{HEAD ~ This inserted layer}, with {TAIL ~ a thickness} that can vary according to needs, has the role of protecting the activated carbon bed against the abrasive action of the grains of sand during filtering or washing 
with
{ HEAD ~ This inserted layer } , with { TAIL ~ a thickness } that can vary according to needs , has the role of protecting the activated carbon bed against the abrasive action of the grains of sand during filtering or washing
HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH