How a city gets rid of its waste
In June 1858, unusually hot and dry weather caused a sharp fall in the level of the River Thames in London. The stench that emanated from the depleted waters was so appalling that Londoners, who christened the phenomenon The Great Stink’, could approach the banks or bridges only with handkerchiefs clamped firmly to their mouths and noses. River traffic was suspended. In the House of Parliament, beside the Thames at Westminster, sittings could not continue until the windows had been draped with curtains soaked in chloride of lime to counteract the smell.
The Great Stink was the final result of centuries of carelessness in waste disposal. Londoners, like the denizens of other crowded towns and cities throughout the world, had been accustomed to treat any handy waterway – often their sole source of drinking water – as a vast open sewer. Daily, urban rivers such as the Thames, the Seine and the Tiber received their loads of human and animal excrement, offal, household refuse, the unwanted byproducts of crafts and trades and all other detritus of city dwelling. As populations grew and the flood of rubbish increased during 19th-century industrialisation, nature and man cried ‘enough’.
Methods of treating raw sewage before discharging it were developed in Britain and copied and improved in Western Europe and the United States.
Unfortunately, as methods of getting rid of excrement and other liquid waste have become more effective, the growth of modern cities has increased the output of solid waste – everything from discarded food packets to old light bulbs and batteries.
The average US household generates nearly 531b (24kg) of solid waste every week; in France, the equivalent figure approaches 371b (17kg) and in Britain it is 351b (16kg). In a single year, the average inhabitant of New York City throws away eight or nine times his own body weight of solid waste.
Into the sewers
The basic treatment of liquid waste, or sewage, in cities such as London and Washington DC, has changed little from the methods developed in the middle of the 19th century, though the volume is constantly increasing.
In Washington, the original capacity of one sewage plant, at Blue Plains, was stepped up from 130 million gallons daily in the 1930s to 290 million gallons by the 1970s, and has been further enlarged since.
A network of sewers, usually underground, carries the waste, either under gravity or by pumping, from household and office lavatories, basins and baths to the sewage works. Originally, and in many cities still, the sewers were also storm drains, with the result that sudden downpours could flood them, bringing raw sewage to the surface. Now, where possible, civic engineers try to separate the two functions.
At the works, the sewage flows through screens which filter out large objects such as rags and wood. These are either torn by machines into small pieces and fed back into the treatment process, or taken for burning or burial elsewhere. Then the sewage is pumped through grit removal channels, where small stones and sand sink to the bottom. This detritus is dredged out and washed. It is widely used to fill holes in roads or on building sites.
The remaining sewage passes into preliminary sedimentation tanks, where the solid material settles to the bottom and is known as ‘crude sludge’.
The sludge and the liquid separated from it then follow different routes. In London, the liquid goes to secondary treatment plants where microbes feed on and destroy the waste matter it contains in about eight hours. It then passes through final sedimentation, when the microbes themselves are separated out and re-used. The water that is left is clean enough to go into a river. Water from the River Thames is re-used several times before flowing into the North Sea.
In the USA, chlorine is added to the water at various stages to purify it.
Meanwhile, the crude sludge is pumped to digestion tanks. There, over three or four weeks, microbes convert part of the sludge into gas containing methane, which is piped off and used to create power to run the sewage works.
The rest of the sludge has more water removed before being sold to farmers as fertiliser. At Blue Mountain, Pennsylvania, for example, sewage sludge has helped to reforest land destroyed by zinc extraction. Any sludge left over is dumped at sea.
Each day, New York City generates between 24,000 and 25,000 tons of solid waste – much of it household refuse carted away in twice-weekly, municipally organised collections. Virtually all of this mountain of rubbish is transferred to one site at Fresh Kills on Staten Island, where it is dumped into what began as a hole in the ground and is now the largest tip in the world, covering 3000 acres (1215 hectares).
Such waste disposal by controlled dumping is called a landfill. On a worldwide basis it is the most widely practised and cheapest way of ridding society of unwanted byproducts that cannot be flushed away through sewage systems.
But as the volume of waste continues to grow, landfills like Fresh Kills are less able to cope. Lack of space creates problems of pollution.
from the rotting tips can contaminate surface and underground water supplies unless the tip is specially equipped to treat it.
When landfills are full or become a sufficiently serious hazard to public health, they have to be closed. Since the 1960s, New York has shut 14 and new sites are not easy to find. If they are too far away from the city they serve, the cost of transporting the rubbish to them becomes too high for municipal budgets.
In many countries, including Sweden, Germany and Japan, burning has long been used as an alternative to dumping. But in the United States incineration accounted, in the mid-1980s, for less than 5 per cent of solid waste disposal. The advantage of burning is that it reduces the volume of rubbish by two-thirds; the heat produced in the process can also be used to generate electricity or provide heating.
Against that, the ash residue may contain toxic chemicals which have been concentrated during incineration. The ash cannot therefore always be safely deposited in ordinary landfills. Other toxic chemicals, such as hydrogen chloride and dioxin, can be released into the atmosphere during burning if the proper gas-cleaning equipment is not used. In Los Angeles, citizens afraid of atmospheric pollution have stopped incinerators from being built.
Another reason against burning rubbish is expense. In the USA, the charge for burying it may be up to $60 a ton. To burn it can cost three times as much.
Health hazard Uncontrolled dumping of waste on land can create hazards to health. But in most Western countries, new laws are tightening up on waste disposal.