The Role of the Support Layer in Ultrapure Water Filtration Process (Part 2)

To prevent the support layer from shifting during backwashing, the United States also employs a "coarse-fine-coarse" gravel stratification method for single-layer and double-layer media filters. The upper layer of coarse gravel prevents the middle layer of fine gravel from moving upwards during backwashing; the middle layer of fine gravel prevents the loss of sand filter media; and the lower layer of coarse gravel serves as the middle layer of fine gravel. This stratification method can also be applied to three-layer media filters. The specific particle size distribution and thickness should be determined based on the type of water distribution system and the filter media gradation. For example, if the support layer consists of 7 layers, then the 1st and 7th layers have the same particle size, with the largest particle size. The 2nd and 6th layers, the 3rd and 5th layers, etc., also have correspondingly equal particle sizes, but progressively smaller, with the 4th layer having the smallest particle size. With this gradation stratification method, the total thickness of the support layer does not necessarily increase; instead, the thickness of the lower layer is appropriately reduced. If a low-resistance water distribution system is used, the support layer may not be necessary, or some coarse sand or fine gravel may be added, depending on the specific water distribution system.