CPV code 45253600 designates Deionisation plant construction work in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
45253600 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 45253000 (Construction work for chemical-processing plant).