How is this done?

Different countries handle this engineering problem in different ways. Experts have developed many production processes which make it possible to reconvert mountains of garbage into substances that are of crucial importance to industry and agriculture. Among such enterprises is the St. Petersburg automatic plant which processes solid domestic waste. It alone “gobbles up” 35-40 per cent of the city’s tips – 900,000 cubic meters.

The garbage is first fed to belt conveyers, where magnetic separators extract ferrous metals from it, which are subsequently pressed into compact briquettes and shipped to metallurgical plants for resmelting. Domestic wastes, in particular tin cans (and other discarded objects as well), also contain valuable materials like tin, copper, zinc and brass. Until recently they were extracted in the metallurgical plants which receive the briquettes, but now this is being done in the automatic plant itself – in special newly built departments.

Refuse relieved of metal then goes into biothermal drums – enormous cylinders up to 60 meters long and four meters in diameter. They slowly rotate, mixing the waste. The air which is fed into the drums provokes an intensive aerobic process. Under the action of microorganisms all the garbage’s organic components decompose. The temperature of what is no longer waste, but is known as the compost mass, rises to 60 – 70 C. As a result, the disease-producing microbes die and the substance in the drums becomes harmless.

In 48 hours the compost mass ripens. Nevertheless, it still contains a wide variety of particles and fragments which biothermal treatment leaves unchanged. In order to dispose them the mass is sifted through giant sieves, where the material processed in the biothermal drums is separated into compost and ballast. Now the main thing is to get rid of glass fragments – enemy number one of compost. Obviously, fertilizer containing glass fragments cannot be introduced into soil. However, if they are reduced to the size of grains of sand and rolled smooth, they can no longer harm the roots of future plants.

To this end, all the material sifted through the giant sieves goes into a vertical shaft in which it gets into an upward air current. The light particles – mostly organic – are carried off. The particles of glass and ceramics which are left go to the grinding zone.

The ballast which fails to pass through the sieve - rubber, plastics and wood fragments had to be taken back to the garbage dumps, and this type of refuse accounts for almost a quarter of all domestic waste. Now even this ballast is processed – by pyrolisis, or the thermal disintegration of complex substances into simpler ones. As a result valuable products are obtained, such as fuel gas, bituminous resins and solid carbonaceous compounds, which in our day find broad application as graphite substitute in metallurgy.

The plant’s final and main product is compost. It looks like ordinary earth. In fact, it is a biologically active substance which can be used far more effectively than an ordinary fertilizer. Due to a high temperature it is possible to employ it as a biological fuel to heat the soil in hot-houses. The compost’s biological energy is sufficient to grow two crops. After that, it can be transformed to open ground as an ordinary fertilizer.

The Leningrad plant was the Soviet Union’s first enterprise which almost wholly restores to the national economy what is contained in city dumps. Not only does it pollute the environment in any way, it does not even consume water on its technological requirements.

Ex.7 Translate the questions and find the answers on them:

1. Чому проблема міських сміттєвих сховищ набуває в наші дні глобальне значення?

2. Про які основні стадії переробки міського сміття ви взнали з тексту?

3. Які цінні матеріали можуть бути отримані в результаті переробки міського сміття?

4. Що є кінцевим продуктом промислової переробки міського сміття?