Refuse lorry

Not wasting waste

How does Switzerland maintain its position as the world's number one recycler of household waste?

A barrel of crude oil contains 42 US gallons or 159 litres. The energy in it is about 1,600kWh, depending on the quality.

A tonne of household rubbish averages an equivalent energy of about 3,000kWh, yet most of it is thrown away.

Every tonne that we landfill is like pouring almost two barrels of crude oil down the drain.

Let's put this in perspective; in the UK and much of Europe, we throw away the energy of almost three barrels of oil per man, woman and child each year. At 90 dollars a barrel, that's worth over £8bn per year in the UK alone, not counting the cost of maintaining landfills!

Let's assume that one-third of the dry rubbish (hopefully, mostly organic, because the glass bottles and aluminium cans will have been sorted for recycling) is carbon. In a landfill, over time, one tonne will decompose roughly into equal parts of 200kg each of carbon dioxide and methane, which will be uselessly emitted into the atmosphere. Methane has a global warming potential of 21 to 50 times that of carbon dioxide. Taking the most conservative figure, this means that the emitted greenhouse gases are the equivalent of 4.4t of carbon dioxide per tonne of rubbish, and we gain not a single joule from the energy.

Worse still, household rubbish is often transported vast distances before it is landfilled emitting further carbon dioxide and other pollution from the trucks. For example, the City of Toronto ships, by truck, 700,000t of rubbish each year as far as Wayne County in Michigan for landfilling - a trans-frontier distance of about 390km in each direction - and pays a not-so-small fortune ($36m) for the privilege of dumping their trash there.

Yet we already have the technology to extract energy from what we tuck into an ordinary plastic bin bag and, at the same time, emit less than 400kg of equivalent carbon dioxide per tonne and almost no other pollution. The cinders from the process take up only one-tenth the volume of landfill and are almost inert, so that pests like rats and insects find no home where they are dumped, nor are there nauseous smells.

The fuel that we recycle into energy is abundant, sustainable, renewable and easy to collect locally.

Why not?

Why don't we all do it then? Well, we do - at least in a few places, such as Japan, Switzerland, some parts of the UK and the USA. As a guide, about 9 or 10 per cent of the electricity requirements for the waste catchment area can be generated from household garbage with industrial organic refuse, such as tyres, non-recyclable plastics and paper, sawdust, wood chips, demolition wood, non-compostable garden refuse, forestry waste, sewage solids, abattoir and medical waste etc, as an extra.

Lausanne, Switzerland, has recently inaugurated a small modern combined heat and power plant to replace an old one. It is run by Tridel S.A., whose shareholders include cities in the catchment area with a total population of 380,000. It produces 80MW of steam, generating 20MW of electricity. The turbine has a special two-stage design.

This allows efficient low power generation in winter, when the heat demand is high, and full power in summer, when the heat demand is low. The heat is used to provide hot water and space heating for a major teaching hospital and apartments housing 18,000 persons. The overall annual efficiency is just over 50 per cent. It is obvious that the fuel-to-electrical output is no higher than for any other steam turbine power station, about 35 per cent, but the thermal efficiency for space and water heating can be as high as 85 per cent.

Transport

One of the unique features of the Tridel power station is that most of the fuel arrives by train. Lausanne is a hilly city, and the plant is about 250m above the lower quarters of the town. Having the waste collection vehicles chugging constantly up the hill would constitute a polluting and noisy nuisance. They therefore discharge their loads at a goods station, at a lower altitude, where the garbage is compacted into special rail trucks.

A 4km-long standard gauge tunnel, buried 50m under Lausanne, effects the transport. This has an additional advantage that waste from distant cities can be loaded into similar trucks and railed directly to the power station, reducing road pollution. It is economical and non-polluting to transport refuse over long distances by rail, and Tridel has accepted it from as far as Germany and Canton Ticino (south-east Switzerland), several hundred kilometres away. Road vehicles supply garbage locally from the higher parts of Lausanne and surrounding communes.

One of the major issues surrounding recycling rubbish into energy is pollution. Many politico-ecologists seem to have a fixation about dioxin from burning lettuce leaves, although I doubt whether any of them could define a dioxin or how it is produced.

There is little uncertainty that burning coal or oil would produce just as much dioxin or its more dangerous derivatives than when extracting the value out of garbage.

However, pollution is an important matter and none of the waste streams - air, water or solids - must be allowed to get away with it.

Air

Obviously, the major product from incineration is gas, mostly nitrogen from the air used for combustion and carbon dioxide but with a myriad of other chemical compounds, including infinitesimal quantities of dioxin and its derivatives.

Subsequent to generating the superheated steam, the gas goes through an electrostatic precipitator to remove the bulk of the fly ash. After that, it is passed through a heat exchanger and then into a water quench which cools it down and removes any residual dust. From there, it goes through a series of scrubbing operations which remove most of the chemicals, including mercury and other heavy metals, and back to the heat exchanger to warm it up again to nearly 300°C. The scrubber also removes some of the carbon dioxide. From there, it goes through a catalytic converter, similar to a scaled-up version of the one on a car. It is then forced up the stack.

Switzerland has very strict laws governing emitted pollutants; in all cases bar one, the gas discharged from the stack has pollutant concentrations of about one-tenth the permitted levels for each chemical (including dioxin).

The one exception is nitrogen oxides, mostly removed in the catalytic converter, which are down to about half the permitted levels. These pollutant levels are continuously monitored (separately from each of the two lines) and, if necessary, an excess will cause automatic corrective action to be taken or even a shut-down of the offending line.

This proves that air pollution from burning organic waste can be kept to negligible levels.

Water

Water is used in the gas and fly ash purification processes. It becomes polluted and must be rendered harmless before disposal. To a large extent it is recycled, but inevitably there is a waste stream.

The water source is mainly rain run-off from roofs and paved areas to reduce the consumption of mains water. The raw waste water is acid and contains heavy metal salts and all the fly ash, including from the precipitators. The acidity is neutralised with lime water, so that most of the acids are converted to insoluble calcium salts which are filtered out, along with the fly ash. These are added to the clinker. The remaining water is treated with sodium hydroxide to precipitate out the heavy metals as hydroxides. They are then separated in a filter press and bagged for despatch by rail to a recycling plant that recovers the cadmium, lead, chromium, nickel and other valuable metals. A final neutralisation and the water conforms to the strict Swiss legislation for discharge into a public sewer.

Solids

This leaves the solid waste stream. It is mostly in the form of clinker to which the washed fly ash and lime products are added. By weight, this is as much as 20 per cent of the compacted waste but, by volume, it is only about 10 per cent.

The size of the landfill to where it is sent by rail is very small compared to what it would have been if the garbage had been sent there without extracting its value. The solid waste is almost inert (there is a small proportion of elemental carbon in it), so that the long-term pollution from the landfill is acceptable.

Of course, the heavy metal precipitates, mentioned in the previous paragraph, are also solid waste. Other solid waste includes iron and steel, separated magnetically from the cinders, and non-ferrous metals, separated by induction detectors. These are sent to scrap metal dealers.

Costs

Economically, the Tridel plant is healthy, even though the construction costs were high, largely because of the rail tunnel and another 1km tunnel for the hot water (200°C) circuit to a 40-year-old secondary plant providing heating to the hospital and apartment blocks.

Its revenue comes from the charges for waste disposal, similar to landfill costs, and the sale of the energy in the form of heat and electricity. It was profitable in its first year of operation.

Recent articles

Info Message

Our sites use cookies to support some functionality, and to collect anonymous user data.

Learn more about IET cookies and how to control them

Close