Normal stress of wastewater sludge

Shear stress and total thrust on the bottom plate for wastewater sludge subjected to shear were measured using an AR1000-N rheometer. Wastewater sludges are demonstrated not only a non-Newtonian fluid, but also a visco-elastic fluid. Before the stress reaching the peak stress, the wastewater sludge is not really flowable at shearing. But the sludge exhibits a rather low normal stress. The unsheared sludge is resistant to shearing but is readily normally elongated. After sufficient shearing, the sheared sludge exhibits a low viscosity, meanwhile, the corresponding normal stress has been increased to a high value. Sufficiently sheared sludge is relatively easy to shear, but very resistant to normal elongation. This occurrence is attributable to the development of elasticity of the sludge network during shearing. Adding cationic polyelectrolyte into sludge could enhance its network strength to resist both plastic and elastic deformation.