Measures for Protecting Water Quality: Current Approaches and Future Developments

An important component in protecting the ecological quality of watercourses is the regulation of point source discharges. These are usually expressed in terms of “end of pipe” limits on certain physical parameters or on the concentrations of specific chemicals. Sometimes discharge licences take the form of Emission Limit Values (ELVs) which are based on Best Available Technology with respect to emissions and apply to all discharges (usually for a specified industry sector) irrespective of the dilution capacity of the receiving watercourse. Alternatively, they may be based on water quality standards (e.g., EQSs) for specific chemicals which describe thresholds below which no adverse impact on the receiving water is predicted and which take explicit account of available dilution at different discharge locations. The strengths and limitations of these approaches are reviewed, along with a consideration of new approaches that address some of the limitations associated with control measures based on ELVs or EQSs. These include approaches for controlling well-defined mixtures by a “Toxic Equivalent” approach, exemplified here for discharges containing alkylphenol ethoxylate surfactants, and a fundamentally different approach for regulating complex effluents based on Direct Toxicity Assessment of whole effluents. Circumstances under which these different approaches to controlling point source emissions to surface waters might be appropriate are discussed.