Seawater Desalination Success Story
Published on by Nikhilesh Mukherjee, Independent - Consultant
This report is a review of the technological advancement in desalination technology which has gained it a feasible alternative for meeting the challenges of the looming global water crisis.
General
The world is adding an extra 80 million people each year. The UN has estimated that the world would require over 30 percent more water between 2012 and 2030. Traditional approach to improve on freshwater availability would no longer be able to keep pace with a swelling population and the worse impact climate change that would be seen in these years.
Why desalination
According to an estimate, 97.5 percent of the earth's water is seawater and 2 per cent is in the form of ice that leaves only 0.5 percent which is fresh water. The supply of desalinated water is possibly the only water resource that does not depend on climate patterns.
Desalination plants operate in more than 150 countries in the world, the key locations are, Saudi Arabia, Oman, United Arab Emirates, USA, Spain, Cyprus, Malta, Gibraltar, Cape Verde, Portugal, Greece, Italy, India, China, Japan, and Australia. Globally, there are more than 17,000 desalination plants working mostly in the locations mentioned above producing over 21.1 billion US gallons of potable water a day. More than 300 million of people around the world today rely on desalinated water for some or all their daily needs
More than half of global installed capacity of desalination is in the Middle East. World's Biggest Desalination Plant is in Saudi Arabia. It has a production capacity of 264 million gallons of drinking water a day. The largest percent of desalinated water used in any country is in Israel, which produces 50% of its domestic water use from seawater desalination
Desalination market
An estimate suggests the $8.6 billion world water desalination industry will grow at 9.3 percent annually through 2015. The Africa/Middle East region will remain the dominant market, while the growth in the Asia/Pacific region would be the fastest. Reverse osmosis and other membrane-based technologies will continue to gain market share over thermal methods.
About, 60% of desalination is used for human consumption. The other areas where desalination application is growing include agricultural and industrial, like petroleum or chemical industries which need large quantities of pure water for boilers or process requirements. They are opting for desalination when local supply cannot meet the requirement.
Desalination technology
Through the mid-1900s, the most commonly used techniques involved evaporation and distillation. The growth of desalination processes took a major step forward in the 1940s during World War II, when military establishments operating in dry areas needed a way to provide their troops with potable water. In the post-war years, scientists began studying osmotic processes to desalinate water. By the 1980s, desalination technology became a fully commercial activity and by the 1990s, the role of desalination technologies for municipal water supplies commenced in some countries.
The major desalination processes employ thermal and membrane technologies. Thermal process is essentially separation of pure water from salt water by distillation and the membrane process is RO technology. Thermal processes are used mainly where power cost is low and waste thermal energy is available from power plant reject. Middle East has the largest numbers of thermal desalination plants. RO gives significant cost advantage in locations where power costs are high. At present, reverse osmosis (RO) accounts for approximately 60 percent of global installed capacity.
Thermal desalination
There are two thermal processes
(1) Multi Stage Flash (MSF) distillation and (2) Multiple effect distillation (MED)
Multi Stage Flash (MSF) distillation
In a MSF unit feed seawater is heated and pressurised. Then it is introduced into different cells maintained at lower pressure and temperature where the feed sea water flashes into steam. The steam is condensed into pure water. The product of the heated water is reheated multiple times, each time functioning on lower pressure than the last.
Multistage flash distillation plants are built alongside power plants in order to use the waste heat. Several large facilities in Saudi Arabia use multistage flash distillation, accounting for around 85% of all desalinated water. The main disadvantages of multistage flash distillation are that it requires more intake of salt water than reverse osmosis and the upfront and maintenance costs are considerably higher because of corrosion.
Multiple effect distillation (MED)
In this case, there are multiple cells. Feed water flows down by gravity in a thin film around horizontal tubes heated by an internal steam flow. Seawater partly evaporates and the vapour generated condenses inside the horizontal tubes of the next cell, thus turning to fresh water. This principle is repeated in multiple cells, the vapour raised in one cell heats up the tubes of the next one.
