A novel approach was developed to enhance the efficiency of the dissolved air flotation (DAF) for biodiesel wastewater by acidification and coagulation. Firstly, Grease & Oil and Chemical oxygen demand (COD) removal efficiencies of biodiesel wastewater using acidification with pure hydrochloric acid and pure sulfuric acid at pH = 3 and 1 day retention time were more than 80%, and 50%, respectively. Secondly, Grease & Oil and COD removal efficiencies of biodiesel wastewater using alum, polyaluminum chloride and ferric chloride coagulants without acidification at 1.0 g/L were more than 90% and 30%. Thirdly, OAF alone and OAF with acidification could not separate Grease & Oil from biodiesel wastewater. Thus, OAF with the acidification and coagulation is suggested for biodiesel wastewater treatment. Biodiesel wastewater treatment by acidification before alum coagulation allowed the alum concentration to be reduced by 60-90% compared to treating without acidification. In addition, the efficiency of Grease & Oil removal from biodiesel wastewater by OAF with alum and acidification was 85-95%. It can be concluded that the efficiency of Grease & Oil removal from biodiesel wastewater using DAF with acidification and coagulation was 10% greater compared to other processes. (C) 2011 Elsevier B.V. All rights reserved.