SODIS - (Solar Water Disinfection) [Download a PDF Copy]
Solar Water Disinfection (SODIS) is the easiest method I know for making naturally
contaminated water safe to drink, using solar energy. Figure 1 shows the basic SODIS
method. The procedure was developed by the Swiss Institute for Aquatic Science and
Technology (EAWAG) and is endorsed by The World Health Organization (WHO),
UNICEF, and the Red Cross as a way to treat household water in developing countries.
The official website of EAWAG for SODIS is Sodis.ch. SODIS can be used as an
alternative to water pasteurization (heating above 150° F), where pasteurization is not
practical or economical. In SODIS clear plastic bottles made of PET (Polyethylene
terephthalate) are filled with contaminated water and left exposed to direct sunlight for
at least 6 hours on sunny days and two consecutive days when the skys are cloudy.
The heat of the sun plus ultraviolet (UV) radiation interact to kill harmful bacteria
virus and microbial cysts that cause diarrhea and other water borne illness. SODIS is
NOT used to treat chemically contaminated water, seawater or brackish water. Also,
SODIS is NOT a filter or purification system. EAWAG recommends filtering turbid
(cloudy, brown) water before applying SODIS. The water used should be clear, free
of sediment and debris. When only turbid water is available - I recommend boiling,
chlorine treatment or solar thermal pasteurization methods like the Solar Kangaroo
instead of SODIS.
SODIS works because UV radiation interferes with the metabolism and destroys
the cell structures of bacteria. UV also reacts with oxygen in the water to create some
oxygen free radicals and hydrogen peroxide that also aid in killing bacteria and virus.
Finally, infrared radiation (IR) heat from the sun works with UV to help kill bacteria
when temperatures rise to at least 120° F (50° C).
SODIS is used in over 30 counties by over 3 million people worldwide as a simple
and inexpensive means of water treatment and storage for individuals and households.
SODIS is practiced in mainly equatorial regions including Brazil, Kenya, India,
Pakistan, and Indonesia. Generally, any location +/- 35° latitude from the equator
is suitable for SODIS. In practice, 1.5 - 2 liter clear plastic bottles are filled with
water. It is recommended to fill the bottles about halfway, shake the contents for about
20 seconds to oxygenate the water, then fill the bottles up to the neck of the bottle.
Next, the bottle(s) are exposed to direct sunlight for 6-10 hours (all day). Ideally, the
bottles are placed on a sloped sun-facing corrugated metal roof or panel. In addition
to maximizing sunlight, the corrugated metal provides infrared heat and reflected UV
radiation. Also, placing the bottle at an angle keeps water from remaining under the
bottle cap where bacteria can hide and re-contaminate the water later (in storage).
However, in actual use, bottles may simply be laid sideways on a flat ground or
surface (c.f., Figure 2). Water remains under the bottle cap where bacteria can hide
to re-contaminate water in storage. Where corrugated roofs or panels are not available,
thatched or other non-metallic roofs are used. Many of these other surfaces may
provide IR heat, but are poor UV reflectors.
SODIS - Solar Jacket
The Solar Jacket is a SODIS tool I invented to help collect, focus and amplify
available sunlight including UV radiation. I developed the Solar Jacket in May, 2010.
Figure 3 shows the amplified SODIS method I developed using the Solar Jacket.
Below, I describe its development, application and construction.
As I see it, there are three main components to the SODIS system: the sun, the surface
where the bottle is placed, and the bottle. The sun is the sun; we can do nothing to change
the sun. We just have to wait for mostly sunny skies to use SODIS and place our bottle(s)
to maximize exposure. The surface where we place our bottle(s), should be 'shiny' so that
the suns' UV and IR rays can be reflected back into the water. Also, the surface should
be capable of absorbing and transferring IR heat. This is why corrugated metal roofs and
panels are so good. Finally, In addition to being made from clear PET plastic or glass, I
believe the bottle can be improved to focus and reflect sunlight thereby amplifying the
available UV/IR radiation that impacts the water.
