An investigation of dual-step ozone induced flotation (DOIF) process for tertiary wastewater treatment: Characteristics of aerated flocs, removal performance and mechanism

A novel dual-step ozone induced flotation (DOIF) process was developed for the application in wastewater reclamation, where ozonation, coagulation and separation occurred simultaneously. Compared with the dissolved-ozone flotation (DOF) process, an ozonation zone was added to further oxidize refractory contaminants by residual ozone gas. In this study, a pilot-scale DOIF system fed by secondary effluent from a municipal wastewater treatment plant in Xi’an, China, was used to evaluate its performance in tertiary wastewater treatment. The effect of main operating parameters (saturator pressure: 0.3, 0.4, 0.5 MPa; recycle ratio: 30%, 40% and 50%) involved were studied. The results showed that the favorable bubbles and aerated flocs were generated at 0.4 MPa and 30% recycle ratio. At the optimal operational condition, the DOIF process exhibited good performance of color, turbidity, UV ₂₅₄ , TP removal and disinfection in general. Moreover, extra ozonation can further improve the color, UV ₂₅₄ removal and disinfection performance. To interpret the mechanism of the DOIF process, para-chlorobenzoic acid (pCBA) removal, a hydroxyl radical ( ^• OH) probe compound, was monitored. It was observed that the DOIF process generated more ^• OH in the presence of polyaluminium chloride. This indicates that the ^• OH oxidation reaction was involved in DOIF process to enhance the dissolved organic matter removal. The application of DOIF system in tertiary wastewater treatment can reduce the waste of ozone and improve the treatment efficiency.