Measurement of surface tension in agricultural waste slurries

Abstract

An experimental method to determine the equilibrium surface tension of cattle and pig slurry is described. The surface tension was determined by using a published modification of the “maximum bubble pressure method”: the pressure inside a spherical bubble in excess of its surrounding is measured just prior to its release into a slurry through a capillary; the surface tension is calculated using the well known relation, P = 2σ/ r where P is the maximum pressure, σ is the surface tension and r is the radius of curvature. The method was found to be valid only at slurry solids concentrations of less than 6% by weight, where measurement errors due to slurry viscosity and the influence of solids were insignificant. Slurry surface tension was found to be 0.06 to 0.07 N/m, only slightly lower than water. This method may be applied to other agricultural waste slurries, fermentation broths and domestic sewage.