Page 191 - ro membanes

174 8. GRANULAR MEDIA FILTRATION
2. Inadequate depth of the gravel packing layer (for filters with nozzle underdrain),
3. Plugged nozzles or underdrain blocks,
4. Hardening of the media due to calcium carbonate buildup, and
5. Plugging of the gravel packing layer with sand due to incompatibility of gravel and
sand sizes.
Filter cells should be backwashed at a rate that is 3e5 times higher than the average filtration
rate to provide complete removal of the solids accumulated in the filtration media over the length of the entire filtration cycle (24e36 h) (Qasim et al., 2000). The lower end of backwash rate range (i.e., approximately 3 times the filtration rate) is applied during periods of low source water turbidity (i.e., turbidity < 2 NTU), while the maximum rate (5 times the filtration rate) is used for periods of heavy solids and organic loads (turbidity > 15e20 NTU).
Under average turbidity conditions the optimum backwash rate is usually 3.0e4.0 times the average filtration rate (i.e., backwash rate of 30e40 m3/m2 h for filtration rate of 10 m3/m2 h). If the filter backwash rate is inadequate, solids not removed with the backwash water will begin to accumulate in the filter cells and occupy a portion of the filtration media, which in turn will gradually reduce the length of the filtration cycle and deteriorate filtered water quality. In addition, balls of mud, coagulant, silt, and other solids will begin forming inside the media and the media grains could begin solidifying in some portions of the bottom, and such solidification will create zones with little to no filtration and backwashing.
In most conventional filter-cell underdrains using nozzles, a gravel packing layer of adequate depth and size would need to be provided to maintain sufficient and uniform back- washing of the filter media. As a rule of thumb, the depth of the gravel packing layer should extend at least one to two times the height of the nozzles. For example, for typical 40-mm nozzles widely used in filter underdrain systems, the gravel pack layer depth should be 80e120 mm. In addition, the size of the pebbles of the top gravel layer that is in contact with the filtration sand layer should be at least two times smaller than the higher end of the sand media size. For example, if the sand media has size of 0.6e0.8 mm, then the size of the underlying gravel should be less than 1.6 mm (2  0.8 mm).
8.5.4.4 Filter-Media “Cratering”
Filter-media cratering (e.g., creation of indentions on the surface of the filter media, which resemble small craters) is usually caused by broken filter nozzles or filter underdrain blocks. Loss of filtration material through the broken nozzles creates indentation on the surface of the filter media above the impacted underdrain, which is clearly visible on the surface of open gravity filters.
8.5.4.5 Filter-Media Mounding
Plugged filter nozzles or inadequate thickness of the underdrain gravel pack covering the nozzles tend to cause more intense vertical flow around the impacted area during backwash, which lifts the filter surface media in this location and forms a mound of media (Fig. 8.8). In such cases, the underdrain gravel pack has to be increased in depth and reinstalled. Other- wise, this condition will result in a permanent reduction of the length of filtration cycles, in extended maturation period and in deterioration of the filtrate water quality over time.