美國工程院院士塞德拉克：反滲透膜的諷刺之處是飲用水太乾淨

美國工程院院士塞德拉克：反滲透膜的諷刺之處是飲用水太乾淨 戴維·塞德拉克和他的論文

不久前，美國《環境科學與技術》（Environmental Science & Technology）發表了戴維·塞德拉克（David L. Sedlak）的一篇文章，這篇文章對目前全球超過10億人口在使用的反滲透淨水技術提出了一個頗為震撼的結論：反滲透技術革命的諷刺之處在於它製造了太過乾淨的飲用水，因為太過純淨，水中天然存在的很多有益礦物質都不存在了，因此導致人體存在著營養不良和增加患病風險。

戴維•塞德拉克是一個美國環境工程師與加州大學伯克利分校柏拉圖·馬洛扎莫夫講座教授，美國化學會《環境科學與技術》與《環境科學與技術通訊》雜誌主編，長期致力於化學汙染物與水資源的研究，他於2016年被選為美國工程院院士。

下為廈門大學水科技與政策研究中心首席科學家藍偉光的譯文。

前言

問道者

半個多世紀以前，當Sidney Loeb和Srinivasa Sourirajan發明反滲透膜時，水革命的種子開始萌芽。

在過去的五十多年中，學術界和製造商降低了反滲透膜的生產成本與反滲透裝置的能源消耗，反滲透淨化水的過程變得更加簡單與可靠。

如今，通過反滲透對海水和苦鹹水進行脫鹽淡化，世界上一些水資源緊張的城市多了一條水源供給的管道。隨著加利福尼亞、德克薩斯州和新加坡的水再生利用設施投入使用，這項技術的影響已經擴充套件到脫鹽以外的領域。

此外，由於公眾對市政部門提供的自來水的質量心有疑慮，反滲透膜在家用淨水裝置和瓶裝水的生產中也被廣泛使用。

2018年，全世界約有1％的人口飲用海水淡化的水。雖然沒有現成的精確估算，但相信會有千萬上億的人使用通過反滲透膜淨化處理的廢水，汙染的河水和傳統上認為不適合使用的水。

目前，通過反滲透膜淨水的固定資產投資每年增長約15％，而且這種增長趨勢並沒有放緩的跡象。根據這些趨勢，可以合理地假設，到二十一世紀中葉，超過十億人口會使用反滲透膜淨化的水。

反滲透革命為人類帶來了福音，但與所有顛覆性的技術一樣，它有可能產生意想不到的後果。根據反滲透膜淨水的現狀，世界飲用水供給的性質已經發生變化。因此，我們需要確認知識有什麼差距、技術應如何改進與採取什麼政策，以便保護為了保護公眾健康和環境安全。

反滲透技術革命的諷刺之處在於它製造了太過乾淨的飲用水。人們早已認識到，長期飲用去離子水會導致營養缺乏。

由此，處理過的水通常在反滲透處理後必須再新增礦物質才適合飲用。考慮到反滲透淨化的水質（呈酸性）會腐蝕供水管網，大型的反滲透淨水廠都會使用便宜且容易獲得的熟石灰再礦化水質（既把反滲透膜淨化的水的pH值調回弱鹼性，又增加了水中的鈣—譯者注）。

不幸的是，由於反滲透淨化的水即使加熟石灰再礦化也幾乎不含有鎂，而人們飲用鎂缺乏的水會增加患心臟疾病的風險。當這個問題第一次被發現時，以色列的供水商開始努力開發一種在再礦化過程中成本可控的新增鎂的方法。

但是，在這些方法成為標準之前，水廠提供未和其他水源混合的反滲透純淨水的社群可能需要膳食補充劑。如今，已經有人把鎂已被新增到反滲透產生的許多瓶裝水中，也有人通過家用反滲透系統新增鎂（問題是在家用反滲透純水機中額外增加新增鎂的裝置非常複雜，經濟上不可行。

且問，你家的反滲透淨水機有調節pH、新增鈣與鎂的步驟嗎？顯然沒有。所以，長期飲用反滲透膜過濾的純淨水必然會給您的家人帶來健康問題—譯者注）。

鎂還不是反滲透膜淨化的水中唯一缺少的重要營養離子。幾十年來，在自然界中氟濃度水平較低的地方是否應向飲用水中新增氟化物的問題一直存在爭議。

由於反滲透膜淨化的純淨水會造成飲用水額外的氟缺乏，公共衛生專家將不得不更加關注增加飲食中氟化物來源的需求。這個問題在低收入的群體中尤其值得關注，因為他們很少使用含氟牙膏。

舉例而言，儘管我們還無法確定反滲透膜把氟化物去除是導致中國兒童身高變矮和齲齒增加的原因，但那兒的許多中小學已經安裝了反滲透膜淨水機卻是一個不爭的事實。

考慮到飲用反滲透純淨水的人數眾多，我們必須相當謹慎地思考這個問題，即人體所需要的其他微量元素也可能需要通過飲用水來攝取。

大約10年前，流行病學家報告說飲用水中鋰濃度較低的群體自殺率較高。雖然並非所有後續研究都支援鋰缺乏性假設，但海水淡化水的鋰濃度幾乎都處於檢測限的低端，據報導飲用這類水的地方自殺率有所增加。

因此，可能的話，應該在反滲透純淨水中新增少量的鋰或其他所需的微量元素，或在缺乏營養的人群中以其他方式補充微量元素，但至今沒有進一步的研究來確定這些主意的有效性，可能沒有人這樣做。

幾乎沒有溶解性離子存在也意味著反滲透處理後的純淨水加強了礦物質溶解速率（此乃純淨水喝多了會導致人體內的礦物質被溶解流失的原因—譯者注）。

因而在工程實踐中，通常都要把反滲透純淨水進行再礦化以降低溶解那些覆蓋在管道內壁的方解石和氧化鐵層的趨勢。在建設反滲透水處理廠的許多地方，此前富含礦物質離子的水已經在水管中流淌多年，一旦呈酸性的純淨水被引入，管壁中長年累月形成的固體層就會被溶解破壞，一些吸附其中的有害元素，如砷，鉻和鉛等，就可能釋放溶解出來，引發飲水安全風險。

因此，反滲透純淨水在與老化管道接觸之前必須新增石灰，以提高反滲透純淨水的pH值，從而最大限度地減少管壁中碳酸鹽和氧化物的溶解。此外，反滲透處理過的水作為飲用水源注進地下加以貯藏時亦存在安全風險，因為它可能導致地下的蓄水層釋放出地質砷。

此外，工程和自然系統中的微生物將受到水化學變化的影響，由此可能改變生物地球化學過程並影響水生病原體的命運。

從現在開始的一個世紀，歷史學家往回看時將把反滲透的普及當作是飲用水供給發展中最重要的事件之一。科研群體面臨的挑戰是確保歷史書中不會腳註說明反滲透革命的意外後果。

下為原文

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.

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