Treatment of Organic sewage: Sewage consists of faeces and urine along with water from kitchens, washrooms, and shops. It may contain industrial waste too. Sewage contains germs and eggs of parasitic worms so it is necessary to dispose it in such a way that the parasites may not contaminate ground drinking water. Moreover the disease vectors like flies, rats and cockroaches can be infected by these pathogens and can be spread the diseases to healthy persons. Treatments of municipal sewage involve the following steps.
Screening:
Large objects such paper, sticks, rags and discarded fruits are removed by passing through series of screens.
Setting in Grit Tanks:
The velocity of screened sewage is reduced by passing it through grits. Particles like sand, silt and small stones settle down.
Sedimentation:
The water is made to remain still for long times. Suspensions and lighter solids settle down as sludge which is taken for dewatering in a separate unit. Where as liquid from tank overflows which is further treated.
Biological Processing:
The liquor of sedimentation tank containing materials is biologically treated. Oxygen of air, microorganisms and particles environments are needed for such decomposition. All this is arranged in a single equipped vessel. The polluted water is pumped into a process tank where it is mixed for hours and hours with air and activated sludge. A fast moving chain sets in operation. Bacteria respire aerobically consuming molecules of proteins, fats and carbohydrates. The protozoa as secondary consumers feed on these bacteria. This decomposition is accelerated by adding fertilizers such as Urea, DAP etc.
Sedimentation:
The treated water is introduced to a sedimentation tank where activated sludge settles out. A part of this sludge is sent back to process tank. Rest of the sludge is taken out for dewatering. The treated water is sometime disinfected by chlorine gas.
After these stages of treatment, the original sewage is changed to harmless liquid which is drained into rivers and seas. The solid sludge obtained maybe used as fertilizer.
20.7 Population
Human population means the number of people living in a particular area. The area may be town, a district, a country or world as a whole. The human population grows when more people are born each year than die. The rapid rate of human population is a matter of great concern for the ecologists.
Ever increasing human population is continually using and depleting the natural resources on the earth and upsetting the balance of nature. The rapid growth of population has led to an increase in demand forests are cleared and trees are cut down. This is known as Deforestation. Whenever the land of any region is cleared for any purpose, the natural ecosystem of that region is destroyed which is return results in the destruction of natural habitats and disturbance of the balance of nature.
Purposes of deforestation
Urban development: L and is cleared for building, houses, roads, factories etc.
Cultivation: for growing crops.
Grazing: Large open grassland have to be provided for grazing by cattle, sheep etc.
Timbers: wood is needed for construction, furniture making, and paper.