They're miraculous in their own way, even if they don’t quite turn water into wine. Personal water treatment plants could soon be recycling our waste water and producing energy on the side.
Last month, Boston-based Cambrian Innovation began field tests of what’s known as a microbial fuel cell at the Naval Surface Warfare Center in Maryland. Called BioVolt, in one day it can convert 2250 litres of sewage into enough clean water for at least 15 people. Not only that, it generates the electricity to power itself – plus a bit left over.
This is a big deal, as conventional treatment plants guzzle energy – typically consuming 1.5 kilowatt-hours for every kilogram of pollutants removed. In the US, this amounts to a whopping 3 per cent of the total energy demand. If the plants could be self-powered, recycling our own waste water could become as commonplace as putting a solar panel on a roof.
Existing treatment plants use bacteria to metabolise the organic material in waste water. “There’s lots of food for them, so they reproduce fast,” says Cambrian chief technology officer Justin Buck. At the end of the process, the microbes can make up a third by weight of the leftovers to be disposed of. Before being put in landfill, this “microbe cake” itself needs to be heat-sterilised and chemically treated, which uses a lot of energy.
Microbial fuel cells have long been touted as the way forward. The idea is that the biochemistry involved in metabolising the contaminants can yield electricity to help power the process. But fuel cells of this kind have been very difficult to scale up outside the lab.
BioVolt uses strains of Geobacter and another microbe called Shewanella oneidensis to process the sludge. Its proprietary mix of organisms has one key advantage – the bacteria liberate some electrons as they respire, effectively turning the whole set-up into a battery. This has the added benefit of slowing bacterial growth, so that at the end of the process you have electricity and no microbe cake.
A number of teams are working on their own versions of these cells. Orianna Bretschger at the J. Craig Venter Institute in San Diego, California, is testing hers at a farm run by the San Pasqual High School in nearby Escondido, using it to process about 630 litres of pig waste per day.
Towards renewable water
Bretschger is in the early stages of building a larger pilot system, to be commissioned in Tijuana, Mexico, later this year. “I think that we will still be on track for commercialisation in the next three to five years,” she says.
Her system goes a step beyond BioVolt and traditional plants in that it can rid water of pharmaceuticals – synthetic oestrogens, for example. Bretschger is now looking at ways to add pain relief drugs to the list.
Cambrian CEO Matt Silver sees a future in which different kinds of microbial fuel cells treat different kinds of waste, perhaps recovering useful by-products. Another of the firm’s designs, EcoVolt, generates methane as it cleans up waste water produced by a Californian brewery. It has also cut the brewery’s energy use by 15 per cent and its water use by 40 per cent.
Cambrian hopes BioVolt will scale up to processing more than 20,000 litres per day. Microbial fuel cells, Silver thinks, will do for renewable water what solar and wind did for renewable energy.