the vat 14 a pulp mat 15 also referred to as a u201c cake u201d forms on the screened surface 16 of the rotary drum 12 the screened surface is porous such that filtrate and other liquids can flow through the surface to the longitudinal drainage channels 18 the screened surface 16 and deck 17 form a cylindrical sheath over the longitudinal drainage channels 18 of the rotary drum 12 the longitudinal drainage channels 18 may extend the length of the rotary drum 12 and are parallel to the axis of the rotary drum 12 the screened surface 16 may be formed of a metal or plastic mesh material that covers a deck 17 of the rotary drum 12 the deck 17 may be a wire structure a perforated plate or corrugated plate the pulp mat 15 and screened surface 16 form a filter that prevents the passage of pulp fibers and allows the passage of water liquor and other liquids the water liquor and other liquids passing through the pulp mat 15 become filtrate which collects in longitudinal drainage channels 18 under the deck 17 and flow through an annular array of drainage channels 20 to a filtrate chamber 21 that may be at the end of the rotary drum 12 the pulp slurry in the vat 14 typically has a low consistency such as 1 5 pulp and 98 5 liquid by weight the pulp is drawn from the vat 14 to the pulp mat on the cylindrical screened surface 16 as the pulp mat 15 and screened surface 16 rise out of the vat 14 the pulp mat 15 has a consistency of about 10 pulp and 90 liquid after rising out of the vat 14 the pulp mat 15 is washed.