Recycling of used resources and waste:

Some of the materials once used need not go waste, these can be  re-used. The process through which the waste resources are again made usable is known as recycling. Scraps and used metals Scrap metal is produced  in  large quantities  in mills and factories. Old used metal of discarded vehicles, machine, aircrafts, ships, buildings etc. can be melted and recycled for useful purposes. Used aluminium utensils, for example, can be collected, melted and shaped into new utensils. We can meet the growing demand of such scarce metals as copper, zinc, lead, platinum etc. by  recycling the used materials. 

Waste water 

Domestic and municipal waste water is rich in organic nutrients. If this kind of water is made free from disease carrying germs and poisonous elements, it can be used for irrigation of farms, gardens and other vegkations. For the removal of germs and toxic elements, the waste water or sewage is treated in a tank or in ponds for several days. In doing so, the heavy particles settle down to the bottom by themselves, while the finer particles are made  to settle down by  adding alumand caustic soda. The clear liquid is then allowed to pass through filters or sand or earth and finally air is blown through it. This treatment not only removes carbon dioxide and hydrogen sulphide which is generally dissolved  in waste water, but also adds oxygen to the filtered water, thus helping in purification. Treatment of water  with appropriate doses of chlorine, known as chlorination, kills all the harmful germs and makes water usable.