I think that the answer to ...

I think that the answer to your problem lies in the multi media filter. The high SDI indicates that not all of the material that should be removed by the MMF  is being removed. This may be due to not adding a coagulant in the right amount for the conditions or by faults in the filter and its backwashing leading to cracks that permit both the anthracite particles and silt particles to break through. What is the size of the anthracite particles on the cartridge? If they are approaching the size of the sand and garnet then the problem is likely to be cracks or failed nozzles. Very fine and it is more likely to be coagulation. A sudden failure points more towards a filter fault.