Key difference between MSF and MED plants
The MSF process requires a large flow of seawater or brine to be circulated in condensers. This results in a specific extra electricity consumption of 3 to 4 kWh/m3 for MSF plants over MED plants which do not require such circulation. This gives a cost advantage of MED plants over MSF plants.
Reverse Osmosis (RO)
It is a pressure driven dissolved solid separation process. A semipermeable membrane acts as a barrier between the feed water and pure water which separates salt and pure water. The natural phenomenon of Osmosis of return of pure water to feed water is prevented by applying pressure on the feed water side of the membrane. One of the main issues with RO is plant downtime arising from membrane fouling and varying membrane life.
Desalination key concerns
- Cost of desalinated water
- Environmental issues
Cost related issues
(1) High capital cost is the first challenge for desalination plants.
The cost of desalinated water taken at the outlet of a plant may vary widely from one site to the other. It mainly depends on the, technology, level of automation, salinity of feed water (Salinity of water is temperature dependent. Some examples of salt content in sea water: Atlantic Ocean :35 g/l, Mediterranean Sea :38 g/l, Arabian Sea :45 g/l, Dead Sea :300 g/l), cost of energy, cost of labour, the plant's total capacity, mode of financing , cost of money, the depreciation period, and concentrate disposal cost.
(2) High energy cost is the second challenge.
Energy consumption in typical desalination processes is given below
Energy consumption | |||
| Thermal process | |||
MSF | MED | RO | |
Electrical energy (kWh/m3) | 4-6.1 | 1.5 - 2.5 | 3-5.5 |
Thermal energy (kWh/m3) | 50-110 | 60-110 | None |
Electrical equivalent of thermal energy (kWh/m3) | 9.5-19.5 | 5-8.5 | None |
Total equivalent electrical energy (kWh/m3) | 13.5-25.6 | 6.5-11 | 3 -5.5 |
"Electrical equivalent" of thermal energy is the electrical energy that cannot be produced in a turbine because of the use of steam consumed by desalination plants. | |||
Source: Wikipedia
Cost reduction: Global trend
There are three clear tends which have emerged over the years to address cost issues:
- Choose thermal processes of desalination where energy costs are low or waste heat source is available.
- Co-locate desalination plants with power generation facilities.
- Invest in big > 20 mgd (million gallons a day) plants rather than too many small plants
Middle East has the largest number of thermal desalination plants. The world's biggest desalination plant is in Saudi Arabia works with a hybrid method that combines MSF (Multi-Stage Flashing) and Reverse Osmosis (RO) technologies.
There has been no major breakthrough in the cost of thermal desalination. The small reduction in cost due to improvement in technology has been mostly offset by increased material and labour costs.
Reverse osmosis technology however has been under great scrutiny to reduce cost over the past decade and there has been a substantial success to achieve a significant reduction of cost of desalinated water.
Brief overview of cost reduction initiatives in RO
The cost component of a typical seawater RO desalination plant is given below:
Capital cost: 31%, Energy cost:26%, Maintenance cost: 14%, Membrane cost (replacement) : 13%, Chemicals / manpower cost /other costs : 16%
Source: Banat
Cost reduction initiatives in RO
A continuous improvement in the membrane materials and membrane module configuration has reduced the cost of RO desalination very significantly over the years. The other breakthrough in RO desalination, which reduced energy cost very significantly, was the innovation of the pressure recovery system. In seawater desalination plants, salts are separated from the fresh water applying pressure to the seawater which is 60 to 70 times higher than the atmospheric pressure. After the salt/water separation is complete, a great portion of this energy can be recovered, and reused to minimize the overall energy cost for seawater desalination.
Nearly all reverse osmosis plants operated for the desalination of sea water are equipped with an energy recovery system based on turbines. These are activated by the concentrate (brine) leaving the plant and transfer the energy contained in the high pressure of this concentrate usually mechanically to the high-pressure pump. In the pressure exchanger the energy contained in the brine is transferred hydraulically and with an efficiency of approximately 98% to the feed.
