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