Reverse osmosis is a process based on pressure through which salt is eliminated from seawater. The process entails placing seawater on one side of a semi-permeable membrane and diluting water on the other side of the membrane. Natural osmosis forces water through the membrane from the dilute side to the concentrated side to ensure equilibrium (Malaeb & Ayoub, 2011). The process overcomes natural osmotic pressure using an external pressure that forces water through the membrane to the less concentrated water from the more concentrated side. During the process, pressurized water flows through a permeable membrane, which separates it from the dissolved salts. The flowing liquid through the membrane is permeated. The pressure difference between the pressurized seawater and the product water encourages the permeate to flow through the membrane. The residual seawater continues to flow as brine through the pressurized side of the unit. Energy is only required to initiate seawater pressurization (Malaeb & Ayoub, 2011). Pressurization of seawater involves pumping it against the membrane and into a closed container. The concentration of the residual brine solution and seawater increases as the product water flows through the membrane. A specific volume of the concentrated seawater and brine solution is removed from the container to decrease the concentration of the remaining dissolved salts (Malaeb & Ayoub, 2011). The processes involved in Seawater reverse osmosis include feed water intake, pretreatment, cartridge filtration, pressurization, membrane separation/reverse osmosis, and post-treatment stabilization (Malaeb & Ayoub, 2011). The feed water intake is the first process that collects seawater and feeds the water to the pretreatment system. The pre-treatment system treats the seawater to eradicate suspended solids, adjust pH, and add a threshold inhibitor to control scaling, which components like calcium sulphate cause. The system removes solids using adsorption, flocculation, sedimentation, impaction, interception and straining mechanisms. Pretreatment ensures that feedwater is compatible with the membranes. A feed pump then pumps the water into a cartridge filtration unit, which removes fine suspended material from the feedwater. The water then flows to a high-pressure pump. The High-pressure pump then increases the pressure of the pretreated water to an adequate operating pressure for the membrane and the feed water salinity.
Reverse osmosis or separation then occurs and includes the core desalination process. The pressurized water is forced across a membrane element in which water permeates the membrane while the dissolved substances flow through the surface of the membrane and exit without permeating the membrane. This produces a stream of fresh water and a stream of brine reject. Brine then flows through the energy recovery equipment and is then returned into the sea while the permeate flows into the post-treatment system.
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Post-treatment system is used to adjust water pH, and remineralize, and disinfect the water. The final product is then delivered to a storage reservoir. While reverse osmosis for seawater desalination is an effective technique that produces high-quality water efficiently, the process can affect the environment, particularly through its intake systems and concentrate disposal. For instance, the intake system impinges and entrains marine organisms. Disposal of brine can affect underwater life, particularly if the discharge is poorly diluted (Missimer, Jones & Maliva, 2015). The effect of the intake system can be reduced by placing it in a location with a low oceanic productivity. Intake systems can also use velocity caps to minimize the number of entrained fish. Subsurface intake systems can also be used. The use of correctly-developed diffuser systems can be used to minimize the effect of brine disposal (Missimer, Jones & Maliva, 2015). The entire environmental impact of reverse osmosis for seawater desalination can also be minimized through properly designed plants following a complete environmental impact evaluation.
References
Malaeb, L., & Ayoub, G. M. (2011). Reverse osmosis technology for water treatment: state of the art review. Desalination , 267 (1), 1-8.
Missimer, T. M., Jones, B., & Maliva, R. G. (Eds.). (2015). Intakes and Outfalls for Seawater Reverse-Osmosis Desalination Facilities: Innovations and Environmental Impacts . Springer.
