Nowadays, every third person has no access to clean drinking water, and this is no longer a problem that only affects developing countries.
Sea water is abundant, but not drinkable because it is saturated with salt. With fresh water it is the other way round: it can be drunk, but availability is very limited. It is estimated that only one percent of it is available to us – the rest is trapped in glaciers or deep underground. Therefore, it is impossible not to ask why we do not simply turn seawater into potable water.
Is the desalination of sea water the answer to the water crisis?
The ocean accounts for 70 percent of the earth’s surface and 96 percent of the planet’s water (more about water crisis ). The problem is that this water cannot be consumed. It is saturated with salt. Desalination converts the salty sea water into drinking water.
If 783 million people do not have access to clean water and more areas are affected by severe droughts, could desalination be the salvation?
Countries like Israel, the Gulf states or countries in Africa with little or no natural freshwater resources depend on the desalination of seawater to produce drinking water. Israel, for example, obtains 40 percent of its industrial water from desalination. These countries still have hardly any groundwater or freshwater sources. In addition, they account for one percent of the world, which is currently dependent on desalination to cover the demand for water.
The supply of fresh water is also problematic in many coastal regions or in South American countries. If freshwater shortages, environmental pollution and climate change make water supply more difficult in the future, desalination of water (sea water, salty groundwater, brackish water) will become increasingly important.
Over the last five years, the capacity of continuous desalination plants has increased by 57%. This is demonstrated by the latest data published by the International Desalination Association (IDA) as well as by Global Water Intelligence (GWI).
The base of desalination plants installed worldwide today has a capacity of 78.4 million cubic meters per day (19.8 billion US gallons), compared with 47.6 million cubic meters per day (12.6 billion US gallons) at the end of 2008 Issue of IDA / GWI Worldwide Desalting Plant Inventory.
The growth of the desalination market reflects the fact that coastal communities are increasingly turning to the sea to meet their drinking water needs, while inland groundwater becomes increasingly brackish over time. Approximately 60% of the desalination capacity is used for sea water. The rest treats brackish and less salty water.
However, the technology is not uncontroversial. The desalination plants are regarded as energy-intensive and because of the highly saline and (chemically) contaminated wastewater also as environmentally harmful.
Critics therefore do not necessarily regard desalination of water in large plants as a suitable instrument for counteracting water scarcity. Rather, they rely on the economical use of water (especially in agriculture), on modern water management, the rehabilitation of leaky water networks or the treatment of wastewater. However, the desalination of water in arid regions is currently the only available method of securing the freshwater supply for the population.
However, the UN predicts that 14 percent of the world will depend on desalination by 2025 to meet its water needs.
How does desalination work?
In short, salt water is converted into drinking water during desalination. In principle, sea water is converted into potable freshwater.
Sounds promising at first. What’s the catch?
Desalination is an industrial process, which requires a lot of resources in terms of chemicals, energy and processing. Due to the focus on large plant types, it often involves considerable capital investment, engineering and infrastructure. The associated costs and potential environmental impacts continue to stand in the way of widespread use of this technology.
There are various ways to remove salt from the water. Reverse osmosis (RO) and distillation are the most common methods of desalinating water. Reverse osmosis water treatment forces water through small filters and leaves the salt behind.
Industrial distillation involves boiling water and collecting steam during the process. Both require a lot of energy, infrastructure and are expensive.
What about the environmental relevance? Are the processes only expensive, or also harmful to the environment?
In addition to the product fresh water, all desalination processes generate large quantities of a salty residue (concentrate), which can, as a rule, contain chemical residues and heavy metals as a result of corrosion and which flows back into the sea, sometimes mixed with chemicals from the purification stages.
In the marine environment, negative impacts can occur in particular when high desalination capacities collide with fragile ecosystems. The physical and chemical properties of wastewater depend on the desalination process and the operation of the individual plants. Wastewater from thermal plants usually has reduced oxygen values.
