Modeling brine discharge dispersion from two adjacent desalination outfalls in coastal waters

A far field mathematical model for two brine outfall discharges that incorporates the effect of an oscillating tidal current is used to study the merging and interaction of two brine plumes in coastal waters. The maximum compounded shoreline concentration is formulated and used as a measure for assessing the long term salinity build-up of brine discharges into the sea. It is found that the modern engineering practice which installs a twoport diffuser at the end of the outfall pipe does produce less potential environmental impact. © 2015 Elsevier B.V. All rights reserved.

A key concern of seawater desalination plants is the concentrate and occasional chemical discharges to the marine environment, which may impair coastal water quality and affect marine life. Moreover, when two or more brine outfalls along a coastline are discharging into shallow coastal waters, the long term impacts are strongly inter-dependent and compounded from neighboring outfalls. Through a simplified mathematical model, we demonstrate how the design parameters of the outfalls, e.g., the separation distance, individual pipe length and discharge rate for two separate outfalls as well as a single outfall with two-port diffusers, can affect the shoreline concentration produced. Specifically, using the maximum value of the compounded concentration at the beach as a measure of how well the brine discharge plumes are diluted, it is found that if two brine outfalls are independently operated, the maximum value can be kept small as long as the new outfall length is larger than doubling the old outfall length, and discharging at a rate smaller than the old outfall. If the integrated total brine discharge load can be shared between two brine outfalls, then it is found that the maximum value is smaller than that of the single brine outfall as long as the new outfall length is larger than the old outfall length. A similar result is also found for the case of a brine outfall with a two-port diffuser. Therefore, the model can be used to help inform how adjustment on the outfall parameters can be made to achieve desired results. The mathematical formulation presented can be extended to the case of multiple brine outfalls discharging into shallow coastal waters, and in particular, the results for brine discharges through a multiport diffuser have been reported in Ref. [14]. However, implementation issues related to the control of discharge rates, reliability and cost effectiveness of the marine outfall are not addressed. The current study was inspired by the densely distributed large-scale desalination facilities in the Gulf region, however, the issue is also relevant globally in view of the existing and emerging desalination hotspots in other parts of the world, e.g., the Mediterranean, China, India, Australia, and the U.S. The advantage of such simple but valid mathematical model is also manifested in this respect. Substituting the values of the dimensionless parameters (V ¼ 0:3, λ ¼ 10, η = 25) relevant to the Gulf of Oman, where the current study is based, with appropriate values will make the model work for another study area, with the model formulation basically unchanged. Acknowledgment D.D. Shao gratefully acknowledges the financial support from the National Key Basic Research Program of China (973 Program) (Grant No. 2013CB430402) and the National Natural Science Foundation of China (Grant No. 51209005) as well as the support from Sultan Qaboos University Internal Grant (IG/SCI/DOMS/14/01) for funding his academic visit to SQU during the course of this work. The authors appreciate the constructive comments and suggestions from the three anonymous reviewers on an earlier version of the manuscript.