What is Desalination?

Types of desalination

Today there are two main types of desalination technologies – membrane (RO) and thermal (MED, MVC and MSF) desalination. Reverse Osmosis (RO) desalination uses the principle of osmosis to remove salt and other impurities, by transferring water through a series of semi-permeable membranes. Thermal desalination uses heat, often waste heat from power plantsor refineries, to evaporate and condense water to purify it. In the most advanced desalination plants, such as those built by IDE, water is pretreated in order to improve the efficiency of the plants.

Thermal Desalination VS Reverse Osmosis

RO desalination technology was initially developed in the late 1950s, and has now evolved into the leading desalination technology globally. There are many different processes used in RO desalination, and a variety of factors come into play when selecting the appropriate solution for each situation – the quality of the source water, the desired quantity and quality of the water produced, pretreatment, energy requirements and disposal of concentrate.

RO can remove many types of molecules and ions, making it suited for potable and industrial uses. Standard osmosis involves a solvent (such as water) naturally moving from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent reduces the free energy of the system by equalizing solute concentrations on each side of the membrane, generating osmotic pressure. Applying an external pressure to reverse the natural flow of solvent is called reverse osmosis. To create clean water using this process, seawater or brackish water is pressurized against one surface of the membrane, causing salt-depleted water to move across the membrane, releasing clean water from the low-pressure side.

Thermal desalination mimics the natural process of the water cycle – evaporation from the ocean, accumulation in the atmosphere, condensation as rain or snow, and collection. Since heat is vital in thermal desalination, the process is often linked to power plants and refineries to use waste heat. There are three main types of thermal desalination – vapour compression (VC), multi-effect distillation (MED) and multi-stage flash distillation (MSF).

IDE develops and provides unique MED & MVC technologies for desalination.
In MED distillation, water is boiled in successive stages (or ‘effects’), each at a lower temperature, to reduce the amount of energy needed, with the boiling/condensation process delivering clean water. This process is the most energy efficient of the thermal processes, negates the need for pretreatment and tolerates variations in the quality of the seawater. It’s highly reliable, simple to operate and has low maintenance costs. Nearly any heat source can be used so it’s very adaptable.

In VC distillation, when heat is delivered by compressed vapour, water evaporates and is collected while the heat is recycled back to the remaining feed water. Vapour compression is usually performed by mechanical means, typically electrically driven (Mechanical Vapour Compression). This is also very reliable and simple to operate.

With the growing demand for fresh water, coupled with increasing cost of traditional sources of fresh water and new, more stringent drinking water quality regulations, desalination is becoming more and more practical and economical. IDE is leading this industry as it grows and plays an important role in the future development of life on earth.

Related Content:
X

The MVC: 38 Years of Experience

This paper describes the development of large capacity high efficiency MVC and the accumulated experience of this process in power utilities, refineries and the industrial sector.

Utilizing Available Coldness from Liquefied Natural Gas LNG Regasification Process for Seawater Desalination

In this article a case study was considered, in which an LNG regasification plant can supply 1750 ton/hr of water-glycol solution at -15°C to be utilized for seawater desalination. The main challenge in this evaluation is being able to provide a desalination solution able to compete with the commonly used RO plants.

Recovery of Osmotic Power in SWRO Plants

The paper analyzes the ways that different researchers have selected to overcome the phenomenon of CP, and presents an overview of the existing RO membranes from the point of view of their suitability for use in Forward Osmosis power generation.

Design Challenges and Operational Experience of a Mega MED Seawater Desalination Plant in Tianjin

This article is a continuation of the article "Sliding Pressure Turbine Integrated with Seawater Desalination Facility (MED)" presented at the IDA 2011 World Congress.

Three Pressure Retarded Osmosis PRO Processes

Pressure retarded osmosis (PRO) can be implemented on a number of water types, using different technologies and achieving various power outcomes. This paper presents the three most practical options.

Implementation of Build Operate Transfer Schemes: Obstacles and Solutions

This paper focuses on these cases, addressing the main causes of the failure of suggested/initiated BOT scheme offerings and what prevented their successful implementation. The paper analyses the main constraints and obstacles characterizing these setbacks, and presents different solutions and alternatives to by-pass and/or overcome these obstacles that, if adopted, could enhance the possibilities of successful implementation of the projects.