Filter Media
Hardness
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The durability of anthracite filter media has been described
by many methods each of which has its strengths and weaknesses.

1 Hardness Moh Scale
2 Grindability Hardgrove Index
3 Crushing Modulus Pressure
4 Attrition Loss Loss per 100 hours Backwashing
  1. Hardness on the Moh Scales compares the hardness of 
    anthracite with other minerals. An average of many comparisons
    are made to obtain a value. The higher the value the harder 
    the anthracite. The Moh's hardness test is a very subjective test.
    The resulting minimum value should be above 2.8.

  2. The hardness of the anthracite may be high on the Moh scale,
    but if it fractures easily it will appear soft. The Hardgrove Index
    of Grindability goes some way to address this. Basically the 
    anthracite is hit a set number of times by steel balls and the
    its degradation measured. An figure is then calculated to give
    the Hardgrove Index. The lower the figure the less friable the
    anthracite. No figure is given by any standard but a reasonable
    result would be between 50 and 80 Hardgrove.

  3. Neither hardness or grindability completely describes the
    condition of anthracite in a filter. The Crushing Modulus was
    developed to over come this. In this test anthracite is crushed
    in a test chamber and a figure calculated from the results.
    The result should have a minimum value of 105 bar. The
    draw back of this method is that a different value is obtained
    for the same anthracite but of different hydraulic size. The
    smaller the hydraulic size the higher the Crushing Modulus.

  4. The backwash test is probably the best way to determine
    the durability of the anthracite as it mimics the actual conditions
    under which the anthracite will operate. This test determines
    the loss of anthracite due to particle breakage over a period
    of 100 hours backwashing [approx 3 years of operation].
    It should be noted, however, that the test cannot be just
    carried out for 1 hour and the results multiplied by 100 as
    during the first period of the test most of the sharp corners and
    edges of the anthracite will break off. This result should not be
    greater than 5% loss over 100 hours