Prediction of organic chemical fates in biological treatment systems

Attention is presently being focused on the fates of organic chemicals in biological treatment systems. Most systems have been designed and operated to remove organic compounds from the aqueous waste stream and performance has been defined in terms of removal of a non-specific parameter from the wastewater. Parameters like total organic carbon, biochemical oxygen demand, and chemical oxygen demand have historically been used in treatability studies, in system design, and in determining system performance. Performance data using these approaches are inadequate for determination of the removal of specific compounds unless (fortuitously) the specific compound's biological activity is approximated by the generalized activity of the non-specific parameter. The resulting assumption that the biokinetics, and therefore the fate of organic compounds, are similar and well-described by the biokinetics of the non-specific parameter does not hold for specific chemicals that possess strong stripping/volatilization and/or biomass sorption tendencies or for compounds that have biotransformation kinetics that differ from the idealized “BOD-like” compound. Some compounds are likely to preferentially strip/volatilize or sorb onto biomass either if generally recalcitrant to biotransformation, or if the biotransformation process is variable and the rates of the competitive fate mechanisms are large relative to the low transient biotransformation rate. All of the above cases have been experimentally documented and will be discussed. The biotransformation rates have the greatest magnitude of variation of any fate-related parameter and therefore have a major influence on the compound fate. An equation coupling the competitive fates of organic chemicals in completely-mixed and steady-state biological treatment systems has been developed and is presented. This equation can be arranged to allow calculation of the fraction or percentage of an organic compound fed to a system that strips, sorbs onto biomass, biotransforms, or remains in the effluent if the stripping, sorption, and biotransformation rates are known. It can be used to correct experimental data to account for the competitive mechanisms and generate more reliable biotransformation data. Finally, it also serves as a unifying approach against which the specific fate performance of a given system may be compared. Therefore it may be used to estimate potential organic air emissions and the potential of toxic organics in sludge generated from the biological waste treatment process.