English abstract
Seawater desalination is a reliable solution to the water shortages in the arid Middle East and North Africa countries. Due to prolonged drought conditions, and a rapid economic development with sharp growth of the population in the industrial coastal areas, limited underground water resources are being used faster than they can be replenished. For Oman, in particular, with the availability of desalination water, it is envisaged that groundwater abstracted from the wells be reduced, and it will also eventually resolve the critical problem of seawater intrusion into the coastal aquifer. However current desalination technology limits the efficiency of producing desalinated water, and up to 60% is lost via unwanted brine product, a concentrate stream (typically double seawater salinity) that must be continuously discharged back to the sea.
Model simulations for the mixing and spreading of desalination brine discharged through outfalls into the sea from coastal seawater desalination plants will be carried out in two parts:
i. in the far field region, where the brine effluent plumes are eventually dispersed in the ambient environment, analytical solutions for a two-dimensional advection-diffusion equation with a point source are obtained and used to investigate the long time effects of brine discharges in a tidally oscillating flow, an
ii. ii. in the near field region, where the initial momentum of discharge and brine buoyancy dominate, commercial PC-based software packages CORMIX and VISJET are used to simulate and graphically visualize the evolution of three-dimensional effluents plume released from marine outfalls into the sea.
Since the unwanted brine stream is primarily seawater but at more concentrated level, in Chapter 2, we will in the far field region ignore the effect of brine density, and a twodimensional advection diffusion equation in a tidally oscillating flow will be used to study the mixing and spreading of brine effluents continuously being discharged into the sea. The solutions are illustrated by plotting contours of concentration to replicate the motion of brine plumes discharged in shallow coastal water. The results will also be applied to assess the potential environmental impacts of brine discharges into environment.
To investigate the effect of effluents discharged density in the near field region, we first run CORMIX model in Chapter 3 to simulate the behaviour of brine discharges from onshore surface open channel, offshore submerged single port and offshore submerged multiport diffusers. For single port and multiport diffusers, two scenarios are considered: heated brine discharges to represent a positively buoyant plume that will eventually rise towards the sea surface, and dense brine discharges to represent a negatively buoyant plume that will be sunk to the seabed. Beyond the near field region, CORMIX is also designed to analyze water quality criteria within regulatory mixing zones and widely used to demonstrate the compliance of marine outfall system with regulations for discharging effluents in marine environment.
Finally, in Chapter 4, we run VISJET model to simulate the details of brine discharged plumes in the near field region. Unlike CORMIX model, VISJET can visually display the evolution of a group brine discharged plumes from multiport diffusers at different angles to the ambient current.
