Water is essential for life, but after use, it becomes dirty and full of impurities. The used and dirty water from homes, industries and other places is called wastewater or sewage. In this chapter we study what sewage contains, how it is treated in sewage treatment plants, and why the disposal of wastewater must be handled carefully to protect our health and the environment.
Every day we use water for drinking, cooking, bathing, washing and flushing, and a large amount of this water comes out as wastewater. If this wastewater is allowed to flow into rivers and lakes untreated, it pollutes the water and spreads diseases. Therefore, treating wastewater before releasing it into water bodies is essential.
The wastewater that is collected and carried from homes and industries is called sewage. Sewage contains impurities and a wide variety of waste materials. It contains solid waste, which may be organic or inorganic, water, and harmful microorganisms. Organic impurities include human faeces, animal waste, oils, fats and urea, while inorganic impurities include nitrates, phosphates and heavy metals. Microorganisms in sewage include bacteria and viruses, many of which cause diseases like cholera, typhoid and dysentery.
Sewage is carried from its source to a treatment plant through a system of underground pipes called sewers. Together, all the sewers of a town or city are called a sewerage system. There are separate pipes for sewage and for rainwater so that the treatment plant is not overloaded during heavy rains. In the sewerage system, manholes are provided at intervals so that workers can clean and inspect the pipes. Before the sewage is treated, it is important that we do not throw solid waste, oils or chemicals into the drains, as they block the pipes and harm the treatment process.
The process of removing impurities from wastewater before it is released into water bodies is called sewage treatment, and it is carried out in a sewage treatment plant (STP). The treatment takes place in several steps. First, the sewage is passed through big screens that remove large solid objects like rags and sticks. This is followed by passing the water through grit and sand removal tanks where the heavy grit settles down.
The clarified water then enters large tanks called clarifiers or sedimentation tanks, where the suspended solids settle at the bottom and are removed with scrappers. This settled solid material is called sludge, and it is further treated to be used as manure. The clear water that remains is called clarified water. Finally, the clarified water is passed through aeration tanks, where air is pumped in to help the aerobic bacteria decompose the remaining organic matter. In some plants, chlorine or ultraviolet light is used to kill harmful germs before the treated water is released into rivers or seas. The treated water can be used for gardening and other non-drinking purposes.
Sanitation means the proper disposal of human excreta and other wastes so that they do not cause pollution and disease. Poor sanitation and the open disposal of sewage are major causes of diseases like cholera, typhoid, jaundice and dysentery. It is important that the sewerage system is properly maintained and that wastewater is treated before it is released.
Good sanitation practices include using proper toilets and washrooms, keeping the surroundings clean, not defecating in the open, and disposing of garbage properly. Drinking water must be clean and safe; it should be boiled or purified before drinking if there is any doubt. People living in areas without a sewer system must use methods like low-cost on-site sanitation systems, for example septic tanks or composting pits, which treat the waste on the spot. Sanitation is closely linked to health, and improving sanitation improves the health of the whole community.
Wastewater must be disposed of carefully, and better housekeeping practices can reduce the amount and toxicity of sewage. We should never throw oils, fats, paints, medicines or chemicals into the drains, as they can block the sewer pipes and are difficult to treat. Used tea leaves, solid food scraps, cotton and sanitary materials should be thrown into the dustbin, not into the sink or toilet. We should not pour pesticides or strong chemicals into the drains.
In better housekeeping, we use the minimum amount of water needed, repair leaking taps, and reuse water wherever possible, such as using the water from washing vegetables for watering plants. The government has built sewage treatment plants under schemes like the Ganga Action Plan, which aims to reduce pollution of the river Ganga. Societies and towns can also install their own treatment plants. By following good housekeeping practices, we reduce the burden on treatment plants and keep our water resources clean.
| Impurity in Sewage | Type |
|---|---|
| Human faeces, animal waste, oils, urea | Organic |
| Nitrates, phosphates, heavy metals | Inorganic |
| Bacteria, viruses (cholera, typhoid) | Microorganisms |
| Step of Sewage Treatment | Process |
|---|---|
| Screening | Removes large solid objects |
| Grit removal | Sand and grit settle |
| Clarifier/sedimentation | Solids settle as sludge |
| Aeration | Aerobic bacteria decompose organics |
| Chlorination | Kills harmful germs |
Wastewater, if not treated, is a serious threat to health and the environment. Sewage contains organic, inorganic and microbial impurities, and it is carried by the sewerage system to sewage treatment plants, where it is purified through screening, grit removal, sedimentation, aeration and chlorination. Good sanitation prevents the spread of water-borne diseases like cholera and typhoid, and better housekeeping practices reduce the burden on treatment systems. By using water wisely, disposing of waste properly and treating sewage, we can protect our rivers, lakes and groundwater for future generations.