Patent ID: 7344644

Claim:
A method of making a water purification system, comprising: burying an underdrain in situ under a filter media bed within a natural body of water; providing a pump for withdrawing water from the underdrain at a sufficient rate to cause a biological matrix filter to proliferate on top of the filter media bed forming a portion of the floor of the body of the water; positioning the underdrain sufficiently below the water body floor by a distance D to allow the top surface of the filter media bed to serve as a mechanical filter of unwanted entrained particles in the water moving toward and into the underdrain, D being equal to between about 0.5 meter and about 1.5 meters; and positioning the filter media bed at a depth d of water between the water body surface and the water body floor so that the upper surface of the filter media bed remains substantially undisturbed by water motion at the surface of the body of water to facilitate the proliferation of the biological matrix filter, wherein the filter media bed is a particulate material bed composed of particles of a size between about 150 microns and about 350 microns.