Environmental Biotechnology: Wastewater and Sewage Treatment
Science & Technology › Biotechnology – Applications · Topic 11.15
Environmental biotechnology uses living organisms to monitor, prevent and clean up pollution, and wastewater and sewage treatment is its largest application. Sewage treatment removes organic matter, nutrients and germs from used water in stages: primary treatment removes solids by settling and screening, secondary or biological treatment uses microbes to break down dissolved organic matter, and tertiary treatment polishes the water by removing nutrients and disinfecting it. The best known secondary method is the activated sludge process, in which air is bubbled through sewage mixed with microbial flocs. The pollution load is measured by biochemical oxygen demand and chemical oxygen demand, and treated water can be reused for industry, gardening and farming.
5 min read · UPSC Prelims · Science & Technology · Biotechnology – Applications
At a Glance
- Stages Primary, secondary, tertiary
- Biological method Activated sludge
- Measure BOD and COD
- Anaerobic reactor UASB
- Indian programme Namami Gange
- Exam link UPSC Prelims, Science & Technology: Biotechnology – Applications
Where Does It Fit?
This topic follows the Biotech-KISAN scheme. After agriculture, the macro turns to environmental uses, beginning with the treatment of wastewater which links to the bioremediation topic. Previous topic: Biotech-KISAN Scheme. Next topic: Landfilling Technologies and Detection/Monitoring of Pollutants.
What Are the Stages of Sewage Treatment?
| Stage | Description |
|---|---|
| Preliminary treatment | Screens and grit chambers remove large objects and sand |
| Primary treatment | Settling tanks remove suspended solids as primary sludge. Removes a part of the organic load |
| Secondary treatment | Microbes consume dissolved organic matter in aeration tanks. The activated sludge process uses air and floc forming bacteria. Trickling filters use a film of microbes on stones or plastic media |
| Settling of flocs | Flocs of microbes settle in a clarifier, and part is returned to the aeration tank as activated sludge |
| Anaerobic treatment | UASB, or upflow anaerobic sludge blanket reactor, digests organic matter without air and gives biogas |
| Tertiary treatment | Removes nitrogen and phosphorus, and kills germs by chlorine, ultraviolet light or ozone |
| Sludge treatment | Digested to make biogas and used as manure after safe processing |
What Measures and Natural Methods Are Used?
- Biochemical oxygen demand: the amount of oxygen microbes need to break down organic matter in water over five days at 20 degrees Celsius. High BOD means high pollution
- Chemical oxygen demand: the oxygen needed to oxidise all organic and inorganic matter chemically
- Dissolved oxygen: fish need dissolved oxygen, which falls when sewage enters a river
- Eutrophication: excess nitrogen and phosphorus cause algal blooms in lakes
- Constructed wetlands: planted beds with reeds and microbes that treat sewage naturally at low cost
- Oxidation ponds: shallow ponds where algae and bacteria clean sewage with sunlight
- Membrane bioreactor: combines activated sludge with membranes to give high quality water
- Phytoremediation and algae: plants and algae remove nutrients and metals from wastewater, as in Bioremediation and Microbial Remediation
What Else Should You Remember?
- Indian numbers: the Central Pollution Control Board reported in 2021 that sewage generation in cities was about 72,000 million litres a day, while installed treatment capacity covered well under half of it. Check the latest report
- Namami Gange: the National Mission for Clean Ganga, launched in 2014 with a large outlay, funds sewage treatment plants along the Ganga and its tributaries
- Hybrid Annuity Model: a model used for sewage plants in Namami Gange in which the Government pays part of the cost and the private partner builds and runs the plant
- One city one operator: an approach under Namami Gange to give a single agency the responsibility for sewage in a city
- AMRUT: the Atal Mission for Rejuvenation and Urban Transformation funds sewerage and water supply in cities, and AMRUT 2.0 aims at universal sewerage coverage in the largest cities
- Treated water reuse: the Government promotes reuse of treated water for industry, construction and horticulture
- Standards: the Central Pollution Control Board and the National Green Tribunal have tightened discharge standards for sewage treatment plants, with limits for BOD, suspended solids and faecal coliform
- Faecal sludge management: septic tanks and pits need safe emptying and treatment, which a national policy on faecal sludge supports
- Common effluent treatment plants: small industries share a plant to treat their wastewater
- Zero liquid discharge: industries such as textiles and tanneries must recycle their wastewater and not discharge any
- Anammox: special bacteria remove nitrogen without air in an energy efficient way
- Link: waste treated by microbes follows the principles in Bio-Composting and Carbon Capture Techniques
- Health risk: untreated sewage spreads cholera, typhoid and hepatitis, so treatment protects public health as well as rivers
Why Does It Matter for the Exam?
