What is the upflow rate/unit area of the backwash? Is there an air scour during the backwash cycle? How many filters are in service? It may be that the anthracite is more visible in the clearwell.

My experience has been with anthracite filters, but when I detected anthracite in my effluent samplers (it was too dense to be scooped up by the dipper); my problem was determining which, of a dozen filters was the source. Your solution is to remove the media from the filter and repair the underdrain.

What are the filters used for? Mine were used for wastewater and I have some tips for returning most of the media to the filter after repairs if you're interested.

grrun

That is why it is called a mixed media filter. The filter media doesn't completely classify into separate and distinct layers during backwash. In addition, the air scour may allow the anthracite to fall into the underdrains by expanding the graduated media covering the breaks. If possible, take the suspicious filter out of service and clean out the clearwell.

grrun

Specific gravity isn't the only factor in hydraulic classification; size and shape are also significant factors. The air scour probably mixes the media completely, but I'm not familiar with the geometry and operational variables of these filters.

We used a 1 m³ tank connected to a 3", double diaphragm pump to pump a slurry of media up into the filter. The filter drain was left open to drain off the water from the slurry. This allowed us to handle the media without having to physically elevate the media 20' to the top of the filter. We used this apparatus to recover used as well as new media for adding to the filters. We still had to manually cover the underdrains with graded gravel; but it really reduced the effort in loading the media.

grrun