U7 exemption allows you to use effluent from water and waste water treatment plants to clean highway gravel beds where high-quality water is not needed.
Source
https://www.gov.uk/guidance/waste-exemption-u7-using-effluent-to-clean-a-highway-gravel-bedCanonical document at the regulator. Always cite this URL — not the Vantage detail page — in compliance evidence.