Blowing hot CO2 bubbles to purify wastewater

Access to clean water is something that many take for granted, but is actually a serious problem across much of the world. Now researchers at the University of New South Wales (UNSW) Canberra have tested a new method for sterilising water using hot bubbles of carbon dioxide, which they’ve found to be both effective and efficient.

In the new method, CO2 gas is first heated to a set temperature, then piped into a tank of wastewater. As these hot bubbles rise up through the water, they transfer heat to the water around them, and the hot surface layer of the bubbles is enough to kill bacteria and viruses in the liquid. Unlike previous attempts at similar systems, the CO2 doesn’t need to be pressurised to do the job.

In experiments, the researchers heated gas to a variety of temperatures, then bubbled it up through a sodium chloride solution that had been laced with E. coli bacteria and MS2 viruses. To test which configurations worked best, the team compared both carbon dioxide and plain old air, heating the gases to temperatures ranging from 7° C to 205° C.

Unsurprisingly, at low temperatures there was basically no antimicrobial effect at all. Both air and CO2 worked better when the heat was cranked up, but CO2 consistently killed the bacteria and viruses faster.

The CO2 was found to be most effective at temperatures between 100° C and 205° C. As might have been expected, the hotter the gas, the faster it managed to destroy the bugs. Meanwhile, the liquid itself stayed at a comfortable 55° C.

The team says the method has several benefits over existing water purification techniques. Since it takes less energy to heat gas than to boil liquids, the method is more energy efficient. It’s also less potentially harmful than chemical methods like adding chlorine, and it’s a simpler system than using UV radiation. And finally, carbon dioxide is a common by-product of industry, so finding uses for it is particularly handy.

“This new technology could become a new sterilisation technology candidate able to compete with the existing ones,” says Adrian Garrido Sanchis, an author of the study. “The fact that the process can use heated CO2 gas instead of heated water and the possibility of reusing exhaust gas from combustion processes makes the new process potentially more energy efficient.”

A small pilot plant for the new wastewater-sterilising technique has been built and tested in a pig farm, with promising results so far.