DETERMINATION OF THE OPTIMUM LOADING STRATEGIES FOR MONOCHLORO-, TRICHLORO-, AND 2,4-DICHLOROPHENOXYACETIC ACIDS TO ANAEROBIC CULTURES E. Guven, T. H. Erguder, G. N. Demirer Middle East Technical University, Department of Environmental Engineering, Inonu Bulvari, 06531 Ankara, Turkey ABSTRACT With respect to their adverse health and environmental effects, halogenated organic compounds (HOCs) are among the most important priority pollutants. HOCs cannot be easily biodegraded. However, if suitable microbial cultures are acclimated to these compounds properly, and the optimum operating conditions are achieved, HOCs can be removed with biological methods. Recently, great interest has recently focused on reductive anaerobic dehalogenation for the removal of these compounds. Because this process yields lower halogenated compounds which are less toxic and more amenable to further biodegradation. Chloroacetic (CAA), trichloroacetic (TCAA), and 2,4-dichlorophenoxyacetic acids (2,4-D) are significant industrial compounds mainly used in herbicide, pesticide, and dye manufacturing. The objective of this study was to (i) investigate the inhibitory effects of CAA, TCAA, and 2,4-D and (ii) determine the optimum loading strategy of these HOCs to anaerobic cultures. To this purpose Anaerobic Toxicity Assay (ATA) and Optimum Loading Strategy experiments were conducted for CAA, TCAA, and 2,4-D. The results of this study are expected to develop an optimum loading strategy of HOCs to anaerobic systems and contribute to more effective use of anaerobic reductive dehalogenation of HOCs. KEYWORDS Anaerobic; acclimation; loading strategy; 2,4-dichlorophenoxyacetic acid; monochloroacetic acid; trichloroacetic acid INTRODUCTION HOCs which cannot be biodegraded easily, are among the most important priority pollutants. It is known that the halogenated groups they contain are severely toxic on microorganisms (Chou et al., 1978). However, if suitable microbial cultures are acclimated to these compounds properly, and the optimum operating conditions are achieved, HOCs can be removed with biological methods. HOCs often exert toxic effects to anaerobic microorganisms as reported by Belay and Daniels (1987), Mikesell and Boyd (1990). Microbial acclimation, which can be defined as the ability of microorganisms to adapt to various environmental stresses through processes of metabolic change, is the most significant parameter in terms of reducing toxicity on anaerobic consortia, thus achieving reductive dehalogenation. It is a process of physiological adjustment to the environmental conditions that make the microorganisms able to grow under conditions unfavorable to the original populations, thus making the culture more fit for survival (Demirer and Speece, 1998).