Abstract: 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.