... filter sand has a high void space, normally, its filtering velocity is 15-20 m3/h, less loss water-head, and suitable ... easy to be cleaned, the back-flush strength is 12-20 l/m2.s; it can save the back-flush water ... -6,Φ6-8,Φ8-10,Φ10-12,Φ13-16,Φ16-20,Φ20-30. Ceramic Sand Filters