Desalination, Water And Climate Change

Desalination refers to the process of separating water from salt in sea water and brackish water. In essence, it is used to obtain freshwater from the vast resources of the sea.

The global ocean covers 70% of all land on Earth and constitutes 97% of all water resources. That’s quite a big figure which makes oceans a viable source of water.

With climate change, the impacts on water are quite significant. Climate change caused droughts are expected to increase water scarcity in many places, by drying out the land. This will affect surface water, because of high evaporation rates caused by consistent rising temperatures. Again, climate change causes changes in precipitation with rainfall quantity, distribution and seasonality affected. All this means that rainfall is no longer a very reliable source of freshwater.

Climate change affects snowpack, with lesser amounts of snow in the cold months, which also melts earlier in the hot months of spring and summer. Snowpack is what feeds rivers in these months. And, because of the changes in climate, more water falls as rain and not as snow.

All this affects freshwater availability especially from surface water. Reduced stocks of surface water (lakes, rivers, dams) cause more exploitation of groundwater, which also faces depletion.

This is where desalination comes in. Desalination is one of the adaptation approaches to climate effects in the water sector.

Desalination is not affected by climate change directly, because the source of water is the ocean not rain. It is however affected by energy prices, which are in turn affected by climate change in many ways.

In light of increasing population sizes and increased demand for water in the present and future, and also because of climate change effects, desalination is becoming part of the mix of the solution in the water sector.

It can be used as a source of drinking water supply and for a few other uses like industry or targeted tourism. It however cannot be used in large scale like irrigated agriculture.

How does it work?

There are many methods of removing salt from seawater but there are two broad categories. One is membrane technologies chiefly reverse osmosis, and thermal desalination is the other.

Both can be carried out on sea water which is more concentrated (30,000 –50,000 mg/L of salt), but reverse osmosis is preferred for brackish water (below 10000 mg/L of salt). Brackish water is found near river mouths (estuaries) and is more diluted by the freshwater from rivers. Brackish water is also the habitat for unique flora and fauna like mangroves. Seawater is just directly from anywhere in the sea.

Osmosis refers to the natural process of water molecules moving across a semi permeable membrane to dilute a salt and form a solution.

In reverse osmosis, instead of water flowing into a salty solution to dilute it, it is forced to flow out of the said solution, in this case seawater. Reverse osmosis forms 2/3rds of all desalination capacity globally. It uses electricity as its energy source and is much more design intensive than thermal technologies.

By design intensive, this means that it is built with slightly more chambers to accommodate its step by step process.

A RO desalination plant. Image courtesy.

It works well in waters that are not as salty (less saline) and that are cooler. Water is first drawn in from the sea (intake), filtered to remove debris, and then pretreated with certain chemicals to remove contaminants and biological material. It is then directed to chamber where it is forced by a pressure higher than osmotic pressure through a rolled up semi permeable membrane. The salts are prevented from going through the membrane while water passes through.

This water is then collected on the other side. After the entire process, this product water is again treated by chemicals to harden it (it’s usually soft), and disinfected by agents such as chlorine and the pH restored to alkaline (high pH) in order to make it consumable. This is called posttreatment.

SWRO (seawater reverse osmosis) has some drawbacks in that it requires frequent cleaning of the membranes because biological agents cling to it, and form a film hampering with the efficiency of the membrane.  This is called biofouling. The method is affected by boron content, organic matter and dissolved solids. It also needs more chemicals for the pretreatment phase to remove all this, and more of labor and careful maintenance.

When sea water is warmer or hotter, there’s more potential for algal blooms which further contaminate the water and make it more difficult to separate. This requires more pretreatment in order to remove these agents and prevent them from plugging (clogging) the membrane.

The same is the case in places of high turbidity e.g. at river mouths because of high nutrient level and dissolved solids. In these places, brackish water reverse osmosis (BWRO) is used which is a bit more detailed.

SWRO is used in clearer and cooler waters and also in places which are a bit inland.

Reverse osmosis has more benefits in that it is able to recover 40-60% of freshwater from the sea water and 50-90% from brackish water.

In the end, it removes viruses, bacteria, dissolved organic solids and mineral salts. Because of this, this technology can be used in treating wastewater too.

SWRO is cheaper at higher capacity, which makes it ideal in many cooler countries.

Thermal desalination refer to multistage flash desalination (MSF) and multieffect distillation (MED). After the initial stage of filtration of water, these two basically work by heating water in a series of big chambers under decreasing pressure, causing evaporation. When this occurs, the vapour is captured and condensed to form freshwater. After collection, this freshwater also undergoes hardening using minerals such as calcite (CaCO3) or lime, and the pH raised in order to make it drinkable.

