The world’s largest waste-to-biogas conversion plant is gearing up for a grand opening next spring in Northwich, north-west England, using advanced enzyme technology to handle unsorted domestic waste.
Around 15 tonnes of trash per hour – 120,000 tonnes a year – will be sorted at the REnescience plant in a process that will power nearly 10,000 homes, according to Dong Energy, which runs the project. The Danish energy company has invested £60m in the Northwich venture, which will use trash collections by local councils in Chester and Wigan to produce 5MW of renewable energy.
Its energy conversion process works by separating organic matter such as paper and foods from other waste streams and then “washing” it free of contaminants using enzymes in a giant reactor. “The enzymes take out all the organic compounds in the waste material and liquefy them so that, after washing, they come out as the perfect sludge, full of fats and oils, that can be converted to biogas,” says Thomas Schrøder, a bio-refining vice-president for Novozymes, which developed the enzymes.
The slurry generates a gas byproduct when eaten by naturally-occurring bacteria, which in turn creates heat and energy when burned in the plant’s on-site engines.
Other waste streams such as plastics and glass are syphoned off for recycling. Digestate from the process – a fibrous plant matter residue – is saved for land remediation, as a substrata top soil on surfaces such as short-rotation energy crop fields. Any non-recyclable materials, such as shoes, wood pieces, textiles and foils, are isolated for incineration – a process that can create energy but also releases compounds that cause air pollution – or use as fuel in cement kilns.
Academics estimate that better use of waste resources could save Europe €600bn by 2030. The environmental case is even more clear cut – the UK currently dumps just over a quarter of all waste in landfill sites, the most ecologically damaging form of waste disposal.
Over time, landfilled TVs, computers and electrical appliances leach poisons such as mercury, arsenic and cadmium into the soil and groundwater. Decomposition processes in compacted landfill sites also create large quantities of methane, a greenhouse gas up to 35 times more potent than carbon dioxide over the course of a century.
Thomas Dalsgaard, an executive vice-president for Dong, describes the plant as “a potential game changer” for the waste industry. “It is novel in its approach, it addresses society’s needs and it has a large commercial potential,” he says.
A demo plant has been up and running in Copenhagen since 2009 and, with sister projects under construction in the Netherlands and Malaysia, Dong is talking up the potential for global expansion.
“The sky is the limit,” says Flemming Kanstrup, project director for REnescience Northwich. “This technology will be applicable anywhere in the world where you have a lot of unsorted municipal waste with a high organic content.”
But, exciting as the REnescience solution sounds, some environmentalists fear that it could damage the green dream of a “circular economy” in which all waste is separated by householders for focused reuse, reaping greater efficiency benefits.
The European Environmental Bureau (EEB) argues that REnescience-style plants may create an industry with its own demand for unsorted rubbish, and an interest in resisting separated municipal collections.
Stephane Arditi, an EEB spokesman, casts doubt on the “game-changing” nature of this technology: “applying well known technologies on mixed municipal waste reduces REnescience’s claims to a simple marketing slogan.”
“REnescience will only recycle a minor part of its mixed waste. The organic residues from the process cannot be spread safely on land and will just feed incinerators or other combustion processes, and eventually landfills. Burning material remains a failure in a circular economy perspective. There are much better solutions that could be implemented.”
Even after incineration, some waste remains – such as bottom ash – are landfilled.
Dalsgaard argues that Dong is already working on ways to improve the technology’s recycling rates, and expects big improvements in the years ahead. For now, he says, “the truly new feature of REnescience is the effectiveness by which it captures the organic part of the waste, ensuring much higher green gas yields than from source sorting.”
As Dong sees it, the technology’s key advantages also include its convenience for householders and compatibility with current market models.
“The higher the household waste organic content – food, papers, cards, all contaminated waste – the higher the gas yield will be,” says Kanstrup. “But we don’t necessarily see this as a direct competitor [to sorted collections]. Source separation also has issues, and I think we will complement each other.”
