The fate and behaviour of priority pollutants in end-of-pipe wastewater treatment plants.

The majority of conventional wastewater treatment plants (WWTPs) are not designed to specifically remove xenobiotic organic compounds or the priority pollutants identified by the Water Framework Directive. The main removal mechanisms which operate during the wastewater treatment process and are capable of effectively reducing the concentrations of conventional pollutants are sorption, volatilisation and biodegradation. These processes can be modelled through the application of the appropriate physico-chemical properties of specific pollutants i.e. the octanol-water partition coefficient (Kow) for sorption, Henry’s Law constant (Kh) for volatilisation losses and either the appropriate decay rate constant or the half-life for the persistence of the pollutant for biodegradation. There is a significant lack of available experimental field data for many of the priority substances and so three different modelling approaches have been use to assess the fates of pollutants through both the primary and secondary treatment stages. Generally good agreement was observed between the models with regard to removal efficiencies during primary treatment. However, significant differences existed for secondary treatment, with the Byrns and STPWIN models being the most reliable when compared to the experimental data which was available. However, high removal is not necessarily consistent with protection of the aquatic environment where effluent concentrations play a significant role. These values need to be determined for priority pollutants leaving wastewater treatment plants.