SODIS Method

SODIS Method: Bottle UV Exposure

Part of Water Purification

This article covers the practical execution of bottle-based solar disinfection — selecting, preparing, and deploying PET bottles for UV water treatment at scale.

From Theory to Bottles in the Sun

The Solar Disinfection article explains why SODIS works. This article focuses on how to execute it consistently, day after day, with maximum throughput and minimum error. The difference between SODIS as an idea and SODIS as a reliable water supply system comes down to bottle management, positioning technique, weather judgment, and workflow discipline.

In a post-collapse scenario, you will be running SODIS not once as an experiment but hundreds of times as a daily necessity. Small inefficiencies compound. A bottle placed in partial shade. A scratched bottle that blocks 40% of UV. Forgetting to oxygenate. These errors do not produce obviously bad water — they produce water that looks clean but harbors enough surviving pathogens to give you giardia in a week. Consistency is survival.

Selecting and Preparing Bottles

Finding PET Bottles

PET (polyethylene terephthalate) bottles are marked with the recycling symbol containing the number 1 on the base. They are the most common disposable beverage container worldwide. In any populated area, even years after collapse, you will find them in:

• Grocery stores and warehouses
• Vending machines
• Vehicles (cup holders, trunks, back seats)
• Trash bins and landfills
• Restaurant supply areas
• Recycling collection points

Ideal bottle characteristics:

• Clear, colorless PET (not green, blue, or brown)
• 0.5L to 2L capacity (1L is the practical sweet spot)
• Round cross-section (maximizes UV exposure angle)
• Intact cap with no cracks
• Minimal scratching

Bottle Inspection Checklist

Before using a bottle for SODIS, check the following:

Check	Pass	Fail
Material	PET, recycling #1	PVC (#3), HDPE (#2), or unknown
Color	Completely clear/transparent	Any tint: green, blue, brown
Scratching	Minimal, mostly smooth	Heavily scratched, opaque patches
Shape integrity	Round, holds shape	Crushed, dented, warped
Cap	Screws tight, no cracks	Missing, cracked, does not seal
Labels	Removed	Blocking light (remove them)
Smell	None after rinsing	Chemical odor (discard)

Scratched Bottles

Surface scratches scatter UV light, dramatically reducing the radiation that penetrates the water. Research shows that heavily scratched bottles can transmit 50% less UV-A than new bottles. Replace any bottle that you cannot read print through clearly. In a long-term camp, rotate your bottle stock every 3-6 months.

Preparing New Bottles

1. Remove all labels — soak in water for 10 minutes and peel, or scrape off with a blade
2. Rinse thoroughly 2-3 times with clean water to remove any residual beverage
3. Inspect for scratches and cracks under sunlight
4. Mark each bottle with a scratch number or symbol for tracking in your rotation system

The SODIS Procedure: Detailed Execution

Morning Setup (Perform Daily by 9:00 AM)

Step 1 — Collect source water. Use the cleanest available source. Pre-filter any turbidity through cloth, a sand filter, or a layered grass bundle. The water must be clear enough that you can read text through the filled bottle.

The Newspaper Test

Hold a filled bottle over printed text at arm’s length. If you can read the words, the water is clear enough for SODIS. If the text is blurred or invisible, filter the water further before proceeding.

Step 2 — Fill each bottle three-quarters full. Leave approximately 25% air space at the top.

Step 3 — Oxygenate. Cap tightly and shake vigorously for 20-30 seconds. You should see small bubbles forming and clinging to the inside of the bottle. This dissolved oxygen reacts with UV to create reactive oxygen species that enhance pathogen killing by 3-5 times compared to un-oxygenated water.

Step 4 — Top off and cap. Remove the cap, fill the bottle completely to the brim, and re-cap tightly. The air space has served its purpose — now you want maximum water volume with the dissolved oxygen trapped inside.

Step 5 — Position bottles. Lay horizontally on your exposure surface. Space them at least 5 cm apart so they do not shade each other. Ensure no shadow from trees, buildings, or other objects will cross the bottles during the exposure period. Think about where the sun will be in 3 hours and in 6 hours.

Positioning for Maximum Effectiveness

Surface Selection (ranked by effectiveness):

Surface	Effectiveness	Why
Corrugated metal roof	Best	Reflects UV from below, heats bottles, natural groove holds bottles stable
Aluminum foil on flat surface	Excellent	Maximum reflection, easy to set up
Car hood/roof (dark color)	Very good	High thermal absorption boosts water temperature
White-painted board	Good	Reflects light without absorbing excess heat
Dark rocks or asphalt	Good	Thermal boost, less UV reflection
Bare ground	Adequate	Minimal enhancement but functional
Grass	Poor	Insulates from heat, may shade bottle bottom

Angle: Horizontal placement exposes the maximum bottle surface area to direct overhead sun. Do NOT stand bottles upright — the small cap area catches almost no UV.

