The k2p blog

There is no shortage of water on earth. There is not even a shortage of fresh water resources. However there is a fundamental mismatch between the availability of fresh water and the centres of population. If sea water (or brackish water) can be converted into fresh water at an acceptable energy and economic cost, the problem vanishes.

Excluding the vast amounts of water bound up within rocks in the earth’s core, the fresh water on earth is less than 1% of all the “free” water. (Note also that when humans consume water, the water is not destroyed. Most of it is discharged somewhat contaminated and a small amount is bound up as hydrocarbons. Water “manufactured” by combustion – whether induced by humans or by natural combustion processes – creates water vapour from bound-up hydrogen but the quantities are not very significant).

The UN estimates that by 2025 up to 15% of the world’s population may be subject to fresh water scarcity. Techniques for conversion of sea water into drinking water have been known for at least 3,000  years (and perhaps even 5,000 years). But desalination as an industrial process for providing fresh water to large populations only started in any significant way in the 1960s and started showing high growth rates from the 1990s on.  There are two basic paths to obtaining fresh water from sea water. Through evaporation followed by condensation (multi-stage flash – MSF) or by filtration (reverse osmosis -RO). Whether as heat for evaporation or pumping energy through semi-permeable membranes, the energy requirements (and cost) have been relatively high. Costs have reduced sharply over the last 30 years and currently the lowest cost of production is at less than $0.5/m³. Note, however, that costs of distribution are in addition to the production cost. The world’s population using desalinated water today is fast approaching 1% (perhaps about 500 million people today). But the growth rate here is currently above 5%/year.

For water scarcity to disappear as a potential problem, the cost to access the water (prior to distribution) needs to be less than about 50% of the cost of distribution. For that situation to arise, current desalination costs have to reduce by a factor of about 20 (production cost < $0.02/m³). It seems unlikely that such a cost reduction can be achieved along the evaporation/condensation path. The filtration path remains the best bet but would require

1. a sharp reduction of the pressure drop across the filtration membrane, and
2. a reduction in the cost of the membrane, and
3. developments in the economic handling or treating of large amounts of the salts and minerals filtered out

The rate of development suggests that it is quite probable that such an advance in filtration technology can be achieved over the next 10 – 20 years. The advent of graphene and the use of graphene oxides to create nano-filters is one path which shows great promise.

 Tunable sieving of ions using graphene oxide membranes, Jijo Abraham et al, Nature Nanotechnology (2017), doi:10.1038/nnano.2017.21

BBC: A UK-based team of researchers has created a graphene-based sieve capable of removing salt from seawater. The sought-after development could aid the millions of people without ready access to clean drinking water. The promising graphene oxide sieve could be highly efficient at filtering salts, and will now be tested against existing desalination membranes.

It has previously been difficult to manufacture graphene-based barriers on an industrial scale. Reporting their results in the journal Nature Nanotechnology, scientists from the University of Manchester, led by Dr Rahul Nair, shows how they solved some of the challenges by using a chemical derivative called graphene oxide.

Isolated and characterised by a University of Manchester-led team in 2004, graphene comprises a single layer of carbon atoms arranged in a hexagonal lattice. Its unusual properties, such as extraordinary tensile strength and electrical conductivity, have earmarked it as one of the most promising materials for future applications. But it has been difficult to produce large quantities of single-layer graphene using existing methods, such as chemical vapour deposition (CVD). Current production routes are also quite costly.

On the other hand, said Dr Nair, “graphene oxide can be produced by simple oxidation in the lab”. He told BBC News: “As an ink or solution, we can compose it on a substrate or porous material. Then we can use it as a membrane. “In terms of scalability and the cost of the material, graphene oxide has a potential advantage over single-layered graphene.”

By 2100 global population will be declining almost everywhere. The water scarcity problem will be solved long before the population pressure reduces the demand for fresh water.

Tags: Desalination, fresh water, graphene oxide, water

This entry was posted on April 4, 2017 at 7:39 am and is filed under Energy, Technology. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.