It is waste form a community containing solid and liquid excreta. It is derived from houses, street and yard washings, factories and industries. Sewage contains 99.9% water and 0.1% solids. The solids contain inorganic and organic matter and also disease producing organisms.
It is the waste water which does not contain human excreta. It includes waste water from kitchens and bathrooms (not from toilets).
Amount of sewage
The amount of sewage which flows into the drainage system depends on:
1. Habit of people e.g if more water is used, sewage will be more.
2. It sewage is combined with rain water, the amount will be more.
3". Time of the day - it is more in the morning and less in the afternoon.
Aims of sewage treatment
The aims of sewage treatment are:
1. To breakdown organic matter by aerobic or anaerobic bacterial action. It results in simple substances which will not decompose further.
2. To produce an effluent which is free from harmful organisms. So it can be safely disposed.
3. To utilise the water and solids without harmful effect on health.
Method of sewage treatment
Modem methods of sewage treatment involve two major steps 1.-Primary' treatment 2. Secondary treatment.
It involves a.screening b. passage through grit chamber c. primary sedimentation
a. Screening: Sewage is passed through a metal screen, it removes large objects like pieces of wood, garbage and dead animals.
b. Grit chamber: Then, the sewage is passed through a grit chamber. Here, heavier solids such as sand and gravel settle down.
c. Primary sedimentation: Later, the sewage passes slowly through a large primary sedimentation tank. Here, organic matter settles down and it is called as sludge. Fat and grease rises to the surface and it is called as scum.
The sewage from the primary sedimentation tank contains organic matter and living organisms. It is treated by either trickling filter method or by activated sludge process.
Trickling filter method:
The trickling filter is a bed of crashed stones. The effluent from primary sedimentation tank is sprinkled on this bed by means of rotating pipes with several rows of holes. The effluent is oxidized by algae, fungi, protozoa and bacteria as it passes through this filter bed.
Activated sludge process:
It involves the use of aeration tank. The effluent from primary sedimentation tank is mixed with sludge from final sedimentation tank. It is aerated in the aeration tank by agitation or by compressed air. Aeration oxidizes the sewage to carbon dioxide, nitrates and water.
The sewage from trickling filter or aeration tank is now lead into final sedimentation tank. The sludge which now settles down is harmless. It can be used as a manure. Part of this sludge is pumped to aeration tank in the activated sludge process (as above).
The effluent that remains after the sludge has seeded can be let out in rivers or streams. Also it can be used for irrigating land.
OTHER METHODS OF SEWAGE DISPOSAL
1. Sea outfall: Towns and cities situated in sea coast can let the sewage into the sea. The solid matter in the sewage may be washed back to the shore.
2. River outfall: Raw sewage should never be discharged into rivers. It is purified before discharging into rivers.
3. Land treatment: The sewage may be let into land for irrigation. Suitable crops can be grown in these lands.
4. Oxidation pond (Redox pond, waste-stabilization pond or sewage lagoon): It is a cheap method of sewage disposal. It is a suitable for small communities. At present there are about 50 ponds in India. It is an open, shallow pool about 3 to 5 feet deep. It has an inlet and an audit. It contains algae and also certain types of bacteria. There must be sufficient sunlight.
Mechanism: The organic matter contained in the sewage is oxidized by bacteria into simple compounds such as Coil, water and ammonia. The algae utilize CO2 and water in presence of sunlight and release oxygen required for bacteria. Thus there is a mutual beneficial effect between bacteria and algae. The effluent may be let out in streams or rivers. Also it can be used for irrigating land.
SOCIAL ASPECTS OF EXCRETA DISPOSAL
In India, most of the people in villages have the habit of open air defecation i.e. they go to the fields for defecation. This habit is deeply routed in the cultural behavior of villagers. These people do not accept latrines because they think that:
1. Latrines are associated with bad smell.
2. They are breeding places for flies.
3. They are costly to construct.
Also, these people are not aware of fecal borne diseases.
The village people must be motivated for accepting sanitary latrines. They should be made to understand the importance of latrines. This can be achieved only be health education. Media like pamphlets, newspapers, films, radio, television, demonstration etc. can be used for this purpose.