4. How weeds clean water

Fanciers of tropical-fish use marine vegetation to help keep the water in their aquariums clean and the same or similar plants are used in many reservoirs to aid the process of water purification. Now engineers are using the same approach to help purify sewage and industrial water wastes.

The "living-filters", which include a number of reeds, rushes and irises cleanse water in a variety of interrelated ways. They absorb inorganic pollutants such as nitrates, phosphates and metals and toxic organic compounds such as phenol. Their roots trap small particles of insoluble pollutants. The plants reduce the-number of' pathogenic bacteria in water, possibly by producing chemicals that destroy the bugs. They add oxygen to dirty water and act as hosts for-various bacteria, insects and small fish that also clean up pollutants.

Sudanese tribesmen have long used green plants to I make the murky waters of the Blue Nile potable and I palatable, but the large-scale use of this natural treatment is a recent innovation. The most advanced-process of this kind is a system used to purify water from the befouled Rhine River for the German town of Krefeld. The Rhine water, containing huge amounts .of municipal and industrial sewage, is first subjected to chemical treatment which removes the bulk of the pollutants, and then sprayed into a lagoon planted with bulrushes. The spraying increases the amount of oxygen in the water, and the rushes remove almost all of the remaining pollutants, including toxic organic chemicals and coliform bacteria. This water infiltrates the soil below the lagoon —^: which purifies it further —- and is then pumped off, through wells dug close to the lagoon into Krefeld’s water system.

Other schemes using green plants are on a somewhat smaller scale. In Holland’s Zuider Zee region, long waterfilled trenches planted with reeds have success-fully cleaned up sewage from summer camp sites, at about a quarter of the cost of conventional plants. Researchers are testing the use of natural and artificial marches to treat municipal effluents and experimenting with lagoons full of water hyacinths for the same purpose.

Experts recognize that the method is not a panacea f or water-treatment problems. The plants require a lot of space, are vulnerable to pollutants that kill plants and cannot work year-round in areas where ponds freeze. Nevertheless green plants could provide-clean water for small communities that cannot afford full-scale purification systems. And in combination with conventional techniques, biological treatment offers relatively cheap way to remove the last traces of the pollutants that now end up in the drinking water of most large cities.

5. Clean water – can it be clean entirely?

“Clean water”, “pure water”, “clear water” are some of the terms we use in describing water of good quality. But what do they mean? Pure water, two parts hydrogen and one part oxygen, is great in a laboratory but not for plants and animals. Scientists have found, in fact, that the water from most streams in their natural state contains the proportions of dissolved minerals necessary for human health.

As we’ve mentioned, water is never entirely pure in nature. Water picks up a broad range of elements as it moves through its cycle of evaporation, condensation, precipitation, and percolation on its way back to stream, lake, or sea. From the air, water picks up dissolved oxygen and other gases. As water passes over and through the rocks and soil, minerals are dissolved into it. Some materials are filtered out, but others remain in solution and are carried along with the water wherever it goes. Generally the deeper the water sinks into the ground the more minerals and other materials it contains.

Large areas of phosphate rock and minerals such as coal, can result in natural water pollution bad enough to effect long stretches of a stream. In addition, natural occurrences, such as earthquakes, volcanic eruptions, and landslides, can cause severe pollution of lakes and streams. Large populations of wild animals can also contribute to high levels of bacteria in a stream, under certain conditions.

Add to the natural sources of pollution the activities of people and truly hazardous conditions that can arise. Just about everything that people cause some pollution. There is no way to avoid this entirely. There always seems to be something left over that can not be used, and that becomes waste. When fuels are burned, they produce smoke and gases of various kinds. These gases and smoke eventually come back to earth and find their way into the water somewhere. There are also ashes or other residues from the burning process that become waste and can contaminate water supplies.