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8.5 FILTER PERFORMANCE 173
FIGURE 8.7 Intermixing of sand and anthracite media.
anthracite cannot be separated easily) and the only practical solution is to replace both the anthracite and the sand media.
This operation’s challenge underlines the critical importance of the selection of the specific weight of anthracite. As a general rule of thumb, the specific weight of the anthracite media should be more than 1000 kg/m3 lighter than the specific weight of the underlying filtration sand media. In addition, when anthracite is replaced or topped off periodically, it is important to closely match the specific weight of the original and new commercial anthracite products. Using overly light product would have the disadvantage of elevated media expansion and excessive loss of media during the filter backwash cycle.
Another common reason for intermixing of sand and anthracite media is the mismatch of the grain size ranges of the sand and anthracite media. Such intermixing will occur even if the spe- cific weight difference of sand and anthracite is within the range specified above. As a practical rule of thumb, the coarsest size of anthracite in its size range should not exceed five times the finest size of the underlying sand media. For example, if the selected anthracite has a media-size range of 0.5e2.0 mm and sand has a range of 0.15e0.90 mm, the two medias are very likely to intermix because the largest size anthracite grains in the anthracite size range (i.e., 2.0 mm) are over 13 times larger than the smallest size in the sand size range (i.e. 0.15 mm), i.e., 2.0 mm/ 0.25 mm 1⁄4 13.3. This particular challenge underlines the necessity to use both anthracite and sand media of low uniformity coefficient (preferably 1.4 or lower), which would allow to avoid the significant discrepancy of media sizes and associated media propagation effect.
8.5.4.3 Filter-Media Solidification and Mud Ball Formation
One of the common reasons for poor filter performance and reduction of the length of the filter cycle over time is the low backwash velocity of the filtration media. Such low backwash velocity could be caused by:
1. Inadequate filter backwash rate due to backwash pump-capacity constraints, capacity limitation of the storage tank for backwash water of the tank storing spent filter backwash, or due to operator error,