Solvatten made sunlight purify drinking water in a folding 10-litre jerrycan

Household water treatment in off-grid regions has always carried a cost: boiling burns fuel that is scarce, chlorine needs a supply chain and a habit, and the do-it-yourself standard, leaving clear bottles in the sun (SODIS), is slow and fails when the water is murky. Petra Wadström, a Swedish inventor, spent eleven years developing an answer and unveiled it as Solvatten — a portable container that from the outside resembles a standard jerrycan but opens out to treat up to 10 litres of water at a time.

The design attacks the two classic failure modes of point-of-use treatment. Turbidity is handled by an in-built filter that removes particles, including faecal coliforms, at different turbidity levels; uncertainty is handled by an indicator that shows a red face when the water is unsafe and a green one when the sun has done its work, so no technical skill is needed to know when to drink. Wadström put the logic plainly: many people live with dirty water that is easily preventable, with plenty of solar energy going unused.

Validation came from the field and the lab. Trials in Kenya gave early positive feedback, and in 2007 Nepal's ENPHO tested the unit under UN-HABITAT across three climatic zones — Solukhumbu in the mountains, Kathmandu's hills and Hetauda in the Tarai — concluding it was feasible for the country, with water disinfected after roughly four hours in a sunny spot. A 2024 microbiological evaluation reported inactivation of 98.77% for coliforms and E. coli and 99.90% for Salmonella, cutting bacterial populations by 2 to 4 log units.

Fold a treatment plant into a jerrycan run by sunlight

Sunlight is free where fuel is scarce, so the recurring cost of safe water falls to zero.

The filter removes turbidity first, preventing shadowed microbes from surviving the dose that should kill them.

The red-green indicator removes the guesswork that makes other treatment methods unreliable in practice.

Compared with bottle-based SODIS, it disinfects faster and carries its own filter and indicator, trading a small manufactured shape for reliability.

Where users lack power, fuel or training, the winning design has no moving parts and a self-explaining interface: let the free resource do the work and let the product say when it is done.

The Nepal study confirmed the unit across mountain, hill and plain, and Solvatten settled into the shortlist of point-of-use treatment options. Swedish university assessments measured its footprint and the value added for families before and after adoption, and laboratory work has continued to confirm its microbiological performance years after launch — 99.9% Salmonella inactivation in 2024, in a unit that still asks nothing of its user except sun.