If the mean cell residence time (MCRT) is adjusted, what would normally appear in the secondary clarifier first?

When the mean cell residence time (MCRT) is adjusted, particularly in a wastewater treatment system, changes in the characteristics and behavior of the biological mass can have significant effects on the secondary clarifier's performance. The correct answer identifies that the first indication of changing conditions will be the presence of clumping and ashing.

Clumping refers to the formation of aggregates of microorganisms, which can occur when there is an increase in the concentration of solids in the system due to adjustments in MCRT. In an environment where the MCRT is manipulated to retain biomass for longer periods, this can lead to a higher density of microorganisms and consequently more significant interactions between them, resulting in clumping.

As for ashing, this term can relate to the accumulation of inert material or inorganic matter that may be associated with the biological flocs. When MCRT is increased, the system might not only see an increase in biological solids but also potential settling issues, leading to the formation of a layer of less active solids that can exhibit ashing.

Understanding the processes at play, the clumping and ashing will typically be visible in the secondary clarifier before other phenomena such as clear water or excess sludge. Clear water may not appear until after the system has effectively processed the