Technical Bulletin No. 0174: Mechanical Pressing of Primary Dewatered Papermill Sludges

With the development of improved pressing devices, considerable interest had arisen in the industry concerning the basic requirements for, and the possible value of, pressing for facilitating the disposal of sludges obtained on clarification of mill effluents. Experiments designed to predict pressing results in respect to sludge consistency and the optimum time and pressure required, were conducted at the National Council’s Central and Lake States Regional Center, then at Kalamazoo College. It was found that vacuum filter cakes, produced on partially dewatering board mill and deinking sludges which contained twenty-one (21) and thirty percent (31) solids, respectively, could be further dewatered to consistencies of thirty-nine (39) and fifty-three (53) percent by pressing. Optimum pressing conditions appear to be in the order of three hundred (300) psi for five minutes. With both the sludges tested, cakes of considerably lower volume, easier to handle and not subject to odorous decomposition, were obtained through pressing. The addition of fiber to the sludges tended to increase rather than decrease the moisture retained after pressing. The water removed was sufficiently low in solids for return to the clarification system when suitable drainage media was employed in the press. It is of interest to note that since these studies were completed five years ago, presses have gained an important position in sludge disposal technology in the paper industry.