Lava For irregular grain, color to red to brown and black, porous light,and it's the good filter material ... : no point granular, the flow resistance small, not easy to be blocked, water distribution cloth gas uniform; Surface roughness, ... speed, back flush when microbial film is not easy to fall off.2, porous: volcanic rock is a natural ... , chemical industry is the ideal product. Lava (Brown, black, red)