Several methods to quantify sludge settleability are reviewed and interrelationships between the zone settling velocity (as defined by Vesilind's (1968) equation), the stirred sludge volume index and the diluted sludge volume index are derived from empirical equations. An experimental investigation showed that activated sludge composition had a marked influence: Sludge settleability increased with increasing sludge age, i.e. as the fraction of active sludge (living organisms) in the sludge decreased.