Tip drainage

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Earlier in this book we dealt with the pollution problems that arise from the disposal of our liquid waste. We also get rid of an enormous amount of solid waste in the form of the refuse that is put into our bin sacks or 'wheelie' bins. It's difficult to visualize the amount of rubbish we throw out but Cover Illustration 5 shows the accumulated waste that a 'typical' family would put into their rubbish bin in one year.

Despite government attempts to get us to recycle our refuse, the great majority is collected by the local authorities and disposed of. Some is incinerated and the heat produced is occasionally used to heat large buildings or to produce steam for generating electricity. Some is converted to a compost for improving the quality of municipal land. Most however is dumped at landfill sites, areas of derelict or poor quality land, worked-out quarries or open-cast coal mines which are filled with waste to convert them into more usable land. Figure 20 shows the relative proportion of waste disposed of by various methods.

If you think about what is put into your waste bin at home, you will notice that it consists of plastics, shopping bags, wrappings, paper (newsEngland and Wales

| RDF manufacture m

Recycled

Incineration with EfW

Incineration without EfW

Incineration without EfW

EfW = Electricity from waste

RDF = Refuse derived fuel

Figure 20. Relative proportion of disposal methods for solid waste Source: The Environment of England and Wales, a snapshot, Bristol, 1996

EfW = Electricity from waste

RDF = Refuse derived fuel

Total reported = 22.41 million tonnes

Figure 20. Relative proportion of disposal methods for solid waste Source: The Environment of England and Wales, a snapshot, Bristol, 1996

paper and packaging), metal (mild steel and aluminium cans), organic matter (waste food, garden waste), and glass from jars and bottles. The quantities of these various types of domestic rubbish have been measured by government scientists. They have found that the relative amounts of solid refuse by weight are as shown in Table 14.1

Table 14. Composition of domestic waste, 1992

Type of waste

Percentage by weight

Plastic film

5

Plastic bottles and packaging

3.5

Glass

10

Rags

4

Ferrous metals

7

Aluminium cans and foil

1

Paper

32.5

Waste food

20

Dust, etc.

10

Miscellaneous items

7

This analysis shows only the weight of the refuse. When you consider that plastics are much lighter than, say, glass, then you can see that plastic waste will occupy a much greater volume of the rubbish.

The composition of our waste today is very different to that of 100 years ago. Plastics hadn't been invented then and most people had coal fires in their homes, from which the ashes were put into the dustbins. The composition of waste from 1890 was as shown in Table 15.

Table 15. Composition of domestic waste, 1890

Type of waste

Percentage by weight

Cinders and ash

64

Fine dust

20

Vegetable & animal waste

4.6

Waste paper

4.3

Straw

3.2

Glass

1.4

Metals

1

Crockery

0.6

Bones

0.5

Rags

0.4

They say you can learn a lot from people's waste and a comparison of these two lists clearly demonstrates how much more food we waste and wrappings we throw away.

At the landfill site (or waste tip) the rubbish brought in by the collection vehicles is first dumped, then spread out evenly and compacted by specially designed vehicles (Cover Illustration 6). Ideally, at the end of each working day, the refuse should be covered with top soil to prevent it being infested with flies, rats and seagulls. Eventually the tip site is completed and is covered with soil and planted with grass. It can then be used for farmland or even for golf courses and play parks.

Throughout the time of the formation of the landfill site, and for many years after its completion, nearby streams and rivers can be polluted by drainage water from it. This drainage water is called leachate and originates from rainfall or spring water which percolates through the decomposing waste. As it does so, it picks up pollutants from the waste. The most common constituents are organic matter from the rotting waste food and garden refuse, and iron from the rusting cans and other pieces of mild steel such as old bicycles, etc. The leachate can also contain high concentrations of inorganic ions such as chloride or toxic metals. Table 16 shows the typical results of the chemical analysis of a sample of leachate from a waste tip.

Table 16. Chemical analysis of leachate from a new waste tip

Chemical determinand Concentration (mg/l except pH)

Biochemical oxygen demand (BOD) 830

This sample was obtained from a tip site where the rubbish had been recently tipped and these results show that the leachate was very polluting. The waste was decomposing rapidly and there was sufficient rain water to wash the pollutants from the site.

The analytical results vary greatly from different tip sites and also from pH

Ammonia nitrogen Oxidized nitrogen Chloride Iron

Table 17. Chemical analysis of leachate from an old tip site

Chemical determinand

Concentration (mg/l except pH)

BOD pH

Ammonia nitrogen Oxidized nitrogen Chloride Iron

the same tip site over time. Eventually, the rate of decomposition slows down and the tip site stabilizes. The results shown in Table 17 are typical of drainage water from a tip site that has been in operation for many years.

Once the leachate reaches a stream or river, the organic matter is broken down by naturally occurring bacteria on the river bed (biodegradation) in the same manner as described earlier for treated sewage effluent (Chapter 2). If the concentration of organic matter is high, and there is little dilution in the receiving stream, the dissolved oxygen levels will be reduced and this can have an adverse effect on the aquatic life. The iron in the leachate is usually present as insoluble ferric hydroxide and appears as fine particles of rust. These particles give the stream a distinctly orange colour. They slowly settle to the stream bed so that, with increasing distance from the source of the leachate, the stream gradually becomes clearer. The accumulated iron has an adverse effect on the stream because, not only is it discoloured, but the particles of rust coat the leaves of water weeds, reducing their photosynthesis. They also clog the gills of insect larvae and fish so they swim away or die because they cannot extract oxygen from the water.

In order to stop this type of pollution occurring, water must be prevented from entering the decomposing waste as much as possible. This is done by constructing drainage channels to collect water and direct it away from the tipping area. If water does get into the rubbish and later emerges as polluted leachate, it should be collected and purified before it enters a stream. The organic matter in the leachate can be treated in the same way as described for sewage and at some tip sites the operators build a small treatment works.

If the concentration of organic matter is relatively small but the leachate contains a great deal of iron, then the treatment method is usually to precipitate the iron and allow it to settle in large shallow settling ponds. The rate of precipitation can be increased by adding lime or special chemicals called coagulants, such as polyelectrolytes.

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Trash To Cash

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