Floating capsule auto-disinfects water without chemicals or battery

An international team of scientists has created a portable water purifier that works simply by shaking it for three seconds and setting it to float in wild water. It could be a breakthrough device for water treatment in emergency situations and, best of all, it’s estimated to cost less than $25 to produce.

This low-cost and simplicity of operation disguise the sophistication of the science involved however: shaking the canister generates enough power for it to auto-analyze the water and activate an appropriate disinfection process. It then floats in the liquid and, as it bobs, movement of water against its surface creates an electrostatic charge that kills microbes.

That extraordinary combination of processes can disinfect a liter of water in 20 minutes, to provide reliable safe drinking water without needing any chemicals, battery or external power source.

The capsule was devised by a collaboration based at Yonsei University in Seoul, South Korea and led by Professor Sang-Woo Kim but has input from scientists at the Korea Institute of Science and Technology, the University of Bath in the UK and Renmin University of China.

The device’s ingenious working process was explained in a recent paper in Nature Water. It is based on the combination of two state-of-the-art systems.

The first system is the shaking: just three seconds is enough to jostle the built-in magnet through a copper coil to generate enough current to power an internal electric field and sensor. This analyzes a small sample of the water in the canister by measuring the amount of dissolved material in the water, mostly ions such as salts and minerals. The device registers this as a warning signal for possible chemical contamination.

If the water measures below 250 milligrams of ‘total dissolved solids’ per liter, that’s the green light. The canister treats the water as low contamination risk and proceeds to microbial disinfection. If the reading is higher, it flags the water as unsuitable and you have to find another patch of water. While it’s good at killing microbes, the system can’t deal with a chemical contaminant, like an arsenic spill, pesticides or industrial pollutants.

Stage two is enacted once the device gives itself the chemical all-clear. This involves letting the canister float right in the water it is going to treat.

As it bobs along, the water laps against the plastic shell. This generates a tiny electrostatic charge, like when you rub a balloon against your jumper. The plastic surface becomes positively charged and the water becomes negatively charged. And here’s the clever bit: the surface on the outside of the capsule is covered with microscopic polymer ‘nanorods’ whose tiny tips concentrate the charge into minuscule but very strong electric fields. They’re powerful enough to damage any microbes in the water nearby. The process of destroying them by breaking open their bacterial membranes is a little-known but established science known as electroporation.

In the Korean-based study the self-powered electroporation destroyed common lab organisms like Escherichia coli, Bacillus subtilis and virus surrogates used to quantify water studies. The capsule removed all E. coli and MS2 in one liter within 20 minutes. Adding B. subtilis took the time to about 25 minutes. In all the tested river and lake water complete disinfection was achieved within 30 minutes. Cleaning four liters of river water took 52 minutes.

The significance of the process is massive. More than two billion people around the world lack the safe normal drinking water that the first world takes for granted. Add in emergency disaster situations and the potential applications of this device are huge.

Normal strategy in disaster situations is boiling water to purify it but this uses valuable fuel resources. Chlorine tablets, if available, have an unpleasant taste and limited effectiveness. Filters are comparatively expensive and clog, break and have limited life span. Purification devices usually depend on chemicals, batteries, expertise and maintenance.

This new canister however maintained its cleaning performance through 120 repeated disinfection cycles in four liters of river water (ie: it purified 480 liters of water just with a few shakes to set it off). Yet the researchers say the estimated manufacturing cost of a canister is under US$25 – and could be a lot less if production scales up.