6 . For the source categories listed in paragraphs 9 to 22 where it is demonstrated to the satisfaction of the competent authority that for an individual installation compliance with the fugitive emission limit value ( ELVf ) is not technically and economically feasible , the competent authority may exempt that installation provided that significant risks to human health or the environment are not expected and that the operator demonstrates to the satisfaction of the competent authority that the best available techniques are used .