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Why mechanical biological treatment is not a magic wand

Bill Swan

Landfill is bad. This is one of the few things that everyone can agree on. But if landfill is bad, what is good in waste disposal? Even with the best recycling schemes, the UK must dispose of millions of tonnes of waste for which material recovery is not practical, cost effective or carbon positive.

Incineration - recovering the energy in the residual waste to produce electricity and sometimes heat - has many proponents. Although widely used in Europe, it is controversial in the UK. Even rebranded as energy from waste,any proposed plant faces a bruising battle to obtain planning consent.

So if we can’t bury or burn our waste, what can we do with it? For local politicians, one increasingly popular solution is mechanical biological treatment (MBT). This takes “black bag” residual waste and, in the “mechanical” phase, conveyors, trommels, shredders and other impressive powerhungry machinery convert the incoming waste into two principal outputs.

“[RDF] turns out to be a fuel that nobody wants. Instead it must be sent for… incineration”

The first is normally refuse-derived fuel (RDF), a high-calorific mix of paper, cardboard and plastics that is dried and pelletised. The second organic-rich fraction is treated biologically, either aerobically or anaerobically. The main output of this stage is a “compost-like output” (CLO). It’s like the alchemist’s trick of turning lead into gold - we have turned black bag waste into fuel (RDF) and compost (CLO). Even better, about a third of the waste by weight typically disappears in the process.

But let’s take it to the next stage. What happens to these two promising-sounding outputs? Surely, in this time of high energy prices, there is demand for RDF? Can it replace coal in boilers, factories and power plants? Unfortunately not, as it turns out to be a fuel that nobody wants. Instead it must be sent for… incineration.

What about CLO? Can we spread it on farmland and grow plants in it? Unfortunately, it turns out to be not much like compost, and can’t be used in agriculture or horticulture either. Instead it must be sent to…landfill. And remember that impressive 30% weight reduction? That’s simply due to water being lost as the waste is dried.

Proponents of MBT will no doubt point out that some materials are recovered. Ferrous materials are easily separated with overband magnets, but that’s also true of the ash of incinerators.

Some MBT plants will have picking stations where some poor souls stand pulling cardboard and plastics from the passing conveyor of filthy waste. But MBT achieves virtually nothing. There is so little demand for RDF that a prominent waste company recently had to seek permission to export 40,000 tonnes to Holland and Germany in the next year, and others may have to follow suit.

As the alchemists learnt, you can’t turn lead into gold. You can, however, create fool’s gold, an illusion that dirty commingled residual waste can somehow be converted by technology into valuable useful materials.

Quick fixes and magic wands don’t work. Instead, responsible, professional waste management requires source separated collections of dry recyclables, an organic food waste stream and then thermal treatment of the residual waste to recover energy and render it inert.

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