How could you possibly fill a tank with sand if it's already full of sewage? I actually took my septic course at University of Missouri, one of the leaders.
You've previously said they don't need pumped, according to MU they do.
"A general rule of thumb is to have a septic tank pumped by a licensed pumper every three to five years. But how often a tank needs to be pumped depends on the size of your tank, the amount of wastewater generated in your household, the amount of solids carried in the wastewater, and the age of the system."