Frequently Asked Questions

What is solar water distillation?

Solar distillation is a method of water purification that relies on the simple and well-known physical phenomenon of evaporation. In a typical solar distiller, the sun shines through a window and with the help of a dark surface inside, the impure source water is heated up to the point where it evaporates. The resulting water vapor, free of impurities, then condenses on the relatively cool inner surface of the window to form droplets that run down the tilted window pane. A trough collects and diverts this distilled water out of the distiller to gradually fill a receiving vessel preferably made of glass or stainless steel.

Why distill water with the sun?

Exceptional water purity - The purity of solar distilled water, especially when combined with an activated carbon filter, is unmatched by any other common filtration method. Chemicals, pathogens, medications, minerals, salts, metals, nitrates, and dissolved solids are all left behind in the evaporation process. Certain volatile molecules (for example VOC’s) can evaporate along with the water, but fortunately carbon filters are very effective at removing these molecules.

Green technology - Solar water distillation requires only the sun to operate, making it a sustainable, environmentally friendly process. Although no external energy source is necessary, the sun does need to be shining. If a solar still is to be used as your sole means of obtaining drinking water, then surplus quantities of water produced in the summer and on very sunny days should be stored away (for example in glass carboys), to be used on those days when the sun is not shining.

Emergency use - Even in emergencies, when there is no running water, a solar still can be used. The input water can be from virtually any source, including sea and sewage-infested water. If the water is particularly murky (high sediment content), then some pre-filtering is recommended.

Why not just use an electric counter-top still?

Counter-top stills use substantial amounts of energy (electricity) to boil water into steam. The energy required is substantial due to the high specific heat and heat of vaporization of water. Assuming no heat losses, it takes about 0.72 kW-hr to vaporize 1 liter of room temperature water. Not only then is the initial cost of an electric counter-top still rather high, but there is also this cost of electricity to run it. In contrast, a solar still uses the freely available energy of the sun. The electricity that is saved essentially pays back the initial cost of a solar still in about 1-2 years, depending on your local electricity rate. Please see our page on Payback Times for more details.

The boiling of water in counter-top stills is a relatively violent process, where some contaminants can be carried away by steam droplets. In contrast, a solar still uses the gentle process of evaporation and consequently does not suffer from this problem.

Nevertheless, a counter-top still can, for its small size, produce a sizeable quantity of distilled water, and can do so anytime, not just when the sun is shining. As such, a counter-top still can be a useful backup system to a solar still. Note, though, that with careful management of surplus water (storing it in glass carboys), a solar still can serve as your sole source of drinking water.

Is distilled water safe to drink?

There are differing views on whether distilled water is 100% safe to drink. On the one hand, the World Health Organization claims that it can potentially deprive the body of essential minerals†, and on the other hand the Canadian Water Quality Association counters such claims by noting that the U.S. Navy uses distilled water on their ships for months at a time with no noticeable health effects‡. Rather than take a side on this contentious issue, we simply point out that it is a simple matter to remineralize distilled water. Doing so alleviates any concerns, and may slightly improve the taste as well. Mineral powders and liquids for remineralizing distilled water are readily available online and in health stores, and Do-Right Solar offers its own inexpensive house blend. Of course, if you feel that distilled water is perfectly safe, then no remineralization is necessary.

One point that is not controversial is that if a water source (typically from the tap) is contaminated with lead, mercury, or other chemicals, then drinking this untreated water is detrimental to the health. One of the best methods to remove such contaminants is solar water distillation in combination with carbon filtration.

Finally, we note that distilled water is mildly corrosive to most materials, so before it is remineralized it should be contained in vessels made of an inert material such as glass or stainless steel, otherwise harmful contaminants can be reintroduced into the water. Do-Right Solar offers 3-gallon glass carboys with silicone stoppers for sale.

† Health Risks From Drinking Demineralised Water
‡ Clean Water: When it is muddied by Misinformation

What is the basic design of a solar still?

The two most common solar still designs are “basin” and “wick”.

In the basin design, a dark, horizontally level basin holds a shallow amount of water. The sun shines through an inclined window and heats up the water to the point of evaporation. The water vapor condenses on the inner surface of the window and forms water droplets that run down the window into a trough. The trough is also inclined, allowing the collected water to be directed out of the still and into a receiving vessel. As the water from the basin evaporates, it must be replenished periodically, usually once a day. As well, the basin must be flushed of accumulated sediments, usually every few days.

In the wick design, the basin is replaced by a dark wick kept moist by a constant slow stream of water. The wick is usually positioned parallel to the window. Due to the small volume of water contained at any one time in the wick, the wick still has a fast response time. This is in contrast to the basin still, where a relatively large volume of water must be heated before evaporation can begin, resulting in a delayed response. Due to the wick being parallel to the window, the wick still can also be tilted directly at the sun in order to enhance production. Finally, production is further enhanced in a wick still due to the fact that the wick is a 3-dimensional surface, effectively creating a larger evaporation surface. Do-Right Solar is proud to offer for sale a solar water still of wick design: Model SWS-200.

There are other less common still designs, for example ones using vacuum tubes or focusing reflectors, but they tend to be out of the price range of most consumers.

Are solar water stills easy to use?

Yes, most solar stills are simple to operate. As with all water filters/purifiers, though, they do require routine maintenance to be kept fully operational. In the case of a basin still, this maintenance takes the form of filling it every day, and flushing out sediment every few days, as well as replacing the collection vessel when full. In the case of a wick still, the input flow of water is slow and continuous. The wick itself requires periodic cleaning and/or replacement, due to calcification and fading. The model SWS-200 wick still offered by Do-Right Solar is especially convenient to use, as it utilizes tried-and-true drip irrigation technology to control the input water flow. A timer can be set to turn the still on/off according to the sun’s movement (modified seasonally). The most frequent maintenance task is simply replacing the collection vessel about once a week.

How do I carbon filter the distilled water?

The SWS-200 solar still incorporates a granular activated carbon (GAC) post-filter, situated just above the carboy. This post-filter removes organic volatiles that may be present in the distilled water. The carbon granules need to be replaced periodically.

Additionally, we sell binchōtan sticks, which are activated carbon twigs that are placed into your water crock or pitcher, and provide further carbon filtration. Both the GAC and binchōtan sticks are plastic-free and fully compostable.

Can the SWS-200 solar water still be used in emergencies?

Yes, even when there is no pressurized tap water, it is still possible to use the SWS-200 still. All that is required is to elevate the source water above the level of the still, to create some head pressure. This can be done using for example a camping shower bag or 5-gallon bucket hung from a ladder, tree, or nearby building. The source water can be virtually anything: sea water, pond water, even sewage water. If the water is particularly murky (high sediment content), then pre-filtering is recommended. Such pre-filtering can be accomplished with a simple DIY sand filter, of which there are many simple designs on the internet, for example:
How to Make a Survival Water Filter
Bio Sand Filter, Part 1,2 & 3