My humble contribution to the field of SODIS is the simple observation that I could
greatly increase the sunlight/UV exposure of water in a bottle by adding up to four
overlapping ~11-inch strips of white plastic or aluminum foil duct tape (1.89 inch) to the
back of a clear SODIS bottle. I cover at least 1/3 but no more than 1/2 of the bottle
with this reflective tape. Figure 4 shows a SODIS bottle wrapped 1/2 with white
plastic duct tape. Next, I lay the bottle on a flat surface with the clear side centered
and facing the sun. Doing this, creates a small parabolic trough which focuses and
reflects the sunlight back into the water - giving bacteria a double whammy! Figure 5
shows a side-by-side comparison of a bright and shiny taped SODIS bottle next to a
basic SODIS bottle. Similarly, I have found that the same principle applies to water
bags. One side of a SODIS water bag can be treated with white plastic or aluminum
foil duct tape to make it more UV reflective. Bottles and bags shine so brightly
- it hurts my eyes. Further, with sunny skies and air temperatures of at least 85° F
(29° C), water in the bottle is heated to ~120° F (50° C) in about 5-6 hours.
Bottles treated with white plastic tape appear to reflect more bright diffuse light while
aluminum is more like a mirror where you can see the sun reflected better. Aluminum
has better parabolic properties and creates a warmer focal point in the center of the
bottle. Aluminum foil tape is more rigid and easier to handle than white plastic duct tape.
However, aluminum will begin to oxidize and discolor if exposed to water, so the surface
needs to be kept clean and dry. Plastic is far more water resistant. I have a hard time
choosing between the two, but plastic duct tape is ~50% less expensive than aluminum.
Moreover, by cutting a 2 liter soda bottle in half, lengthwise, and applying eight strips
of white or aluminum duct tape, I can make two reusable Solar Jackets which fit on any
other 2-liter SODIS bottle of the same dimension. Applying the tape this way allows us
to use the more shiny and bright side of the tape instead of the less shiny adhesive side.
The jacket has a durable reflective surface on the inside and a hard plastic shell on the
outside to protect it from surface friction. I also find that it is easier to maintain a few
reusable jackets than dozens of taped bottles. This is a savings over treating every bottle
with tape. Furthermore, I can think of no better use for a discarded plastic soda/water
bottle. About 85% of used plastic bottles are discarded and end up in landfills. This
includes millions of bottles, annually. The production of SODIS bottles and Solar Jackets
from recycled PET bottles will help ease both our safe water crisis and plastic pollution
problem. Figure 6 shows a comparison of jacketed aluminum and white plastic SODIS
bottles (left) with a basic SODIS bottle (right).
Another advantage of this method is that the surface where we place our bottles becomes
less relevant. Our main concern becomes exposure to sunlight. I have found that bottles
laid flat on the ground get the best exposure when positioned East to West. Bottles on a
roof or other sloped surface should be facing South (if north of the equator) or North
(if south of the equator). Using our Solar Jacket, we can increase the water temperature
by placing our bottles on dark (black) surfaces like asphalt, old concrete slabs or dark
soil. In some parts of the world, pasteurization temperatures could be reached. At this
time, I am not advocating that sunlight exposure time can be cut using this method, but
the time spent will be far more productive in killing harmful bacteria and virus. Morever,
this method could expand the use of SODIS to countries well beyond the equator.
Another recommendation is that when bottles must be laid flat, we can avoid bacteria
hiding under the bottle cap by replacing the cap with a recycled wine cork or soda bottle
stopper. A cork or stopper pushes most of the water out from the neck and into the
main body of the bottle. The goal is to spare bacteria no quarter, no safe haven. Below,
I show how to make a simple yet effective SODIS bottle stopper.
Finally, if we place jacketed or taped bottles on an uneven surface, the bottles may roll
around and block the sunlight. We can prevent this by placing multiple bottles together or
placing small pebbles or twigs to the underside of troublesome bottles/jackets. I have also
taped a couple of small pebbles to the underside of jackets for stability.
