Food and Rural Affairs, with reference to the Environment Agency guidance entitled Water companies: environmental permits for storm overflows and emergency overflows, published on 13 September 2018, how exceptional circumstances, in which the use of storm overflows is permitted, are defined.
The guidance document referred to in the question does not define ‘exceptional circumstances’ for storm overflows. ‘Exceptional circumstances’ were not referenced in either the Urban Waste Water Treatment Directive or the regulations which transposed this Directive in England and Wales: the Urban Waste Water Treatment (England and Wales) Regulations 1994. The term ‘exceptional circumstances’ was, however, used in relation to storm overflows by the Court of Justice of the European Union in the European Commission v UK (Re Storm Water Overflows) case, where ‘situations such as unusually heavy rainfall’ were characterised as ‘exceptional circumstances’. In its 2018 Storm Overflow Assessment Framework guidance, the Environment Agency (EA) sets out a methodology for assessing ‘exceptional rainfall’. Defra, the EA and Ofwat are currently reviewing and updating the guidance on the regulation of storm overflows and sewerage. This will include further clarification on how 'exceptional circumstances' may be defined.