S19A of the Environment Protection Act 1970 requires a scheduled premises to apply a works approval prior to construction and operation of the facility.
The Environment Protection (Scheduled Premises) Regulations 2017 specifies what scheduled premises are.
A wastewater treatment plant with a design capacity of more than 5000 litres/day is a scheduled premises, whereas a wastewater storage facility of any capacity is not a scheduled premises. Storage of waste water generated on site and tankered away does not require a works approval.