Rick: the millbank bag is a canvas "sock" that is filled with suspect water and then the water is allowed to drain through the canvas into another container. It is a purely mechanical filter driven by gravity, and will remove organic solids, mud, and suspended clays, improving the water prior to chemical treatment. It was developed by the British Army (WW2?) and is named after the place it was developed, the School of Tropical Hygiene, in the London suburb of Millbank.