The Principles Behind Reverse Osmosis Water Purifiers and an Analysis of the Nutritional Value of Purified Water

The Principles Behind Reverse Osmosis Water Purifiers and an Analysis of the Nutritional Value of Purified Water

In recent years, issues surrounding the safety of drinking water have frequently made the headlines, leading to a surge in popularity for water purifiers.

However, whilst many wish to purchase a water purifier to tackle the problem of ‘contaminated water’, they are also concerned that the purified water produced may be harmful to their health. With a constant stream of news stories and rumours circulating, it can be a real source of stress for the average consumer.

To address this, we will focus on three key questions to explain the principles of water purification, guide you on how to select a water purifier based on your specific circumstances, and clarify whether purified water is truly ‘devoid of nutrients’.

Question 1: Some people believe that regularly drinking ‘pure water’ filtered by a water purifier can be harmful to health. To cater to this consumer perception, water purifier manufacturers have launched products that claim to retain ‘minerals’. Is drinking mineral water really better than drinking pure water? Can drinking pure water lead to malnutrition?

Rebuttal:
The *Chinese Dietary Reference Intakes* (2016 edition) recommends a daily intake of macrominerals (such as calcium, phosphorus and potassium) and trace elements (such as iron, copper and manganese), which are essential for building human tissue and maintaining physiological functions.

However, it is regrettable that current separation technologies make it difficult to completely distinguish between these beneficial elements required by the human body and certain toxic heavy metals (such as mercury, lead and cadmium). The choice is either to retain everything or to filter everything out.

When selecting a water purifier, you may notice that models promoting ‘mineral water’ generally cost less than 1,000 yuan, whilst those promoting ‘pure water’ typically cost several thousand yuan or more.

In fact, water purifiers that emphasise mineral content typically utilise ultrafiltration or nanofiltration membranes—which have a relatively low filtration grade and are less expensive—as their core filtration components. Manufacturers, however, often fail to mention that whilst this method retains minerals such as calcium, phosphorus and potassium, it also retains harmful substances like mercury, lead, chromium and cadmium.

As for the concern that drinking purified water may lead to nutritional deficiencies, Zhang Nengwei, Deputy Director of Beijing Shijitan Hospital, has publicly stated: ‘The primary source of nutrients for the human body is food. Cases of malnutrition among people who regularly drink purified water are entirely unrelated to their drinking water.’

Furthermore, taking ‘calcium’ as an example: the manual lists the recommended daily intake of calcium as 800 milligrams. Generally, mineral water available on the market contains 4 milligrams of calcium per litre. Assuming an adult drinks 2 litres of water a day, they would only be able to obtain 8 milligrams of calcium from water alone. In contrast, 1 litre of milk contains as much as 1,000 milligrams of calcium; a single serving of stir-fried wood ear mushrooms with eggs also contains over 500 milligrams of calcium. Therefore, for people with a normal diet, drinking water serves only metabolic purposes; relying on water to supplement trace elements is no better than taking a few breaths of smog.
▲ Every 100 millilitres of milk contains 120 milligrams of calcium
In summary, Master Xiao Dao advises consumers to ‘adapt to local conditions’ when selecting water purifiers.

Question 2: Is it really reliable to rely on a TDS pen—which costs around twenty yuan including postage—to test water quality when selecting a water purifier?

Fact: Firstly, pure water containing no impurities is virtually non-conductive. Water conducts electricity because it contains certain conductive impurities.
The principle behind a TDS pen’s water quality measurement is to first test the water’s electrical conductivity, and then calculate the total content of all dissolved charged substances in the water to derive the TDS value (Total Dissolved Solids).
In theory, the lower the TDS value, the better, as this indicates relatively fewer conductive impurities in the water. However, it is important to note that TDS does not reflect the concentration of non-conductive substances such as bacteria and organic matter; therefore, a low TDS value provides only a partial indication of water quality.

Water with a high TDS value requires further analysis. As mentioned earlier, conductive ‘impurities’ can be either beneficial or toxic: some water may contain elements essential to the human body, and as these elements are conductive, the water’s TDS value will be elevated; whilst other waters may contain toxic impurities; in such cases, a high TDS value indicates poor water quality.
Overall, TDS pens are suitable for households to conduct preliminary water quality analyses; however, their readings are not entirely accurate, as certain pollutants (such as bacteria and organic matter) do not cause the TDS value to rise.
It is recommended that consumers whose tap water has a TDS value above 100 consider using a reverse osmosis water purifier.

Question 3: Why do reverse osmosis water purifiers require electricity, and why do they produce a certain proportion of wastewater? What is the principle behind reverse osmosis water purifiers?
Answer: First, let me pose an interesting question: seawater is undrinkable, yet seagulls fly over the sea – so what do seagulls drink?
The answer is seawater. This is because seagulls possess a membrane within their bodies that filters out impurities from seawater; humans have therefore modelled reverse osmosis membranes on this principle.

The original design was motivated by the fact that, due to limitations in space technology, it was impossible to carry large quantities of drinking water into space. To achieve the recycling of drinking water, reverse osmosis water purifiers were used to purify urine and domestic wastewater. It is worth noting that the faecal water Bill Gates once drank actually utilised reverse osmosis filtration technology as well!

　　As pressure is required, reverse osmosis water purifiers must be connected to a power source to operate; the concentrated wastewater discharged is undrinkable and is known as ‘wastewater’, which can be used for washing clothes, mopping floors and similar purposes.