The conceptual design of Lopar agglomeration wastewater treatment plant on island Rab is presented in this graduate thesis. Based on data taken from the „Feasibility Study for the agglomerations of Rab, Supetarska Draga and Lopar“ and conducted analysis, respecting the regulations, it follows that for the agglomeration of Lopar it is necessary to build a new wastewater treatment plant with a capacity of 9,550 PE with the first stage treatment and offshore outfall. It is envisaged that the device will capture sanitary wastewater via pressure pipeline from the wastewater drainage distribution system of the Lopar agglomeration. The wastewater is then carried out through pre-purification process (coarse and fine grates) to remove large solids and through removal procedures of dispersed substance, sand and grease, with disposal of the separated waste. After the purification on the first stage , through the deposition process in the primary settling tank, the amount of suspended solid is reduced by at least 50% and the biochemical oxygen demand index BOD5 is reduced by at least 20%. Purified water is discharged by an outlet gravity pipeline and an underwater outlet to the receiver (coastal sea). The primary sludge formed in the deposition process is treated by thickening, stabilization and dehydration processes and is adequately disposed of. As a part of this graduate thesis an analysis of input data is conducted, a hydraulic calculation for the design of basic objects in a device is made, and also the technical solution with graphic attachments and an approximate cost estimate is given.