Domestic wastewater marine outfall diffusers can be set up with sharp-edged circular ports. These simple and cylindrical ports opened on diffuser pipe wall have two main types: (1) sharp-edged port in thin-walled diffuser pipe (TN) and sharp-edged port in thick-walled diffuser pipe (TK). Because of less minor head loss, TK has 20% higher flow rate capacity than TN in same hydraulic head condition. No matter how TKs are better than TN type, TK type ports are still open-ended holes in very dynamic seawater ambient. TK ports suffer from clogging risk because of sediment transportation and seawater intrusion into diffuser pipe during less wastewater flow rate periods. In order to protect TK ports, an elastomer and one-way flow providing "duckbill" check valve (DBV) with same nominal size can be mounted on circular ports. This rehabilitation method could specifically be applied on domestic wastewater discharging marine outfalls after start of their operational period. However, additional DBV part could change original hydraulic and initial dilution conditions, due to increased minor head losses and jet velocities. In this study, TK to DBV nozzle type conversion effects on initial dilution were discussed. Q = 0.5 m(3)/s design flow rate, t = 30 years lifespan, H = 20-50 m port depths, and rho = 1,015-1,035 kg/m(3) receiving water densities, 12 different diffusers with d = 150 mm diameter sized TK circular ports were designed and converted to 150 mm regular (not wide type) DBV nozzles. All hydraulic parameters and initial dilutions of before and after DBV usage were recalculated from literature data for line source in this study. Initial dilution comparisons between original and DBV converted conditions as Delta S (%) were varied from -10.77 to 6.53 for at the end of lifespan, t = 30 years with 33% positive values. For the first days of life span, t = 0 year, Delta S (%) were varied from -9.10 to 10.94 range with 50% positive values.