Strength Evaluation of Low Solids Content Pulp and Paper Mill Sludge by Factorial Design, 1993 Environmental Conference Proceedings

Sludge from pulp and paper mills typically requires some form of dewatering to improve strength characteristics prior to disposal in a landfill. Because of increased economical and environmental pressure to minimize waste generation and to reduce fiber losses, the strength characteristics of pulp and paper mill sludge can vary. Therefore, the physio-chemical aspects of these wastes need to be carefully and continually evaluated for engineering purposes.

Sludge with low-strength characteristics can have a negative effect on landfill operations. Low-strength sludge hinders the operation of landfill equipment and may limit the placement or “stacking” of the sludge to desired heights and slopes. Low-strength sludge may also preclude the placement of final cover materials such as compacted clay.

Strength characteristics of two types of pulp and paper mill sludge from two different pulp and paper mills have been considered. One sludge is derived from a mill that contains a kraft process, while the other is from a mill that contains a groundwood process only. The strength testing program used a statistical factorial design, which is intended to evaluate various effects on sludge strength due to variation in solids content, compaction efforts (as measured by dry density), and type of test (vane shear and triaxial compression). The testing for this project included consolidated-undrained triaxial compression tests of small (approximately 7.6 centimeters in diameter and 15.2 centimeters in height) samples, using traditional American Society for Testing and Materials (ASTM)-approved equipment, and vane shear strength tests of large (approximately 29.7 centimeters in diameter and in height) samples using laboratory bench-scale equipment.

The evaluation of the results revealed that the sludge type, test type, initial moisture content, and initial dry density were all statistically significant. Conclusions drawn from the research indicate that sludge

strength is a function of the source pulping process, and traditional triaxial compression tests tend to underestimate strength characteristics of sludge with large fiber content.