Downflow reed beds consist of a shallow excavation made waterproof but the media is fine at the top of the filter and coarse at the bottom. Unlike the horizontal flow reed bed the downflow is free draining and not constantly flooded.

Downflow reed beds, although not as numerous in the UK as horizontal flow beds at present, are being used increasingly in applications where horizontal flow beds would formerly have been specified.

Reed beds are a cost-effective method of sewage treatment. The systems are robust and well-proven, requiring only a fraction of the maintenance of traditional methods of treatment. Chemicals are not required and, provided that just over one metre of hydraulic head is available between the process and final discharge point for the treated effluent, no power is required.