In wastewater treatment redox can be changed due to physical, chemical and biological agents.
Concentration of electrolytes in solution, concentration of metal cations in solution and of course, when talking of electrochemical treatment, the amount of electrons moving from anode to cathode due to the external power source. In general low pH and high ORP create oxidizing conditions whereas high pH and low (negative) ORP stand for reducing conditions.
according to Nernst'law the only factor (except temperature) affecting ORP is the ratio (ox activity/red activity) of the dominant oxydo-reduction couple in the solution. For instance in denitrification ORP is given by the ratio nitrate concentration/carbonaceous substrate concentration when this ratio is not too far from 1. But as ORP is commonly measured with Pt/AgCl electrodes, every compound in the solution that may interfere either with Pt or AgCl, will modify the result (for instance sulfide interferes strongly with Pt)
Next to all these abiotic factors, living organisms will for sure influence your redox potential. The microbes need electron receptors, and will actively change your Eh.
Your question is very general, are you looking for some references to text books?
Many factors such as solution temperature and pH, irreversible reactions, slow electrode kinetics, non-equilibrium, presence of multiple redox couples, electrode poisoning and ...
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