Index >> Waste Water Microbiology >>Percolation Processes

Typically, a Scholler plant has six or eight 50-cubic-metre digesters constructed of steel and lined with acid-resistant tile. The diameter of these digesters is 2.4 metres, and the overall height about 13 metres. The top of a digester, or percolator, has steam and vent lines and a line for the introduction of hot dilute acid. The bottom is equipped with a filter cone and a quick opening discharge valve for removing the lignin residue. The digester is loaded with 9 to 10 metric tons of sawdust and chips to a density of 180 to 200 kilograms of dry wood substance per cubic metre. A charge of dilute acid is then injected at a temperature lower than that of the percolator contents, and the injected acid is heated by steam from the bottom until the desired temperature is reached. The solution is then pressed from the percolator by applying steam to the top of the charge. This operation is repeated for a total of up to 20 cycles, with 0.8 percent sulphuric acid at temperatures increasing to a maximum of 184° C. From 10 metric tons of wood approximately 120 metric tons of liquor with a concentration of 5 to 6 percent sugar is obtained.