Investigating COD and Nitrate–Nitrogen Flow and Distribution Variations in the MUCT Process Using ORP as a Control Parameter

[Image: see text] This study aimed to reveal the flow and distribution lows of chemical oxygen demand (COD) and nitrate–nitrogen under different main anoxic stage oxidation–reduction potential (ORP(an)) conditions based on the analysis of material balance in each reaction stage of the modified university of cape town (MUCT) process, combined with the biochemical reaction principles of activated sludge. The rule of the carbon source saving effect was also clarified. The study adopted the programmable logic controller automatic control system and the feedback control structure using the inner circulation flow of nitrate as the controlled variable. The ORP(an) setting values of control parameters were −140, −125, −110, −95, −70, and −60 mV, respectively. The results showed that when the ORP(an) setting value was −95 mV, COD distribution ratios of phosphorus-accumulating bacteria reached the highest in the anaerobic stage and preanoxic stage, with the values of 51.74 and 7.70%, respectively. The COD was distributed between heterotrophic bacteria and denitrifying bacteria in the main anoxic stage, and the distribution ratios were 4.40 and 7.19%, respectively, when the ORP(an) setting value was −95 mV. The study also showed the distribution of nitrate–nitrogen between denitrifying bacteria and denitrifying phosphorus-accumulating bacteria in the main anoxic stage, and when ORP(an) increased from −140 to −60 mV, the distribution ratios of denitrifying phosphate-accumulating bacteria increased from 76.46 to 86.32%. When there was no denitrification and phosphorus absorption, the acetic acid dosage increased from 20.33 g/d at −140 mV to 24.76 g/d at −95 mV, and the carbon source saving rate increased from 23.19 to 26.56% under similar conditions. Therefore, in the MUCT process, the regulation of ORP(an) changed the material flow direction and mass quality distribution of COD and nitrate nitrogen. When ORP(an) set value was −95 mV, COD and nitrate–nitrogen got the best distribution and the carbon source saving effect was the most significant.