The Geotechnical and Environmental Properties of Amended Biosolids

Abstract

The shear strength and stiffness properties of biosolids are of great concern when used for land reclamation, rehabilitation, or monofilling. In this study, in order to enhance the shear strength and stiffness properties, several waste by-products from local industries were mixed in low quantities with biosolids from a local wastewater treatment plant. The highest strength found was for biosolids mixed with 20% fly ash + 20% lime followed by 20% lime only (based on dry mass) after 12 weeks' loading under 225kPa. This yielded an increase in strength of over eight times and five times the strength of unamended biosolids, respectively. The next best additives for strength increase were two smelter slags (KOBM, works debris) followed by lime kiln dust, which exhibited the lowest increase in strength (8.0kPa compared to 5.1kPa for unamended biosolids with 12 weeks' loading under 225kPa). Additionally, measurements of the environmental characteristics such as pH, carbohydrates, proteins, lipids, dissolved organic carbon and metal concentration (copper, nickel and zinc) were carried out and compared with the shear strength measurements. In general, either only indirect or no detectable relationships were found. The consolidation and effective stress characteristics of biosolids amended with 20% lime were found to be unlike those of natural fine grained soils. Results from the consolidometer showed large strains (>30%, first pressure increment of 47kPa), low coefficient of consolidation (0.015m2/year) and low permeability (0.4-5.2x10-11m/s). Using step wise modelling, it was shown, both experimentally and theoretically, that the consolidation parameters were highly influenced by the viscosity of the leachate. The effective stress path analysis resulted in an angle of shearing resistance of 55.5° and an effective cohesion of zero. To conclude, amended biosolids have been shown to be a complex three phase material (gas, liquid, solid). Their strength and deformation characteristics were highly influenced by the additives used, the chemical reaction induced and the amount of overburden pressure applied. Especially, the viscous characteristics of the material proved to be of particular importance, in addition to the cementitious properties of the amended biosolids.