DENSITY OF ACTIVATED-SLUDGE SOLIDS

A technique for measuring the density of activated sludge solids is reported in this paper. Along with particle size and shape, solids density is a factor in the removal efficiency of secondary clarifiers. Density gradient centrifugation, a technique widely used in the biological sciences, was adapted to measure the density of activated sludge solids. Density gradient centrifugation consists of allowing the solid particles to settle in a fluid of continuously increasing density until the particles become stationary. At this point, particle and the fluid densities are equal. This technique was used to measure the density of activated sludge solids at several treatment plants in the Sacramento area. The density of activated sludge solids at a given treatment plant over a period of time was relatively constant. However, at different treatment plants densities ranged from a low of 1.02 to a high of 1.06 g/ml, indicating that plant operations may influence solids density. Using the measured density of secondary solids, variation in settling velocities was estimated using the following assumptions, (1) discrete particle settling; (2) an average particle diameter of 100-mu-m; and (3) laminar flow conditions. The corresponding settling velocities for particles with densities of 1.02 and 1.06 g/ml are 16.9-26.8 m/day.