A Sewage Treatment Plant (STP) is a facility designed to remove contaminants from domestic and municipal wastewater before it is safely discharged into the environment or reused. As urbanisation accelerates across India, STPs have become a legal requirement and an environmental necessity — not just for municipalities but for residential complexes, hospitals, hotels, and industrial townships that generate significant volumes of sewage every day.
Why Sewage Treatment Is Essential
Untreated sewage is one of the most significant sources of water pollution in India. It contains disease-causing pathogens, heavy metals, nitrogen, phosphorus, and biodegradable organic matter. When discharged into rivers, lakes, or groundwater without treatment, it depletes dissolved oxygen, creates toxic algal blooms, spreads waterborne diseases like cholera and typhoid, and permanently degrades aquatic ecosystems. Effective sewage treatment protects public health, satisfies pollution control norms set by the CPCB and SPCBs, and — increasingly — enables water recycling for flushing, gardening, and industrial cooling.
How an STP Works: The Three-Stage Treatment Process
A standard sewage treatment plant operates in three sequential stages. Each stage targets a progressively finer category of pollutant. The table below summarises the entire treatment chain at a glance:
Treatment Stage | Mechanism | Pollutants Removed | Key Equipment |
|---|---|---|---|
Stage 1 — Preliminary & Primary | Physical (screening, settling) | Large solids, grit, floating debris, 30–40% suspended solids | Bar screens, grit chamber, primary clarifier |
Stage 2 — Secondary (Biological) | Biological (aerobic bacteria) | Dissolved organics, BOD (80–90%), COD, suspended solids | Aeration tank, activated sludge unit, MBBR, SBR, secondary clarifier |
Stage 3 — Tertiary (Advanced) | Chemical & physical polishing | Residual pathogens, nutrients (N, P), colour, turbidity | Sand filter, UV/chlorination unit, membrane filter, RO |
Stage 1: Preliminary and Primary Treatment
Raw sewage enters the STP through an inlet chamber. Before any biological treatment can begin, large solids and inorganic materials must be removed — if allowed to pass further, they damage equipment and poison the biological processes downstream. Preliminary treatment uses mechanical screens and grit removal; primary treatment uses gravity sedimentation in a clarifier to settle out 30–40% of suspended solids. The settled material (primary sludge) is drawn off for further treatment or biogas generation.
Bar Screen & Fine Screen — Removes rags, plastics, and large floating solids to protect downstream equipment
Grit Chamber — Allows sand, gravel, eggshells and other heavy inorganic particles to settle before they can abrade pumps and pipes
Primary Clarifier (Settling Tank) — Reduces flow velocity so settleable organic solids sink to the bottom as primary sludge, and scum floats to the surface for skimming
Stage 2: Secondary (Biological) Treatment
This is the most critical — and most complex — stage of an STP. It harnesses naturally occurring aerobic bacteria to consume dissolved organic matter that physical treatment cannot remove. Oxygen is continuously pumped into the aeration tank to sustain bacterial activity. The bacteria break down BOD (Biochemical Oxygen Demand) and COD (Chemical Oxygen Demand), dramatically reducing the pollutant load. Different process configurations are used depending on the site area, influent load, and desired effluent quality:
Activated Sludge Process (ASP) — The conventional method; a mixed liquor of bacteria and wastewater is aerated, then settled in a secondary clarifier. Proven and cost-effective for large flows
Moving Bed Biofilm Reactor (MBBR) — Bacteria grow on plastic media suspended in the aeration tank. Compact, robust, and well-suited for land-constrained sites like apartments and commercial buildings
Sequencing Batch Reactor (SBR) — A single tank performs aeration and settling in timed cycles. Highly flexible, easy to automate, and suitable for variable flow rates
Membrane Bioreactor (MBR) — Combines biological treatment with ultrafiltration membranes for extremely high-quality effluent. Chosen when treated water must be reused for irrigation or toilet flushing
Stage 3: Tertiary Treatment and Disinfection
Tertiary treatment is required when the effluent must meet very stringent quality standards — particularly for water reuse, sensitive receiving water bodies, or when CPCB designates the discharge point as ecologically critical. It removes what secondary treatment leaves behind: residual turbidity, nutrients like nitrogen and phosphorus, colour, and pathogens.
