FAQ : SOL Bags


How does the SOL Water disinfection technology work?

A number of factors play a role in solar water disinfection, or SODIS as it is commonly called.  The effectiveness of these factors are enhanced by the design and plastics technology of the SOL Water bags.

First, and the most important factor, UV light is absorbed by microorganisms causing their DNA and/or cellular membranes to become damaged. The DNA absorbance of UV light is strongest in the UV-C region, peaking at about 265nm (Setlow), but a sufficient dose of UV-A light, which ranges from 320-400nm, can still inactivate microorganisms.  Cellular membranes are damaged to the point that they can no longer function to transport needed materials in and out of the cell required for life and reproduction. (Acra et al.)(Brock et al.)  This effect is greatly increased by the SOL Water bag’s effeicient design and highly reflective back panel.

Second, when the water’s naturally occurring dissolved organic matter absorbs UV light, photochemical reactions produce highly oxidized compounds, such as hydroxyl radicals (OH), superoxides (O2-), and hydrogen peroxide (H2O2) (Reed; Stum).  These compounds oxidize cellular components of microorganisms, thereby damaging or killing them (McGuigen et al; Reed).

Third, red and infrared light is absorbed by the water, which raises the water temperature.  Beyond the maximum growth temperature, additional heat causes denaturation, impeding protein function and often killing the organism (Brock et al). This heat energy has a synergistic effect with the UV mechanisms at and above 113°F (45°C) (McGuigen et al), a temperature which is commonly reached within the SOL bags.  When the temperature exceeds 122°F  (50°C) only one third of the fluence (treatment time) is required for SODIS to work effectively, compared to solar disinfection at lower temperatures (Wegelin et al).  These temperatures are also commonly reached within the SOL bags during treatment on a hot day.  The rule of thumb is that below these temperatures 3 to 5 hours of solar radiation above 500 W/m2 is adequate to render microorganisms inactivated (EAWAG/SANDEC).  These treatment times are even further reduced by using the SOL Water bag.


Acra et al., Water disinfection by solar radiation: assessment and applications. International Development Research Center. (1984).

Brock et al., Biology of microorganisms. Prentice Hall. (2000).

EAWAG/SANDEC. SODIS news no. 1. Internal Report. (1997).

EAWAG. Solar disinfection of drinking water. Accessed from the World Wide Web on January 15, 2009. http://www.eawag.ch/…

McGuigan et al., Solar disinfection of drinking water contained in transparent plastic bottles: characterizing the bacterial inactivation process. J. Appl. Microbiol. (1998).

Reed. Sol-air water treatment. 22nd WEDC Conference Discussion Paper. (1996).

Setlow. The wavelengths of sunlight effective in producing skin cancer: a theoretical analysis. Proc Natl. Acad. Sci. (1974).

Stumm et al., Chemical equilibria and rates in natural waters 3rd Edition. Aquatic Chemistry. (1995).

Wegelin et al., Solar water disinfection: scope of the process and analysis of radiation experiments. J Water SRT – Aqua. (1994).

Is the SOL Water treatment method safe?

Yes, but don’t just take our word for it.  In fact, solar water disinfection (SODIS as it is known) is one of the few approved and recommended water disinfection treatments by the World Health Organization (WHO).  It is also recognized and recommended by the Center for Disease Control (CDC), Swiss Federal Institute of Aquatic Sciences and Technology (Eawag), UNICEF,  and the Red Cross.  Many studies have been conducted on the effectiveness of solar water disinfection showing a 99.9% decrease, or greater, of illness causing waterborne organisms when done properly.  Currently over 3 million people in 30 countries are already using SODIS to reduce their risk of water-borne disease with a very high success rate, despite the fact that they are using inefficient containers often scavenged from waste areas.

WHO:  http://www.who.int/water_sanitation_health/dwq/wsh0207/en/index4.html

CDC: http://www.cdc.gov/safewater/publications_pages/options-sodis.pdf

Eawag: http://www.sodis.ch/methode/index_EN

UNICEF: http://www.unicef.org/wash/files/Scaling_up_HWTS_Jan_25th_with_comments.pdf

Red Cross: https://www.redcross.ch/de/data/info/press/114_de.pdf

How effective is the SOL Water treatment method, what organisms can it kill?

Very effective.  With proper treatment time solar water disinfection kills or deactivates 99.9% of most waterborne bacteria, viruses, and protozoa.  Micro organisms in numbers smaller than this are very difficult to measure which is why no studies show a complete 100% kill / deactivation rate, and numbers that small are not great enough to cause illness.

Reference: http://www.sodis.ch/methode/forschung/mikrobio/index_EN

How are the SOL Water bags superior to other solar disinfection devices?

