Model development for biological treatment of toxic industrial wastewaters: an easy tool for linking pilot results and field data∗

Highly toxic industrial effluents are often difficult to treat in biological wastewater treatment plants (BWWTP). Results obtained during a previous pilot-plant study allowed the identification of key parameters needed for the degradation of high concentrations of cyanides, thiocyanates and ammonium[13]. Kinetic models and parameter values were given for the pilot conditions using a synthetic effluent and commercial bacterial inoculums (Ecosynergie Inc.). In the present paper, these results have been applied to an existing full-scale BWWTP that was treating wastewaters from the coke and steel industry. The full-scale BWWTP contained the same contaminants and bacterial inoculums that were used in the pilot study. A conceptual hydraulic model of the BWWTP was built using STELLA software (ISEE Systems) and was adapted to the main contaminants undergoing degradation in the waste stream, using the same kinetics and parameters determined during the pilot study for the degradation of cyanide, thiocyanate, ammonium concentrations, and pH. Verification of the models using field data gave a close fit between simulated outlet concentrations and field outlet concentrations with only one significant parameter adjustment. The pilot studies were thus considered to be representative and conditions for this and for the observed discrepancies are discussed. The interest in alternative plant management policies such as pH regulation or new recycling flow patterns have been evaluated by adjusting parameters as variables in the model frame. The STELLA diagrams built for cyanide, thiocyanate and ammonia degradation are given, and examples of field cases are discussed with analysis of pH adjustments and flow recycling for process optimization The use of such software appears so to be very attractive for the simple representation of a complex BWWTP.