Sludge volume index settleability measures: Effect of solids characteristics and test parameters

Sludge volume index (SVI) type activated-sludge settleability measures are used in the design and control of secondary settling tanks. Considerable confusion exists as to the best parameter and experimental technique to use. The common settleability measures (SVI, diluted sludge volume index, stirred specific volume index [SSVI], and SSVI at 3.5 g/L) are outlined. This discussion is used as the basis for commenting on the validity of these measures and their relation to zone settling velocity measurements. A simple mechanistic model is used to evaluate the effects of solids characteristics and test parameters on SVI-type indices. Solids settleability and compactability, settling column height, and solids concentration in the test have an interactive effect on the measured SVI. The applied model explains many of the artifacts associated with SVI and questions the validity of correlations for zone settling parameters based on SVI-type measurements.