A dispersed-ozone flotation(DOF) separator for tertiary wastewater treatment P.K.Jin, X C Wang and G. Hu School of Environmental and Municipal Engineering, Xi'an University of Architecture and Technology, No 13 Yanta Road, Xi'an 710055, China(E-mail: pkjin @xauat edu cn; rcwang @xauat edu cn) Abstract A dispersed-ozone flotation(DOF)separator was devised for a pilot study of tertiary wastewater treatment for re-use purposes. As a compact device combining coagulation, ozonation and flotation in an integrated unit, the DOF separator achieved a very high removal of SS, ToC, Uv254 and colour, as well as effective inactivation of coliform and total bacteria within a short hydraulic retention time of 30 min. The finished water quality is comparable to or better than that by a conventional tertiary treatment process using coagulation, sedimentation, filtration and chlorine disinfection, and meets the quality standards for non- drinkable domestic reuse Keywords Dispersed-ozone flotation; tertiary treatment; water reuse A typical process for tertiary treatment of the secondary effuent usually consists of oagulation, solid/liquid separation and disinfection units for a removal of residual Ss. organic matter, colour, offensive odour and microorganisms to meet the requirement of water re-use. Regarding solid/liquid separation, sedimentation/filtration and flotation/ filtration are common options, and regarding disinfection, chlorine and ozone are among the disinfectants widely applied. For onsite or decentralised wastewater treatment and re-use, systems with compact configuration and ease of control and operation are expected (Lens et al., 2001) Comparing with sedimentation, dissolved or dispersed air flotation is often considered be more suitable for the separation of pollutants with densities similar to or smaller than water. Many studies have shown the advantages of flotation for the treatment of algal rich reservoir stored lowland water(Bauer et aL, 1998), low turbidity surface water (Johnson et aL., 1995), and taste and odour control(Hargesheimer and Watson, 1996). To enhance pollutants removal by flotation, pre-ozonation has been widely used (Yu et aL., 1994 Schmidt et aL., 1995). There have also been practices of ozone-induced flotation for water and wastewater treatment(Betzer et al. 1980: Rashid et al., 1990) Generally speaking, the residual pollutants in the secondary effluent are composed of low concentration suspended solids and organic substances which are less biodegradable and usually cause the problems of colour and offensive odour( Gogate and Pandit, 2004). One idea for effective removal of these pollutants is to seek a combination of coagu- lation, ozonation and flotation in one treatment unit. This leads to the development of a dispersed-ozone flotation(DOF) separator as is introduced in this paper. Materials and methods Dispersed-ozone flotation system Figure I shows the schematic flow chart of the DOF separator for the pilot study at Beishiqiao Wastewater Purification Centre, Xi'an, China. The main body of the DOF doi:10.2166/wst2006.263A dispersed-ozone flotation (DOF) separator for tertiary wastewater treatment P.K. Jin, X.C. Wang and G. Hu School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, No 13 Yanta Road, Xi’an 710055, China (E-mail: pkjin@xauat.edu.cn; xcwang@xauat.edu.cn) Abstract A dispersed-ozone flotation (DOF) separator was devised for a pilot study of tertiary wastewater treatment for re-use purposes. As a compact device combining coagulation, ozonation and flotation in an integrated unit, the DOF separator achieved a very high removal of SS, TOC, UV254 and colour, as well as effective inactivation of coliform and total bacteria within a short hydraulic retention time of 30 min. The finished water quality is comparable to or better than that by a conventional tertiary treatment process using coagulation, sedimentation, filtration and chlorine disinfection, and meets the quality standards for non￾drinkable domestic reuse. Keywords Dispersed-ozone flotation; tertiary treatment; water reuse Introduction A typical process for tertiary treatment of the secondary effluent usually consists of coagulation, solid/liquid separation and disinfection units for a removal of residual SS, organic matter, colour, offensive odour and microorganisms to meet the requirement of water re-use. Regarding solid/liquid separation, sedimentation/filtration and flotation/ filtration are common options, and regarding disinfection, chlorine and ozone are among the disinfectants widely applied. For onsite or decentralised wastewater treatment and re-use, systems with compact configuration and ease of control and operation are expected (Lens et al., 2001). Comparing with sedimentation, dissolved or dispersed air flotation is often considered to be more suitable for the separation of pollutants with densities similar to or smaller than water. Many studies have shown the advantages of flotation for the treatment of algal rich reservoir stored lowland water (Bauer et al., 1998), low turbidity surface water (Johnson et al., 1995), and taste and odour control (Hargesheimer and Watson, 1996). To enhance pollutants removal by flotation, pre-ozonation has been widely used (Yu et al., 1994; Schmidt et al., 1995). There have also been practices of ozone-induced flotation for water and wastewater treatment (Betzer et al., 1980; Rashid et al., 1990). Generally speaking, the residual pollutants in the secondary effluent are composed of low concentration suspended solids and organic substances which are less biodegradable and usually cause the problems of colour and offensive odour (Gogate and Pandit, 2004). One idea for effective removal of these pollutants is to seek a combination of coagu￾lation, ozonation and flotation in one treatment unit. This leads to the development of a dispersed-ozone flotation (DOF) separator as is introduced in this paper. Materials and methods Dispersed-ozone flotation system Figure 1 shows the schematic flow chart of the DOF separator for the pilot study at Beishiqiao Wastewater Purification Centre, Xi’an, China. The main body of the DOF Water Science & Technology Vol 53 No 9 pp 151–157 Q IWA Publishing 2006 doi: 10.2166/wst.2006.263 151