a wastewater in a pond or lagoon containing a volume of water. The system comprises at least one enclosure comprising growth media and/or substrate therein for growing bacteria and/or biomass. The at least one enclosure is configured in the pond or lagoon to receive a volume of wastewater flowing therethrough from the pond or lagoon over a treatment period, and is configured to treat substantially the entire volume of water flowing through the pond or lagoon over time. A plurality of growth media and/or substrate can be provided in a given enclosure. Different enclosures may comprise different growth media and substrate therein.