at a location just below the cooking zone 13 and above a wash zone 24 which is disposed at the bottom end of the digester liquor containing dissolved solids is extracted from the interior of the digester through the screens the liquor extracted through the individual screens is conveyed to a discharge header 28 which encircles the girth of the digester externally of the digester in the region of the screens and is conveyed as by a pump system 30 to a chemical recovery station 32 or is selectively returned in part to the digester via a downcomer 54 as desired a heater may be interposed within the piping between the pump station and the downcomer to heat the filtrate prior to its return to the digester the downcomer is located centrally of the digester and includes discharge ports 38 adjacent the lowermost end of the downcomer as depicted in fig1 these ports are disposed substantially radially equidistant from the surrounding screens such that the filtrate liquor discharged through the ports is directed substantially radially outwardly see arrows of fig1 from the downcomer ports thereby ensuring that the filtrate liquor discharged from the downcomer flows simultaneously and substantially uniformly radially toward all of the screens when the filtrate liquor discharged into the chip mass adjacent the wash screens is heated to about the cook filtrate liquor temperature and by reason of the radially lateral flow of the discharge filtrate liquor upflow or downflow of the liquor through the chip mass in the area of the screens is prevented or discouraged as needed or desired black liquor from.