High level of automation in RO has also helped reduce overhead costs significantly.
Today, the RO membrane technology, in terms of economic value is highly standardized with respect to size, productivity, durability and useful lifespan.
The present trend of building fewer large capacity seawater desalination plants and co-locate them with power generation facilities has been a great step in cost reduction. Typically, the economy of scale of facilities larger than 20 mgd (million gallons / day) yields additional cost of water reduction in a range of 5 to 10 %. The point to note is the savings are large when moving from small to medium-sized plants and not as important when moving from medium to large. Benefits are not significant for plants larger than 50 mgd.
Other sensitive areas which impact cost (1) membrane fouling is a major issue for sea water desalination by the RO. The cost of replacement of membrane is about 13% of the overall RO desalination cost. Any decrease in membrane life from 5 to 3 years can increase desalination costs by over 3% (2) the concentrate (rejected brine solution from RO plant) management is another sensitive area and it has huge impact on the product cost.
Economies of Desalination
Thermal desalination | ||||
MSF | MED | RO | ||
Investment costs($/m3/day) | 1200-1500 | 900-1000 | 700- 900 | |
Total cost product($/m3) | 1.10-1.25 | 0.75-0.85 | 0.68- 0.82 | |
Assumptions | ||||
Plant capacity | 30, 000 m3/day | |||
Interest rate | 7% | |||
Project life | 20 years | |||
Electricity cost | 0.065 US $/kwh |
Source: Kaufler
Singapore's first seawater reverse osmosis desalination plant, the largest of its kind in Asia, ranks among the most energy efficient ever constructed. It has achieved, the lowest desalinated seawater cost at 0.49 US$ / m3.
Environmental issues
The three main ways that a RO desalination plant causes environmental harm are (1) loss of aquatic life at the intake of sea water to RO plant (2) continuous discharge of the rejected concentrated salt solution, called brine into the sea impacts marine life and (3) greenhouse gas emissions (GHG) from power plant.
Seawater Intake
Seawater desalination plants can receive feed water from different sources, but open seawater intakes are the most common option. The use of open intakes may result in losses of aquatic life when they collide with intake screens (impingement) or are drawn into the plant with the source water (entrainment). The construction of the intake structure and piping causes an initial disturbance of the seabed, which results in the re-suspension of sediments, nutrients or pollutants into the water column.
Brine discharge into sea implications
The desalination plant typically uses three kilograms of seawater to produce 1 kilogram of fresh water. The salt content therefore, gets concentrated in the reject water, which is returned to the sea. In a typical RO plant the salt content in the reject gets concentrated by about 50%. For a feed water with 4% salt (40g/lit) the RO reject carries 6% salt (60 mg/lit).
The high temperature concentrated brine salt solution, returned to sea also contains various residual chemicals used in the desalination process along with heavy metals generated from corrosion of the plant. On an average, the temperature of sea water rises by 6-8 degc in the vicinity where desalination plant effluent is discharged.
The constant discharge of huge quantity of reject streams from RO desalination plants with high salinity, chemicals and high temperature levels can be fatal for marine life.
GHG Emission
The majority of desalination plants in operation (and planned future plants) use energy from fossil fuels or nuclear power. For countries in the Middle East, which have a huge quantity of domestic petroleum sources that seems the obvious choice. Energy generation from both these sources have serious environmental concerns (nuclear power plants do produce GHG indirectly). Therefore, growth in the number of desalination plants could result in an increase in greenhouse gas emissions that contribute to climate change. Globally, the use of renewable energy technology in power plants is becoming popular and hopefully, this concern would be addressed.
In summary, while significant efforts have gone to address cost related issues, the potential environmental impacts of desalination projects need more focus in order to safeguard a sustainable use of this technology.
Advancements in new technologies like Forward osmosis, Low temperature distillation, Low fouling membrane , Graphene membranes are some of the new innovations in the pipeline which are expected to address many of the existing issues in the near future.