Unravelling the Influence of Surface Water Waves on Desalination Plant Discharges

Climate change combined with human population growth has led to increasing water scarcity, with potable water demands increasingly met by seawater reverse osmosis (SWRO) desalination technologies. With the expansion of the SWRO industry, the need to safely dispose of residual hypersaline by-products has also increased. These effluents are typically denser than their receiving marine discharge environments and may have consequences for the endemic benthic biota. The accepted strategy of disposal is via submerged inclined jet diffusers, which aim to rapidly dilute the SWRO effluent to near-background levels. While the transport and mixing mechanisms are well understood under zero-current conditions, the behaviour of inclined dense jets under dynamic forcing remains a significant knowledge gap, with implications for understanding diffuser performance in the field. In particular, the effect of regular waves upon inclined dense jets is a relatively sparsely covered topic for designers and managers of SWRO systems.

An experimental investigation into the behavior of inclined dense jets in the presence of regular surface waves is presented. Discharge dynamics from a single diffuser port are systematically considered under various wave scenarios, for counter-propagating wave-discharge regimes. High resolution, two-dimensional spatiotemporal measurements of discharge dynamics were obtained in a laboratory flume using a light attenuation system. The time-averaged data are presented in dimensionless form as a functions of the newly defined wave-Froude number, ( = horizontal wave-amplitude velocity to jet velocity ratio; = jet densimetric Froude number). Experimental results revealed distinct variations between discharge- and wave-governed regimes. Under low wave-induced velocities, the jet behavior was similar to a quiescent discharge regime however under large orbital wave-forcing conditions deflections over the ascending jet phase and spiral flow paths occurred, causing significant reductions in both the trajectory and dilution. For > ~0.5, trajectory properties are wave-governed, while reductions in dilution result for > ~1.0. The decay of both trajectory and dilution entities arise due to the complex interactions of cyclic deflection over the jet ascent phase, arrested flow development, re-entrainment and emergence of the self-Coanda effect. These outcomes have implications for the design and operation of desalination outfalls in coastal environments.