Bod/Cod Ratio as an Optimizing Tool for Coking Wastewater Treatment in the O/H/O Process

Biodegradation of coking wastewater (CWW) containing high concentrations of toxic pollutants with nutritional imbalance is challenging. Biodegradability estimated as BOD5 to COD ratio (B/C) was studied along with influential operational factors in a full scale oxic / hydrolytic & denitrification / oxic (O/H/O) plant for CWW treatment. Experimental and theoretical BOD5 in the O/H/O units provided the B/C ratios clarifying the influential factors. Batch experiment with various sludge inoculation settings showed 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 the reduction in the CWW toxicity. In the H reactor, the B/C ratio increased to 0.28±0.04 because 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 B/C ratio. In a whole, the monitoring of the B/C ratio along different stages of the CWW treatment is a tool suitable to follow the efficacy of functional bacterial groups and their response to the key pollutants. 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 to optimize the operation parameters of biological processes established for the treatment extent.