Desalination

Approximately two thirds of the earth’s surface is covered with water. We call it Earth, but we truly live on the “Water Planet.” But the media keeps reminding us that there is a growing problem with water shortages throughout the world. How can that be?

As you may know, we don’t have a problem due to a lack of water. We have a fresh water distribution problem. Only 3% of the world’s water is fresh. And of that, approximately 90% of the fresh water is locked up in the ice in Greenland and Antarctica. Further, 20% of the available fresh water is contained in the Great Lakes region of the U.S. The remainder of the world must obtain their fresh water from local sources such as wells (ground water) and rivers or lakes (surface water). And with the growing population in the more arid parts of the world, this puts a tremendous strain on local and regional supplies.

That is unless we can tap into the unlimited supply of seawater. The vast oceans can satisfy mankind’s thirst for water for an eternity. But, as is, seawater is unfit for human consumption and agriculture. Seawater contains a high concentration of dissolved salts that make it toxic to drink and unfit for irrigation.

As far back as the 4th century BC, Aristotle designed a successive series of filters to remove salt from seawater. Later, around 200 AD, sailors removed salt using simple boilers and condensers aboard their ships. Today, we have much more efficient and effective ways to desalinate seawater by distillation and membrane separation. And yet, only 15 billion gallons (about two-tenths of the fresh water consumed in the world) is desalinated salt water. As the world’s growing population continues to put increased pressure on local water supplies, however, the incentive to produce more fresh water from the sea continues to grow.

CHEMICAL COMPOSION OF SEAWATER

Seawater differs in quality from fresh water in that it contains a high concentration of dissolved salts. These salts alter the properties of water. The freezing point of seawater, for example, is -1.8 C as compared to fresh water at 0 C. The salt also causes the density to increase to the freezing point, which drives the circulation mode of the oceans and is completely different from freshwater in lakes.

We frequently refer to the amount of dissolved salt in seawater as salinity. Salinity is roughly the number of grams of dissolved matter per kilogram of seawater. From about 1900 into the 1960’s, salinity was calculated from chlorinity, Cl, as determined by titration with silver nitrate.

Salinity = 1.80655 Cl

Where Cl = 19 parts per thousand.

As of 1978, it became standard to calculate “practical salinity”, S, from measured specific conductance. Note, however, that practical salinity is unit-less and not a SI (Systeme International d’Units) quantity.

Standard Mean Ocean Water (SMOW): S ~ 35

Going forward, a salinity measure in g-salt per kg-seawater is needed that is more accurate than the conductivity-based Practical Salinity.

The chemical analysis of seawater reveals the major ions present for water having a salinity of 35 as indicated in the following chart.

Name

Symbol

gms/kg

% of total

Chloride

Cl

19.353

55.29

Sodium

Na

10.760

30.74

Magnesium

Mg

1.292

3.69

Sulfate

SO4

2.712

7.75

Calcium

Ca

0.412

1.18

Potassium

K

0.399

1.14

Total

34.928

99.79

From this we see that of the major salt ions dissolved in seawater, about 86.2% is sodium chloride. The remainder is chloride and sulfate salts of calcium, magnesium and potassium.

We can also refer to the elements in seawater as either conservative or nonconservative. Conservative elements are non-reactive meaning that they remain in the ocean for long periods of time. These include sodium, potassium, sulfur, chloride, bromide, strontium and boron. Non-conservative ions are those that are biologically or chemically reactive such as carbon, phosphorous and iron.

DESALINATING SEAWATER

Two methods are commonly used for desalination: membrane separation and distillation. As of June 2011, 15,988 desalination plants were in operation worldwide. The total global capacity for the plants in operation was 17.6 billion gallons per day. As mentioned previously, this represents approximately 0.2% of the global demand for fresh water.

At present, reverse osmosis represents almost 60% of installed capacity with multi-stage flash distillation at 26% and multi-effect distillation at 8.2%. The remaining processes include electrodialysis (a membrane separation process) at 3.4%, hybrid processes that incorporate membrane separation with power generation at 0.7% and electrodeionization at 0.4%.

The cost to produce fresh water from seawater is a critical factor. It is still less costly for municipalities to obtain water from ground and surface water sources. The cost for desalinating seawater are generally higher when energy, infrastructure and maintenance of the desalination plant are factored in. Surveys in 2013 indicate that costs range from $2 to $4 per 1000 gallons.

The energy required for desalination is significant. The minimum theoretical energy consumption is 3.79 kilowatt-hours (kWh) per 1000 gallons, which does not include supplemental pumping requirements. The theoretical rate is not achievable in actual practice, however. In some plants, the energy consumption can be as low as 11.4 kWh per 1000 gallons. But this is much higher than drawing from local fresh water supplies that use 0.76 kWh per 1000 gallons or less. Overall, reverse osmosis (RO) consumes less energy than other desalination processes like multi-stage flash distillation, multi-effect distillation because no thermal energy is needed.

Because of the energy required to produce potable water from the sea, desalinated water is not a viable alternative for water-stress regions that are far inland or at high elevation. This includes some of the parts of the world that have the greatest problem obtaining sufficient fresh water. On an equivalent basis, one can pump water nearly 1000 miles or lift it 6,600 feet before the cost equals the desalination cost.

Another hurdle that must be cleared for desalination plants is the impact the process has on the environment. In 2011, a court ruling under the Clean Water Act decreed that ocean water intakes are not acceptable without reducing the impact on marine life. This requires that alternatives be employed such as beach wells, which consume more energy at higher installed costs.

The desalination process produces large volumes of concentrated waste. Reverse osmosis, for example, produces two streams: purified drinking water and a concentrated waste. The concentrated waste stream may contain, in addition to the salts removed from the seawater, residual cleaning and pretreatment chemicals, reaction byproducts and certain dissolved metals. Therefore, finding an environmentally acceptable method of disposal is an ongoing concern.

The concentrated brine is denser than seawater. If discharged directly into the ocean, this brine sinks to the bottom. Here it can remain long enough to damage the ecosystem. One option is to mix the concentrated waste brine with a more-dilute flow of water such as the treated water from a wastewater treatment plant. Some plants install a diffuser system in a mixing zone on the ocean floor that minimizes the concentration and residence time of the brine as it is re-introduced into the ocean.

CONCLUSION

It is clear that desalination will play an ever-increasing role in producing clean, fresh water to satisfy the needs of a growing world-wide population. The responsible production of fresh water from the sea presents many challenges, however, as engineers and scientists work to find ways to minimize energy consumption and reduce the overall impact desalination has on the environment.

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