A regulatory framework for controlling effluent discharges using toxicity testing in the UK.

Legislation designed to regulate effluent discharges is expressed in terms of minimising the risk of harm of that discharge to the environment. Practical implementation is usually based on the control and measurement of a limited number of individual chemicals. However, this is not always effective because of the complex nature of some discharges or the lack of data for some chemicals, preventing the definition of acceptable levels in the environment. Direct Toxicity Assessment (DTA) clearly has a role in such circumstances because it avoids the need for a comprehensive chemical characterisation of effluent discharges. Several scenarios in which DTA can play a useful role in water quality management under current legislation have been identified. A 'generic' framework for the implementation of bioassays for effluent control is proposed. This provides a number of options in terms of the level of environmental protection afforded and the 'starting point' for a DTA programme (i.e. whether it is based on local measurable impacts on water quality or directed from the outset towards the risk assessment of particular discharges). A DTA programme can be developed that meets both the technical and regulatory requirements of these different scenarios. Guidance on the design of such programmes for different scenarios is provided.