Nowadays, the lack of freshwater is expanding with increasing the world population, urbanization, contamination, and environmental change. To keep up the equilibrium between water supply and demand; some strategies should be implemented. The best system to treat saline water is desalination. Desalination is a process of removing dissolved minerals and pollutants from seawater, brackish water, and treated wastewater Different desalination technology has been developed based on evaporation and membrane technology Table 1. 1 2
Most of the desalination plants are built on the coastline, where seawater is used to produce fresh water. The brine water (BW) is the byproduct of the desalination process and is discharged into the sea again by channels or pipelines. It has a high concentration of dissolved minerals, salts, and temperature compared to seawater. The brine characteristic depends usually on the type of feed water and desalination process, recovery percentage, and chemical used in the desalination process 2 3.
The BW has two different behaviors when it is dumped into seawater. The first behavior is near the discharge point, and it is called the near field region behavior. The second behavior is far from the discharge point, and it’s called the far-field region behavior. The Near-field region depends on the design of the brine discharge system, which is intended to increase the dilution rate at this region. The Far-field region is located far from the discharge point, where the brine is flowing down the seabed due to gravity current. In far regions, the brine mixing with seawater depends on ambient conditions and density differences between the Hypersaline brine and the receiving seawaters.
BW discharge to seawater can cause a change to seawater salinity, alkalinity averages temperature, and can cause a change in the marine environment. The BW characteristic usually depends on the type of feed water, desalination process, recovery percentage, and chemicals used in the desalination process. Also, Brine discharge into seawater has several environmental impacts on marine life.4
The List of the environmental Impacts of a desalination plant on the seawater environment includes salinity, Temperature, Dissolved Oxygen, Chlorine Concentration, unionized ammonia, and Seawater Intake.
BW Salinity can lead to an increase in the seawater salinity level near the discharge point. The negative effect of brine discharge occurs with the sensitive marine ecosystems 3 4. According to the Medeazza study, changes in the salinity of seawater have a potential to heavily affect marine biota. Changes in Seawater salinity affect the propagation activity of the marine species, their development, and growth rate. “Larval stages are very crucial transition periods for marine species and increasing salinity disrupts that period significantly” 5 6.
Also, some of the desalination plants are combined with power plants. Power plants produce water effluent at high temperatures. These water effluents are mixed with BW from the desalination plant and discharged into seawater causing an increasing the seawater temperature between 7 °C to 8 °C. The raises in the seawater temperature leading to a change in species composition and abundance in the discharge region. 4 6
Seawater dissolved oxygen level is a very important factor and depends on the temperature. By increasing the temperature of the seawater near the discharge point, the amount of dissolved oxygen will decrease. That causes Hypoxia, which is resulting from the low level of dissolved oxygen concentration in water, and it can cause serious damage for the marine life 6 4
Moreover, the presence of high chlorine concentration in BW can lead to the formation of hypochlorite and hypobromite in seawater, which affect seawater quality and the ecological system. Chlorine affects the fish and marine invertebrates can cause burns in both of them and resulting in serious damage to the marine organisms (Boumis). 4
Ammonia substances and un-ionized ammonia are very toxic to marine life. The ratio of ammonia substance and un-ionized ammonia are PH dependence. With increasing the PH, the concentration of both ammonia substance and un-ionized ammonia increases. The concentration of both ammonia substance and un-ionized ammonia in the discharge region depends on the size of the plant and the ambient seawater conditions. Usually, the concentration of ammonia substance and un-ionized ammonia should meet the water quality standard because they are very toxic and harmful for marine life 4 (Mohamed 2009).
Seawater Intake has the most impact on marine life. Substantial Seawater intake can be very harmful to marine life, by causing Entrainment & impingement. Impingement happens when large organisms such as fish are pulled into the water intake pipe and looked in intake pipe mesh, causing death or injury of these organisms. Entrainment happens when small organisms pulled into the water intake pipe and pass the intake pipe mesh to feed the water tank, causing the death of these organisms in a pre-Chlorination process 6
Nowadays, the continuous increase of water desalination plant numbers led to the development of methods and processes to minimize the negative impact of BW discharge. There are many strategies for decreasing the negative impact of brine discharge. The main strategies include BW treatment before discharge and redesigning the desalination plant.6
desalination plant brine can be treated before discharging into seawater to reduce the impact on the marine environments. In brine discharge into the seawater surface, outfall diffusion devices such as diffusion nozzles can be used to dilute the brine with the surface seawater. Also, the brine can be diluted with treated wastewater before the discharge into seawater. Further, the brine can be evaporated naturally by spreading it in pools and reused the solids lefts. These strategies are suitable for small and medium-sized desalination plants. It reduces the salinity of the brine and decreases the negative impact on the marine environment 6 7 8
the environmental impact for desalination plants can also decrease by designing them in a sustainable way and by installation mechanisms reduces their negative impact. Beach wells or infiltration galleries are one of the mechanisms that are used to reduce the negative impact. Beach wells or infiltration galleries work as a natural filter for feed seawater. It keeps out the marine organism, increases the quality of feed water, reduces the cost of pretreatment and Impingement, and entrainment. Jet brine release is another mechanism that can be added, where the brine released at an angle of 30-45⁰ can enhance brine mixing and offshore transport in coastal water. This leads to a decrease in the intensity of brine plumes resulting in a decrease in the salinity level 8 9.
However, BW management is on from the biggest challenge economically and environmentally. But Still, BW monitoring and quality standards should be applied in the desalination plant. More research and development in the field of brine impact on the environment and processes to reduce this impact should be further investigated. Research and comprehensive studies to minimize the negative impact of desalination plants should be carried out in water research centers and water authorities.
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