Solar Water Disinfection (SODIS) purifies water using UV-A radiation from sunlight — no filter, no chemicals, no equipment beyond a clear plastic bottle. Fill a clear PET bottle (type 1 recyclable, the recycling number stamped on the bottom), lay it in full sunlight, and wait. The WHO field validation protocol: 6 hours in direct sun with a UV Index above 2 reduces bacteria and protozoan counts to safe drinking levels. This article is a companion to water purification without boiling — it covers SODIS in full detail.
The Science: UV-A Radiation and Pathogen Inactivation
SODIS works through two simultaneous mechanisms. UV-A radiation (wavelengths 320–400 nanometers) damages pathogen DNA and RNA, preventing replication. In parallel, solar heating — when water temperature exceeds 50°C (122°F) — enhances UV-A efficacy synergistically. A 2008 study published in Applied and Environmental Microbiology confirmed that the combination of UV-A and heat above 50°C reduces E. coli O157:H7 to undetectable levels 3× faster than UV-A alone.
SODIS inactivates bacteria and protozoa — including Cryptosporidium parvum and Giardia lamblia — through UV-A exposure. Unlike chemical treatments, SODIS kills Crypto without extended contact times, because the inactivation mechanism (DNA damage) is not the same as the chlorine-resistance mechanism of Crypto oocysts. The WHO endorses SODIS for household water treatment in resource-limited settings based on field trials in over 50 countries. SODIS does not reliably inactivate viruses in the standard 6-hour protocol — in scenarios with high viral contamination risk (post-flooding, sewage intrusion), pair SODIS with chlorine dioxide treatment.
Bottle Selection: PET vs Glass vs Other Plastics
Not all clear bottles work for SODIS. The bottle must transmit UV-A wavelengths to the water — and several common bottle materials block them:
| Bottle type | SODIS effective? | Why |
|---|---|---|
| Clear PET plastic (type 1) | Yes | High UV-A transmittance, widely available |
| Clear glass | Marginal | Blocks significant UV-A; requires 2× exposure time |
| Green/blue/tinted PET | No | Tint absorbs UV-A wavelengths needed for disinfection |
| PVC plastic | No | Low UV-A transmittance, may leach chemicals |
| HDPE (opaque white/colored) | No | Opaque — blocks all UV-A |
| Polycarbonate (hard Nalgene) | No | Contains UV stabilizers that block UV-A |
The standard single-use water bottle — Dasani, Aquafina, Smartwater, and most store-brand water bottles — is clear PET (type 1) and works for SODIS. Bottles 2 liters or smaller provide adequate UV-A penetration to the center of the water column. Bottles larger than 2 liters have insufficient UV-A depth penetration and are not recommended for SODIS.
Scratched or cloudy PET bottles reduce UV-A transmittance. Inspect bottles before use — replace any bottle with significant surface scratching or yellowing. A bottle that has gone opaque from UV degradation over repeated outdoor use no longer works for SODIS.
Exposure Time: Clear Days, Overcast Days, and UV Index
SODIS exposure time depends on UV Index and sky condition:
| Condition | Required exposure |
|---|---|
| Sunny — UV Index ≥ 2, full direct sun | 6 hours |
| Partly cloudy — >50% sunshine | 6 hours |
| Overcast — <50% sunshine | 2 full days |
| Rain or heavy overcast | Not reliable — use chemical or filter treatment |
UV Index 2 is the minimum threshold for effective SODIS in the standard 6-hour protocol. At UV Index 2, direct sun at midday feels warm but not intense — early spring and late fall sun in the continental US typically exceeds UV Index 2 between 10 AM and 2 PM. Check the UV Index via a weather app before relying on SODIS; in deep winter at northern latitudes, midday UV Index may fall below 2 even on clear days.
Practical shadow test for UV Index sufficiency: if you can see a well-defined shadow from an object in direct sun, UV Index is likely above 2. If your shadow is faint or absent on a bright overcast day, plan for the 2-day protocol.
Turbidity: The Hard Limit
SODIS requires clear water. Suspended particles scatter UV-A radiation before it reaches pathogens, reducing efficacy proportionally to turbidity. The WHO limit for reliable SODIS is 30 NTU (Nephelometric Turbidity Units). At turbidity above 30 NTU, UV-A penetration to the center of a 1.5-liter bottle is insufficient for reliable pathogen inactivation.
Field assessment without a turbidimeter: pour water into a clear 1-liter bottle and hold it to the sky with your hand behind it. If you can see your hand clearly through the bottle, turbidity is likely below 30 NTU and SODIS is appropriate. If the water is visibly cloudy, murky, or discolored, pre-treat before SODIS:
- Settle first: Let turbid water stand undisturbed for 1–2 hours. Most suspended sediment settles; carefully pour the clear upper layer into the SODIS bottle without disturbing the sediment.
- Pre-filter: Pour through a bandana, coffee filter, or hollow-fiber filter like the Sawyer Squeeze to remove suspended particles before SODIS.
Optimization: Reflective Surfaces and Bottle Placement
Three techniques increase SODIS effectiveness:
- Reflective surface: Place bottles on aluminum foil, a car hood, metal roofing, or a Mylar emergency blanket. Reflected UV-A from below doubles the effective irradiance on the bottle. A Mylar emergency blanket (approximately $2 and 1 oz weight) provides the best available reflective surface in a field kit.
- Horizontal orientation: Lay bottles on their side rather than standing upright. Horizontal orientation maximizes the water surface area exposed to direct UV-A and increases the heat-absorption surface.
- Dark background below: A black plastic bag, dark rock surface, or asphalt beneath a Mylar reflector can raise water temperature toward the 50°C synergy threshold, increasing efficacy beyond UV-A alone.
What SODIS Does Not Cover
SODIS does not remove or inactivate: heavy metals, nitrates, pesticides, chemical contamination, or turbidity itself. It is a biological disinfection method for bacteria and protozoa in clear water. For water from sources near agricultural runoff, industrial sites, or post-flood conditions with chemical contamination, SODIS alone is insufficient. In those scenarios, activated carbon filtration or distillation are the appropriate methods — beyond the scope of field SODIS.
Where to Go Next
The full comparison of all seven water treatment methods is in water purification without boiling — the parent article for this series. For chemical treatment options that cover viruses and provide faster treatment times, see chemical water treatment dosages. For mechanical filtration that handles turbid water and removes Crypto without extended wait times, see best survival water filters: Sawyer vs Berkey vs Lifestraw.
