Garbology: Talking trash

Thanks to new regulations, the emission of such toxic chemicals from waste processing has been reduced a thousandfold.

Today, the total emission of dioxins and furans produced by all the incinerators in America is less than ten grams a year, according to the Environmental Protection Agency (EPA). By contrast, homeowners burning rubbish in their backyards are reckoned to contribute up to 500 grams a year. Some of the worst emitters are the fireworks used to celebrate the Fourth of July.

Even so, municipal incinerators—especially the new waste-to-energy (WTE) plants that use rubbish as a fuel for generating electricity and heat for local distribution—continue to have an image problem. In America, most communities prefer their waste to be composted—provided, of course, the landfills are nowhere near their own backyards. Yet, without costly plumbing, landfills produce copious quantities of methane from their decomposing waste. As a greenhouse gas, methane does more than 20 times the damage to the environment as comparable emissions of carbon dioxide.

At some of the larger municipal landfills, the methane produced by anaerobic decomposition is captured and used to generate electricity. The mountain of rubbish at the Puente Hills Landfill in Los Angeles, the largest of the 1,900 municipal landfills in America, is over 500-feet high—taller than most of the skyscrapers in the city’s downtown area to the west. With 60 years’ worth of decomposing rubbish, Puente Hills produces enough methane to generate electricity for 70,000 homes.

Not all local authorities can afford such luxuries. New York City transports much of its garbage by truck and train to landfills hundreds of miles out of state. It is not the only one. More and more places are even shipping their rubbish abroad to countries willing to process it at a price. While China exports computers, mobile phones and other manufactured goods to America, the largest export from the United States to China (as measured by the number of cargo containers) is now trash, reports the Journal of Commerce.

The problem is exacerbated by the growing shortage of landfill space everywhere. As the rubbish piles up, officials are having to reconsider the sensitive issue of incineration. Modern incinerators are not your grandfather’s dirty burners. Almost all nowadays capture the energy in the solid waste as well as the emissions from the combustion. Such WTE plants burn garbage at temperatures high enough (over 850°C) to break the molecular bonds in dioxin and other toxic chemicals and thus render them harmless. The flue gases are then cooled in heat exchangers that raise steam to drive the electricity-generating turbines.

The flue gases are then passed to a cleaning system that filters fine particles from the flow and scrubs the gas to remove sulphur dioxide as well as various acids and heavy metals. As in a car exhaust, the flue gases next pass through a catalytic converter, where the nitrogen oxides are chemically reduced with ammonia. Finally, volatile heavy metals remaining in the flue are absorbed on activated-carbon powder.

The volume of the ash left after combustion amounts to around 5% of the waste ingested. The ash at the bottom of the combustion chamber is either buried in municipal landfills or recycled as aggregate for the construction industry. The fly ash that rises up the flue needs further processing to remove any remaining toxic particles that might be clinging to its surface. The result is a remarkably clean and efficient process for disposing of garbage.

Economist