Ozonation of Landfill Leachates: Treatment Optimization by Factorial Design Zacharias Frontistis, Nikolaos P. Xekoukoulotakis, Evan Diamadopoulos, and Dionissios Mantzavinos* Department of Environmental Engineering, Technical University of Crete, GR-73100 Chania, Greece Abstract: The ozonation of landfill leachates in a bubble column was investigated concerning the effect of operating parameters, such as initial organic loading (from 550 to 5500 mg/L dissolved COD), reaction time (from 60 to 360 min) and ozone gas phase concentration (from 19 to 38 mg/L) on treatment efficiency implementing a factorial experimental design. All three parameters tested, as well the second order interaction between initial COD and ozone had a statistically significant, positive impact on COD removal. At increased COD loadings and a maximum ozone concentration of 38 mg/L, COD was the single most important factor affecting both COD and phenols removal. Treatment of the raw leachate (5500 mg/L COD) for 360 min and maximum ozone concentration led to 50% COD removal with a first-order kinetic constant of 2.2 10 -3 min -1 , quantitative elimination of color, henols and acute ecotoxicity and increase of aerobic biodegradability. p Introduction Landfill leachates are defined as those aqueous streams generated as a consequence of rainwater per- colation through wastes, biochemical processes in waste’s cells and the inherent water content of wastes themselves. Leachates may contain large amounts of organic matter, where humic-type constituents con- stitute an important group, as well as ammonia-nitro- gen, heavy metals, chlorinated organic and inorganic salts. The removal of organic material based on chemi- cal oxygen demand (COD), biochemical oxygen demand and ammonium from leachate is the usual prerequisite before discharging the leachates into natural waters. Conventional landfill leachate treat- ments can be classified into three major groups: (a) leachate transfer to municipal wastewater treatment plant where it is subject to combined treatment with domestic sewage, (b) biodegradation (aerobic and ana- erobic) processes and (c) chemical and physical methods such as chemical oxidation, adsorption, chemi- cal precipitation, coagulation/flocculation, sedimenta- tion/flotation and air stripping (1). Over the past several years, advanced oxidation technologies (AOTs) have been widely employed for the treatment of recalcitrant industrial effluents (2), including, amongst others, landfill leachates. Several processes such as ozonation, Fenton and photo-Fenton reactions, and hydrogen peroxide alongside with ultra- violet irradiation have traditionally been employed to treat leachates as reported in detail elsewhere (3). In Keywords: ozonation; landfill leachates; factorial design; waste- water *Corresponding author; E-mail address: mantzavi@mred.tuc.gr Tel.: +30 28210 37797; Fax : +30 28210 37852 most cases, ozone oxidation is capable of achieving moderate to satisfactory COD reduction, the extent of which is actually dependent of the operating conditions employed, i.e. initial COD concentration, effluent pH, ozone dose and contact time (1, 3). In general though, treatment efficiency is improved when ozone is com- bined with another oxidant (i.e. hydrogen peroxide) or a source of irradiation. Given the relatively high organics concentration of the landfill leachates (up to several thousand mg/L COD) and complex composi- tion, process integration may be needed to achieve efficient treatment. Wu et al (4) employed FeCl 3 co- agulation as a pre-treatment stage to remove large organic molecules followed by several ozone-based AOTs to improve the aerobic biodegradability of the untreated leachate. Ntampou et al (5) also employed coagulation with FeCl 3 or PAC as a pre- or post- treatment before or after ozonation respectively and compared the efficiencies of the two processes. Silva et al (6) employed coagulation-flocculation (using Al 2 (SO 4 ) 3 and a cationic polyelectrolyte) to remove leachate color prior to ozonation or ammonia stripping and assessed the acute ecotoxicity of the treated effluents to various microorganisms. Di Iaconi et al (7) proposed a treatment train comprising ammonia remo- val by struvite precipitation, biological oxidation in a periodic biofilter with granular biomass and chemical oxidation by ozone or the Fenton reagent for the complete treatment of a mature leachate. A three-step process consisting of coagulation-flocculation, ozona- tion and aerobic oxidation was proposed by Bila et al (8) for the complete treatment of a saline leachate. Rivas et al (9) reported that ozonation combined with activated carbon adsorption was capable of de- contaminating nearly completely landfill leachates. ISSN 1203-8407 © 2008 Science & Technology Network, Inc. J. Adv. Oxid. Technol. Vol. 11, No. 2, 2008 370