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As mentioned in Desalting the Sea: Part 1, the Salton Sea is undergoing increasing salinization. Desalination, or “desal” for short, is a commonly proposed option to restore habitat and ecosystem health, and its role in the Salton Sea Restoration and Renewable Energy Initiative has been discussed. Part 1 explained the details of thermal distillation, and this accompanying post will introduce membrane filtration, another common desal technique.

Membrane filtration is used in a wide array of industries, including biotechnology, pharmaceuticals, beverage, food and drinking water applications. The general concept entails using pressure to remove particles from a solvent (like water) through a porous membrane. Depending on the pore size of the membrane, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) or reverse osmosis (RO) membranes, as shown in Figure 1. As water passes through the membrane pores, the pore size determines what constituents get “stuck” and are therefore left behind and removed from the solution that comes out the other side. MF membranes are able to remove suspended solids, bacteria, and some viruses from water. UF and NF membranes can remove all those things, as well as emulsified oils, natural organic matter, divalent ions, and pharmaceuticals. RO membranes have the smallest pores (0.001 – 0.01 micrometers), which gives them the ability to remove monovalent ions and minerals, like salts, thereby desalinating the water [1].

1Figure 1. Classification of membrane filtration by pore size [2]

In the process of osmosis, water naturally diffuses from areas of low solute concentration to those of high solute concentration through a semi-permeable membrane [3]. In contrast, the reverse osmosis process works by applying an external pressure to reverse the natural flow of water across the membrane, as shown in Figure 2. Taking saltwater as an example, this means that we end up with freshwater on one side of the membrane, and a highly concentrated salt “brine” solution left on the other.

2Figure 2. Reverse osmosis flow diagram

As discussed in Part 1, the major concerns with both distillation and membrane filtration process are high energy costs and large brine waste discharge. In 1998, the U.S. Bureau of Reclamation presented a pre-appraisal alternative report that included a design of a reverse osmosis desalting plant with pump-out/pump-in system for remediation of the Salton Sea. It was estimated to require 27.8 kWh of electricity per 1000 gallons of water, or $2.03 per 1000 gallons of freshwater, to desalt 60 million gallons per day [4]. For perspective, a typical ocean water desalination plant operates at about half the cost of this proposed plant, requiring 13.25 kWh per 1000 gallons of water, due to the much greater salt concentration of water in the Salton Sea. Additionally, high concentrations of impurities in the water cause membranes to get clogged or foul over time. Therefore, the membranes would need to be cleaned or replaced regularly keep energy costs down [5]. Another issue was the water pump-in and pump-out locations. Most of the suggested pipeline routes were from the Pacific Ocean or the Gulf of California, which would have tremendous installation costs. Depending on the location, the pumps would need to lift the water over elevations of 82ft to 3636ft and the length of the pipelines could vary from 49 to 165 miles. Additionally, any pipelines flowing back to the ocean from the Salton Sea must overcome the Sea’s low surface elevation of 227ft below sea level [6].

Other concerns about membrane filtration in general include the impact on marine life. Depending on how and from where water is brought into the desalination plant, it may suck in tiny larvae and fish eggs that can trap and kill marine life [7]. This is a concern at the Salton Sea shores, as well as in ocean habitats like the Gulf of California, where the endangered vaquita porpoise lives in protected wildlife refuges.

As the 2017 implementation date of the Quantification Settlement Agreement approaches, the Salton Sea is attracting more attention and new, innovative proposals for addressing its problems. Advanced technologies, including desalination, are exciting and attractive when considering their immense opportunities at the Salton Sea. However, no technology is without its drawbacks and potential for unintended consequences. Desalination technology should be carefully evaluated before deciding its role in the larger, multi-faceted strategy for saving the Salton Sea.

Written by Caroline Kim


 

[1] Safe Drinking Water Foundation. “Ultrafiltration, nanofiltraion and reverse osmosis”. Available from: http://www.hinesburg.org/water-project/safewaterdotorg-info-nano-and-ultrafiltration-reverse-osmosis.pdf

[2] Peng Wu and Masanao Imai (2012). “Novel Biopolymer Composite Membrane Involved with Selective Mass Transfer and Excellent Water Permeability”. Advancing Desalination. Available from: http://www.intechopen.com/books/advancing-desalination/novel-biopolymer-composite-membrane-involved-with-selective-mass-transfer-and-excellent-water-permea

[3] H2O distributors. “How Reverse Osmosis Works”. Available from: https://www.h2odistributors.com/how-reverse-osmosis-works.asp

[4] Bureau of Reclamation. Salton Sea Alternative Preappraisal Report. Available from: http://www.usbr.gov/lc/region/saltnsea/pdf_files/ssafpr/c-6.pdf

[5] Chris Johnston. The Guardian.” Desalination: the quest to quench the world’s thirst for water”. Available from: http://www.theguardian.com/technology/2015/may/27/desalination-quest-quench-worlds-thirst-water

[6] Bureau of Reclamation. Salton Sea Alternative Preappraisal Report. Available from: http://www.usbr.gov/lc/region/saltnsea/pdf_files/ssafpr/c-5.pdf

[7] Daniel Potter. KQED Science “Why Isn’t Desalination the Answer to All California’s Water Problems?”. Available from: http://ww2.kqed.org/science/2015/03/30/why-isnt-desalination-the-answer-to-all-californias-water-problems