The reconstructed and upgraded waste water collection and purification system of the island of Krk is planned to include all waters whose physical, biological and chemical characteristics have been changed, i.e. those whose original composition has been changed by use. Wastewater is all potentially polluted technological and domestic water, rainwater and other. They also participate in the hydrological cycle, i.e., after use, the water is taken away through the sewage system for treatment, i.e. purification, and is then returned to the natural environment. In this work as part of the project, total investments of HRK 649,000,000.00 are planned in order to bring the existing island wastewater treatment system up to satisfactory European standards. The project is co-financed from sources of funds of the European Union within the Operational Program for "Competitiveness and Cohesion 2014-2020". The surface of the island of Krk is 409 square kilometers, and the coastline is 190 km long. Considering the highly permeable karst soil and almost 20,000 inhabitants in the island's 68 inhabited places, waste water disposal is extremely important, especially from an ecological point of view. Until now, the island's wastewater and stormwater drainage from settlements and economic zones has not been solved according to the criteria that would meet today's modern technology, but the existing state of the public wastewater and stormwater drainage system was carried out according to the concept of one settlement - one system, so that the island of Krk has seven differently developed system of the public wastewater drainage. The goal of this work is to unify the existing systems into a single system and to increase the control over purification and its quality.