Desalination: Overview of the Reverse Osmosis Process
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A desalination plant is a facility that converts salt water from the sea into water suitable for human consumption, as well as for industrial or irrigation purposes.
Desalination can be achieved by two types of processes: thermal or transmembrane. Semi-permeable membrane desalination is the most widely used technology in the industry because it requires much less energy than thermal desalination and is therefore more cost-effective.
This article will focus on this second type. This is the most widely used reverse waterproofing process in use today in many countries around the world.
The high demand for drinking water and the high energy potential available have enabled the development of sustainable methods of energy production based on seawater desalination, making it an increasingly attractive and necessary technology.
There are several technologies used on an industrial scale to desalinate seawater. Among all the technologies, seawater reverse osmosis is the most widely used worldwide.
It is now a mature technology and can be found in many coastal areas of the world where natural water resources are limited. Advances in research and development related to this technology are ongoing. One of the latest innovations aims to significantly reduce energy consumption and minimise adverse effects on membranes.
To understand the purpose and process of reverse osmosis, we must first understand the natural process of osmosis.
Osmosis is a natural phenomenon. It is one of the most important processes in nature and occurs when two solutions with different solute concentrations are separated by a semi-permeable membrane (allowing only the solvent to pass through). This phenomenon occurs spontaneously without any energy input.
Thus, two solutions with different salt concentrations tend to equalise their concentrations. The solvent from the lower-concentration solution tends to flow through the semi-permeable membrane into the higher-concentration solution until the concentration is uniform.
The driving force of this solvent flow depends on the osmotic pressure and is related to the difference in solute concentration between the two solutions. Specifically, it is defined as the equilibrium pressure that is established between solution and solute. Therefore, the osmotic pressure of a pure solvent is zero. When equilibrium is established between two solutions of different concentrations, the equilibrium pressure is equal to the osmotic pressure difference.
The process of reverse osmosis involves osmosis in the opposite direction. Osmosis occurs spontaneously without energy, so energy must be supplied to make it occur in the opposite direction.
Solvent transfer occurs from the concentrated solution to the more dilute solution. Reproducing this phenomenon in seawater on an industrial scale requires a pump (needed to generate pressure) and a semi-permeable membrane to allow the solvent to pass through.
A reverse osmosis membrane is a semi-permeable membrane that allows water molecules to pass through, but not various compounds that are not required for the end use of the water. In addition, it can also contain bacteria.
The buoyancy must be higher than osmotic for the water to be desalinated. When the concentrated solution is pressurised, water molecules pass through the semi-permeable membrane while contaminants are retained.
A desalination plant can be divided into four stations.
- Pumping, storage, and screening.
- Pre-treatment and high-pressure pump
- Process: Reverse Osmosis.
Seawater recovery systems for desalination plants include open or closed intakes.
Open water intakes draw water directly from the sea. They are the most vulnerable to overflows and, for obvious reasons, present a higher risk of contamination. Therefore, the quality of the water obtained may vary.
In contrast, water from a closed intake (a well) is not an inexhaustible source of water, and although it is more uniform and of higher quality, it can be assumed that its capacity decreases significantly with relative frequency.
Thus, from the point of view of flow assurance, the open intake has an obvious advantage, as the difficulty of securing the production flow in well water is high.
A disadvantage of large-capacity plants is that they require large areas for permeable marine boreholes, which limits the supply rate. Therefore, the first step in the water treatment process is to pump water from the natural environment to the plant.
If the natural soil is not suitable, water must be pumped to the plants. The function of the pumping station is to send water to the desalination plant. The factor that ensures this pressure peak for the water supply to the plant is the hydraulic pump, whose operating point is determined by the flow rate required for the installation process as well as the pressure.
Figure 1. Seawater pumping
Reverse osmosis desalination requires a very thorough pre-treatment of the seawater to prevent suspended solids from settling on the membrane. This rapidly reduces the flux produced.
The purpose of pre-treatment is varied. First, it keeps the module free of blockages from suspended solids, microorganisms, and salt deposits, among others. On the other hand, the water must be given characteristics compatible with the nature of the membranes: chlorine content, pH, temperature, etc.
Depending on the type of reverse osmosis module, all particles larger than 10-50 µm must be retained. This is done by using coarse pre-filtration followed by sand filtration to remove larger suspended solids.
Subsequently, biocide treatment and acidification are required to prevent microbial growth on the membrane and to avoid carbonate precipitation. Finally, the cartridge filtration method can retain particles as small as tens of microns, which cannot be retained by sand filters.
The process used is appropriately selected according to the water quality of the feed water. Nowadays, the removal of contaminants by ultrafiltration (UF) membranes is the most widely used process, as it offers the best balance between contaminant removal and product penetration. The main pre-treatment steps are:
- Clarification: Coagulation Flocculation Sedimentation
- Prevention of scaling
- Cartridge filtration
Figure 2. Ultrafiltration
These procedures are carried out using self-cleaning filters, ultrafiltration, and chemical dosing systems.
Ferric chlorine: Coagulation of small particles at the UF inlet.
Sulphuric acid: UF and RO washing systems
Bisulphite: Chlorine neutralisation at the RO inlet.
Antiscalant: Prevents precipitation on RO membranes.
Sodium hypochlorite: Drinking water tank disinfection
Sodium hydroxide: PH adjustment for drinking water
A high-pressure pump pumps seawater into the RO module where the membrane is located.