Sewage sludge is a by-product of the wastewater treatment process at wastewater treatment plants (WWTPs). The sludge produced potentially poses an environmental problem when disposed of on agricultural soils, and the goal of the profession is to find the optimal way to dispose of sewage sludge with minimal environmental hazard. The amount of sewage sludge produced is increasing with the number of new sewage treatment plants being built, which is increasingly polluting the environment. Sludge samples were collected at the Velika Gorica, Karlovac and Sisak and geochemical analyses were carried out. Using sequential extraction according to the BCR protocol, the chemical composition of the sludge was determined. The level of heavy metals in the sludge complies with the legal requirements for sewage sludge used in agriculture. Other analyses of the sludge samples showed that the composition of the sludge can vary depending on the source, i.e. the incoming wastewater at the treatment plant. Apart from the input medium, the quality and composition of the sludge depend on the degree of purification of the wastewater in that treatment plant.