The removal of COD from coking wastewater using extraction replacement–biodegradation coupling Original Research Article

Using the extraction replacement–biodegradation coupling technique, the removal of chemical oxygen demand (CODcr) from coking wastewater was successfully carried out, and the appropriate extractant type, extractant/diluter ratio, extraction time, and the extractant/wastewater phase ratio were also determined. The results show that when n-octanol and cyclohexane, in a ratio of 1:1, were selected as the mixed extractant, nitrogen-containing heterocyclic compounds (NHCs), such as pyridines and quinolines, were effectively extracted and replaced by the extractant from coking wastewater. As a result, the biodegradable nature of coking wastewater could be improved significantly. After pretreating coking wastewater with the extraction replacement method for 5 min using a 1:1 ratio of extractant to wastewater and following with biodegradation treatment, the biochemical oxygen demand for five days (BOD5)/CODcr values and the CODcr removal efficiencies of the coking wastewater effluent were improved from 0.06 to 0.29 and from 68.81% to 88.63%, respectively. The result of economic feasibility analysis shows that, by controlling the treaded quantity and concentration of coking wastewater, the extraction replacement–biodegradation coupling process can become a profitable project.