Filter gravel does not filtrate directly, but it acts as a supporting layer, preventing filter sand from entering into an underdrain. It also plays a very important role to provide backwashing water evenly. To make the best use of the filter media, it is important to use filter gravel with the selected quality. Non-standard filter gravel are removed by hand, such as flat, cracked, and fragile gravel with seashells. Therefore, we can always provide high-quality rounded gravel in the market.