Residual chemical oxygen demand (COD) fractionation in bio-treated coking wastewater integrating solution property characterization

The refractory nature of residual COD in bio-treated coking wastewater (BTCW) creates barriers for its further treatment and reclamation. It is necessary to fractionate the residual COD in BTCW associated with characterization of solution properties. In this paper, a stepwise process composed of membrane filtration, coagulation, adsorption and ozonation was proposed to fractionate residual COD in the BTCW, in which the COD was stepwise reduced to near zero. In addition, the correlation between COD and water quality indexes as well as solution properties were discussed together with a safety assessment of the water quality. Results showed that the residual COD fractionation percentage contributed by suspended solids, colloids, dissolved organics and reductive inorganic substances in the BTCW was 43.7%, 22.1%, 26.2% and 4.9%, respectively. By stepwise fractionating of these substances, the residual COD was reduced from 168.8 to 5.2 mg L-1, and the UV254 value decreased from 1.90 to 0.15 cm(-1). In addition, the particle size of the dominant substances contributing to the residual COD was smaller than 450 nm. Among these substances, the hydrophobic fraction accounted for 78.66% (in the term of TOC). Three-dimensional excitation-emission matrix (3D-EEM) analysis showed that hydrophobic neutral substances (HON) were the main fluorescence constituent in the BTCW, which was highly removable by adsorption. The residual COD after adsorption was mainly composed of reductive inorganic substances. Apart from pursuit of high COD removal rates, more emphasis should be given to the removal of toxic COD. Correlations were observed between the residual COD and water quality indicators as well as solution properties, providing a guideline for optimized removal of residual COD in the BTCW. In summary, these results gave a referential information about the nature of residual COD in the BTCW for the selection of advanced treatment technologies and the management of water quality safety.