Let's have a look at how a filter press functions. The slurry, a mixture of liquid and solid, is pumped into the presse. As pressure increases, it is pumped through straining plates. The solid portion of the mixture begins to accumulate on the filter cloth. Initially, the filter cloth isn't capable of capturing all the solids passing through. However, as more solid is deposited on the cloth and a layers of filter cake begins to grow and form a layer of particles. The layer that is formed does the bulk of the purifying as it is able to capture more solids as the slurry flows through. After the cloths have been filled of cakes, the whole process is completed and you'll end up with solids that have been dewatered, and water that has been purified.