The filtration plant of the water company Vodoopskrba Kupa d.o.o. that supplies the cities Sisak and Petrinja (Croatia) and their surroundings with potable water is located in Petrinja in the Sisak - Moslavina County. The filtration room within the plant contains eight gravity sand filters (basins) where the water is filtered by passing through one meter thick layer of quartz sand. The process of replacing the sand is currently done manually and therefore is slow and inefficient. The old and new sand are transported by means of trolleys. The main aim of this project is the development of a technical solution and the mechanism for replacing the filter media (sand) so as to reduce the physical work of the maintenance crew and to speed up the process.As a part of this project, the mechanism for replacing the sand has been developed. It enables the old sand to be extracted from the mechanical filter and conveyed to the nearby dumping ground while the new sand is delivered and distributed to the basin of the filter. This thesis provides a detail description of the steps involved in the design process, seven conceptual solutions were taken into account. Each of them has been elaborated and discussed and as a result, the best solution, that proceeded with detail design, was chosen. As the final outcome of the project, a technical solution for the replacing sand mechanism was proposed. Furthermore, the benefits, as well as disadvantages of the proposed solution, are discussed. As a part of the thesis, a way of transporting the mechanism between filter basins was also determined. Finally, comprehensive technical drawings were produced and can be found in the appendix of this manuscript.Chapter two gives a brief description of the water treatment plant and explains in detail the water purification process of the plant. Filter basins and the room where they are located are thoroughly defined in chapter three. This chapter also contains technical sketches of basins as well as the drawing of the space available for the mechanism. All essential dimensions are provided in these sketches. Chapter four outlines the main objective of this project and summarises the design requirements. Additionally, this chapter discusses several potentially suitable locations to position the mechanism. Transport features and equipment are introduced in chapter five where every potential piece of equipment used in the design is explained in detail.Seven conceptual designs, that are discussed in subsequent two chapters, were developed for this project. Based on the given requirements, the most suitable conceptual design was finally chosen in chapter 7. This chapter also contains detail description and the basic working principle of the chosen technical solution.Mathematical modelling and control calculations are described in chapter eight. All assembly parts are shown here along with working principle, adjustment and transportation methods of the mechanism that are also described in this chapter. All calculations are conducted analytically in a semi-empirical manner. The structural strength of some design features was calculated numerically using the commercial finite element method (FEM) based software ABAQUS. All CAD models and comprehensive technical documentation were produced in CATIA V5.