Blog

Embracing Renewable Desalination

There has been a lot of discussion on the topic of desalination in California lately as a source of water to combat drought (see News10 ABC “drought buster”) and it’s apropos to the ongoing dialogue about the environmental consequences typically associated with desalination, namely what to do with the salt?

It’s important to first point out the recent technology advances at play for desalination, as well as the growing opportunity to handle and even monetize the salt and mineral byproducts, between conventional seawater desalination and the type of solar desalination being demonstrated by WaterFX at the Panoche Water & Drainage District. Seawater desalination most commonly relies on reverse osmosis (RO), which requires high pressure to drive freshwater through a membrane. Up to 1,000 psi of driving force is required to overcome the natural osmotic pressure of ocean water. Because of this, the practical limit for RO seawater desalination is 50% water recovery – for every 100 gallons of water processed, 50 gallons of freshwater are recovered and 50 gallons of brine are disposed. To exceed 50% recovery requires increasing the pressure of the system, which from a practical standpoint makes the equipment too costly and substantially increases the electricity consumption. The result is a large volume of high salinity brine that must be sent back to the ocean and as many environmentalists correctly point out, this can have negative consequences to the local marine ecosystem.

Solar desalination, like the Aqua4™ Concentrated Solar Still in use today at California’s Panoche Water District, does not use RO to desalinate seawater and instead uses evaporation (referred to as multi-effect distillation) to clean brackish drainage water. As a result, rather than only 50% recovery, the process achieves greater than 93% recovery. This means that for 100 gallons of water treated, only 7 gallons of salt brine are produced. In addition, the concentration of this brine is greater than 200,000 ppm of total dissolved solids (20% salt by weight). With this high concentration, solids can then be precipitated out of solution using salt separators and processed for a variety of uses. This is something that is not economically feasible with RO seawater desalination due to the high volume of brine. It is also important to note that agricultural drainage water has an inherent disposal cost, so there is substantially more economic incentive to treat this water in its entirety.

In phase two of the solar desalination project with Panoche (to be completed in September 2014), we plan to demonstrate the types of solid byproducts that can be produced and sold into the market without harming the environment. For instance, gypsum (a calcium based salt) is one example of a valuable compound that can be recovered. It is used to make drywall and plaster, so with a concerted effort, the Central Valley could use a portion of the salts as locally sourced raw material for the built environment. Magnesium salts are also present in the drainage water and compounds such as magnesium sulfate (epsom salt) are used in the medical industry to treat pain and complications during pregnancy. It is also true there are naturally occurring materials like selenium that in high concentration are toxic, but if dried and separated, selenium is a highly valuable commodity. It is a health supplement (critical for life in animals), it is a semiconductor for photo-sensors and it is also a key ingredient for glass making. There are nitrates present in drainage water that can be recycled back into fertilizer for growers and calcium compounds that can be used to make cement. Boron is another highly valuable material present at 50 mg/L in drainage water and if extracted sells for $5,000/kg. For a small solar desalination plant this could generate $2.1M per day of revenue. As for motor oil and antifreeze, we have not observed these compounds in the drainage water specifically, but many natural water bodies will contain trace amounts of a variety of these commonly used chemicals.

While the Central Valley is not currently a source of these raw materials, there is no reason to think it can’t be. In Canada, potash (potassium carbonate) and soda-ash (sodium carbonate) salts are refined directly from naturally occurring brines and sold into the fertilizer and chemical industries, which support over $7B per year of exports to countries like China. In Israel, over 2 million tons of salts are produced every year from the Dead Sea. A new industry can be created in the Central Valley that not only removes salt from some of the most pristine farmland in the world, but also creates thousands of new jobs.

Solar desalination, if implemented thoughtfully, serves as a new source of water in California, and can alleviate the salt accumulation that is causing thousands of acres of valuable farmland to be fallowed every year. But why hasn’t this “feature” of desalination been implemented before? The answer lies in the electricity consumption required to separate salt from water. Electricity is a costly source of energy for desalination and even more costly if salts are to be fully processed into solid form. Running a desalination plant at high recovery (> 90%), reliably and in a way that doesn’t require significant upfront pretreatment is the key to unlocking the salt removal. By removing all of the recoverable water the cost of post-processing salts is economical. But it is uneconomical if the cost of the energy source is expensive and this is where solar desalination (pun) shines. Using solar energy as the primary fuel source relaxes the total energy requirements for desalination and enables robust, high recovery thermal technologies like distillation to be used. Without tapping into widely available solar energy, scaling up desalination as a solution to the water problem in California could require over 5,000 MW of additional electric power. Conversely, by efficiently harnessing solar energy, we can return hundreds if not thousands of megawatts back to the grid by displacing freshwater imports delivered through energy-intensive pumping stations. As solar desalination scales up, the cost will continue to fall with improvements in process technology and manufacturing.

This transformation won’t happen overnight and it is not without its challenges. But Californians don’t need to look very far to find incredible examples of industries that have sprung up seemingly overnight. In the midst of the worst drought in recorded history we should give renewable desalination a chance and apply the same innovative thinking to our water shortage that has produced so many disruptive industries throughout California’s history.

- Aaron