Page 188 - ro membanes

8.5 FILTER PERFORMANCE 171 TABLE 8.3 Granular Media FiltersdTypical Problems and Solutionsdcont'd
Indicator/Problem
• Mud ball formation and solidification of some portions of the filter media
• Filter media “cratering” or geyser formation during backwashing
• Filter media mounding
• Filter media siltation at the end of the filtration cycle
• Excessive amount of backwash water (>5%)
Potential Causes Solution
• Inadequate backwash rate • Nozzle clogging
• Broken filter nozzles
• Plugged filter nozzles
• Inadequate thickness of the
underdrain gravel pack on the nozzles
• High content of fine silt in source water
• Excessive length of backwash
• Excessively high solid content in
source water
• Check/adjust backwash rates of water and air, and apply recovery cleaning
• If uniform backwashing is not possible, empty the filter media, clean the nozzles, and replace filter media with material and layers with adequate specifications
• Empty the filter media and replace broken nozzles
• Empty the filter and start a recovery cleaning procedure
• Empty the filter media and replace with adequate depth media to cover the nozzles
• Check the SDI; if >30 for long periods, apply periodic dredging of the intake area
• If the problem persists, add pretreatment using lamella clarification or microscreens
• Improve upstream treatment (coagulation and settling).
• Reduce backwash cycle length
8.5.4.1 Iron-Coagulant Overdosing
Iron-coagulant overdosing is the most commonly observed problem in saline water pretreatment systems with surface water intakes applying granular media filters. Good operations’ practices usually involve frequent parallel measurement of SDI of the source water, filtered water, and the RO-feed water after cartridge filtration and comparison of the test results to identify changes in the SDI value and appearance of the SDI pads.
A practical criterion for well-operating pretreatment system is the difference of SDI15 of the filtered water entering and exiting the cartridge filters. If the pretreatment system is working well, this difference is usually less than 0.2 units (typical accuracy of the SDI test), which means that the cartridge filters are not retaining solids because they have already been adequately removed by the pretreatment system.
If the difference between the SDI15 values of the cartridge filter inlet and outlet is higher than 0.5 units, and SDI15 of the filtered water exceeds 3, this is an indication that the pretreatment system could be further optimized. If the SDI level in the filtered water frequently exceeds 5, then the pretreatment system operation has to be improved by reevaluation of the source-water quality and the applied chemical-conditioning strategy.