BOD/COD ratio as a probing index in the O/H/O process for coking wastewater treatment

Biodegradation of coking wastewater (CWW) containing high concentrations of toxic pollutants with nutritional imbalance is challenging. Biodegradability estimated as the ratio of 5-day biochemical oxygen demand (BOD5) to chemical oxygen demand (COD) was studied along with the influential operational factors in a full-scale oxic/ hydrolytic & denitrification/oxic (O/H/O) plant for CWW treatment. The influential factors for the B/C ratio in each unit of the O/H/O provided by the experimental and theoretical BOD5 were elucidated. Batch experiment with various sludge inoculation settings showed that the B/C ratio of raw CWW increased from 0.38 +/- 0.04 to 0.47 +/- 0.05 after flocculation pretreatment due to the removal of sulfide and cyanide. In the O1 reactor, quickly depleted biodegradable pollutants resulted in a drastic decrease of the B/C ratio to 0.1 +/- 0.02 together with a reduction in the CWW toxicity. In the H reactor, the B/C ratio increased to 0.28 +/- 0.04 because of the hydrolysis of residual macromolecular organic matter and nitrite accumulation from the denitrification process. The B/C ratio in the O2 reactor dropped below 0.06 along with the complete nitrification. The relative abundance of organic pollutants, the structure and function of bacterial community were analyzed in each reactor to explain the changes in the B/C ratio. On the whole, the monitoring of the B/C ratio along different stages of the CWW treatment is an approach suitable to follow the efficacy of functional bacterial groups and their responses to the water characteristics. The B/C ratio thus becomes a marker of the extent of nitrogen transformations, CWW detoxification and mineralization of organic compounds. More broadly, the B/C ratio helps optimize the operation parameters of biological processes and indicate the functionality of each unit.