Sand Filtration — Passes effluent through a sand bed to capture residual suspended particles and colloidal matter
UV Disinfection — Ultraviolet light destroys bacteria and viruses without introducing chemicals, making the effluent safe for discharge into watercourses
Chlorination — Widely used, cost-effective disinfection method; requires careful dose control to prevent harmful disinfection by-products
Nutrient Removal (Biological or Chemical) — Reduces nitrogen and phosphorus levels that cause eutrophication in lakes and rivers
Key Components of a Standard STP and Their Functions
Component | Function in Treatment Process |
|---|---|
Bar Screen | Intercepts large floating solids — rags, sticks, plastics — before they damage pumps |
Grit Chamber | Settles sand, gravel, and heavy inorganic material that would otherwise abrade mechanical parts |
Primary Clarifier | Gravity settling tank that removes 30–50% of suspended organic solids before biological treatment |
Aeration Tank | Maintains aerobic conditions for bacteria to oxidise dissolved organic matter; central to BOD/COD removal |
Secondary Clarifier | Separates biological sludge from the treated effluent; return sludge is recycled to aeration tank |
Sludge Thickener | Reduces water volume of sludge for cost-effective disposal; thickened sludge may be processed for biogas |
UV / Chlorination Unit | Final disinfection step to eliminate pathogens and bring coliform count within CPCB discharge limits |
CPCB Discharge Standards for Sewage Treatment Plants in India
The Central Pollution Control Board (CPCB) has mandated minimum effluent quality standards that all STPs must comply with before discharging into natural water bodies or municipal drains. State Pollution Control Boards (SPCBs) may impose stricter norms. Non-compliance can result in plant closure, penalties, and legal liability for the responsible party.
Parameter | Permitted Limit (CPCB) | Significance |
|---|---|---|
BOD (Biochemical Oxygen Demand) | ≤ 30 mg/L | Primary indicator of organic pollution; high BOD depletes oxygen in receiving water bodies |
COD (Chemical Oxygen Demand) | ≤ 250 mg/L | Broader measure of total chemical oxygen demand including non-biodegradable compounds |
TSS (Total Suspended Solids) | ≤ 100 mg/L | Particulates that cause turbidity, block sunlight, and smother aquatic habitats |
pH | 6.5 – 9.0 | Ensures the effluent is neither highly acidic nor alkaline before entering natural water bodies |
Faecal Coliform | ≤ 1000 MPN/100 mL | Critical indicator of biological contamination; high counts indicate untreated sewage is present |
Where Are STPs Used? Common Applications in India
Residential Housing Societies & Apartments — Mandatory for complexes above a certain area or occupancy threshold under RERA and environmental clearance conditions
Municipalities and Urban Local Bodies (ULBs) — Large-capacity STPs serving entire townships, discharging into rivers or irrigation channels
Hospitals and Healthcare Facilities — Require specialised STPs capable of treating pharmaceutical residues and pathogen-rich wastewater
Hotels, Resorts and Convention Centres — High-flow, variable-load operations where treated water is often recycled for landscape irrigation and flushing
Industrial Townships and SEZs — Manage sanitary sewage separately from industrial effluent, often reclaiming treated water for utilities
Designing the Right STP for Your Site
Selecting the correct STP technology — ASP, MBBR, SBR, or MBR — depends on your daily flow volume, available footprint, effluent reuse goals, and long-term operation budget. At Arvicons Enviro, we have designed, supplied, installed, and commissioned over 1,500 water and wastewater treatment systems across India. Our STP designs are engineered for reliable performance, minimum operator intervention, and guaranteed compliance with CPCB and SPCB norms. Contact us to discuss your sewage treatment requirement, and our engineers will recommend the most appropriate system for your site.