0011-9164/04/$– See front matter © 2004 Elsevier B.V. All rights reserved Desalination 169 (2004) 1–10 Factorial evaluation of operational variables of a DAF process to improve PHC removal efficiency Berrin Tansel*, Beth Pascual Civil and Environmental Engineering Department, Florida International University, Miami, Florida 33199, USA Tel. +1 (305) 348-2928; Fax: +1 (305) 348-2802; email: tanselb@fiu.edu Received 1 August 2003; accepted 25 November 2003 Abstract A dissolved air flotation (DAF) process is used for removing oil from oil–water emulsions. Operational conditions that increase shearing and disturbance of liquid during the operation of large-scale DAF systems can significantly reduce oil removal efficiency. Objectives of this study were to evaluate the individual and interactive effects of the operational variables of the DAF process on removal efficiency of petroleum hydrocarbons from water sources contaminated with fuel oils. A series of batch and continuous experiments (utilizing full pressurization and effluent recirculation) were conducted using a 60-L DAF system which could be operated either in batch or continuous modes. The experiments were conducted as a factorial design to evaluate both the individual effects and the interactions of the operational variables which included oil concentration, detention time, water type (brackish and pond), coagulant use, and operational mode. The factorial analysis showed that for the batch mode of operation, oil concentration, detention time, coagulant use, and water type had a significant effect on PHC removal. However, for the continuous DAF runs, the only variable that was significant at the 95% confidence level was detention time. Coagulant use did not have a significant effect on PHC removal efficiency for the continuous runs due to shearing of the flocs. Keywords: Dissolved air flotation; PHC removal; Coagulation; Oil–water separation; Factorial design 1. Introduction Dissolved air flotation (DAF) is a physico- chemical process used for treating oily waste- waters in petrochemical and metal cutting industries. During the DAF process, air is intro- duced into the oil-contaminated water under *Corresponding author. pressure and the water becomes supersaturated. As the excess air is released in the form of bubbles within the water column, the emulsified oil droplets adhere to the bubbles and rise to the surface forming a float layer that can be removed by a skimmer. In addition to removal of droplets by attachment to the air bubbles, volatilization of petroleum hydrocarbons (PHCs) can be signi- ficant during treatment of water contaminated with fuel oils [1].