The purpose of the collection and calculation of solids settling data is to obtain representative flux curves with which to predict secondary clarifier performance.  To accomplish this goal, accurate settling velocity data must be obtained. Obtaining accurate data, in turn, requires determining interfacial settling velocities of the system activated sludge at several initial concentrations. 

Theory
The gravitational flux, which describes the solids flux curve, is the product of the solids concentration, its settling velocity and a conversion constant.  For a given sludge, settling velocity decreases with increasing solids concentration due to hindered settling caused by particle-particle interactions. 
Because the decrease in settling velocity is not linear, a maximum product of concentration and settling velocity occurs at intermediate solids concentrations and this is what gives the flux curve its characteristic shape.  The more settling velocities that can be determined, the more accurate the resulting flux curve will be.  It is important that only the linear (middle) portion of the curve be used to calculate the slope because the initial portion is often difficult to measure and the latter stages reflect compression settling.

Two methods are available for determining sludge settling velocities.  The settling velocity can be determined by measuring the height of the interface between the settling solids and the overlying supernatant in a measuring column over time.

The setting velocity is the slope of a plot of interfacial height (in inches) versus time (in minutes).  This procedure is standard and is described in section C.1.2, Direct Measurement of the Interfacial Settling Velocity.  The second method involves estimation of the settling velocity from sludge volume index (SVI) measurements.  This method is described in section C.1.3, Settling Velocity from the Sludge Volume Index.