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8.5 FILTER PERFORMANCE 175
FIGURE 8.8 Filter media mounding.
8.5.4.6 High Content of Fine Silt in the Source Water
Use of intake lagoons could result in increase of fine silt content in source water over time because silt naturally contained in the water tends to accumulate at the bottom of the lagoon and as it exceeds certain threshold depth it tends to migrate into the intake pump wells and to be conveyed to the desalination plant’s pretreatment system. Typically, this effect of migration of fine silt from the intake source water can be monitored by measuring the SDI5 of the source water (or SDI2.5 if silt content increase is so high that SDI5 cannot be measured).
When the intake lagoon depth is such that there is at least 3e5 m (10e16 ft) from the bottom of the lagoon sediments to the entrance of the intake pump wells, this buffer distance prevents fine silt from entering the intake, and the SDI5 of the source water is usually between 10 and 20. As sediments accumulate at the bottom of the lagoon over time, some of them are conveyed to the intake and typically result in a slow increase in source water SDI. Usually, when SDI5 ex- ceeds 25e30, the performance of the desalination plant granular filtration system would begin to be impacted because the fine silt would begin to accumulate on the surface of the filter media and decrease its ability to filter the source water, which in turn would reduce the length of the filtration cycle.
The most cost-effective troubleshooting measure in this case is the periodic dredging of the bottom of the intake area when the SDI5 of the source water exceeds 30, if SDI increase is caused by siltation. Sometimes, similar SDI increase could also be caused by algal blooms or by strong winds that resuspend bottom sediments and silt. However, well-designed gran- ular media filters would usually be able to handle such SDI increase up to a level of 30 and algal content of up to 20,000 cells/L.
Filter surface siltation of the anthracite or pumice media could also be triggered by naturally occurring events that cause resuspension of large quantities of fine silt and residuals from the bottom of the ocean floor such as high intensity winds, storms, or waves especially if the plant has an open onshore intake or shallow offshore intake located in the tidal zone (e.g., 200e300 m