- Stages: primary, secondary, tertiary
- Terms: BOD, COD, activated sludge, UASB
- Schemes: Namami Gange, AMRUT
- Current affairs link: river pollution and treated water reuse
- Common errors: saying that primary treatment is biological or that high BOD means clean water
As an analytical point, India’s rivers will not be clean unless collection networks and treatment keep pace with growing cities. Reuse of treated water also reduces pressure on freshwater.
Why Is This Topic Important?
- Process Three stages
- Terms BOD and sludge
- Rivers Namami Gange
What Came Next?
The next topic covers landfilling technologies and the monitoring of pollutants. Next topic: Landfilling Technologies and Detection/Monitoring of Pollutants.
Key People and Terms
- BOD Oxygen demand of microbes
- COD Chemical oxygen demand
- Activated sludge Microbial floc in aeration
- UASB Anaerobic sludge reactor
- Effluent Treated outflow
- Sludge Settled solids
Exam Corner
Points to Remember
- BOD test: 5 days at 20 degrees.
- Secondary treatment is biological.
- UASB produces biogas.
- Namami Gange launched 2014.
- High BOD = more pollution.
Do Not Confuse With
- BOD and COD Biological versus chemical oxygen demand
- Primary and secondary treatment Physical settling versus microbial
- Activated sludge and UASB Aerobic versus anaerobic
Memory Hook
Screen and settle, then microbes eat, then polish and kill; low BOD means the river will heal.
Mains Angle
Questions ask about stages, terms and river schemes. These points help.
- Process Stages
- Gaps Collection and treatment
- Schemes Namami Gange
A Question You May Face
An original practice question, not a past paper question.
“Explain the stages of biological sewage treatment and discuss the challenges in cleaning Indian rivers.”
How to Answer
- Process Three stages.
- Terms BOD, sludge.
- Challenges Gaps in capacity.
- Way Reuse and operation.
GS Relevance
Frequently Asked Questions
What are the stages of sewage treatment?
Sewage treatment has primary, secondary and tertiary stages. Primary treatment removes solids by screening and settling, secondary treatment uses microbes, as in the activated sludge process, to break down dissolved organic matter, and tertiary treatment removes nutrients and kills germs by chlorine or ultraviolet light before the water is released or reused.
What is the activated sludge process?
The activated sludge process is a secondary treatment method in which sewage is mixed with a culture of microbes in an aeration tank supplied with air. The microbes form flocs that consume organic matter, then settle in a clarifier. Part of the settled sludge is returned to the tank to keep the microbial population high.
What is biochemical oxygen demand?
Biochemical oxygen demand, or BOD, is the amount of dissolved oxygen needed by microbes to break down organic matter in a water sample over five days at 20 degrees Celsius. A high BOD means that the water has a high load of organic pollution and that fish and other aquatic life may suffer from oxygen shortage.
What is a UASB reactor?
A UASB, or upflow anaerobic sludge blanket reactor, is a tank in which sewage flows upward through a blanket of anaerobic microbes that digest organic matter without air. The process needs no aeration energy and produces biogas, but the effluent usually needs further treatment to meet discharge standards.
What is Namami Gange?
Namami Gange is the Government of India's flagship programme, launched in 2014, to clean and rejuvenate the river Ganga. Its main component is building sewage treatment plants and sewer networks in towns along the river and its tributaries, along with industrial pollution control, river front development and public awareness.
PYQ Practice — Statement Analysis
1 Secondary sewage treatment involves microbial breakdown of organic matter.
As in activated sludge.
2 A high BOD value indicates cleaner water.
It indicates more organic pollution.
3 UASB reactors work without oxygen and produce biogas.
Anaerobic digestion.
4 Namami Gange was launched in 2014.
To clean the river Ganga.
Past Mains Questions on This Topic
This topic comes under Biotechnology – Applications. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
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