Thermal methods are quite energy intensive because they use heat more than electricity. They also require more heat resistant materials which can be expensive. These materials, like titanium, also needs to be resistant to corrosion. These materials are more expensive than the membranes used in RO technology. Because evaporation leaves solid “gunk” behind, antiscalant chemicals are needed to remove these “scales.” This adds to the cost. However, thermal methods are simpler and can be used anywhere by the sea especially in waters that are highly saline and warmer.  This system also produces water of higher quality than RO. MSF and MSD collectively form a third of all capacity globally.

Thermal desalination is used widely in many countries of the Middle East and parts of North Africa I.e. MENA, because groundwater and surface water is increasingly scarce, there is plenty of cheap energy and these waters are more saline than the Pacific, Atlantic and sections of the Indian Ocean. Higher temperatures in the Arabian Gulf increase evaporation rates of seawater and so increase salinity of the same.

This system recovers 20-30% of water but uses almost twice the amount used in reverse osmosis.

The third method is hybrid desalinationwhich combines RO and thermal, with the first taking up a third and the rest of the plant’s capacity coming from thermal. This type is growing in popularity because the two methods feed off each other and make up for the deficiencies of the other.

Now, all desalination methods result in the production of brine. Brine is the waste product once the water is separated and freshwater obtained. Brine from membrane plants is little but more concentrated while thermal plants release warmer and larger volumes of brine.  Known as “concentrate”, brine contains a high level of chemicals used in the pretreatment and cleaning phase, heavy metals and all the waste left behind. It is poisonous and toxic to marine organisms and because it is discharged into the sea is a big environmental hazard.

Brine is more dense and settles directly onto the sea floor immediately making the entire area more saline. This negatively interferes with organisms at the lowest level and hurts the ecosystem.

Some of the solutions to this includes diluting brine with treated wastewater (wastewater is also discharged into waterbodies and the sea), or with water flows used to cool power plants (also discharged into the sea). A sanitary sewer or subsurface discharge can be employed.

Another way is to use a diffuser, which is a pipe with many holes, to spread the brine over a larger area rather than discharging it at one point.

There are possibilities for repurposing brine for other uses e.g. to make sea salt, to extract metals and minerals commercially and for use in the fishing industry.

Another environmental hazard is the destruction of marine organisms which are caught up by the force of the water intake. These creatures can be stuck against the intake chamber (impingement) or sucked into it (entrapped), unable to swim out, which would be against the current.

Desalination plants however can only be built by the coast, not too far inland. They therefore supply water to coastal regions. Geographically, they need to be at the same level as the area they are supplying water to or a little bit higher because this lowers the cost of pumping and distribution. Pumping water uphill further adds to the high cost of this type of water extraction.

Because of its high cost, this method can only be used as part, not whole of the water supply solution.

Desalination however affects climate change because it is very energy intensive and can only be carried out in highly energy secure countries like in the Gulf. It can compete with other energy uses. Thermal desalination especially uses fossil fuels to provide heat energy and so has a considerable carbon footprint, releasing quite the amount of greenhouse gases.

Reverse osmosis also uses electricity, the majority of which is generated from burning fossil fuels.

One of the mitigation measures of this is building thermal desalination plants near power plants so that the waste heat from the power plant can be used in the water plant. This is known as colocation.

Another way is to build solar power plants near reverse osmosis plants so that the water plant uses the renewable electricity from the solar plant. It can also use wind power for the same.

There are also many other methods of desalinating water that are in development and others used in small scale. Such include forward osmosis, electrodialysis, electrodialysis reversal, electrochemical desalination, capacitive deionization, dewvaporation, adsorption desalination, membrane distillation and carbon nanotubes and many others.

Desalination lies under the technology part of the response to climate change as reported by the IPCC and covered by climate change agreements.

Environmental regulation for such plants is yet to be fully standardized globally, but different regions have their own framework. To be very clear, because of the harmful effects this technology has on the natural ecosystems and marine life, it has to be regulated and enforcement mechanisms put in place.

By itself, desalination cannot solve the entire issue of water scarcity, but is increasingly becoming one of the several methods. It can produce hundreds of thousands of cubic metres of water a day.

Continuing research and improvements also continue to lower the costs of this method.

In terms of health, a reported concern is the lack of certain nutrients or minerals in desalinated water which leads to deficiencies.

Desalination is one of the responses to climate change and water, because adaptation is now a must. But by far the most consequential action would be to cut emissions towards a low carbon pathway.