The Pentek DGD-2501-20 Sediment Water Filter is dual gradient meaning the outer layer acts as a pre-filter, filtering out larger particles than the inner layer of the filter. The outer layer is a 25 micron filter and the inner layer filters down to 1 micron, reducing very fine sediment--prolonging the useful life of the cartridge by filtering more efficiently. High quality construction and durability make these filters perfect for a variety of applications. These filters are a great value for various applications, on both city and well water.