SOL Water bags utilize technology that is superior to other solar disinfection containers.  Our flexible plastic is a special laminate blend that allows maximum passage of UV rays through the material without quickly degrading it.  Our plastic is completely free of plasticisers and other chemicals known to be toxic to humans.  Every SOL Water bag has a built in reflector back that reflects and focuses UV energy back into the water, making far more effective and efficient use of collected sun energy.  The shape of each SOL Water bag design is specially calculated to allow for maximum solar exposure per volume, while maintaining a ergonomic and useable packing and carrying shape.  Bottles or other rigid solar disinfection containers are far from optimum shape to allow solar energy absorbtion.  In fact, in a recent study “SODIS bags were found to yield as much as 74% higher treatment efficiencies than SODIS bottles”.  Additional studies have shown that adding a reflective back onto a solar disinfecton bag increases effectiveness by at least 20%.

Overall, the SOL Water technology makes our containers is at least 95% more effective than other solar water disinfection containers! 

Reference to study article showing 74% higher bag efficiencies: https://circle.ubc.ca/bitstream/handle/2429/39864/ubc_2012_spring_pierik_bradley.pdf?sequence=1

Reference to study showing 20% efficiency boost with reflective back: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC383040/

How long does it take to disinfect the water with a SOL bag?

Though studies have shown that the SOL bag can achieve effective disinfection in as little as one hour with the right factors, we instruct that the full recommended treatment times be used to compensate for different environmental factors that could be present.

Typically a full 6 hour sun treatment time is recommended to give ample allowance time for different factors such as the water cloudiness, clarity of the sky, and time of day.  This is a generous time allowance for SOL bags because it is based upon treatment times with traditional containers which are far less efficient.   

If the sky is up to 50% cloudy the 6 hour treatment time is still effective.  If the sky is completely overcast but without rain or heavy cloud cover treatment will still be effective if 24 hours daylight treatment time is allowed.

If it is a hot, clear, sunny, summer day, and the water collected is crystal clear, we recommend a treatment time of 3 hours when done between the hours of 9am-3pm.

What if there is cloud cover, will the SOL bag still work?

In many cases, yes.  If the sky is up to 50% overcast, the recomended 6 hour sunlight treatment time is adequate.  If the sky is 100% overcast with thin clouds, and a sun spot can still be seen throgh the clouds then we recommend a 24 hour sunlight treatment time.  If the sun is completely obscured or it is raining then the SOL disinfection bag will not work.

Will the SOL bag work with murky water?

Yes, to a degree.  The large SOL symbol inside the bag is designed to be a perfect water turbulence meter.  As long as all of the rays on the SOL symbol can be clearly distinguished through the water, then it is clear enough to work.  If the water is too cloudy, some water can be let out to make the bag flatter until the rays on the SOL symbol can be distinguished, this ensures that an adequate amount of UV rays can penetrate clear through the water.  If the water is very cloudy it can often be settled first inside the SOL bag.  This can be done by hanging the bag so that the lowest point is near the opening.  The particulates will settle out over time to the bottom and can be carefully flushed out of the bag opening.  Otherwise a filter can be used before treating the water.  If a commercial filter is not available, there are many types of makeshift filters that can be made to clarify water.

Why do your instructions say to shake the bag with some air?

When the water’s naturally occurring dissolved organic matter absorbs UV light, photochemical reactions produce highly oxidized compounds, such as hydroxyl radicals (OH), superoxides (O2-), and hydrogen peroxide (H2O2) (Reed; Stum).  These compounds oxidize cellular components of microorganisms, thereby damaging or killing them (McGuigen et al; Reed).

Shaking the bag with a little bit of air helps to oxygenate the water.  The more oxygen in the water, the greater amount of photochemical oxidization happens, and treatment becomes more effective.


McGuigan et al., Solar disinfection of drinking water contained in transparent plastic bottles: characterizing the bacterial inactivation process. J. Appl. Microbiol. (1998).

Will the SOL bag work to disinfect water anywhere in the world?

Generally, latitudes greater than 35N° or 35S will not have an adequate amount of sunlight in the winter time for the SOL bag to disinfect properly with the given treatment times.

How does SOL Water bag disinfection effectiveness compare with other water treatments?

  • SOL Water bag.  Treats bacteria, viruses, and protozoa.  Cannot remove toxins or filter debris.
  • Water filter.  Treats bacteria, protozoa.  Does NOT remove viruses.  Some can remove toxins.  Can filter debris.
  • UV sterilizer pen. Treats bacteria, viruses, and protozoa.  Cannot remove toxins or filter debris.
  • Chemical water treatment.  More effective treatments such as chlorine dioxide can treat bacteria, viruses, and protozoa.  Often actually adds toxins to the water.  Cannot filter debris.

How does the cost of SOL Water disinfection compare with other methods?

  • SOL Water bag (2 liter bag) disinfection cost/liter of treated water = approx.  $0.017/liter.  Does not require separate container.     
  • Typical water filter cost/liter of treated water = approx.  $0.045/liter + requires separate water container.
  • Typical UV sterilizer pen cost/liter of treated water = approx $0.056/liter + requires separate water container.    
  • Typical effective chemical water treatment cost/liter of treated water = approx. $0.066/liter + requires separate water container.    

What CAN’T the SOL Water bag do?

Our SOL Water bags are NOT filters, therefore they cannot remove debris from the water.  If many debris are present, larger than a pin head, then a separate filtering device is needed or water should be settled towards the opening to let settled debris out before treatment.

