CPV code 90513800 designates Sludge treatment services in the Common Procurement Vocabulary (CPV 2008), the EU classification used to describe the subject of public contracts.
90513800 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 90513000 (Non-hazardous refuse and waste treatment and disposal services).