Filtration device and method

A new filtration device which floats in a body of the liquid to be filtered and which makes use of a bed of granular buoyant filtration media. The device comprises buoyancy means for giving the device sufficient buoyancy to float in the body of liquid; flow-defining structure attached to the buoyancy means and defining a flow path through which liquid may flow upwardly from the body of liquid; porous retaining means disposed across the defined flow path for holding a bed of buoyant filtration media against the force of liquid flowing upwardly through the path; and outlet means in communication with said flow path for conveying filtered liquid away from the filtration device. In use, the device is placed in the body of liquid; the buoyancy of the buoyant structure maintained such that the device floats in the body of liquid with the porous retaining means at a level lower than the surface level of the body of liquid; and a bed of buoyant granular filter media disposed within the defined flow path under the porous retaining means. Liquid is filtered through the bed of buoyant media and collected. The buoyancy of the device can be adjusted to move it through a backwashing cycle.