A filtration barrier comprises first elements that are fibers, strips, or bristles wherein apertures between elements provide passages for a fluid transporting granules to the filtration barrier, and wherein the elements (11) are supported against deflection by second elements (12) when the fluid is flowing in a forwards direction whereby granules are prevented from accompanying the fluid through said passages by the barrier, but whereby the second elements do not support the first elements against deflection when the fluid flows in a (back-wash) direction opposite the forwards direction, when the elements (11a, 11b, 11c) become deflected by the fluid to increase the size of the apertures. In various embodiments of the invention the elements are supported at one end or both ends, and elements are arranged to deflect differently to further increase the apertures between neighboring elements as shown below.