What is desalination?
Desalination is a process which is used to extract minerals from saline water. In this case, it refers to the removal of salt from salt or brackish water to produce water suitable for industrial application or human consumption. This article looks at the concepts of desalination and the application to the Caribbean.
How does desalination work?
There are many different methods of desalination. For the most part, in the Caribbean, there are two main methods employed: Multi-stage Flash Distillation and the more widely used, Reverse Osmosis.
Basic Concept of Multi-stage Flash Distillation, MSFD
In MSFD, the plant has a series of units called stages. Each stage contains a heat exchanger and condensate collector. The sequence ranges from a hot to a cold end with each stage having different pressures corresponding to the boiling points of the water in that stage. Feed water at the cold inlet temperature is pumped in through the heat exchangers in the stages and warms up. Once the water has reached the brine heater at nearly maximum temperature additional heat is added.
The water, which is now called brine, flows through valves into stages with lower temperatures and pressures and since its temperature is above the boiling point at the pressure of the stage, and a small percentage of the brine water flashes (boils rapidly) to steam. The resulting steam is a little hotter than the feed water in the heat exchanger. The steam cools and condenses against the heat exchanger tubes, which helps to heat the inlet water initially but more importantly allows the product water to be collected.
Schematic of a ‘once-through’ multi-stage flash desalinator
A – Steam in
B – Seawater in
C – Potable water out
D – Waste out
E – Steam out
F – Heat exchange
G – Condensation collection
H – Brine heater
Basic Concept of Reverse Osmosis, RO
Reverse Osmosis is the process of Osmosis in reverse. Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline solution will tend to migrate to a strong saline solution. Examples of osmosis are when plant roots absorb water from the soil and our kidneys absorb water from our blood.
Whereas Osmosis occurs naturally without energy required, to reverse the process of osmosis you need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-permeable membrane that allows the passage of water molecules but not most dissolved salts, organics, bacteria and pyrogens. The water needs to be ‘pushed’ through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure to desalinate (demineralize or deionize) the water, allowing pure water through while holding back most contaminants.
General Desalination in the Caribbean
The vast majority of Caribbean islands who require desalination to provide for their population use RO. Some of these countries include Antigua & Barbuda, Bahamas, Barbados, BVI, Trinidad & Tobago, St. Lucia and Grenada. These respective governments have set up authorities, partnerships between authorities and private companies as well as hotels and resorts to invest in the commissioning of these plants.
The reasoning for their commissioning tends to be slightly different in each case. For example, Barbados, an island with one of the highest population densities in the world, decided in the mid 1990’s that, even after their attempts to designate protected catchment areas, more needed to be done to satisfy the demand for water for its general population as well as for its dependence on its tourism industry. This lead to a partnership between a pair of local and international companies to build and operate under Ionic Freshwater Ltd. who run the brackish water RO plant. All the water produced is sold exclusively to the Barbados Water Authority and Ionics is in a contractual agreement to hand over ownership to the BWA 20 years after its commissioning in 2000.
Comparatively, in Trinidad, a country with a relatively large source of water for its population, the reasoning is more to do with its demand for water suitable for industry. In 1999, the largest RO plant in the Caribbean at the time was built on the Point Lisas Industrial Estate.
Reverse Osmosis Plant in Trinidad at Point Lisas Industrial Estate. (Photo via Seven Seas Water)
That’s not to say that MSFD or similar distillation processes do not exist in the Caribbean. Although it is much less popular, it has been and still is used in different places across the Caribbean. In Antigua, the Crabbs power plant combines the production of both water and electricity. This is beneficial in general as waste heat from the power plant can be used to heat the sea water, reducing energy needs by half to two-thirds and thus saving money. Both the Bahamas and the BVI have used or continue to use flash distillation as well.
Reverse Osmosis vs. Multi-stage Flash Distillation
Before the RO process, contaminants must be filtered out as the membranes are very delicate. This is not required in MSFD.
Output Quality MSFD
MSFD is slightly better but RO can be improved if more than one set of membranes are used.
RO is much more efficient than MSFD because it has a much higher yield less energy is required. However, MSFD is competitive if used in energy cogeneration.
MSFD plants tend to be a lot larger and require special materials as well as infrastructure meaning it is more difficult and thus more expensive to maintain.
Water in Construction
Desalination is one efficient way of providing clean water however it is also important that sensible use of water is practised since this has a key part to play in making construction sustainable. It is important that we work towards minimising waste. Water supplies are a growing concern for the construction industry since it is required in the production process for many materials such as concrete and steel.
As well as the actual construction process, water is an important part of the design of each project too. Factoring in usage required within the building, as well as how to encourage people to be less wasteful of water through the design, is important. Some buildings can even be constructed to catch and use rainwater, which is cost-effective in the long-run but saves energy and overall water consumption. Proactive water efficiency planning from the beginning design process can identify areas of potential savings, and work out the best solutions for usage, management and maintenance.
Water auditing is a great way to help companies manage their water consumption, and reduce it in areas where it is possible to do so. It can save money, minimise waste and create a more sustainable business. With conservation now more important than ever, water is not a utility to be taken for granted. Auditing of water use and footprinting is now increasingly common on construction sites, to enable them to manage usage over time.
Read more here at Construction Caribbean.