18. — (1) A person shall be deemed not to have contravened section 16 (1) in relation to an existing discharge of trade effluent or other matter made before he is granted or refused a licence under section 16 if, before the relevant date, he applies for such a licence and complies with the requirements of any regulations under section 19 regarding the furnishing of information to a sanitary authority.

(2) In a prosecution for an offence under section 16 it shall be presumed, until the contrary is shown by the person charged, that the discharge concerned is not an existing discharge of trade effluent or other matter made in the circumstances specified in subsection (1).

(3) Where regulations under section 26 relate to an existing discharge of trade effluent being made in the circumstances specified in subsection (1), the sanitary authority concerned shall grant or refuse (as the case may require) a licence under section 16 in respect of the discharge as soon as may be after the regulations are made.

(4) In this section—

“ existing discharge ” means a discharge which is similar in nature, composition and temperature to, and is of a similar volume and is made at a similar rate to, a discharge made during any corresponding period in the period of twelve months ending on the relevant date;

“ relevant date ” means the date fixed by order under section 16 (1).