Laboratory simulation of belt press dewatering: Application of the Darcy equation to gravity drainage

Belt-thickening processes are used for preliminary dewatering of flocculent sludges such as polymer-conditioned wastewater sludges, industrial sludges, and water treatment silts. These processes are also directly or indirectly used as the lead-in step in belt-pressing dewatering of these types of sludges. In the belt press application, improperly drained sludge can yield failure of the subsequent pressing steps. This paper is Part I of a series on the laboratory simulation of belt-pressing dynamics and addresses the mathematics of the gravity drainage of conditioned sludges through short depths on fabric filter cloth. The rate of water drainage through the fabric filter for sludge from a given source and treated with constant polymer doses per dry weight of sludge solids is a function of the total volume of the sample, the solids fraction within the sample, the specific resistance of the forming cake, and the specific resistance of the filter cloth. A drainage rate model that fits the general shape of drainage rate from numerous sludges is derived. Methods for using the proper test volume in batch tests in relation to full-scale dynamic presses are presented. Finally, a simple test method for determining the resistance through the filter cloth on full-scale presses is suggested. Data from 12 test conditions including five types of sludges are analyzed. The model appears to adequately account for volumetric changes, sludge concentration changes, and changes in belt resistance in laboratory drainage tests.