Aldol condensation followed by ozonation to reduce phenolic compounds in coking wastewater

To treat the coking wastewater with phenolic compounds over 3000 mg/L, the biological method cannot be directly employed due to the toxicity to microorganisms and other methods are usually expensive. In this study, the aldol condensation followed by ozonation was developed to reduce the phenolic compounds in the wastewater from a coal coking plant in Gansu Province of China. For the aldol condensation reaction, the removal efficiency of phenolic compounds increases with increasing the molar ratio of formaldehyde to phenol (F/P), reaction time, reaction temperature, and acidity or alkalinity. Under alkaline conditions the removal efficiency of phenolic compounds is higher than under acid conditions. If the reaction temperature, reaction time, F/P and pH values of the aldol condensation are respectively 95 degrees C, 3 hours, 2.0 and 10.5, the concentration of phenolic compounds decreases from 9018.3 mg/L to 1901.2 mg/L after precipitation separation of phenolic resins. Subsequently adjusting the pH value to 7 further reduces the concentration of phenolic compounds to 764.7 mg/L. Thereafter 2 hours ozonation further reduces the concentration of phenolic compounds to 22 mg/L, resulting in total removal efficiency of 99.75%. The ozonation removes phenolic compounds with methyl/ethyl substituent on benzene ring easier than phenol.