|
Why is desalination important?
Fresh water is the most precious resource on the planet. Water is used in every facet of life from the food we eat to the products we use. The global demand for freshwater is outpacing supply. According to the United Nations, two-thirds of the world's population will face a lack of water in less than 20 years if current trends in climate change, population growth, rural to urban migration and consumption continue. Water scarcity also threatens food security. Food production is the largest use of water worldwide, accounting for 70 percent of all water withdrawal.
Desalination offers the potential to convert the world's inexhaustible supply of seawater and vast quantities of brackish groundwater into fresh water. In conjunction with water conservation, re-use and watershed protection, desalination is viable option towards a drought resistant water supply.
What technologies exist for desalination?
Commercially available desalination technologies include reverse osmosis, multi-stage flash and multi-effect distillation, vapour recompression, electrodialysis, and electrodeionization.
Reverse osmosis (RO) is generally considered to be the most energy efficient commercial desalination technology. RO is a separation process in which salt water is pressurized and forced through a semi-permeable membrane that filters salt ions from the pressurized solution.
Pre-treatment of the feed water is critical to ensure that membrane surfaces remain clean. Pre-treatment for seawater desalination commonly involves the addition of anti-scalant chemicals, ultrafiltration for fine particle removal, chlorination to control organics, and de-chlorination to prevent membrane damage. RO represents over half of the global desalination market with most new plants using this technology.
Multi-stage flash (MSF) and multiple-effect distillation (MED) and are thermal processes that boil salt water and condense fresh water in several stages each at a successively lower pressure. MED and MSF are very reliable; however, scaling and corrosion can be a concern. In terms of volume, MSF plants produce over 85 percent of all desalinated water in the world, primarily in the Middle East.
Vapour Compression (VC) is another thermal process that uses the same principles of reduced boiling points with lower pressure. The heat for evaporating the water comes from the compression of vapour rather than the direct exchange of heat from steam produced in a boiler.
Electrodialysis (ED) and Electrodionization (EDI) are ion exchange processes that force ions from salt water using an electric field. ED and EDI devices consist of a cell of compartments separated by alternating positive and negative ion exchange membranes which allow only ions with the same charge to pass through them. An applied electric forces positive and negative ions in opposite directions through the membranes. Ions are trapped in every other compartment and flushed while alternate compartments are desalted. EDI is commonly used to make ultrapure water for industrial processes and differs in that ion exchange resins are used to enhance the transfer of ions.
|
Why is brine treatment important?
All desalination methods produce salt water reject, or brine, as a by-product in addition to fresh water. For desalination plants in coastal locations, this brine is typically discharged into the ocean. However, for inland operations, this brine must be treated and is not easily disposed. Inland desalination from underground brackish water sources is increasing to meet growing water demand as fresh water aquifers are depleted and deeper, saltier sources are extracted.
Many industrial processes, such as oil & gas and mining, require large volumes of freshwater for extraction or processing. They also produce salt water as a byproduct. With tightening regulations around water usage, many industries must desalinate brackish water and fully treat any salt water byproduct.
What technologies exist for brine treatment?
Commercially available brine treatment technologies consist of evaporation ponds, deep well injection, concentrators, and crystallizers.
Evaporation ponds use solar energy in the form of heat to evaporate water from the brine in large area, shallow ponds. Evaporation ponds are simple with low operating costs.
Deep well injection involves injecting brine deep underground, typically below any freshwater aquifers. Deep well injection can be a low cost solution for disposal of large brine volumes if geology is compatible.
Concentrators are thermal mechanical devices that evaporate brine to a lower volume. Crystallizers are also thermal mechanical devices, incorporating vapour recompression, which are often used in conjunction with concentrators as a final step to produce solid salt from the concentrator output. Concentrators and crystallizers are well proven, typically involve titanium or stainless steel construction, and are very energy intensive.
|
