The Viability of Desalination
by: Wilson Moyer
In the hot, parched suburbs of Carlsbad, California, a water facility works tenaciously to draw up enough water to turn on sprinklers, sinks, and showers for over 3 million people.i On the surface level, the consumption of such a magnitude of water from a single body appears irresponsible: the surrounding area is wrought with drought.ii But the Carlsbad facility is not prone to running out soon, because it is drawing water from the Pacific Ocean. Using oceans as a water source is possible due to desalination, which is the process of converting saltwater to freshwater. This paper explores the energy, infrastructure, and environmental costs of desalination, as well as why it may be a critical technology for alleviating drought in coastal regions.
Desalination begins with ocean water filtering through a plant which purifies the water, ridding it of larger particles which could clog the desalination process. Then, the water is pumped at high pressures through membranes with 0.1μm holes. These holes are so small that they block all of the salt which is later discarded as a brine.1 The concept of desalination is certainly compelling: it’s hard to have anxiety about droughts when the entire ocean is a source of water. However, there are limitations to desalination which are critical to consider while striving for a drought-prepared future.
The energy required to operate reverse osmosis is one of the biggest challenges facing desalination plants. Rather than simply bringing the water up from the ground, as would occur in freshwater pumping, the desalination pump needs to bring the water to high pressures in order to pump it through the tiny holes. Consequently, every cubic meter of water coming through the plant expends about 33 Watt-hours (Wh), or 0.12 Wh per gallon of water coming through the plant. The energy cost associated with desalination is actually quite small: a plant that produces 8 million gallons of water per day would consume about a Megawatt-Hour (MWh) of energy per day. One MWh is equivalent to the daily energy consumption of about 30 households, iii which is a minor cost considering that the plant could serve over 25,000 households’ daily water consumption.iv
It takes energy to pump groundwater as well. However, it is hard to define the energy consumption of groundwater because the energy consumption depends greatly on the depth of the water. Nonetheless, the benefits of reverse-osmosis water desalination likely outweigh the energy cost, especially with new advancements in solar technology which are improving the feasibility of carbon-neutral desalination plants.v
As with any large facility, the implementation of desalination comes at a cost. Building a desalination plant with a capacity of 100,000 m3/day of water (26,000,000 gallons/day) would cost about $50,000,000 and would serve enough water to supply about 80,000 households.vi $50 million is a substantial cost for capital, but in comparison to personal residential wells, it is much cheaper. A personal well to service a single family can cost an average of $9,000, so supplying the same 80,000 households with personal wells would equate to a total cost of $720,000,000, a much larger price tag than the original $50 million.vii The costs decrease for pumping groundwater on a larger scale, but the low cost of desalination is still present. Desalinated water is usually priced between $0.5-1.5/m3 ($1.9-5.7 per 1000 gallons),viii while ordinary water can carry a price tag of between $0.26-1.98/m3 ($1-7.5 per 1000 gallons).ix Desalinated water is very similarly priced to groundwater, further bolstering its viability in a modern sustainable economy. However, a major limiting factor for the economic viability of desalinated water is proximity of the consumers to the coast. It can cost up to $2 million/mile to lay major water pipes, meaning that it is unlikely that desalinated water would be suitable to service households more than a few miles inland.x
Another major factor to consider for desalination is the environmental cost of each plant. Although it is possible to offset the energy cost with renewable power sources, the damages to local ecosystems could also be detrimental to the environment. Desalination plants need to intake large volumes of water from the ocean, which can also suck local wildlife into the pipes. The State Water Resources Control Board estimates that desalination plants can kill nearly 70 billion larvae per year in California. Additionally, the process of treating saltwater leaves behind a waste product, brine. Although there are ways of treating brine to make it less toxic, the current disposal of brine can cause detrimental effects wherever desalination plants dispose of it.xi Overall, the impacts of desalination can result in decreased biodiversity and damages to the ocean’s overall health.
The Need for Desalination
Although the negative impacts of desalination are sobering, converting salt water to freshwater is nonetheless a critical service in California. Aquifers and groundwater supply up to 46% of California’s water needs, but these sources are far from inexhaustible. When consumers draw up too much water from aquifers, the water no longer supports the layers above it, and the land begins to sink in a process known as subsidence. Subsidence damages local infrastructure including flood structures and aqueducts and can hinder the future usability of the aquifer.xii Desalination can circumvent the issue of aquifer drainage, because the oceans are unlikely to run out soon — rather, they are projected to rise due to global warming.xiii Therefore, desalination can reduce overexertion of water sources, and proves to be a critical service in many areas.
In a world wrought with climate change and increasing water uncertainty, desalination is a very tantalizing prospect. Drought will only increase in severity as increased temperatures lead to lower snowpack levels and higher rates of evaporation. However, desalination has a set of costs to overcome. The energy cost, although relatively small, can add up as the demand for desalinated water increases. Monetary costs for building infrastructure will pose a challenge to the widespread implementation of desalination, and pipeline costs will limit the scope of consumers of desalinated water. Finally, the environmental degradation of desalination should not be ignored. With water quality issues already impacting countless lives around the world, the desalination industry must act with increasing caution about the traces they leave behind. Despite all of the negatives, desalination holds promise for people suffering from lack of water globally. As the oceans rise from global warming, sticking some straws into them may just be the way to alleviate water pressures from millions around the world.
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