Access to pure drinking water is an invaluable part of being fit and healthy. Yet, getting pure drinking water makes for some interesting discussion, for a multitude of reasons. From the perspective of the chemist, simply trying to define what makes drinking water “pure” is rather fascinating. It’s also has social connotations, since at the time of this writing, about 1.1 billion people around the world lack access to pure water. So, from a social perspective, arguing about contaminants in drinking water takes on some interesting implications. For me personally, I consumed rainwater harvested from the roof of my parents home for most of my childhood. It wasn’t until I was well into my teenage years that our family had access to a municipal water supply. Is that drinking water more, or less, pure than what I consume today?
These things notwithstanding, the discussion is worth having, and the purpose of this post is to examine four of the tools available for making pure drinking water. Perhaps more important, I want to understand exactly how effective they are in comparison to one another.
Making Pure Drinking Water
SODIS Method
Solar distillation, known by the acronym SODIS, is one of the preferred methods for making pure drinking water in developing countries. The technique is most widely used in areas that lack access to municipal water supplies, to minimize the risk of illness from impure water. Users place water in PET bottles, shake them up to saturate the water with oxygen, and allow the bottles to sit in direct sunlight for at least 6-hours (more on cloudy days). Since the PET plastic is transparent to UV radiation, the ultraviolet light passes through the plastic where it then destroys the bacteria responsible for human illness.
Successful use of this technique for making pure drinking water begins by starting with the right plastic bottles. Bottles must be made of colorless PET that does not contain UV absorbers. Bottles must also be free of labels, scratches, or other defects that will interfere with the ability of the ultraviolet radiation to pass through the plastic. Water must have a minimum clarity, since dirt and other particulate matter interferes with the ability of the ultraviolet radiation to destroy bacteria and parasites.
This method has been successfully used to provide drinking water that is free from disease causing parasites to people in Africa, South Asia, and South America who would otherwise have no access to safe drinking water. It does not remove particulate contaminants or dissolved minerals, but it provides water that is free of disease causing microorganisms. Since the consumption of impure drinking water in developed countries is a contributor to many deaths, particularly among infants and children, the social ramifications of this method are tremendous.
If you’re looking for a method of making small quantities of water that is safe for consumption, but don’t have 6 hours to wait, you may look to consider something like the Lifesaver 4000, which is a portable filtration system that removes parasites and other disease causing organisms. The system is widely used by hikers and outdoor enthusiasts, and has been reportedly been used in emergency situations by military personnel. It won’t filter out salts, but it will make water safe from parasites.
Municipal Purification
Moving to a more industrial scale, most of the developed world obtains its drinking water from a municipal water purification source. Water purification on a large scale is generally a multi-step process that includes the following:
- Physical Filtration
- Chemical Purification
- Biological Purification
There are no big secrets here. The process of physical filtration is removing dissolved materials by a physical process. That process may include some combination of a settling process, simple screening, filtering through a sand bed, or even reverse osmosis. Chemical purification includes treating with chemicals to further reduce the effect of dissolved hard water minerals (i.e. calcium or magnesium), or disinfection. The final step, biological purification, is the process of allowing microbes to decompose residual dissolved organic compounds.
It’s important to understand that municipal systems are generally built with an eye toward the concerns specific to a given region, and are less about making “pure” drinking water, than they are about making acceptable, safe drinking water. Contaminants that may be considered nuisance materials, or aesthetically objectionable (such as dissolved iron or calcium), but aren’t regulated or considered a risk by health experts, are generally allowed to pass through the system, without any special effort taken to reduce their concentrations. The goal is to reduce substances that are considered a health risk by “the experts” to safe levels, so things like artificial fertilizers and metals are dramatically reduced, but low levels may remain.
Making The Purest Drinking Water
If one wants to make their drinking water as pure as possible, be prepared to get into a multi-step process, and understand that simply purchasing “bottled water,” “distilled water,” or “R.O. water” is no guarantee.
Generally considered the most advanced methods of purifying water, both distillation and reverse osmosis rely on separating water from its contaminants by physical phenomenon. In the case of distillation, source water is heated to its boiling point, and the vapor passes up into a column. Materials of significantly lower boiling points escape before water, and contaminants of significantly higher boiling points are left behind, with the quality of separation depending heavily on the length of the column. Contaminants with boiling points close to that of water can, and likely will, be collected along with the “pure” water.
Reverse osmosis relies on molecular size to separate water from contaminants. If one chooses to view a reverse osmosis membrane as a filter, the pores of the filter are approximately the same size of a water molecule. In an ideal situation, pure drinking water passes through the filter, while contaminants are excluded, and left behind. Again, the separation is based on molecular size, so contaminants that have a size similar to that of a water molecule, like many of the trace pharmaceuticals in drinking water, can be allowed to pass through the filter. In the case of R.O. filtration, the purity of the filtered water is a direct function of the size of the pores in the R.O. filter. Smaller pore size, higher quality filters do a more efficient job of separating out contaminants. But, of course, the cost of making these gains rises precipitously, as demonstrated by the R.O. purification plant in Orange County, California, where costs are 15-times that of traditional municipal treatment methods. Purification by R.O. usually makes water mildly acidic, so treatment plants generally adjust the pH up to 6.5-8.5 for practical reasons. Sodium hydroxide (lye) or potassium hydroxide (potash) are the preferred chemicals for this purpose.
As you can see, both R.O. and distillation are exceptionally effective for removing much of the trace inorganic materials like dissolved metals, salts, and minerals from source water. Naturally, distillation, which heats water to its boiling point, also functions as a disinfectant, while R.O. purification usually requires subsequent chemical purification, with ozone being the preferred method. Bubbling ozone into the filtered water kills pathogens while not leaving residual compounds behind.
The final step in making water as pure as we can usually includes passing the filtered, disinfected water through activated carbon. Activated carbon has a strong affinity for organic molecules, which are the material most likely to be carried along after purification by distillation or reverse osmosis. This final step will usually remove any residual organic materials that have escaped the other techniques.
The Bottom Line
The pursuit of pure drinking water isn’t easy or simple. If the goal is to get completely pure water, simply buying bottled water does not offer any guarantees, since a large fraction of bottled water is simply repackaged municipal supplies. Suppliers of distilled or R.O. water can certainly reduce, or perhaps even completely eliminate, dissolved minerals and some organic contaminants, depending on their exact processing conditions. But, even this makes no guarantees when it comes to trace levels of pharmaceuticals or personal care products in your drinking water, which are present at vanishingly low levels. The next step beyond this is activated carbon filtration, which will certainly further reduce any trace contaminants, but improper maintenance of the filter cartridge could eventually re-release those contaminants.
So back to the question of what it takes to make pure drinking water. Municipal water facilities certainly don’t provide the purest drinking water to be found on the planet. But in retrospect, that isn’t really their goal. The goal of municipal water suppliers is to provide safe drinking water in a cost effective manner to the greatest number of people possible. Generally speaking, they’re very effective at achieving that goal. But, for those who want the cleanest, purest drinking water available, it can be obtained.
With enough effort.
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