Orientation: Align the bottle axis roughly east-west so the broadest face catches sun as it arcs from east to west through the day.

Timing Decision Matrix

Conditions	Action	Exposure Time
Clear sky, strong sun, warm day	Standard SODIS	6 hours
Partly cloudy, sun breaking through	Standard SODIS	6 hours
Overcast but bright (can see shadows)	Extended SODIS	2 consecutive days
Dark overcast, no shadows visible	Abort SODIS	Use boiling or chemical treatment
Rain	Abort SODIS	Use boiling or chemical treatment

The shadow test: If you can see your shadow on the ground, there is enough UV for SODIS to work. If you cannot see your shadow, UV levels are too low.

Retrieval and Storage

Step 6 — Retrieve bottles after exposure period. Do not retrieve early. If you placed bottles at 9 AM, they are ready at 3 PM on a full-sun day. If clouds moved in at noon, extend to the next morning or switch methods.

Step 7 — Label or group by date. Mark the treatment date on the bottle with a scratch, marker, or rubber band system. You need to know which bottles are treated and which are waiting.

Step 8 — Store in shade. Once treated, move bottles out of direct sun to prevent algae growth. Store upright with caps tightly sealed. Consume within 24 hours — SODIS provides no residual disinfectant, so recontamination is possible over time.

Scaling SODIS for Groups

Daily Water Requirements

Group Size	Bottles Needed Daily (1L)	Total Bottles in Rotation
1 person	3-4	8-10
Family of 4	12-16	30-40
Group of 10	30-40	80-100
Community of 50	150-200	400-500

The rotation column accounts for bottles in sun (today’s batch), bottles cooling/being consumed (yesterday’s batch), and spare bottles for cloudy-day buffer stock.

Camp SODIS Station Design

For a permanent or semi-permanent camp, build a dedicated SODIS station:

1. Exposure rack: A south-facing (Northern Hemisphere) or north-facing (Southern Hemisphere) angled surface at roughly 30 degrees from horizontal, covered with reflective material. Corrugated metal sheeting is ideal. Nail or wire small wooden stops to prevent bottles rolling off.

2. Pre-filter station: A sand/charcoal filter gravity-feeding into a collection vessel where bottles are filled.

3. Labeling system: Use colored rubber bands, string ties, or a scratch code. Monday = 1 scratch, Tuesday = 2, etc.

4. Buffer stock: Keep 2 days’ worth of treated water in sealed bottles stored in shade. This covers cloudy days when SODIS output drops to zero.

Troubleshooting

Problem	Cause	Fix
Water still cloudy after treatment	Insufficient pre-filtration	Filter again; never SODIS cloudy water
Bottles fog up inside	Temperature differential (normal condensation)	Not a problem — disinfection still works
Green tint developing in stored bottles	Algae growth from sunlight during storage	Store treated bottles in darkness; consume within 24h
People getting sick despite SODIS	Insufficient exposure, scratched bottles, or cloudy water	Audit every step; replace old bottles; enforce clarity test
Not enough bottles for the group	Supply shortage	Scavenge more; consider a mix of SODIS and boiling
Caps degrading	UV breaks down plastic caps faster than bottles	Replace caps from discarded bottles; stockpile extras

Combining SODIS with Other Methods

SODIS alone is sufficient for most bacterial and viral pathogens in clear water under good sun conditions. For higher confidence:

• Pre-filter + SODIS: Standard protocol, handles all common scenarios
• Pre-filter + SODIS + lime/lemon juice: Adding 30ml of citrus juice per liter before sun exposure accelerates pathogen killing (the acidity synergizes with UV). Effective if you have citrus access.
• SODIS + chlorine: Treat with a half-dose of chlorine (1 drop/liter) before sun exposure. The chlorine provides residual protection, and the UV handles organisms that resist low-dose chlorine. Good for higher-risk water sources.

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Key Takeaways

• Use only clear, minimally scratched PET bottles (recycling #1), 0.5L-2L, labels removed
• Oxygenate by shaking 20-30 seconds with air space, then fill completely and cap
• Lay horizontal on a reflective surface; ensure no shadow will cross bottles during exposure
• The shadow test: if you can see your shadow, UV is strong enough for SODIS
• 6 hours full sun, 2 days overcast, abort in rain or dark cloud cover
• Consume within 24 hours — no residual disinfectant means recontamination is possible
• For groups, maintain a 2-day buffer stock of treated bottles for cloudy-day coverage
• Replace scratched or aged bottles every 3-6 months — UV transmission degrades with wear