Country Profile – Israeli Desalination Plant Strategy
Israel is a leader in designing, building and operating desalination power plants. The climate in Israel is very dry with a low amount of rainfall, which means access to potable water is very limited. Due to its geographical location, Israel has an abundance of salt water that it can covert using desalination into drinkable water. In addition the country also has access to cheap supplies of coal, oil and gas, which makes the desalination process cost effective.
In 1999, the Israeli Government initiated a long term, large-scale desalination program based on reverse osmosis technology. The reason for this decision was due to large periods of droughts during the mid 1990s. Having gone through a requirements phase, it subsequently revisited targets and decided to push for fresh water capacity of 750 million m3 by 2020.
Summary of Key Facts – Hadera Desalination Plant
As of 2012, desalination contributes 349 million m3 of potable water to Israel, with the Hadera plant currently providing the largest amount (127 million m3), which is currently about 20% of the total requirement. This plant which was completed in December 2009, is to date the largest salt-water reverse osmosis (RO) plant in the world. However another RO plant is currently being built at Sorek, Israel and when complete (end of 2013), will overtake Hadera as the biggest in the world.
The Hadera plant is about 50km from the capital Tel Aviv and situated along the Mediterranean coast. It has the ability to produce about half a million cubic metres of potable water per day. The plant takes in seawater that is firstly pre-treated, and is then pushed through fine pored membranes under high pressure. In post-treatment water is adjusted for pH levels to make sure it is suitable for drinking.
The plant supplies water at a cost of $0.57 per cubic metre. It is operated by IDE Technologies and Shikun & Binui, for a period of 25years.
Hadera Desalination Plant and the Environment
The Hadera plant uses significant amount of electricity, with most of this energy being supplied from the nearby Orot Rabin coal fire powered station. From this point of view, the desalination plant doesn’t get a high score for environmental sustainability. However the plant uses state of the art technology and energy recovery systems, which mitigate the fossil fuel supplied electrical energy.
Hadera desalination plant uses the latest ERI PX Pressure Exchanger devices, which operate at high efficiency and also cost less electricity to run. For a similar sized RO desalination plant these PX devices reduce energy cost the exchanger used by approximately 60% (700MW) and saves an equivalent 2.3m tonnes of CO2 per year.
The desalination plant can be further improved by making sure electrical energy is sourced from renewable technologies. A Solar PV farm would complement a desalination plant very well, as shown by similar projects being operated in Saudi Arabia.
There is an estimated 300m miles3 of water present on the earth. Of this, 96% is found in oceans, another 2% of water is tied up in glaciers and ice caps, and 1% sits in the atmosphere. This leaves only 1% of the water present on earth available for human and animal consumption, and even a large percentage of this is inaccessible. Demand is growing as a direct result of an increasing population and increased economic development, and has tripled over the last 50 years alone. To cater for this increasing demand desalination may be the only answer in the years to come.
What is desalination?
Desalination describes a range of processes which are used to reduce the amount of dissolved solids in water. It is most often used to describe the process of converting salt water (e.g. sea water) to fresh water that can then be used for drinking (potable water) and irrigation. Used for sometime on ships and submarines, this process now has new focus to provide fresh water for human use in areas where it is currently limited.
Large scale desalinisation plants tend to use large amounts of energy to produce the water as well as costly infrastructure, therefore when compared to drawing fresh water from rivers and groundwater, desalinated water is very expensive. However, you tend to find desalination plants associated with electricity generating plants, from which electricity and waste heat are readily available making them more cost effective. This combined use of resources is explored more in CHP cogeneration.
The quantity of dissolved solids in a liquid is known as total dissolved solids (TDS) and is measured in mg/l. Typically sea water has a TDS value of over 30,000mg/l, while drinking water sits within the range of 0-1000mg/l.
What are some of the desalination techniques?
There are many techniques involved in desalination, each with their own advantages and disadvantages, but broadly speaking most techniques sit within two camps.
1. Thermal distillation – evaporating the pure water from salt water using heat. The processes that fall in this category are as follows and explored below in more detail: a) multistage flash distillation, b) multiple effect distillation, c) vapour compression and d) solar humidification-dehumidification
2. The use of semi permeable membranes – processes explored that fall in this category are reverse osmosis and electro-dialysis reversal.
