Organic waste from farming

The large numbers of livestock on Britain's farms all produce waste. Some livestock, such as poultry and pigs, are in most cases kept indoors all of their lives, i.e. they are raised intensively. The waste products have to be removed from the rearing buildings and disposed of, usually by spreading on the land. Winter weather is unsuitable for cattle to spend their days out in the fields; there is also a lack of grass for grazing and the weight of the animals walking over the ground in wet weather quickly turns fields into mud. For these reasons, the animals are brought indoors in the late autumn and don't return to the fields until the grass starts to grow again in the spring. As with poultry and pigs, the farmer has to dispose of the waste from the cattle for all the months they are confined to their stalls.

The spreading of animal manure onto fields can be carried out only in favourable weather conditions. This is usually on frosty days because the ground is frozen and can withstand the weight of the tractor and muck-spreader. In a mild damp winter, farmers have a frustrating time because they cannot get the waste spread and instead have to store it in large tanks or middens until the ground is firm enough to dispose of it. Care has to be taken when spreading animal waste onto the ground so as to avoid it entering water courses. We learned in Chapter 1, in the section on sewage treatment, that the amount of organic matter is measured as biochemical oxygen demand (BOD) and that untreated sewage has a BOD of about 300 mg/l. Animal slurry can have a BOD as high as 20,000! Even a small amount of waste entering a stream or being washed in by unexpected rainfall can have a disastrous effect on the water. The dissolved oxygen in the water (which may be only about 10 mg/l) is rapidly depleted and aquatic life suffocated.

A particular problem arising from cattle farming is that caused by silage liquor. All the time that cattle are indoors they have to be fed. They receive specially prepared cattle cake, hay, maybe some turnips, and silage. Silage is degraded grass and is prepared in the late spring and summer by cutting grass and allowing it to wilt for only a day or two. It is then collected with special equipment as shown in Cover Illustration 3 and put into a silo or a silage pit.

The grass is compressed by the tractor in the silage pit or by its own weight in the silo, and then covered with a waterproof membrane so that it degrades in the absence of oxygen. After a few months, the grass is converted into a moist solid which has the appearance rather like tobacco. During the degradation process, juices are squeezed out of the grass and

Figure 16. Water pollution caused by different agricultural activities in

Scotland, 1982-90

Source: Scottish Agricultural Pollution Group, Pollution Review, 11, August 1994, Scottish Environment Protection Agency, Stirling

Figure 16. Water pollution caused by different agricultural activities in

Scotland, 1982-90

Source: Scottish Agricultural Pollution Group, Pollution Review, 11, August 1994, Scottish Environment Protection Agency, Stirling emerge from the silo or silage pit. This is called silage liquor; it has a very high BOD value, typically about 50-60,000. If this liquid accidentally gets into a stream it has an even worse effect than slurry. Of all the water pollution events that occur in the UK, probably the most serious, because of the numbers of fish killed, are caused by silage liquor. Figure 16 shows the proportion of different types of water pollution caused by agriculture in Scotland.

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