Water is an important factor in human health and an essential ingredient of living organisms and necessary for the life of all living organisms. With the increase in the population and the increase of sliving standard, there was an increase in water consumption, which led to a large increase in the amount of wastewater. Prior to discharge into the environment, the wastewater must be treated with a wastewater treatment device. Wastewater treatment of the public mixed-drainage system of the city of Čakovec in Croatia and suburban settlements is carried out by mechanical and biological procedures, with the final treatment of the separated sludge. The paper will analyze the values of input and output time series of chemical oxygen demand (COD), biological oxygen demand (BOD₅), and suspended solids (SS) on the wastewater treatment plant using the RAPS method (Rescaled Adjusted Partial Sums). The results showed that the input series contain more pronounced subseries with respect to their mean values and trends of increase and decrease, respectively. By comparing the input and output subseries, it can be seen that the output subseries do not oscillate to a large extent given that they express the output quality of wastewater. A significant reduction in the output values of the indicators determines the quality treatment of incoming wastewater.