CPV code 71800000 designates Consulting services for water-supply and waste consultancy in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
71800000 is a leaf code — the most specific level of its branch — so it can be used directly as the main CPV code of a contract notice. If it does not quite fit, check the neighbouring codes under 71000000 (Architectural, construction, engineering and inspection services).