In addition to low concentrations of copper and nickel from the corrosion of heat exchanger surfaces, residual amounts of chlorine, which is added to reduce the growth of bacteria, halogenated organic reaction products such as trihalomethanes, as well as so-called anti-calcifiers and anti-foaming agents are often present in wastewater.
These are organic substances such as polymaleic acid used to prevent deposits in pipelines or polyglycol used to reduce foam formation on the water surface. Thermal systems are cleaned by rinsing with acid solutions to which corrosion inhibitors are added.
In contrast to thermal plants with a salt content of up to 80 g/l and unchanged temperature, wastewater from RO plants is heavier than the surrounding seawater and can sink to the seabed in shallow, unmixed coastal areas.
In order to prevent this, diffusers are used in larger plants to introduce the concentrate through a larger number of nozzles. Chlorine is typically added at the inlet of the plant, which is removed again by chemical reaction just in front of the oxidation-sensitive membranes. As with thermal processes, anti-calcifiers (antiscalants) are used.
As the RO plants are constructed of plastic and durable stainless steel in contrast to thermal plants, heavy metals in wastewater are rarely a problem. However, as the fine-pored membranes react sensitively to suspended material, iron salts are used as flocculants and the water is filtered through sand-anthracite beds before it reaches the membranes.
In the past, the sludge resulting from the backwashing of the filters used to be discharged into the sea with the concentrate. In large plants in Europe, Israel, Australia and the USA, this sludge is now generally dewatered and – due to the salt content – disposed of in special landfills in order to avoid turbidity plumes in the sea and the resulting possible environmental effects.
The cleaning solutions for the membranes are either acidic (pH 2-3) to remove calcium deposits and metal oxides or alkaline (pH 11-12) against biofilms and may contain detergents, oxidants, complexing agents and biocides to improve the effect. These are partly comparable with substances used in conventional household cleaners, but the quantities applied locally are much higher than in standard household application.
Depending on their composition and the environmental legislation, the cleaning solutions are either treated or discharged untreated into the sea often with devastating effects on the ecosystem.
In addition to environmental toxins, there is also the need for energy as an environmentally relevant factor. The energy demand is so high that the costs are too high for many countries. Therefore, it is mainly used in regions without freshwater, on ships and military ships. Furthermore, the whole process is still very expensive compared to the use of freshwater sources. In 2005, for example, Israel invested in a large desalination plant to supply half of the country. The construction of desalination plants is very costly ($1 billion for the largest plant in the U.S.), but is a safety net for places where droughts persist and freshwater is limited or completely absent.
How can desalination become more environmentally friendly?
Desalination can only be a good option to solve the water crisis if renewable energies are used, costs are reduced and environmental protection measures are also taken for marine life.
For example, Saudi Arabia has promoted the use of solar energy to operate desalination plants. In California, the California Coast Keepers Alliance is working with desalination plants on a plan to ensure that marine life is minimally harmed by using techniques that prevent the absorption of water below the surface rather than sucking water from the surface where marine life predominates.
The following sources were used for this blog post
GWI and IDA (2007): Desalination markets 2007, a global industry forecast (CD Rom) und IDA worldwide desalting plant inventory, no. 20 in MS Excel, Global Water Intelligence, Media Analytics Ltd., The Jam Factory, Park End St, Oxford OX11HU, United Kingdom, www.globalwaterintel.com.
SHANNON M., P. BOHN, M. ELIMELECH, J. GEORGIADIS, B. MARINAS & A. MAYES (2008): Science and technology for water purification in the coming decades. Nature, 452(20): 301–310.
LATTEMANN S. (2010): Development of an environmental impact assessment and decision support system for seawater desalination plants (PhD thesis), CRC Press/Balkema. http://repository.tudelft.nl/. 276 pp.
NRC (2008): Committee on Advancing Desalination Technology. Desalination: A national perspective. Water Science and Technology Board, National Research Council of the National Academies, National Academies Press, Washington D.C.
WILF M., L. AWERBUCH, C. BARTELS, M. MICKLEY, G.PEARCE & N. VOUTCHKOV (2007): The guidebook to membrane desalination technology. Balaban Desalination Publications, L’Aquila.