The objective of the diploma thesis is to analyse legislative frameworks regarding the design and construction of water supply and sewerage systems with an emphasis on water treatment plants and water treatment processes. The diploma thesis consists of a theoretical part and a practical part referring to the implementation of laws in practice, with the accompanying documentation. Websites, printed publications, documents received by the designers, supervising engineers, infrastructural engineering companies, contractors and investors were used in the process of drafting the thesis. The purpose of using different sources for a diploma thesis is to show the implementation of legal provisions, their amendments protecting the natural wealth of water from its sources, its use, waste water creation and treatment, the process of returning the water to its natural recipients with the goal of using it, after it has been conditioned, as drinking water (as healthy water in terms of technical and technological factors) without any risks to the users' health. The intention of this thesis is to present the types of waste water, their composition, processes and degrees of treatment in order to be able to use it as drinking water and for the needs of everyday living and working environments, after it has been conditioned, without any (or minimum) risks to further contamination of the natural recipient. The legal provisions must be improved through control and surveillance, their implementation must be ensured and a feedback system regarding the surveillance conducted by the investors' inspection teams and end users must be put in place.