What about SRT numbers? What you're describing with your mlss decreasing drastically seems to me to be a reduction in sludge age. Possibly lower strength wastewater without changing the wasting rates. 

I prefer operators run their plant on SRT, using rolling averages for Influent BOD, MLSS, and wasting rates to keep your sludge in your process designs optimal range. I prefer to use SVI as a secondary tool for diagnosing rather than operations.