The Pentek DGD-7525 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 75 micron filter and the inner layer filters down to 25 micron reducing smaller 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. Depending on use and water quality these filters can last 6-12 months. When pressure drops below 15 psi it s probably time to replace your cartridge. These filters are a great value for various applications on both city and well water.