Believe  your multi-media ...

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Believe  your multi-media filter consists of antracite-sand and possibly garnet. Anthracite is normally with size 0.8 to 1.6mm or more. Unless your filter bed is seriously cracked (or has developed spiral holes) it is difficult for the particles to enter underdrain and then into filtered water outlet. If the Anthracite is not of requisite hardness then there is a possibility that crushed powder may have entered into filtered water outlet through the cracks.(A remote possibility is that underdrain nozzles have got damaged too). Best thing is to open the filters and have a physical inspection.    

However normally it is inadequacies of pre-treatment which results in higher filtered water turbidity. This results in increased SDI. Need to check your feed turbidity & other parameters along with the chemical doses.(Is the pretreatment only limited to multimedia filters, what is the source of raw feed water?)

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