I. Physical characteristics of biological filtering materials of volcanic rock
    1.Appearance

No sharp particle, small water flow resistance, not easy to be blocked, water distribution and air distribution is even; rough surface and rapid biofilm formation speed, microbial membrane is not easy to fall off during the back flush.
    2.Porosity

Volcanic rock is a natural porous honeycomb and best growth environment for zoogloea.
    3.Mechanical strength

General Administration of Quality Supervision, Inspection and Quarantine of the People’s Republic of China proves that the mechanical strength is 5.08Mpa; practice has proved that it can endure different strength of hydraulic shearing action and service life is far more than other filtering materials.
    4.Density

Moderate density, easy to suspend and no material escaping during the back flush, can save energy and reduce consumption.

    II. Chemical characteristic of biological filtering materials of volcanic rock

Chemical composition is as follows:
Chemical composition SiO2 CaO MgO Fe2O3 FeO Al2O3 TiO2 K2O Na2O
Content % 53.82 8.36 2.46 9.08 1.12 16.89 0.06 2.30 2.55
    1.Biochemical stability

Biological filtering materials of volcanic rock are anticorrosive, inert and don’t participate in the biochemical reaction of bio-film under the environ

Filtering materials of Volcanic Rock
ment.
    2.Surface electrical property and hydrophilic property

Biological filtering materials of volcanic rock have positive charge on the surface, beneficial to the attached growth of microorganism, strong hydrophilic property; there is a great amount of attached bio-film and very quick.
    3.Impact of bio-film activity

As the bio-film carrier, biological filtering materials of volcanic rock do no harm and no inhibitory action to the fixed microorganism; practice has proved that it will not affect the microorganism activity.

    III. Hydraulic characteristics of biological filtering materials of volcanic rock
    1.Void ratio: Internal and external average porosity is about 40%, small water resistance, comparing with same kind of filtering materials; required filtering material amount is small and can meet the expected filter target.
    2.Specific surface area  

Large specific surface area, high open hole ratio and inert, beneficial to the contact, biofilm formation and growth of microorganisms, keep a large amount of microbial biomass, beneficial to the mass transfer process of oxygen and nutrients required for microbial metabolism and wastes produced from metabolism.
    3.Shape of filtering materials and water pattern 

  Since the biological filtering materials of volcanic rock is no sharp particle, most of hole diameters are larger than ceramic particles; therefore, it has small water flow resistance during the use and save the energy consumption.