不久前，美国《环境科学与技术》（Environmental Science & Technology）发表了戴维·塞德拉克（David L. Sedlak）的一篇文章，这篇文章对目前全球超过10亿人口在使用的反渗透净水技术提出了一个颇为震撼的结论：反渗透技术革命的讽刺之处在于它制造了太过干净的饮用水，因为太过纯净，水中天然存在的很多有益矿物质都不存在了，因此导致人体存在着营养不良和增加患病风险。
半个多世纪以前，当Sidney Loeb和Srinivasa Sourirajan发明反渗透膜时，水革命的种子开始萌芽。
Over half a century ago, the seeds of a water revolution were sewn when Sidney Loeb and Srinivasa Sourirajan invented the reverse osmosis membrane. Over the last five decades, academics and manufacturers have reduced the cost of producing membranes, improved their energy efficiency and made their operation simpler and more reliable. Today, desalination of seawater and brackish groundwater by reverse osmosis provides water to some of the world’s most water-stressed cities. The impact of the technology is now being extended beyond desalination as potable water reuse facilities are coming online in California, Texas, and Singapore. Reverse osmosis also has become popular in household-scale water treatment and in the production of bottled water consumed in places where the public believes that their tap water is unsafe.
In 2018, about 1% of the world’s population drank desalinated seawater. Although precise estimates are not readily available, millions more used reverse osmosis to purify treated wastewater, polluted river water, and water that was deemed unsuitable for consumption. The growth in this practice shows no sign of slowing, with capital investments in reverse osmosis growing by approximately 15% per year. On the basis of these trends, it is reasonable to assume that over a billion people could be consuming reverse osmosis-treated water by the middle of the twenty-first century. The reverse osmosis revolution benefits humanity, but like all disruptive technologies, it has the potential to create unintended consequences. By considering current practices used for reverse osmosis treatment, we can identify the knowledge gaps, technology improvements, and policies needed to protect public health and the environment as the nature of the world’s drinking water supply changes.
The great irony of the reverse osmosis revolution is that it has created drinking water that may be too clean. It has long been recognized that, over the long-term, consumption of ion-free water can lead to nutritional deficiencies. For this and other reasons, treated water is typically remineralized after reverse osmosis treatment. At full-scale water treatment plants, where corrosion of water distribution pipes is a major concern, lime (i.e., Ca(OH)2(s)) is used for remineralization because it is inexpensive and readily available. Unfortunately, the near absence of magnesium in water produced by this process has resulted in deficiencies in magnesium that increase the risks of heart disease. When this problem first came to light, water providers in Israel initiated an effort to develop cost-effective and reliable approaches for introducing magnesium during remineralization. But until such systems become the norm, dietary supplements may be needed in communities where treatment plants deliver reverse osmosis-treated water that has not blended with water from other sources. (Magnesium is already added to many bottled waters produced by reverse osmosis. It is also added by some household reverse osmosis systems.)
Magnesium may not be the only nutritionally important ion missing from reverse-osmosis-treated water. The issue of whether or not to add fluoride to drinking water in places where the naturally occurring levels are low has been a matter of controversy for decades. As reverse osmosis creates additional fluoride-deficient drinking water supplies, public health experts will have to pay more attention to the need to augment dietary fluoride sources. This issue is particularly important in lower income communities, where fluoride-containing toothpaste is less common. For example, failure to appreciate the impact of reverse osmosis on fluoride led to decreases in height and increases in caries among children in communities in China where reverse osmosis systems had been installed at primary schools.
Considering the number of people who rely upon reverse osmosis-treated water, it is prudent to look more carefully at the possibility that other trace elements are derived from drinking water. About 10 years ago, epidemiologists reported increased rates of suicide in communities where lithium concentrations in drinking water are low. Although not all of the subsequent studies supported the lithium deficiency hypothesis, the concentrations of lithium in desalinated seawater are at the low end of the range reported in places where increased suicide rates have been observed. It would be possible to add a small amount of lithium, or other needed trace elements, to reverse osmosis water or to supplement diets in other ways in deficient populations, but without additional research to establish the validity of these ideas, this is unlikely to happen.
The near absence of dissolved ions also means that reverse osmosis-treated water enhances rates of mineral dissolution. The remineralization process, which decreases the tendency of reverse osmosis-treated water to dissolve the calcite and iron oxide layers that coat the inner walls of pipes, was adapted from engineering practices developed in places where the local water supply contained low concentrations of dissolved ions. In many of the locations where reverse osmosis treatment plants are being installed, water from ion-rich sources had been flowing through the pipes for decades prior to introduction of the desalinated water. Adding lime and raising the pH of reverse osmosis-treated water prior to its contact with the aged pipes may minimize dissolution of carbonates and oxides, but exposure to remineralized water could still release adsorbed trace elements, like arsenic, chromium, and lead. Reverse-osmosis-treated water could also pose risks during water storage as illustrated by the release of geogenic arsenic from an aquifer where remineralized water was used to recharge a drinking water aquifer. Furthermore, the microbes in engineered and natural systems will be affected by the change in water chemistry in a manner that could alter biogeochemical processes and affect the fate of waterborne pathogens.
A century from now, historians will look back on the popularization of reverse osmosis as one of the most significant events in the development of drinking water supplies. The challenge for the research community is to make certain that the history books do not include a footnote about the unintended consequences of the reverse osmosis revolution.