When the scriptwriters of the cult ’80s movie Back to Future dreamed up the idea of sending a waste-fuelled car to 2015, they could hardly have predicted that the technology would actually be in existence in that year.

Although the waste-to-fuel formula is a little different (the DeLorean’s use of valuable metal cans for energy feedstock is hardly environmentally or economically sensible!), transport companies in the UK are increasingly utilising organic waste to fuel buses. Indeed, they seem to be doing it well; Reading Buses’ Bus Hound, which runs on biomethane from cow manure, broke the bus land speed record (75 miles per hour) earlier this year!

However, waste recycling and renewable energy firm GENeco, a subsidiary of Wessex Water, is taking the technology one step further, by utilising sewage waste in the UK’s first (human) ‘poo bus’.

The aptly-named ‘Number 2’ bus, officially called the Bio-Bus, can now be seen transporting up to 40 passengers across the city of Bristol. It’s hard to miss, seeing as the side of the vehicle is decorated with images of people sitting on toilets. But it’s the gas recovered from human waste, not the poo itself, that fills the fuel tank.

Surprisingly though, some of the passengers I meet riding the Bio-Bus seem disappointed by this: “I thought there was actual poo in the fuel tank – but you’re saying it’s just gas? That’s less exciting than I thought”, says Tom Barnfield, 23. When I highlight that it’s the gas from processing food and human waste, Barnfield allows that the technology is “cool”, and he “would never have thought you could do that with all that gross stuff”.

So where did the idea for all this come from? Certainly not from Back to Future’s Dr Emmett Brown, as Mohammed Saddiq, Managing Director of GENeco, tells me: “I’m afraid to say the car in Back to the Future did not feature in our thinking… though now you mention it, the link is plausible!

“It all started when we were researching ways to make the most of what we were producing. We’re operating within the philosophy of a circular economy, which not only helps us to reduce our impact on the environment (thus contributing less to climate change), but also reduce the amount of waste we are producing, consume [fewer] resources and make us more productive – which in turn reduces our unit costs and increases our competitiveness. I like to think of this place as a resource factory – we’re producing a range of resources that people need from materials that they don’t want anymore.

“In terms of the biogas, we knew that biomethane could be used as a fuel, and when we started researching the applications in which it could be used, we found that it could be used in buses.”

In fact, Saddiq says that the team was “surprised and shocked” to find that UK cities are in breach of the EU Air Quality Directive, and that around 30,000 deaths occur in the UK each year due to problems associated with air pollution. This is exacerbated by particulate emissions from diesel buses, which repeatedly stop-start on the same routes in inner city areas.

He adds: “Biomethane can reduce particulate emissions by up to 97 per cent, so the idea was to provide thought leadership to some of the air quality issues we face in our cities. We wanted to demonstrate, in a whacky way, that running a bus on waste could not only reduce the carbon impact of public transport by about 30 per cent [compared to fossil fuels], but also improve air quality through the significantly reduced emissions. It helps that the fuel is ultra reliable and very quiet too!”

Harnessing the power of poo

So how does it all work? Receiving a guided tour from Saddiq when I visit GENeco’s Bristol sewage treatment works, I learn that the process of generating the biomethane is astonishingly simple. It’s largely about harnessing natural processes to produce energy (and a biofertiliser byproduct) from two waste sources: sewage and food.

Around 300 million litres of Bristol’s sewage waste is pumped to the GENeco site every day. First, the sewage is screened to remove grit, and larger objects, such as plastic waste and paper, are sent for composting in GENeco’s patented CompAer process. This process produces a biofertiliser from the organic waste, and a refuse-derived fuel from the non-organics.

The sewage is then pumped into settlement tanks, where clear water is siphoned off for further purification (with the water effluent then sent to a nearby power plant to be used in cooling processes), and the remaining sludge is collected, thickened, and sent to enzymic hydrolysis tanks before being treated through anaerobic digestion (AD).

Once in the AD tanks, the sludge is heated to around 35 degrees and – in the absence of oxygen – fermented by microorganisms into biogas (comprising methane and carbon dioxide), as well as a digestate for use as
a biofertiliser.

Biogas from this process, and that produced from anaerobically digesting Bristol’s 10,000 tonnes of food waste, is then upgraded into purified biomethane using a piece of ‘water scrubbing’ kit from Swedish cleantech company Malmberg. This equipment sees biogas pumped up huge silver columns alongside water, which absorbs the carbon dioxide and other remnant gases. The gas is then purified by passing through activated carbon before being cleaned up with propane and compressed into tanks that are stored at the Bio-Bus refuelling station at GENeco’s Bristol site. Around 17 million cubic metres of biomethane also go to the gas grid for heat and electricity generation.

According to Saddiq, the bus, run by First Group, can cover around 300 kilometres (or around 186 miles) with a full tank – more than enough to complete its 16-mile round trip from the shopping centre at Cribbs Causeway in South Gloucestershire to Stockwood in south Bristol.

Although the Bio-Bus is currently only operating on a trial basis, First Group has said it will ‘certainly look [into] running more of the buses in future, if this trial goes well’. However, it added that it would need to take into consideration a new site for refuelling, as it would be ‘impractical’ to send several buses to GENeco’s Avonmouth site, eight miles from the centre of Bristol, each day.

James Freeman, Managing Director of First Bristol & West of England, adds: “Since its original unveiling last year, the Bio-Bus has generated worldwide attention, and so it’s our great privilege to bring it to the city, to operate – quite rightly – on Service 2.

“The very fact that it’s running in the city should help to open up a serious debate about how buses are best fuelled, and what is good for the environment. In this, Bristol [European] Green Capital year, that conversation is more welcome than ever.”

In keeping with the city’s green credentials, Saddiq tells me that GENeco aims to eventually become a ‘closed-loop’ company, utilising all materials and gases on site for “something effective, efficient and environmentally and economically beneficial”.

With aspirations such as this, and for putting unpleasant wastes to good use, there is surely little reason to pooh-pooh GENeco. Here’s to the future…