Densification of Paper Mill Sludge Materials

Dewatered paper industry primary sludges have a consistency similar to very soft remolded clays. Densification involves removal of additional water, usually by placement of a surcharge load after deposition in a landfill. Solids composition includes a noncombustible fraction, primarily kaolin clays, and the organic constituents which include wood fibers and other components. Use of kaolinite-pulp fiber mixtures to simulate these sludges permitted control over organic content, fiber type and size, and mineral type. Decomposition was accelerated by the addition of nutrients and seed microorganisms. Use of the ignition test to determine current organic fractions permitted changes in the degree of decomposition to be monitored concurrent to changes in the model sludge compressibility. The effects of organic content, pressure, and decomposition on sludge densification was shown by compression tests. Experimental data permitted development of a method for computation of void ratio as a function of pressure and organic content. Excellent agreement was observed with limited field data. Using the degree of decomposition, procedures for settlement prediction of decomposing paper mill sludges were formulated. Examples are included.