Flowing sewage includes human excreta disposed from public and domestic latrines. It also contains carbonic as well as non-carbonic materials.
Due to increasing population, people are becoming concentrated in small places in cities and towns, where in the absence of suitable sewage system for disposal of human excreta, it is disposed in nearby water sources.
Some of this water also goes underground. Excreta disposed in water sources pollute the surface water while the underground part pollutes groundwater. Hence, its successful disposal is the demand of the day. This problem did not exist in the past when the population was lesser. At that time people thought that water sources flowing in natural way would automatically get cleaned. But today this thought has become a curse for them.
Sewage of solid nature is mainly carbonic, which is nutritive for disease bearing micro saprophyte, and bacteria. Bacteria, virus, protozoa, algae, fungus etc. develop and multiply. Such sewage pouring in water sources without treatment would pollute the water. Besides, human beings as well as animals dispose excreta in open places which also flow with rain water and mixes in water sources.
Thus, polluted water does not remain fit even for irrigation because such water would pollute groundwater through infiltration during irrigation. Apart from production of disease carrying bacteria, some big parasites also develop, systomomus being the main parasite among them. This parasite lays eggs after entering in the large intestine of human body with water. The eggs spread infectious diseases after being released with urine or excreta. Such diseases develop more in tropical regions.
Except sewage, dead persons and dead animals are also flown in water sources which have become an important problem of the present world. Developed countries are producing manure and energy from sewage whereas developing countries have yet to combat this problem.
According to National Environment Engineering Research Institute (NEERI), Nagpur, only eight out of 3,119 cities of India have arrangement for fully developed sewerage system, whereas 209 cities have partial arrangement. Out of 142 metropolitan cities, 52 cities have partial arrangement and 82 cities have no arrangement.
On the banks of river Ganga, more than 50,000 people in 114 cities dispose untreated excreta in the river water. Waste material of Kolkata city is being disposed in Hoogly River through 361 drains, due to which the number of fish is continuously decreasing.
In Egypt, fish production in Maryug Lake near Alexandria city is in danger due to domestic sewage disposal in it. Tutee River in Brazil is being polluted due to disposal of 300 ton refuse of Sao Palo city. Present world is facing the problem of searching methods for solving this problem caused due to increasing population. If disposal can be made for irrigation after making it fit for use in agriculture.