Evaluation of engineering properties of water treatment plant (WTP) residuals is a critical part in predicting their behaviors in monofills. In this study, ten WTP residual samples were collected from ten water treatment plants across the United States. Their general chemical/physical and geotechnical properties including liquid and plastic limits, compaction) shear strength, permeability and durability characteristics were determined.

It was observed that the geotechnical properties of WTP residuals are influenced by various factors such as mineral particle composition, organic content, type and amount of chemical additives) and time of aging, etc. Based on this study, the behavior of WTP residual can be primarily attributed to a unique floc structure developed due to their colloidal properties and high ion concentrations. This highly oriented floc structure plays an important role in the development of cementation and thixotropic hardening and has significant influence on the geotechnical properties of WTP residuals.

WTP residuals exhibit low durability characteristics. They usually have low strength at low solids content. Aging, which is believed to be a process in which floc water escapes with the reorientation of floc particles and the development of cementation, has a tendency to improve the strength. The levels of development of strength of different residuals have a wide range which is also believed to be related to thixotropy and cementation developing properties. Coefficients of permeability are all low, of the order of 10^-6 - 10^-7 cm/s. The compressibility is very high due to the high water retaining ability and the deformation of filoc structure. Low swelling ability and the high pseudo pre-shy;consolidation pressure of these residuals, may be caused by the development of the inter particle bond due to thixotropic hardening.