SODIS — Solar Water Disinfection

Use clear plastic bottles (PET plastic, food-grade) with a volume of 0.3 to 2 liters. Do NOT use green, blue, or any colored plastic bottles—they block essential UV-A radiation. Do NOT use glass bottles as they also block UV. The plastic must be transparent; you should be able to clearly see through the bottle when filled with water. Look for bottles that are scratch-free or have minimal scratches, as cloudiness from scratches reduces UV penetration. Empty water bottles, soda bottles, and clear juice bottles work well. Avoid opaque or translucent containers.

SODIS only works on clear water. If your water is cloudy, murky, or visibly turbid (muddy), you must filter it first. Use a clean cloth, coffee filter, or improvised cloth filter to remove particles and sediment. Hold the bottle up to light and perform the newsprint test: you should be able to read newsprint clearly through the filled bottle. If you cannot read text through it, the water is too turbid. Water with turbidity above 30 NTU (Nephelometric Turbidity Units) will not be effectively disinfected by SODIS. If the water is extremely muddy or contains visible sediment, filter it through multiple layers of cloth before proceeding.

Turbid water cannot be effectively treated with SODIS—clarity is essential.

Fill your clear PET bottles completely to the top with filtered water. Leave minimal air space—ideally just a small gap. Screw the cap on tightly to prevent contamination and evaporation. If you are using SODIS with citrus (optional but recommended), squeeze a quarter to half of a fresh lime or lemon into each bottle before sealing. The citrus juice acts as a photosensitizer, dramatically accelerating the kill rate of pathogens and reducing required exposure time from 6 hours to as little as 2–3 hours. Mix gently by turning the bottle a few times.

To maximize UV-A radiation exposure, place bottles on a reflective surface that bounces sunlight upward and sideways. Ideal surfaces include corrugated iron roofing, aluminum foil, white plastic sheeting, or a light-colored metal roof. If you have no reflective surface, bare ground or dark roof will still work, but exposure time will be longer. Ensure the surface is clean and dry. Position bottles so they receive direct, unobstructed sunlight throughout the day. Avoid placing them in shade or under trees.

Place sealed bottles horizontally (laying flat) on your reflective surface, directly in strong sunlight. In clear, sunny conditions with full UV-A radiation, bottles need a minimum of 6 hours of continuous exposure. In cloudy conditions, the UV intensity is much lower, so bottles must be left exposed for 2 consecutive days (48 hours) to achieve adequate pathogen inactivation. The bottles can be positioned upright or horizontal; horizontal placement may increase lateral UV exposure. Do not move bottles frequently; leave them undisturbed on the reflective surface for the required duration.

Cloudy conditions require 48 hours (2 full days) of exposure instead of 6 hours.

After the exposure period is complete, your water has been treated. UV-A radiation from sunlight has inactivated bacteria, viruses, and protozoa in the water. You can verify treatment by checking that the water has been exposed to adequate sunlight: in sunny conditions, 6+ hours of direct exposure is sufficient; in cloudy conditions, confirm 48 hours have passed. Water treated with SODIS will look and smell the same as before treatment, which is normal—UV disinfection does not change the appearance of water.

After SODIS treatment, keep the bottle sealed and store it in a cool, shaded location away from direct sunlight. Treated water remains safe to drink for approximately 24 hours if kept sealed and protected from contamination. Do not pour treated water into an open, unsanitized container, as this will recontaminate it. Consume the water within 24 hours of completing treatment for best results. If you need water for longer storage, you can repeat the SODIS process with fresh water.

Consume SODIS-treated water within 24 hours of completing exposure to prevent microbial regrowth.

SODIS is highly effective at killing bacteria, viruses, and protozoa, but it has important limitations. SODIS does NOT remove chemical contaminants, heavy metals, salt, or pesticides from water. It only inactivates living pathogens through UV-A radiation. If your water source is contaminated with chemicals, industrial pollutants, or salt water, SODIS alone is insufficient. For contaminated groundwater or water near industrial sites, use SODIS in combination with other filtration methods or seek uncontaminated water sources. SODIS is designed for pathogenic microorganism reduction, not for chemical detoxification.

SODIS does not remove chemical contaminants, heavy metals, or salt—only biological pathogens.