SOL Water bags are NOT purifiers, they cannot remove chemicals from water such as mercury, pesticides, and other poisonus substances.  Use caution when choosing a water source.

If the water is so cloudy that it obscures the SOL logo indicator, even when the water volume is adjusted, then it must be filtered or settled first.

Though they are metallic, SOL Water bags are not designed to take the heat of open flame, do not attempt to place it over an open fire.

The SOL bag cannot disinfect water if the sun is completely obscured by clouds or other objects.

Have you actually tested the SOL Water bags to make sure they work as you say?

Yes, we have performed careful laboratory testing on our SOL Water product to be sure that it meets AND exceeds previous testing results obtained in SODIS studies with conventional containers.  Here is a summary of our testing method, conditions, and results:

Calculations from previous testing of our special laminate plastics technology and bag design showed that we could likely achieve 94% greater efficiency than a conventional solar water treatment container with optimal treatment conditions.

Here comes the technical jargon…

We wanted to test this hypothesis by super inoculating a water sample with coliforms (bad bacteria like E. Coli and enterococci that cause illness) to the point where we would not be able to kill all of the organisms.  This way we could measure how much more effective our SOL technology is compared to the conventionally tested containers.  Also, we chose to do our testing with adequate, but far less than optimal treatment conditions to be sure that we wouldn’t kill all of the organisms in the super inoculated water samples.


Our seeder inoculant was mixed into a master water batch and this mixture was then divided into separate testing containers.  The inoculant was so potent that the water was almost too cloudy to see the sun rays on the SOL logo, at which point we would have had to dilute the sample.  To put this in more simple terms, our water test batches were so potent that they were basically like diluted feces.  This is a FAR greater coliform concentration than would be found in any natural lake, stream, or river.  It would actually be like taking water from a sewer tank.


Our test conditions were far less than optimal.  It was performed in the fall season and we are located at a fairly high latitude so the sun is very low on the horizon compared to Summer conditions.  The UV index for our testing day was only 2-4.  The air temperatures for the test day peaked at only 77 degrees F.  We were unable to get in the full 6 hours treatment time due to shading of the test samples that occurred at around 5 hours of treatment time due to the sun going behind the trees.  We had 5-10mph winds that create a cooling effect on the samples.  Our test platform on which the samples were laid was not perpendicular to the sun so we did not achieve full potential solar exposure.  There was a mild amount of smoke in the air due to some control burns nearby.  We did have a nice cloudless sky.


Control sample: We filled a SOL bag with water from the master water batch and left it outside in a shaded area with no sun exposure.  In the laboratory test our control sample had so many living coliform that they were too numerous to count.  The laboratory had to dilute the control sample 100 times (maximum dilution), to get an E. Coli number of approx 242K (top end of readability) and other coliforms were still too numerous to count.  To put this into perspective, previous solar treatment studies with less efficient containers showed that samples with coliform numbers above 242K organisms could not obtain 100% kill rates even with optimal conditions.

New SOL bag samples:  We filled brand new SOL bags with water from the master water batch and exposed them to the solar treatment conditions.  Due to the extremely high coliform count and less than optimal conditions we were expecting to get a number of organisms found after testing, but we were hoping the number would still be fairly low.          The laboratory test results on this water sample showed… drumroll please… ZERO living coliforms, not even a single one!          To be honest, we were a little bit surprised.  This means that our SOL bag technology is SO effective at killing microorganisms that we could not even inoculate the water enough to cause a less than 100% kill rate!  As long as the sun rays on the SOL logo were visible, we could not put enough organisms in to overcome even a fairly inefficient solar treatment.  DO WE SOUND A LITTLE EXCITED!?  WE ARE!  But it gets better…

Old SOL bag samples:  We also took some older SOL bags that have been left outside for months of continuous solar exposure cycles.  These exhibited some yellowing and wear due to prolonged UV exposure.  These were filled with the same water batch and exposed to the same test.  The results… absolutely ZERO organisms were found!  Even older worn SOL bags were so efficient that they achieved a 100% kill rate!

Conventional control sample:  To check effectiveness against known solar treatment containers we used the typically tested water containers for this control.  The container used is only 500ml as compared with the SOL bag which is 1000ml.  This lower water volume allows for greater treatment efficiency with a bottle shape due to a thinner water column allowing for greater UV penetration and less water mass to allow for more rapid heating.  A bottle with a volume of 1000ml would not have been viable for this experiment as the inoculant was so numerous as to make the water too cloudy for a bottle of that thickness.  This sample was exposed to the exact same conditions as the SOL bag samples.  Laboratory testing results of this sample showed that there were numerous E. Coli organisms remaining per 100ml and numerous other coliforms.


Laboratory testing showed that our SOL water treatment technology was FAR more efficient than conventional solar water treatment methods.  Since nothing living was found in our SOL bag samples, it is difficult to calculate exact percentages of efficiency as compared to conventional, but were guessing its probably over 100% more efficient!

Other testing…

Samples were sent to test for the presence of heavy metals and other contaminants in our bag materials.  The results from these tests showed that there were no heavy metals, contaminants, or toxins present in the water from our bags.





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