When testing for filter media, specifically for the uniformity coefficient (UC) and nominal effective size, are you allowed to combine the different sizes of the sand or do you test each sand individually as per the layering on the filter bed?
If you mix the different UC and nominal effective size of the different sands, then you will obtain a different UC and a different nominal effective size for the mixed sand.
If you want have a multilayer filter, and you can use antracite, then I recommend you use different densities media as sand and antracite with different size. The you can obtain different layers with different CU and different nominal effective size, and I think a better filtration system.
I would recommend that if different filtration layers are specified the layers must be classified separately. My perception is also that there is generally a trend away from layering, unless there is a specific requirement for a dual media filter. Depending on the application, I would go for a single layer layer suited for the selected nozzles and then sand must definitely be classified for the specific single media on it's own.
If you have a stratified filter (multi-media) you should be checking each layer individually. If you dissimilar materials like anthracite & sand you need to do each one individually.