Dear John, Your points of view are welcomed! ( a little less the  form chosen....though). You've missed the point: we're dealing with wastewater not pure water. To be biological, we need bacteria. To have live bacteria You need Oxygen, either free or bound. Anaerobic bacteria use chemically bound type to breathe and substrate as food. At those ORPs (-400 mV) free oxygen is zero, and chemically bound oxygen is consumed; nitrates become N2 and SO4 becomes H2S or elementar S (that's the scope of the "anaerobic path chosen"). I assume You're not into WWTP, whereas UASB, EGSB, ABR and HEABR are in use ever since. Such process is probably one of that myriad of processes You mention. BTW, thanks for taking time to read and comment.

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Dear John, Your points of view are welcomed! ( a little less the  form chosen....though).

You've missed the point: we're dealing with wastewater not pure water. To be biological, we need bacteria. To have live bacteria You need Oxygen, either free or bound. Anaerobic bacteria use chemically bound type to breathe and substrate as food. At those ORPs (-400 mV) free oxygen is zero, and chemically bound oxygen is consumed; nitrates become N2 and SO4 becomes H2S or elementar S (that's the scope of the "anaerobic path chosen"). I assume You're not into WWTP, whereas UASB, EGSB, ABR and HEABR are in use ever since. Such process is probably one of that myriad of processes You mention.
BTW, thanks for taking time to read and comment.