Grow your own water filter?

A byproduct of fermented tea could find a new use in industrial water treatment, researchers at Montana Technological University have found, after taking the SCOBY membranes that would normally be thrown away, and using them to make water filters that are cheaper, last longer, and can be grown in your kitchen…

Kombucha, a popular fermented tea drink, is made by mixing tea, sugar, vinegar and a SCOBY: a symbiotic culture of bacteria and yeast. The living bacteria and yeasts are responsible for making the drink fizzy and acidic, and while they munch away at the sweet tea, they spread across the surface forming a network of cellulose fibres. Once finished, the fermented tea is bottled up to be sold, but the surface film is discarded. But researchers at Montana Technological University now believe these SCOBY films, which they are calling ‘living filtration membranes’, could be more useful than we thought.

The fine mesh of cellulose fibres make SCOBYs a suitable candidate for helping to filter out bacteria and other harmful microorganisms. One way to remove the bacteria is by pushing the water through ‘ultrafiltration membranes' that act like sieves, only allowing tiny particles through, and blocking larger ones. Katherine Zodrow explains, “they're very good at removing things like bacteria, but they're not going to remove things like salts, so you couldn't filter salt water and get something drinkable out of it.”

One major problem in the ultrafiltration process is biofouling: the gradual buildup of bacteria on the filter as the water is pushed through at high pressure. “When you have a lot of bacteria built up on that membrane surface, you have to press harder and harder, in order to get the clean water to come out. This ends up being a cost for the water treatment plant [...] and it also tends to decrease the life of the membrane.”

But living filtration membranes have the potential to overcome this challenge with ease: the bacteria still living inside the cellulose sheet reduce biofouling and clogging. Katherine thinks this is because a species of bacteria in the SCOBY, Acetobacter, produces acetic acid. This acidic environment is “a little more hostile to other types of bacteria that might want to live on your membrane,” favouring our friendly SCOBYs and preventing the rapid colonisation of harmful bacteria.

SCOBYs are also more environmentally friendly. In the commonly used membranes, Katherine explains that “the chemical solvents that are used are quite harsh and quite hazardous.” SCOBYs are grown in a liquid that is mostly water. “From a safety standpoint, that is fabulous.” They’re safe and simple enough that you can grow them in your kitchen. “They grow at room temperature, and really all you need is water, some black tea, sugar, some vinegar, and then the culture itself.”

One last word of warning from Katherine, it’s best not to try using home-grown SCOBYs to filter the water from your local river. “From a safety and ethical standpoint, I wouldn't go out somewhere where they didn't have easy access to water filtration and say, ‘Hey, just grow these and use them!’ Because we haven't seen how they perform in all sorts of different environments with all sorts of different water.” But Katherine is hopeful: “I think that with a little more research, [living filtration membranes] could be a form of accessible water treatment for many people.”