Steps for Making a Solar Jacket
Materials (brands in parenthesis)
All these materials may be purchased in the USA at most large hardware stores and
supermarkets. I found everything between Loews, Home Depot and any food market
that sells soda in 2 liter bottles. Everything you need is shown in Figure 7.
A clear PET plastic 2 liter bottle (recycled soda bottle).
Aluminum Foil Tape (Shurtape at Loews or Nashua 322 Aluminum Foil Tape, 1.89"
at Home Depot hardware)
White plastic duct tape (Shurtape white duct tape 1.88" at Loews hardware).
Wine cork (recycled)
.104"x 2 1/16" screw eye (at Loews hardware).
Tools (for cutting)
Razor knife, sheet metal sheers, and scissor
Step 1: Cut the bottle in half
You will be making two Solar Jackets from one 2 liter bottle. The bottle can be in poor
condition, including a discarded SODIS bottle. We are interested in its' frame not its'
appearance or ability to hold water. Remove the cap and any plastic rings about the
Step 2: Apply the reflective tape.
Apply four ~11 inch white plastic or aluminum reflective duct tape strips to the inside
of each half bottle section - one strip at a time. Avoid taping the neck and base parts.
After you finish applying the tape, cut the neck and base parts of the half sections away.
Begin cutting the bottle at the opening using your sheet metal sheers. Try to line up
with any existing seams, and if possible, sheer the bottle in half in one continuous cut.
You should end up with two equal half sections as shown in Figure 8.
The finished Solar Jackets are shown in Figure 9.
Step 3: Make a bottle stopper (optional)
Figure 10 shows how we make an inexpensive and effective SODIS bottle stopper with a
recycled wine cork and a .104 x 2 1/16 inch metal screw eye. The cork may be natural or
synthetic. The screw eye can be installed manually without tools. A small dab of glue may
be used on the threads of the screw for better security. Inserting the screw eye makes the
cork fit nice and snug into our bottle and provides a convenient handle to pull the cork out
later. I've held filled bottles upside-down and shaken them, without incident, using these
simple, handmade bottle stoppers. Later after disinfection, we may carefully replace the
stopper with a clean bottle cap for storage.
The Solar Jacket is a simple do-it-yourself tool for water disinfection using solar
energy. It is based upon the proven and reliable SODIS method that was developed
by EWAG and is endorsed by the The World Health Organization (WHO), UNICEF,
and the Red Cross. I have shown how we may amplify the germ killing power of
SODIS in about ten minutes and at a cost of ~50 cents with the Solar Jacket.
Figures 3 and 6 show the significant sunlight amplification that is possible using a
Solar Jacket. A snug, secure bottle stopper instead of a cap gives fewer places
for germs to hide. Further, with a few minutes instruction, the Solar Jacket can be
hand-made by any able-bodied person. Given the materials and simple tools, one
household in a developing country can make all the jackets they need in about an
hour. It may also be a good idea to encourage household cottage industries to
produce and sell Solar Jackets to other people in communities where they live.
The Solar Jacket and stopper will help provide both safe drinking water and an extra
income for enterprising people in developing countries. Finally, the Solar Jacket
may allow the application of SODIS well beyond the current +/- 35° latitude from
the equator. Many more lives may be saved worldwide.
Figure 1. Basic SODIS method = water bottle + 6 hours of sunlight.
Figure 2. Indonesian SODIS users (Source: SODIS Eawag)
Figure 3. Amplified SODIS method = water bottle + solar jacket +
bottle stopper + 6 hours of sunlight.
Figure 4. SODIS bottle wrapped 1/2 with white plastic duct tape.
Figure 5. Taped SODIS bottlle (left) compared to basic SODIS
Figure 6. Comparison of jacketed aluminum and white plastic SODIS bottles (left) with a basic SODIS bottle (right).
Figure 7. Materials and tools used in making a SODIS Solar Jacket.
Figure 8. Two equal half bottle sections.
Figure 9. Completed Solar Jackets - aluminum foil tape
(left) and white plastic duct tape (right)..