Wastewater treatment is a process of reducing the amount of harmful substances to such an extent that water that is filled back into nature has no harmful effects on the environment, living beings in it and on human health. Sludge is formed as a by-product in a wastewater treatment process. Waste sludge is a mixture of organic and inorganic substances isolated from wastewater, and may contain parasites, pathogenic microorganisms, viruses and many potential toxic elements and harmful compounds. The treated wastewater is discharged into a natural receiver, and separated sludge requires further treatment and disposal so as not to pose a danger to the environment and human health, all in accordance with environmentally and economically acceptable methods. A sustainable sludge management approach implies the application of an appropriate sludge treatment technology, which would enable its reuse with satisfactory financial and environmental conditions. The problem regarding the smart disposal and treatment of waste sludge according to certain instructions, guidelines and regulations. has not been clearly defined or regulated in Croatia so far. Recently, the idea of usefulness and costeffectiveness of building a quality wastewater treatment plant that would include and process the recovery of produced sludge in Croatia and in the world is becoming more interesting. It is generally known that as the standard of living increases, the amount of wastewater increases and as the degree of wastewater treatment increases so does the amount of sludge that needs to be treated. All of this increases the operating costs of utility companies engaged in drainage and wastewater treatment. This paper presents the issue of wastewater treatment and the possibility of further use of sludge in an environmentally friendly and economically advantageous way, especially in the field of energy.