Which is NOT listed as a factor affecting settling?

• A. Concentration of sludge
• B. Temp
• C. DT
• D. pH

Answer: (D)

Settling in a clarifier is driven by the physics of particles falling through a fluid. The speed at which sludge settles depends on the concentration of solids (how crowded the mix is), the temperature (which changes the water’s viscosity and how easily particles can move), and how long the solids are allowed to settle (detention time or residence time in the tank). These factors directly influence how quickly particles reach the bottom and how clear the effluent becomes.

pH, while important for chemical processes and for how well flocculation might occur when chemicals are added, does not directly set the rate of gravity-driven settling. It can affect whether particles clump together effectively, but the basic settling velocity is governed by particle size, density difference, viscosity, and time. Therefore pH isn’t listed as a direct factor affecting settling.