The air around us may be the greatest source of available drinking water. Virtually all air has moisture in it, even in the middle of a desert at noon. Extracting that moisture from the air – through a process called atmospheric water generation (AWG) – may provide a possible solution to safe drinking water needs.

First developed for the military to produce reliable drinking water in dry battlefield settings, AWG is now also seen as a good medium- to long-term drinking water source for communities following natural disasters like hurricanes and infrastructure failures as in Flint, Michigan. Installation of AWG systems is often less expensive than delivery and distribution of bottled water.

The AWG system draws air into the device. A layer of air filters removes dust and dirt. The cleaned air is directed to a cooling chamber where the air is chilled to the Dew Point, the temperature at which condensation begins. As with dew in the morning or water that appears on the outside of a chilled glass on a hot summer day, the AWG cooling process produces water inside of the device. That water is then moved within the AWG generator to another chamber where the just-created water is further cleaned via both chemical and biological processes. When necessary, minerals can be added in at this point, as well. Now that drinkable water has been produced, the water is kept circulating to keep microorganisms from growing in stagnant water.

In recent tests by the Environmental Protection Agency (EPA) of the Water-Gen AWG system, there was zero E. coli, a common biological contaminant. Likewise, levels of arsenic, lead, copper and several other metals were either at undetectable levels or at zero.

There were two potential health risks from AWG that were highlighted by the EPA. First, if the air being used has contaminants, those contaminants would be pulled into the AWG generator. If not removed in the filtration process, the water produced could have those contaminants in the drinking water. Second, depending on how it is stored and how long it sits before use, there could be microbial growth in the stored water. Both of those challenges can be addressed with current technology.

Industrial-size AWG units can produce up to 2,500 gallons a day of drinking water. With the average person needing no more than five gallons a day for drinking water and food preparation, one large AWG unit can provide drinking water for up to 500 people. In ordinary civilian settings outside of disasters,

existing freshwater supplies can be used for sanitation, washing, agriculture, and firefighting.

AWG systems are energy intensive, but greater efficiencies are being developed. Depending on the temperature and the level of humidity, the systems produce more (or less) drinkable water. Existing AWG systems are agnostic as to energy source. Power to run the AWG units can come from solar, wind, gas-powered generators, or a traditional power grid. The cost of AWG-generated water is about eight cents a gallon, or 40 cents per person per day.

The disinfection of water by chemical or mechanical means that began around the turn of the 20th century has resulted in the end of epidemics like cholera, dysentery, and typhoid fever. These techniques neutralize the waterborne microscopic pathogens that can cause often-fatal diseases. Countless lives have been saved.

What these forms of purification do not do is to remove all contaminants or even, necessarily, kill all microorganisms. They are ineffective against inorganic chemicals like pesticides or heavy metals, both of which can cause long-term health problems. Still, the widespread use of disinfectants essentially ended the fear of a quick death from drinking water, something of obvious importance and great value for society.

The three main forms of drinking water disinfectants are chlorination, ozonation and ultraviolet, or UV, light. The first two are chemical; the third is mechanical. All three have advantages and disadvantages, and, when it can be done, a combination of these techniques yields the safest drinking water possible.

Chlorination has been in use in the United States since around the turn of the 20th century. Chlorine or a chlorine derivative (such as chloramine or chlorine dioxide) is added in very small quantities to drinking water. The chlorine mixes rapidly with the water and creates a reaction which destroys the structure of any bacteria and viruses found there.

Drinking water utilities use chlorination for many reasons. It is highly effective with consistent outcomes, inexpensive, relatively easy to use (so costly staff isn't needed), does not require the construction or maintenance of complex infrastructure, and chlorine is widely available. The chlorine also stays in the drinking water for at least several hours after it is added, and the purification process continues long after first application offering an additional layer of safety.

But there are negatives, too. Aside from legitimate concerns about operator safety in transporting or handling the chlorine, the largest worry is that when chlorine mixes with organic matter in the drinking water supply (like bits of dead leaves), disinfection byproducts (DBPs) are created. Several of these DBPs have

been linked to cancer, especially bladder cancer, but also interference with the proper functioning of the liver, kidneys, central nervous system, and/or reproductive systems. As a result, utilities making use of chlorine need to balance the risks of pathogenic microorganisms that must be eliminated against the potential longer-term risks to public health. Drinking water disinfection must stop what can kill people fast, but that disinfection shouldn't become a public health menace either.

Much less frequently used techniques are ozonation and the use of ultra-violet light. Neither of these more recently adopted methods are deployed by even 10 percent of utilities. Many utilities are satisfied with the outcomes provided by chlorination and see no need to replace it or to augment it as a disinfectant tool.

Ozonation mixes ozone – a toxic gas – with the drinking water to disinfect it in a way similar to chlorination. As one benefit over chlorination, ozonation is also effective in water of widely differing temperatures and acidity levels. Thus, constant modifications of chlorine levels are not required. Ozonation also helps to improve the taste and smell of drinking water making it more appealing.

As with chlorination, there are negatives to ozonation. The ozone used is created at the point of use via an energy-intensive process. That makes energy costs high and may add to use of carbon fuels. It isn't effective against inorganic chemicals or heavy metals and some microorganisms. Aside from the expense of the creation of the ozone, ozonation also requires greater budget resources to build, buy and maintain the equipment that produces the ozone gas. Ozone gas dissipates very rapidly and there is no ongoing disinfection value as with chlorination. Most troubling of all, when ozone mixes with bromide found in some drinking water, it can produce bromite. Bromite has been found to be a carcinogen in laboratory animals.

