From the beginnings of civilization sewage and domestic rubbish have been disposed of mainly by spreading them over the ground or discharging them into rivers, lakes or the sea. Hitherto, these simple methods of waste disposal have usually been fairly satisfactory because much of this refuse has been rapidly broken down to inoffensive forms by natural processes of decay; indeed the procedure has been generally beneficial by returning to land and water the constituents necessary for the maintenance of fertility. However, growing towns have always tended to pollute themselves with their own wastes, especially by overtaxing the natural capacity of their streams and rivers to disperse, degrade and recycle the excrement and rubbish poured into them. Open sewers in the crowded cities of mediaeval Europe were a serious health hazard causing massive outbreaks of disease. This is still the case in some developing countries. In later years the much improved health of urban populations in developed countries has been attributable in considerable measure to cleaner towns with much safer methods of sewage and waste disposal.
In contrast to land or fresh water, the enormous volume of the sea has a huge capacity for mineralizing sewage or organic refuse. To the extent that the sea can accept wastes without detriment, it is obviously sensible to take full advantage of this natural sink. It has always been assumed that any noxious substances which are not readily inactivated in the sea by natural processes soon become so diluted as to be quite harmless. Many coastal towns still take advantage of this by pouring their untreated sewage directly into the sea. Industries which produce large volumes of waste have often selected sites on the coast because, in addition to the many other advantages of ready access to the sea, they obtained a simple and inexpensive outlet for their effluents. Advantages of waste disposal at sea include the following:
1 The great capacity of the sea to degrade many substances, especially natural organic wastes.
2 Extreme dilution of toxic materials can be effected in course of time.
3 Solid wastes may be permanently lost within sediments.
4 Dumping at sea is often the cheapest method of disposal.
5 It may sometimes be preferable to pollute the sea rather than land. Sewage farms occupy large areas which may not be available where needed. Poisonous wastes placed on rubbish tips, however rigorous the precautions, may present much greater hazards than if discharged at sea.
However, in recent years it has become increasingly obvious that in many cases we are exceeding the capacity of our seas to cope with our various inputs. Recent rapid growth of populations together with industrialization and new technologies has produced wastes in much greater quantities and variety than ever before. Public health problems have arisen from raw sewage discharges into the sea (see
Section 10.1.5), more land has been needed for rubbish tips and industrial spoil, and more effluents are reaching the sea via rivers and estuaries. An additional problem is that some of the new materials produced in large amounts, notably many plastics, accumulate because they are extremely long lasting, being little if at all subject to biological decay.
Despite the great size of the oceans and their thorough intermixing, water circulation is mostly slow and dispersion of materials in the sea is sometimes a very gradual process. Consequently where large amounts of wastes are discharged into shallow water, especially into enclosed areas like the Baltic, the North Sea and Mediterranean, concentrations have now been reached which present a variety of problems. Even if wastes are disposed of by tipping far out at sea, pollution may become obvious if persistent floating substances drift back to the coast, or if solids which sink to the bottom are later carried inshore by shifting of sands along the sea floor.
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