If the sludge concentration in the return sludge line from the secondary clarifier were the same as the MLVSS concentration (XV), would recycle of return sludge increase the SRT?

It would decrease the SRT

Explanation:

The SRT is the average time it will take for the activated-sludge solids system to decompose. The concept is expressed in days. The retention time is a very important design and operational parameter in the design of an activated sludge digester. The make-up stream from the second clarifier contains a considerable amount of bacteria which are already active. Therefore, the synergistic action of the bacteria makes the sludge to be digested faster. This, In turn, reduces the time it takes to digest the sludge. Thus, the make-up stream lowers the retention time.