An investigation of the effects of duckbill valve installation on bellmouth-port marine outfall diffusers in terms of initial dilution

Marine outfall systems are crucial infrastructure solutions that enable the safe and effective discharge of domestic wastewater into marine environments. Among these systems, the most commonly used diffuser type is the multiport diffuser, consisting of open circular orifices. However, since an increase in flow rate is expected over the design lifespan of the system, these systems do not operate at full capacity during the initial years. Under low-flow rate conditions, this can lead to seawater intrusion into the diffuser and subsequent sediment accumulation, eventually causing blockages over time. To prevent such issues, duckbill check valves can be mounted into the diffuser orifices to block reverse flow and enhance overall system efficiency. In this study, 250 different bell-mouthed ports having diffuser design calculations were performed using both constant, and variable diameter diffusers, for a marine outfall system with an assumed project life of 30 years, considering combinations of different flow rates (0.50–1.00–1.50–2.00–2.50 m³/s), receiving water densities (1015–1020–1025 1030–1035 kg/m³), and discharge depths (10–20–30–40–50 m). The orifice diameter was fixed at 0.15 m, and the effluent was adopted as a line source. For each design, initial dilution values were calculated and compared for t=0, t=10, t=20, and t=30 years to evaluate changes throughout the system's lifespan. These baseline values were then compared with the initial dilution values obtained under the same conditions after the installation of duckbill check valves. According to the results, the use of duckbill check valves resulted in changes in initial dilution ranging from -1.04% to 21.94% in variable diameter diffusers and from -0.62% to 21.78% in constant diameter diffusers. These differences were predominantly positive, indicating improved initial dilution following valve integration. The enhancement in dilution was particularly significant at shallower discharge depths and under lower receiving water densitities, for which threshold values were identified. Furthermore, predictive equations were developed to estimate the expected change in initial dilution based on known discharge depth or receiving water density. Furthermore, it was found that both the use of different design flow rates and the temporal variation of inflow throughout the project years had only a negligble effect on the results, as did the use of either constant or variable diameter diffuser pipes.