I have 2 clarifying questions:
1) are your process tanks outdoors, exposed to sunlight?
2) do you notice a higher alkalinity testing in the morning versus testing in the afternoon, and/or do you notice your pH being lower after an overnight period?
Algae can cause alkalinity issues. During the day when exposed to sunlight, they photosynthesize and reproduce, which will extract co2 and calcium carbonate alkalinity from the water, increasing the pH mainly and possible lowering the alkalinity.
Conversely, after nightfall, cut off from sunlight, the algae undergo respiration which adds co2/carbonate alkalinity/hardness back into the water and can seriously depress pH.
I had this issue in the secondary sedimentation basin of my water plant, indoors. We were colonized with an algae that was getting sunlight at just the right angle for just the right bit of time through the windows, at the right temp, and it would bloom during the day. We would notice it because in the morning when we'd start the plant that stage of the process would have a higher than normal pH and lower alkalinity after the night respiration, and once the plant flow was running a while it would even out, then we'd catch the photosynth daytime period and the pH would crash, often a full point or more, but we might not lose as much alkalinity because in that basin we were feeding slaked lime (but only during the day) for pH control. Mix that lime with co2 in the water and it forms calcium carbonate which the algae eat.
Algae may be small, but they have the numbers to wreak serious havok with process chemistry at surprising speeds if left uncontrolled, and though not a WWTP issue, can cause major taste and odor problems, not to mention overwhelm your chlorine dose rapidly and completely kill your residual.
PS EDIT: Sodium Hydroxideis hygroscopic and absorbs carbon dioxide from air. This has the potential to increase alkalinity as well. Best of luck!