In ultra-violet (UV) disinfection, light wavelengths are absorbed by the DNA in microorganisms, rendering them unable to reproduce. While still alive, the pathogens pose no threat as disease results when the microorganisms in the human host begin to rapidly reproduce. In addition, while chlorination and ozonation aren't effective against Cryptosporidium, Giardia, and other protozoa, UV is.

UV pulses are applied to drinking water as it flows through a UV reactor. Because this is a mechanical process requiring no chemicals, there are no disinfection byproducts potentially causing health concerns or residual chemicals left in the water. In addition, once the UV equipment is installed, it is easy to operate the necessary equipment.

Negatives of UV are few other than the energy cost, the expense of installation, and the risk of the system going offline for even a few moments in the event of a power surge. Most concerning is that there is no residual disinfection from the use of UV. This has led many of the utilities that use UV to follow the light treatment by adding in small amounts of chlorine after UV treatment to keep the water free of microorganisms for as long as possible.

Point-of-use drinking water systems are increasingly common in US households. These provide peace-of-mind for many consumers concerned about the safety (or taste) of their drinking water, but these come a price usually borne by the homeowner.

Individualized solutions to drinking water quality is an outgrowth of the acceptance by Congress of a large number of small drinking water utilities. Rather than demand that the utility provide drinking water that is free of regulated (and other) contaminants, Congress and the Environmental Protection Agency (EPA) agreed to allow smaller utilities to filter water by installing point-of-use and point-of-entry treatment devices under individual sinks and/or at the entry of water to the home.

Now, even in areas serviced by larger drinking water utilities, consumers are opting to take the health of their families into their own hands by using a wide range of devices, each with a particular benefit – but also at a different price point. Some people choose to purify all of the water in their home, regardless of whether it is used for drinking and food preparation, washing, running dishwashers and washing machines, or even for flushing of toilets. This obviously requires more comprehensive (and more expensive) devices. Others prefer devices that only purify the water that will be used for drinking and cooking, and, to achieve that, install them for use with the kitchen sink, or a limited number of taps.

Aside from under-the-sink and on-the-tap devices, there are also many other kinds of drinking water filters available. While cost, ease of use, and available space are all relevant to the choice made, most important is for the consumer to know what contaminant(s) is/are in the water. Once that is known, choice of filter gets easier.

Contaminants in drinking water can be detected by testing. A simple internet search will provide a variety of home-based and lab-based tests. Testing companies usually suggest the kind of treatment technology that is best for each contaminant found.

Alternatively, consumers can rely on publicly available information to tell the kinds of regulated contaminants found by each drinking water utility. The Consumer Confidence Report prepared by each local drinking water utility is often

confusing, but may be of some value. One of the most important drinking water not-for-profit organizations, Environmental Working Group (EWG), provides a far easier listing of local regulated contaminants broken out by ZIP Code.

But whether using the utility’s report or EWG’s Tap Water Database, it is important to ensure that the treatment technology chosen actually removes the contaminants present. While the removal of some contaminants require major investments, such as pipe replacement or full-home advanced filtration systems, others can be removed from drinking water with a simple filtered pitcher.

The EWG Tap Water Database and related information on treatment technology can be found here.

What is commonly called reverse osmosis, or RO, is a catch all term for technologies in which contaminated water is forced through a membrane with very small holes. Contaminants remain on the inbound side of the membrane and pure, or purer, water is pushed to the reverse side.

Because contaminants come in a variety of sizes and because it takes more energy and finer membranes to get every contaminant, membranes of several sizes have been created. The less energy needed and the larger the holes in the membrane, the lower the cost. It makes no sense to use more energy or a more expensive membrane than is needed to do the job.

In a sense, the membrane selected is “right sized” for the contaminant sought to be addressed. For example, some particles are larger than others. A grain of sand needs a less fine membrane than does salt dissolved in the water. Likewise, bacteria are larger than viruses. So, if the goal is to rid the water of bacteria alone, a coarser membrane using less energy is possible. With vast amounts of water needing to be purified, the lower the energy load, the better for the environment and for budgets.

To be sure, at all levels of filtration, the holes in the membrane are incomprehensibly small, but the difference between the largest – called microfiltration, or MF – and the smallest – called reverse osmosis – is significant.

The four levels of filtration, the approximate pore size, and how each is most commonly used is:

1. Microfiltration (MF) – for removal of particles, some bacteria and some protozoa [membrane pores are 1/254,000 of an inch]

2. Ultrafiltration (UF) – for removal of all bacteria and protozoa, and some viruses [1/2,540,000 of an inch]

3. Nanofiltration (NF) – for removal of all viruses and also to improve the taste profile of the drinking water [1/25,400,000 of an inch]

4. Reverse Osmosis (RO) – for removal of all inorganic contaminants regardless of size [1/254,000,000 of an inch]

RO is most commonly used in desalination as the membrane can separate out even dissolved salt and minerals found in the water. In all of these systems, the removal of the microorganisms means that less chemical treatment is needed in the water to kill off bacteria, protozoa and viruses.