it can be done using potassium permanganate KMnO4 .
I did it once in a sand filter. SInce the filter can't be stopped, it took few month to get the effective coating because the dosage of potassium permanganate can't be very in order to have no residual at exit of the filter. I am pretty sure that with a high dosage the coating would be done much faster. As for use in your system, you would have to get the proper size, weight, uniformity,etc. of sand to be coated. Be careful however if you try that, KmNO4 stains everything
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