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