BAT is defined [in the Environmental Permitting (England and Wales) Regulations 2010] as:
The use of the best available techniques will emphasise the use of non-waste technology, if available.
The term "best available techniques" means the latest stage of development (state of the art) of processes, of facilities or of methods of operation which indicate the practical suitability of a particular measure for limiting discharges, emissions and waste. In determining whether a set of processes, facilities and methods of operation constitute the best available techniques in general or individual cases, special consideration will be given to:
a) comparable processes, facilities or methods of operation which have recently been successfully tried out;
b) technological advances and changes in scientific knowledge and understanding;
c) the economic feasibility of such techniques;
d) time limits for installation in both new and existing plants;
e) the nature and volume of the discharges and emissions concerned.
It therefore follows that what is "best available techniques" for a particular process will change with time in the light of technological advances, economic and social factors, as well as changes in scientific knowledge and understanding.
If the reduction of discharges and emissions resulting from the use of best available techniques does not lead to environmentally acceptable results, additional measures have to be applied.
"Techniques" include both the technology used and the way in which the installation is designed, built, maintained, operated and dismantled.