Journal of Hazardous Materials 42 (1995) 71-86 JOURNALOF HAZIIRDOIJI mnTERlAW Anaerobic toxic wastes treatment: dilution effects G.F. Nakhla”,*, M.T. Suidanb a Department of Civil Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH 45221, USA Received 21 January 1994; accepted in revised form 30 December 1995 Abstract The impact of waste strength on the treatability of toxic wastes such as coal gasification wastewater by the anaerobic GAC reactor operating with periodic GAC replacement was assessed by operating three units treating 30%, 60% and full strength wastewater. At a COD loading of 4.7 kg/m3 d performance was unimpacted by dilution at all the GAC replacement rates investigated in this study to define the minimum adsorptive capacity required to overcome the wastewater toxicity, with all three reactors achieving over 94% COD removal, more than 99.9% phenol removal, 98-99% cresols removal, and virtually complete removal of dimethyl phenols. The full strength wastewater was not treatable at COD loading rates higher than 10 kg/m3 d while the reactors treating the diluted wastewater maintained a COD removal efficiency of about 90% and over 99.5% reduction of phenolic compounds at a loading of 32-34 kg COD/m3 d. At this loading with a biological solids residence time of less than 21 d and hydraulic retention times of 4-8 h, biological activity was not affected at all by the toxicity of the waste. 1. Introduction Many industrial operations such as coal gasification, oil refining, manufacturing of petrochemicals, pharmaceuticals, and steel mills generate wastes that contain a myr- iad of toxic organics. The promising technology of converting coals to gaseous fuels is often associated with the production of wastewaters that are highly polluted with phenolics, ammonia, cyanide, thiocyanate and a variety of suspected mutagenic substances. Although the biological treatment of phenolic compounds is favored over physicochemical treatment, the other components of the coal gasification wastewater (CGWW) typically impair the performance of biological treatment processes [l, 23. *Corresponding author. Fax: 966-3-860-2879. 0304-3894/95/$09.50 0 1995 Elsevier Science B.V., All rights reserved SSDI 0304-3894(95)00007-0