Since this is a wastewater ...
Published by Terry Scheurman, President at Applied Specialties, Inc.
Since this is a wastewater application, carbon dioxide is the only option that makes sense. It is easy and safe to use and far less expensive overall. The bacteria will be happy with the change from HCl to CO2 as well. If you have any heavy metals, you are far more likely to decrease their concentration using CO2 than with HCl. You will get added buffering with the CO2 something mineral acids will not do. You can augment your current aeration with CO2 if you have an aerobic system or just feed CO2 in an anaerobic one.