Biological wastewater treatment plants are a suitable solution for wastewater treatment for various settlement sizes and are suitable for areas where a greater degree of recipient protection is required. This master's thesis presents the results of a seven-year test (2012-2018) of the physical and chemical indicators of wastewater quality on a continental and coastal wastewater treatment plant. The purpose was to determine whether there is a difference in the efficiency of the device ( continental and coastal) due to the impact of the tourist season, which is expressed in the coastal areas. The inlet concentrations of all analyzed indicators were higher for the coastal device, while for the continental device there was a statistically significant higher load of the device due to the difference in wastewater flow; which also causes a higher load on the recipient. The results show a higher effect of removal of total nitrogen and phosphorus by a continental device, with a statistically significant difference for nitrogen. The proportion of samples that did not meet the criterion of the Ordinance was highest for total nitrogen and phosphorus for both devices, however it was higher for the continental device (for nitrogen by 21%, for phosphorus by 36%), compared to coastal (for nitrogen by 10%, for phosphorus 30%). The only difference in effectiveness between the continental and coastal device during the summer season is greater efficiency of coastal device for COD, which indicates the good functioning of the device under the greatest load. In order to protect the environment and human health in the Republic of Croatia, it is necessary to set higher standards for wastewater treatment, since untreated wastewater is a major pollutant of natural recipients and groundwater. Also, investments are needed to build a sewage network in smaller settlements to reduce uncontrolled wastewater discharges.