Desalination using Multistage Flash Distillation
Approximately 85% of desalination worldwide is completed by multi-stage flash distillation (MSF) – this is a type of thermal desalination. This process involves distilling sea water by flashing a portion of the water into steam in multiple evaporating chambers (known as stages) of what are essentially counter current heat exchanges. The process is as follows.
- The sea water enters the system, and via heat exchanges it is heated as the water travels through the top of each of the main evaporating chambers (stages).
- Eventually, once the water has travelled through all of the stages, it enters a brine heater which heats it up further to the optimum temperature for the process to take place (heating it first in the evaporating chambers limits the energy required by the brine heater).
- This water then enters the first stage, where lower ambient pressure causes the seawater to instantaneously boil, releasing heat and water vapour until reaching equilibrium with the conditions in the chamber.
- This water then enters the next stages one after another, where successively lower ambient pressure in each of these causes the seawater to again instantaneously boil as soon as it enters without reheating each time.
- The vapour from each of the stages is condensed against the heat exchanger tubes (with the heat being used in step 1), creating the freshwater which is pumped away and then the remaining brine enters the next stage and this process gets repeated.
- The sea water increases in salt concentration from stage to stage as distilled water leaves the solution. This can simply be pumped back into the original source.
Desalination using reverse osmosis
Another popular method for desalination is reverse osmosis, which involves the use of a semi-permeable membrane. Osmosis is a phenomenon used by plants to absorb water and move it within the plant itself.
Osmosis involves the movement of a solvent across a semi-permeable membrane into a solution of higher solute concentration. It results in equilibrium being met, where the two solutions on each side of the membrane have equal solutes. The difference in the concentrations is known as osmotic pressure, and the higher this is, the quicker the solvent will move. This process can be reversed if the pressure applied to solution with the greater solute concentration is higher than the osmotic pressure.
In reverse osmosis, pressure is applied to the feedwater, forcing the water molecules through a semi-permeable membrane. The water that has passed through the membrane leaves the unit as product water, and most of the dissolved impurities remain behind and are discharged in a waste stream.
There are however major problems associated with this, firstly the process is very slow, and the membranes are very delicate so can tear easily. In addition the water needs to be filtered first so that large particles don’t damage the membrane, and additives may need to be added to prevent build up of salts on the membrane.
These two methods account for most of desalination that takes place across the world. There are other methods which are described very briefly below.
Desalination using Multiple Effect Distillation
In multiple-effect distillation, evaporators are situated in series, so the energy in the steam from one series is used to evaporate water in the next one. The saline water is usually applied to evaporator tubes in the form of a thin film so that it will evaporate easily.
Desalination using Vapour Compression
The technique of vapour compression uses a mechanical energy source, such as an engine or electric motor, to power a compression turbine. The feedwater is evaporated and the turbine compresses this raising the temperature of the exhaust vapour. The vapour is then passed over a heat exchanging condenser, where it returns to the liquid state as product water. The heat removed during condensation is returned to the raw water to assist in the production of more vapour.
Desalination using Electrodialysis Reversal (EDR)
In EDR, an electrical current is used to separate out salt and impurities in the intake water. Most of the impurities in water are present in an ionized (electrically charged) state and will conduct electric current. When direct current is applied, the impurities migrate towards the positive and negative electrodes; these ions are pulled through a semi permeable membrane resulting in two streams, a desalinated stream (which is tapped off as potable water) and a salt water stream. These membranes can become blocked by ions and other impurities; however by reversing the current the solutes that attach themselves to the membrane dissolve back into the water, so this combats efficiency reductions. However, this process is only possible for brackish water; it does not work effectively on purifying seawater. Click here for an EDR example.
Desalination using Solar Humidification-Dehumidification Method
This process mimics the natural water cycle, however takes place over a much shorter period. The simplest example is using a solar still, where the sun enters a glass covered box heating water held in the bottom of the box (which is black to absorb more heat). This then causes the water to evaporate, and this then condenses on the glass cover where it gets collected. More sophisticated designs separate the solar heat gain section from the evaporation-condensation chamber. An optimised design comprises separated evaporation and condensation sections.
