Some scientists have developed ways to force clouds to rain. Planes sprinkle a chemical into the clouds, making the raindrops grow. Only certain types of clouds can be seeded in this way, however - they have to be the right temperature.

Agricultural Water Supply

Agricultural activities are a major source of serious water pollution, but at the same time agriculture imposes a heavy demand for supplies of clean water. The seriousness of such problems depends, to a large extent, on the varying conditions which prevail in different parts of the world, especially in relation to the climate, the availability of water and the extent to which waste treatment facilities and general public health provisions are established. The disposal of waste waters, whether treated or not, and of sludges from treatment processes to agricultural land has many economic and agricultural benefit, but there are a number of hazards associated with these practices. Even where the deliberate reuse of water is not practiced, it is increasingly likely that agricultural water supplies will be drawn from water bodies which have previously received waste matter.

The most obvious adverse effects of polluted water in agriculture relate to he presence of toxic matter, especially heavy metals, and of pathogenic organisms. Agricultural animals probably do not differ much from humans in their sensitivity to toxic heavy metals, and heavily-contaminated water is no more acceptable to them than it would be as a potable supply for humans. The sensitivity of plant species to heavy metals in their environment is well known. Although there is some possibility that crops may accumulate sufficient heavy metal to be hazardous to consumers, in practice the effects of toxic metals are initially economic; some crops will give a reduced yield, or fail altogether, if the levels of toxic metals in the soil or irrigation water are too high. Thus water pollution can restrict the uses to which land can usefully be put, or impose extra costs relating to the supply of water of adequate quality. Boron, for example, is like many other elements an essential requirement in trace amounts for plants; in excess, however, it is toxic. The widespread use of perborates in detergent formulation has led to concern that domestic sewage effluent, without any contamination from industrial sources, could so elevate the boron content of receiving waters as to exert an adverse effect on crops. Consequently it has been necessary for national and international agricultural agencies to formulate detailed recommendations concerning the chemical quality of water used for various agricultural purposes.

Pathogenic organisms in water or sludges applied to agricultural land present obvious potential dangers. Again, the magnitude of the hazard depends greatly upon the eliminatic, agricultural and general public health situation of the geographical area concerned. Infective life stages of parasites can be ingested by animals contaminated water and transmitted directly to human consumers, particularly from salad vegetables or other crops which are eaten uncooked. Also, microbial contamination of arable crops through irrigation with contaminated water can contribute to the spread of disease.

Industrial Water Supply

Industrial usage accounts for a very large proportion of the total demand for water, and about two-thirds of the water used in industrial processes is used for cooling. The use in some industrial processes of sewage or other effluents, or of water which is too polluted for other water supply purpose, would appear to offer some advantages. In practice, the use of polluted water in industrial processes is subject to certain constraints. For example, the diary and food-processing industries obviously require water of the highest quality, and it is not always economically feasible to construct and operate safely water distribution systems in which water of different quality is supplied to different factories within an area.

Many of the problems which can arise are not of a strictly biological nature; for example, the presence of ammonia and other chemicals can exacerbate corrosion problems in pipework. A common difficulty is the growth of bacterial slimes and the accumulation of organic detritus in cooling systems; this can be controlled by chlorination, provided that the presence of free chlorine at relatively high levels does not interfere with the normal operation of the factory. A new and recent pressure is the demand from some very modern industries, such as the manufacture of electronic components, for water of a very high standard of purity. This could lead to the siting of manufacturing plants, often dealing with highly toxic chemicals, in the vincinity of the least-polluted water bodies of greatest utility and conservation value. In many countries planning considerations prevent this, and the alternative approach is to require potable water of the highest quality, and then to subject it to even further advanced treatment at the point of use.