Water Purification — Solar and Vacuum Distillation

What is it?

Methods for turning water you already have — but which is contaminated, salty, or laden with pathogens — into clean drinking water via evaporation and condensation. (This is not about obtaining water — for that, see Dew Harvesting and Fog Net — but about purifying the water you have.)

What is it good for?

According to the WHO/UNICEF Joint Monitoring Programme, an estimated 2 billion people worldwide lack access to safely managed drinking water — often because the available water is contaminated or salty, not because there is no water nearby at all. Distillation solves exactly this problem: it produces clean distillate from any water source (seawater, polluted surface water, even urine).

The physics behind it

When water is heated to its boiling point, it turns to vapor, but the dissolved salts, pathogens, and most contaminants do not evaporate with it — they stay behind in the liquid phase. Condensing the vapor on a separate, cooler surface yields clean water.

The trick of vacuum distillation follows from the Clausius–Clapeyron relation: water's boiling point depends on the surrounding pressure — the lower the pressure, the lower the temperature at which water boils. At about 0.1 atmosphere (roughly 10 kPa), water boils at ~45-46 °C (113–115 °F) instead of 100 °C (212 °F). This drastically reduces the energy required — in exchange, you need a vacuum pump and an airtight system.

History

The principle of distillation is ancient: Aristotle (4th century BC) already described desalination based on evaporation. Medieval Arab alchemists refined the distillation apparatus (the alembic), the ancestor of today's distillation equipment.

Simple version

The solar still. Dig a pit, place moist soil or vegetation at the bottom (or contaminated water in a container), cover the pit with clear plastic sheeting, and set a stone at its center so the film slopes down to a single point. The sun heats the pit, the moisture evaporates, condenses on the underside of the film, and the slope makes it trickle into a collection vessel placed at the center. Its daily yield is small (about 1 liter/day), but it needs zero energy input and can be built from nothing but materials.

Advanced version

The solar still's efficiency can be raised with a larger collection area, double glazing (thermal insulation), and insulation of the basin. Another direction: a small-scale, fuel-heated still (a simple "pot still"-style build with a condenser coil) — higher yield, but now requiring energy input.

Industrial version

At industrial scale, vacuum distillation (multi-stage flash distillation, vacuum multi-effect distillation) is one of the standard desalination processes — thanks to the lower boiling point, it requires significantly less energy than atmospheric distillation. As an alternative, reverse osmosis is a membrane-based, non-thermal solution — worth noting that it is a fundamentally different method, not a refinement of distillation.

Building your own

The basic layout of a home vacuum distiller: a sealed, heatable tank (the "boiler side"), a vacuum pump connected to it (even a simple diaphragm pump or aspirator is enough for a partial vacuum), and a condenser coil where the vapor cools back into liquid. The critical point is the airtight seal — if air leaks in, the vacuum is lost and the system reverts to the atmospheric boiling point.

(This is a design/reference description — the actual build, measurements, and efficiency documentation are still to come. Once complete, the real build log, photos, and measurement data will go here — that is what makes the "Proof of Knowledge" difference over a mere write-up.)

Common mistakes

• Solar still: the film is not sealed airtight; a bad slope angle that lets condensed water drip back into the pit instead of the collection vessel; without UV-resistant film, the sheeting cracks apart within a few weeks
• Vacuum distiller: poor sealing (in-leaking air kills the vacuum); recontamination of the distillate through inadequately cleaned tubing; underestimating the energy demand (the vacuum pump's own power consumption)

How to measure

TDS meter (dissolved solids content, ppm), electrical conductivity measurement, a simple pH test, microbiological quick-test strips (pathogen presence), and measuring the evaporation/condensation yield (ml per m² of surface per day).

Videos

(TODO)

Downloadable PDF

(TODO)