By Samantha Northcott and Connor Greenwood (edited by DocOnMontereyBay)
The Monterey Peninsula hosts millions of tourists every year who come to enjoy the sights and attractions of our beautiful ocean coastline. However, visitors have no idea Peninsula residents are facing a looming water crisis. By January 1st, 2022, the we must replace roughly two thirds of our primary water supply from the Carmel River with alternative sources (click HERE for graph of Monterey Peninsula’s Water Supply Gap). One solution to this dilemna is seawater desalination.
For over one hundred years, the Carmel River has served as the Monterey Peninsula’s main water supply. But as the community grew, so did our demand for water. Today, steelhead trout struggle to migrate upstream and spawn. Red-legged frogs are forced to abandon dry river beds in summer. As a result, both frogs and fish are now protected under the U.S. Endangered Species Act. In order to restore these species to sustainable populations, the community must look elsewhere for water. By law, we have no choice.
For coastal communties like ours facing looming water scarcity in the future, tapping the ocean as a water supply certainly sees like a logical solution. However, desalintion is expensive. Initial costs to build a plant are huge. Land zoned for industrial use on the Monterey Peninsula is difficult to find and pricey. To date, the Peninsula has had to look toward Marina or Moss Landing for sites to build their plant, which means there are additional costs to pipe water ten to twenty miles back to Monterey (see details of the proposed Water Supply Project HERE).
Removing salt from seawater is energy-intensive. Reverse osmosis (RO) is currently considered the most cost effective method. Even so, nearly half the cost of water generated by RO plants comes from electricity needed to run them (Cooley and Heberger 2013). In the future, electricity prices are expect to rise. As they do, so will the cost of water (for information on water prices from the California Public Utilities Commission, click HERE).
Despite problems of cost and energy consumption, we think seawater desalination could have a place in our water future if it could be made more efficient. Here is how reverse osmosis works. Seawater is forced at high pressure though thin membranes containing microscopic pores. Freshwater molecules are squeezed through these pores while salt is left behind as brine. RO facilities proposed for Monterey Bay have a typical water conversion efficiency of 44%. That means less than half of the seawater processed becomes freshwater. The rest (56%) is left as brine. RO brine has twice the salt of natural seawater. When discharged offshore, it can sink like syrup to the seafloor. Layers of brine above animals living on the seafloor can cause environmental problems not unlike what happened to fish and frogs in the Carmel River .
To address the biological effects of brine, Voorhees et al. (2013) measured salt toxicity thresholds for nine California marine species. Some were found to be more senstive than others to brine. In this study, red abalone, purple sea urchins, and sand dollars were the most sensitive. Because these animals live on the seafloor, they are vulnerable to the effects of sinking brine discharged seawater desalination plants.
Figure 1: Examples of marine species sensitive to brine.
Aside from protecting marine animals on the seafloor, there are other reasons to rethink brine disposal. Consider this: In the 1800’s, Monterey Bay was rimmed by salt ponds contructed for the purpose of drying and selling salt to fish canneries. In fact, Monterey Salt Company, located in Moss Landing, had 200 acres of solar salt ponds before closing down in 1974 (Click HERE for more information). Today, salt prices are increasing in world markets. Thus we wondered if we could not only generate water but produce salt for sale using a better designed, more efficient desalination design for Monterey. Below are three scenarios we considered. Details and the value of salt they could produce a found in Table 1.
Three Desalination Scenarios and their Potential Value in Salt:
1.) Monterey Peninsula needs 10,000 acre-feet of freshwater to replace what will soon be lost from the Carmel River. NOTE: 1 acre foot is defined as the amount of water that covers 1 acre in 1 foot of water. There are 12 million liters in 1 acre foot. To produce that much water using current RO conversion ratios, we would need to desalinate 22,700 acre feet of seawater (or 28 billion liters).
We know seawater in Monterey Bay contains 34 grams of salt per liter. Thus, 28 billion liters of seawater holds 949,310 metric tons of salt. However, this salt is concentrated in 16 billion liters (12,700 acre feet) of brine. Currently, there is no place around the Peninsula for large evaporative solar salt ponds needed to hold this much brine. Thus, recovery of “solar” (or sea) salt becomes impractical in this scenario. But based on 2016 prices for solar salt ($90/ metric ton, USGS 2017), we would be throwing away $85 M of salt. We believe there is strong financial incentive for improving desalination efficiency in order to recovery plant expenses with salt (click HERE for U.S. salt prices from USGS).
Table 1: Three Scenarios. Better desalination efficiency leads to profits in salt.
2.) Because water shortages are a global concern, research is underway to improve desalination efficiency (Morillo et al. 2014). In Japan, for example, Toray Industries developed a two stage reverse osmosis plant that runs brine through a second reverse osmosis unit in order to extract 60% of the freshwater from seawater. If Monterey used 2-Stage technology, less seawater would be needed to produce 10,000 acre-feet of freshwater. That would save electricity because less seawater would need to be processed. The amount of brine would be smaller too, but still too large for solar ponds. In this case, we would leave $63 M behind in discarded salt.
3.) Today are a growing number of ways to desalinate seawater besides reverse osmosis (Morillo et al. 2013). Here, we propose a hybrid combination (2-Stage RO – thermal reduction – solar evaporation) that we believe could recover 80% of the potable water while leaving a highly concentrated brine in a much smaller volume for salt recovery. Our design relies on being located near the Moss Landing Power Plant to take advantage of excess heat energy for brine reduction. Figure 1 illustrates our idea.
As brine leaves the 2-Stage RO facility, it would be piped to the Power Plant where heat produced during electricity genration would be transferred via radiative heating to turn half of the brine into freshwater steam. This steam could be captured with simple condensation traps thus adding an extra 20% more freshwater for municipal use. Meanwhile, brine volume drops dramatically to 2,500 acre feet (see Table 1, line 3). In the Moss Landing area, we believe local evaporation rates are sufficient for solar salt ponds to recover $47 M of salt.
Figure 2: Illustration of Hybrid Design.
Imagine an ocean-friendly desalination plant that produces no brine at all. Envision efficient technology needing less energy and smaller seawater intakes. Envision a water supply that generates valuable salt to offset future costs associated with rising energy prices and plant maintenance.
There are many ways we can solve a water shortage. What are your ideas? What would you do differently?
Cooley, H and Heberger, M. 2013. Key Issues for Seawater Desalination in California: Energy and Greenhouse Gas Emission. Pacific Institute. Oakland, CA. Online document HERE.
Morillo JCA, Usero J, Rosado D, Bakouri HE, Riaza A, Bernaola F-J. Comparative study of brine management technologies for desalination plants. Desalination. 2014;336:32–49.
Voorhees JP, Phillips BM, Anderson BS, Siegler K, Katz S, Jennings L, Tjeerdema RS, Jensen J, Carpio-Obeso MDLP. Hypersalinity Toxicity Thresholds for Nine California Ocean Plan Toxicity Test Protocols. Archives of Environmental Contamination and Toxicology. 2013;65(4):665–670.
USGS (United States Geological Survey). 2017. Statistics and information on the worldwide supply of, demand for, and flow of minerals and materials essential to the U.S. economy, the national security, and protection of the environment (Salt). Online document HERE.