In this paper, elimination of color from the effluents of the secondary stage biological treatment plant of an integrated pulp and paper mill located at a tourism resort was investigated by using a marine outfall system. The investigated mill has a full-scale aerated lagoon system treating effluents from a chemical pre-treatment plant. The colored effluents from the treatment plant have been discharged to a river ending in the Mediterranean and this has resulted in deterioration of the natural appearance of the coastal waters used for recreational purposes. In this study, a marine outfall system with a diameter of 1200 mm and a multiport diffuser of 290 m were planned to eliminate the color problem from the lagoon effluents. The required initial dilution to reduce the color to unnoticeable levels in the marine environment was determined 70 by laboratory-scale dilution experiments. In addition to these investigations, long-term full scale treatment results of the related industry are presented. Oceanographic studies and water quality measurements were performed to evaluate the effects of effluents from the industry on the marine environment. Bioassay tests were also conducted to assess the toxic impacts of the secondary treatment effluents on the macro fauna using Lapistes retucularis as a test species. Considering these findings, a marine outfall system was designed. The designed marine outfall system is currently under construction.