pH is an extremely important parameter for all biological treatment systems, aerobic and anaerobic. Both aerobic and anaerobic systems operate best at a neutral or pH of 7. For activated sludge systems, optimal operation occurs between the relatively narrow pH range of 6.5 to 8.5. In my experience an activated sludge system can better handle a pH deviation above 8.5 than it can one below 6.5. When the pH entering a biological treatment system is above 9 the system does tend to be self-bufffering to a pH around 8 by microbial respiration and the production of carbon dioxide (CO2). And when the pH entering a biological reactor falls below 6 acids will be biologically oxidized to CO2 and water. The CO2 will be air-stripped from the process with the result being a reduction in the acidity of the wastewater and return to a more neutral pH.
An alkalinity is an important parameter for two aspects of wastewater treatment: 1) nitrification and 2) anaerobic digestion. Alkalinity is also a critical parameter for evaluating the water treatment process of lime softening. Two charts are provided below. And a PDF version that has both charts can be obtained here.