Increasing the operational challenge is the reliance on living organisms that are in a constant state of flux to transform the wastes via multiple metabolic pathways into harmless forms. As any home baker or home brewer can tell you - even adding exact yeast cultures to set carbon sources can have a lot of variation in results.
The great amount of variation in both the influent makeup and biological population along with the plethora of system setups (referring to layout, retention times, aeration efficiency, clarifiers etc), results in no perfect, universal target monitoring test numbers that work on all systems. When I work with a new system, I give 'rule of thumb' ranges for normal values based on facilities with similar influent and systems (which is some cases is not easy to find).
It is this lack of a single ideal number or indicator that makes frequent monitoring important for predicting effluent quality based on collected test data. The minimum tests I recommend for activated sludge include SV30, MLSS, secondary clarifier bed depth, microscopic exam of MLSS, influent/effluent BOD/COD, and checking dissolved oxygen in biological unit. Other tests such as ammonia, nitrate/nitrite, phosphate, or specific pollutants can be done.
In starting a new system or treating a new influent, the more data collected the better we can understand how additions, modifications, or changes in influent strength will impact the system. All of this can result in lower, long term treatment costs.