Is Desalinated Water Safe?

Red Tide at Half Moon Bay, 2007

The algae, Pseudo-nitzschia australis, has received a great deal of attention as marine mammals wash up on the beaches of Monterey Bay. The algae blooms from late spring to early fall and can produce domoic acid, a potent neurotoxin that can cause neural damage, disorientation, short-term memory loss and even seizures and brain damage in vertebrates. Red tides occur seasonally in Monterey Bay and have been more frequent in the Bay since 2004 with large blooms lasting several months occurring in 2004, 2005, 2006, and 2007.1

California has strict standards for water agencies concerning their source water. However up until recently that source water has always been on land. There is no existing regulatory standard for domoic acid or saxitoxin in drinking water set by the Environmental Protection Agency (EPA) or the California Department of Public Health (DPH).2

The report from the SCWD2 pilot desalination plant that operated at Long Marine Lab expresses confidence in the ability of the desalination filters to eliminate these toxins.
“Although there is limited published information on the operations of existing facilities during these types of blooms, discussions with industry professionals indicate that toxins are generally not a concern because the algae and toxins are removed by treatment process.”3

Because there was no red tide event during the pilot plant testing, the SCWD2 pilot plant performed a study to determine the ability of the desalination process to eliminate the neurotoxins. Instead of using domoic acid or saxitoxin, they used kainic acid, a safer surrogate which has a similar molecular weight and structure. The results show that pre-filters have a low rate of rejection of kainic acid, but that the reverse osmosis filters have an excellent rate of rejection, with 99.8% of the kainic acid removed.

Is reverse osmosis as effective in removing toxins as it is in removing kainic acid? A 2009 paper, Harmful algae and their potential impacts on desalination operations off southern California, published by a team led by USC biologist, David Caron, suggests that a study be conducted to test if that is true:
“To our knowledge, there are no published reports on the effectiveness of reverse osmosis for removing dissolved algal toxins from seawater. Some of these toxin molecules (e.g. domoic acid) are near the theoretical molecular size of molecules rejected by reverse osmosis membranes, but experimental studies are required to validate the effective of this process on toxin removal.”

Even if reverse osmosis proves to be very effective in removing toxins such as domoic acid, the biggest risk is failure in plant equipment. A paper by UCLA researchers reports, “Examples of faults include valve failure, membrane scaling or fouling, sensor data loss…and pump failure. Because reverse osmosis plants run at high pressures, these failures may cause immediate safety risks to plant personnel. These failures can also lead to a decline in product water quality, rendering it unsafe for public consumption.”4

Failure of reverse osmosis filters can go undetected. Bio-fouling of filter membranes can occur when there is algae in the water, as happened during the pilot desal plant testing. When organic matter fouls reverse osmosis membranes, it must be quickly cleaned in order to avoid permanent damage. Otherwise, bacteria grow on the layer of organic matter and create holes in the membrane. Seawater, and its contaminants pass through the holes. Even with vigilant maintenance, over the course of time, biofouling requires membranes to be replaced.

Listen to former CalOSHA Public Health Medical Officer, Dr. Larry Rose, on the public health risks of desalination. For more information on impact of algae blooms, download: “Impact on Desalination of Harmful Algae Blooms in Monterey Bay”, by Cecile Mioni

Boron and Bromine
Boron and bromine are two elements that aren’t normally of concern in drinking water sources on land. Boron is not an EPA regulated contaminant.5 Boron from desalination water in Israel was found to have an adverse affect on plants. Boron has also been shown to have harmful effects on development in lab animals. So the pilot desal plant study set goals for boron removal.

The report of the pilot desal plant recommends strict goals for bromine removal. “Once the seawater desalination plant is brought on-line, the desalinated water, which contains bromide, will mix with Graham Hill treatment plant water in the distribution system. This may increase the challenge of meeting the Stage 2 Disinfection Byproduct regulations.”6 The pilot study performed a test in which desalinated water was mixed with City water. The study found that bromine reacts with organic molecules in the City’s water to form brominated tri-halo-methanes (THM), a carcinogen. “The results indicate that bromide concentrations greater than 0.5 mg/L may significantly increase total THM formation after blending with treated surface water within the distribution system. For this reason, it is recommended that the reverse osmosis system be selected to reduce the bromide concentration in the desalinated water to 0.5 mg/L or less.”7 In other words, the proposed desal plant needs to exceed the performance of the pilot desalination plant in order to achieve a safe level of bromides. That means using high-performance reverse osmosis filters (which also have higher energy demands). As filters become older, their perf0rmance in eliminating salts declines. So achieving safe levels of bromides may be a challenge.

We’re not sure how the City will determine in what neighborhoods mixing of desalinated water with City water will take place, in order to test for tri-halomethanes. The points of mixing will be a continuously moving target, depending on rates of water consumption. How will the City find the locations where mixed water presents the highest risk?

(Footnotes)
1 Desalination Pilot Test Program Final Report, p 3-18

2 Desalination Pilot Test Program Final Report, Appendix A, TM-4-4

3 ibid TM-4-2

4 McFall, et al, “Fault-tolerant control of a reverse osmosis desalination process”

5 http://www.epa.gov/safewater/ccl/reg_determine2.html
6 Desalination Pilot Test Program Final Report p 4-9
7 ibid p 4-11

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