Effect of pre-coagulation on catalytic ozonation in the tertiary treatment of coking wastewater: Kinetic and ozone consumption analysis

The ozone-based advanced oxidation process in wastewater effluent degrades refractory organics efficiently and significantly. Nevertheless, high energy consumption is a challenge that hinders its widespread application. In this work, the pre-coagulation enhanced catalytic ozonation process for the treatment of biologically treated coking wastewater (BTCW) was investigated. The chemical oxygen demand (COD) removal results indicated that catalytic ozonation has an inadequate removal effect on the organic pollutants in BTCW. Moreover, the two-stage degradation occurred in the COD ozonation process, of which the slow reaction rate in the second stage was the main reason that limited its overall effect. The pre-coagulation overcame the obstacle of the limited effect of catalytic ozonation treatment of BTCW because of its ability to remove hydrophobic organics. The combination of pre-coagulation with catalytic ozonation could increase the COD removal efficiency from 45% to 69% compared with catalytic ozonation alone. There was a linear relationship between specific ozone consumption and ozone dosage regardless of whether BTCW was subjected to pre-coagulation. The fact that the slope of this linear relationship with pre-coagulation was much smaller than that without pre-coagulation. It indicates that pre-coagulation could significantly reduce ozone consumption. The primary role of pre-coagulation was to reduce ozone consumption considerably in the second stage of the ozonation process. The ozone consumption per unit mass of COD in the second stage reached 24.3 gO(3)/gCOD without pre-coagulation, while that was only 6.8 gO(3)/gCOD with pre-coagulation.