As a licensed WWTP Operator in PA, USA let me offer the following:
The scope of your inquiry is too broad to effectively answer. It is also too broad for you to effectively research.
DO, retention time, pH and MLSS are the most crucial parameters for operational control. But the optimum values for all depends upon the plant size (both capacity and actual influent level) and type of treatment.
Prior to searching the 'net, I recommend obtaining:
1) Metcalf & Eddy's Wastewater Engineering - a popular and straightforward textbook (presuming you have had some preliminary coursework).
2)The California State University - Sacramento Operation of Wastewater Treatment Plants Volumes I & II - a popular and straightforward operator's training guide (even if you haven't had previous coursework, you can follow these). Sacramento Volume III is also good, and may be what you ultimately need for troubleshooting and remediation, but you will never understand what you are reading without a primer.
Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”
-Steve
