On top of the traditional diesel models, hybrid, electric and even hydrogen powered buses are now a common sight on the world's roads. But in the south-west of England, one scheme has taken things a step further, and started to fuel buses with food – and human – waste.
Manufactured by Scania, the Bio-Bus seats 40 and runs on biomethane gas, "generated through the treatment of sewage and food waste that's unfit for human consumption," according to the website of GENeco, the company behind the bus' power supply.
GENeco – a subsidiary of British utilities company Wessex Water – runs the Bristol sewage treatment plant that produces the gas. The Bio-Bus was preceded by GENeco's Bio-Bug, a VW Beetle that runs on human waste.
Late last year, the Bio-Bus made its debut, transporting passengers from Bristol Airport to the city of Bath. At the end of January, bus operator First West of England showcased the Bio-Bus in the center of Bristol, European Green Capital for 2015. The bus took passengers on short journeys through the city.
"People think it's a really wonderful idea: this thought that you can do something positive with human waste," James Freeman, Managing Director of First West of England, told CNBC.com in a phone interview.
"Of course it's food waste as well – and people do get that – but the thought you can do something with all that sludge really appeals to people," Freeman added.
GENeco say that 75 million cubic meters of sewage and 35,000 tons of 'inedible food waste' are treated at Bristol sewage treatment works per year.
This waste is processed by GENeco over 12 to 18 days through a process of anaerobic digestion. The result is a biogas 'rich in methane'. This biogas is further processed before being 'upgraded to be the same composition as natural gas'.
Sustainable and powered by renewable energy, the bus can, according to GENeco, travel a maximum distance of up to 300 kilometers on a full tank.
According to Mohammed Saddiq, Managing Director of GENeco, the environmental benefits of using the Bio-Bus are clear. "If you are to run a vehicle on biomethane, you will see a reduction in harmful emissions to the tune of 97 percent," he told CNBC.com in a phone interview.
"So quite a staggering reduction in the pollutants that are emitted in the biomethane bus when compared to diesel," Saddiq added. "That's primarily because it's a clean fuel; it has no particulates associated with it."
The source of the fuel used on the Bio-Bus is, of course, naturally abundant. According to GENeco, the sewage and food waste produced by just one passenger in a year "would fuel our Bio-Bus for 55km."
For Saddiq, the sustainable nature of the bus is another obvious plus point. "From a sustainability perspective, what was clearly very important was that, by utilizing what was in essence a renewable source of fuel, you were therefore actively displacing the use of fossil fuels," he said.
The Bio-Bus is set to return to Bristol soon, with plans to run the bus on a paid, city center route currently being finalized. "It's an extra bus over and above the normal service, and we want to try and run it over weekends and at times when people can go and have a ride on it," Freeman said.
"We're going to run it on the number two route in Bristol, because that seems to sort of ring a bell with people," he added.