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