Thermal distillation vs semi permeable membranes for desalination
- The performance of distillation technologies is relatively unaffected by feedwater salinity
- Distillation technologies have a higher total energy consumption than membranes
- For low salinity feedwater, membrane technologies have a higher rate of recovery
- Reverse Osmosis requires pre-treatment before the desalination process, in which all solid and suspended particles are removed, its pH value is adjusted and appropriate chemical inhibitors are added to ensure scale deposits
- Distillation suffers from higher rates of corrosion and scale formation due to the higher temperatures involved.
Desalination industry development
The scarcity of fresh water is already critical in many arid regions of the world and this will increase in importance in the future. It is also highly likely that the availability of fresh water (along with fossil fuels) will be a determining factor in world stability in the future.
According to the International Desalination Association in 2009 there were over 14,000 desalination plants, producing 60million m3/day of potable water. Approximately 70% of desalinated water is currently produced and used in the Middle East, the largest plant currently in operation is the Jebel Ali Desalination Plant producing 60million m3/year, and this is a MSF plant.
Closer to home, during 2010, Thames Water opened a £250m desalination plant in Beckton, East London. The plant is the first of its kind to be built in the UK, and will be able to supply 150m litres of potable water each day. The Beckton plant is run using 100% renewable energy and uses Reverse Osmosis to produce the drinking water. It is due only to be used in times of drought and to maintain supplies in the event of an incident at another water treatment facility, but will be able to service 400,000 homes in London and the surrounding areas.
We have to wait and see whether desalination will prove to be profitable for investment in the UK, given that recent water shortages have subsided.
Background – Beckton Desalination Plant
Thames Water first submitted plans to build a desalination plant in June 2004, however despite receiving approval from the London Borough of Newham, the then Mayor of London, Ken Livingstone, directed the council to reject the application because he claimed desalination was ‘unnecessary and unsustainable for the UK’
In 2006, Thames Water appealed this decision and took the case to court, which they won. Mr Livingstone again appealed this decision, however when Boris Johnson was elected shortly after this appeal he dropped the legal action.
In 2008, construction commenced on Beckton Desalination Plant by Acciona Agua and was completed in June 2010, when it was handed back to Thames Water for operation.
The argument for a desalination plant in London was based largely on London being categorised as ‘seriously water stressed’ and with the 8.5m population expected to increase by an estimated 700,000 by 2021, Thames Water felt that in times of drought, the current water infrastructure would simply not be able to handle the increased demand. Beckton is therefore not going to be in operation continuously; only in times of drought to help make up the shortfall that would otherwise result in a loss of supply to customers.
Summary of Key Facts – Beckton Power Plant
The plant itself operates using a four stage reverse osmosis process, firstly the water is taken from the Thames river. The water is collected when the tide is going out in an effort to minimise the salt content. This water is then held in large storage tanks where a coagulant is added which causes large suspended material within the water to drop to the bottom of the storage tanks.
Once any large pieces of debris have been removed, the water is passed through a fine sand filter, which removes smaller particles, readying the water for the reverse osmosis part of the filtration process. The water is then passed in turn through each of the reverse osmosis filters, where pressure is increased each turn to force more water against the osmotic gradient. Once the clean water comes out of the other side of this process, minerals are added, that have been removed during the process.
In the Beckton plant there is a total of 10,460 membranes used throughout the entire reverse osmosis process. One of the major issues with this type of plant is the damage to the membranes due to the high pressure being used. However in the case of this plant, which won’t be used on a continuous basis, this should be less of an issue.
Desalination is relatively costly way of producing potable (drinking) water; it is certainly more costly than treating wastewater; treating seawater with desalination currently uses about 3kWh of energy to transform 1 cubic meter. Beckton is treating a mix of seawater and river water, so it uses about 1.67kWh to transform 1 cubic meter. To transform wastewater into drinking water currently uses 1kWh for transforming the same volume. However, there is still a stigma attached with treating wastewater which Thames Water were looking to avoid.
The plant itself can produce about 150 million litres of water per day, which is enough to supply 400,000 houses in North London. Some of the energy needed to run the desalination plants is obtained using biofuels to help minimise the environmental impact of the plant.
UK Desalination Plants – Next Steps
Some critics of the scheme highlight Thames Water and other water utility suppliers have incredibly high rates of leakage (Thames Water – 25.7%), so the money would be better spent on fixing their existing infrastructure. While in the longer term it is key that the leakage issue be addressed, it is paramount to keep customers in a constant supply of water, so plants like Beckton may become more commonplace across the UK.