Sewage Treatment Plant (STP)
A Sewage Treatment Plant is an essential facility that treats wastewater from domestic, commercial, or industrial sources to remove contaminants before safely releasing it into the environment or reusing it.
Key processes typically include:
- Primary treatment: Removal of large solids and sediment through screening and settling.
- Secondary treatment: Biological treatment using microorganisms to break down organic matter.
- Tertiary treatment (if required): Advanced processes to remove remaining nutrients, pathogens, or pollutants.
Purpose:
- Protects public health and the environment.
- Enables reuse of treated water for gardening, flushing, or industrial applications.
- Helps meet legal and environmental regulations.
STPs play a vital role in sustainable urban infrastructure, supporting cleaner water bodies and a healthier ecosystem.
Sewaf Energy: 🌱 Types of Sewage Treatment Plants (STPs)
Conventional Activated Sludge Process (ASP)
- Widely used for municipal and industrial applications.
- Uses aeration tanks and biological sludge recycling to treat sewage effectively.
Sequential Batch Reactor (SBR)
- A fill-and-draw type system treating wastewater in batches.
- Combines equalization, aeration, and sedimentation in a single tank.
- Compact and flexible, suitable for variable loads.
Moving Bed Biofilm Reactor (MBBR)
- Uses free-floating plastic media as carriers for biofilm.
- Compact design, high efficiency, and easy to upgrade.
Membrane Bio-Reactor (MBR)
- Combines biological treatment with membrane filtration.
- Produces high-quality treated water suitable for reuse.
- Compact footprint, ideal for space-limited areas.
Extended Aeration
- A variation of ASP with a longer retention time.
- Produces less excess sludge and is simpler to operate.
Trickling Filter
- Wastewater is distributed over a bed of stones or synthetic media covered with microbial biofilm.
- Simple design, low energy consumption, mainly for small-scale plants.
Rotating Biological Contactor (RBC)
- Discs with biofilm rotate partially submerged in sewage.
- Energy efficient and suitable for small to medium communities.
Got it 👍 Here’s a clear Do’s and Don’ts list for STP (Sewage Treatment Plant) operations — helpful for operators, staff training, or awareness posters:
Do’s for STP Plant
Do regular maintenance
- Clean screens, grit chambers, and tanks periodically.
- Check pumps, blowers, and aeration systems.
Do follow safety protocols
- Ensure workers use PPE (gloves, masks, boots, safety harness where needed).
- Provide gas detectors (Hâ‚‚S, methane, chlorine).
Do monitor water quality
Regularly test influent and effluent for BOD, COD, pH, TSS, etc.
Do proper sludge management
Dewater sludge and dispose of as per environmental norms.
Do maintain records
Keep logs of operations, maintenance schedules, and laboratory tests.
Do ensure an uninterrupted power supply
Use backup generators or solar for critical components.
Do train staff
Regularly update operators on O&M procedures and emergency handling.
Do follow environmental compliance
Discharge only treated water that meets regulatory standards.
Don’ts for STP Plant
Don’t discharge untreated sewage
- Never bypass treatment stages before disposal.
Don’t allow toxic/industrial waste mixing
Chemicals, oils, or hazardous waste damage biological treatment.
Don’t ignore odor control
Use proper ventilation, biofilters, or odor control systems.
Don’t delay desludging
Excess sludge reduces treatment efficiency and damages equipment.
Don’t neglect instrumentation
Faulty flow meters, DO meters, or sensors can mislead operations.
Don’t allow unauthorized access
Restrict entry to treatment tanks, electrical panels, and blower rooms.
Don’t overfeed the system
Sudden hydraulic or organic overload reduces performance.
Don’t skip preventive maintenance
Waiting for breakdowns increases downtime and repair costs.
STP Plant Equipment List
Preliminary Treatment
- Bar Screen / Coarse Screen – Remove large solids, plastics, rags
- Fine Screen / Mechanical Screen – Remove finer particles
- Grit Chamber / Grit Separator – Remove sand & grit
- Oil & Grease Trap / Skimmer – Remove oil & grease
Primary Treatment
- Primary Clarifier / Settling Tank – Sedimentation of suspended solids
- Sludge Scraper & Collection Mechanism – Sludge removal from clarifier bottom
Secondary Treatment (Biological)
- Aeration Tank / Bioreactor – Biological treatment of sewage
- Diffused Aeration System / Blowers & Diffusers – Provide oxygen for microbes
- MBBR Media / Biofilm Carriers – Biofilm growth for MBBR
- Membrane Modules (UF/MF) – Membrane filtration (MBR system)
- Decanter System (SBR) – Decant treated water (SBR system)
- Secondary Clarifier – Biomass separation
Tertiary / Advanced Treatment
- Sand Filter (Pressure/Gravity) – Remove turbidity & suspended matter
- Activated Carbon Filter (ACF) – Remove odor, color & organics
Advanced Treatment
- Chlorination / Hypochlorite Dosing System – Disinfection with chlorine
- UV Disinfection / Ozonation Unit – Disinfection with UV/Ozone
- Softener / RO Plant (Optional) – Polishing for reuse water
Sludge Treatment
- Sludge Holding Tank – Temporary sludge storage
- Filter Press / Belt Press / Centrifuge – Sludge dewatering
- Drying Beds (Optional) – Sludge drying (small plants)
- Sludge Pumps – Sludge transfer
Pumps & Piping System
- Raw Sewage Transfer Pumps – Transfer of raw sewage
- Return Activated Sludge (RAS) Pumps – Recycle activated sludge
- Waste Sludge Pumps – Transfer excess sludge
- Treated Water Pumps – Pump treated water
- Chemical Dosing Pumps – Chemical dosing
Electrical & Control Systems
- MCC Panel / Control Panel – Electrical distribution & control
- Level Sensors & Flow Meters – Measurement & monitoring
- SCADA / PLC Automation System – Automation & remote control
- Power Backup (DG Set / UPS) – Emergency power supply
Basic Questions
What is an STP?
A Sewage Treatment Plant (STP) is a facility designed to remove contaminants from wastewater (domestic or industrial sewage) so that the treated water can be safely discharged or reused.
Why do we need STPs?
To protect the environment, prevent water pollution, safeguard public health, and comply with government norms (CPCB / SPCB).
Process Related Questions
What are the main stages of sewage treatment?
- Preliminary Treatment: Removal of large solids (screening, grit chamber).
- Primary Treatment: Sedimentation of suspended solids.
- Secondary Treatment: Biological treatment (aeration, activated sludge, MBBR, SBR, etc.).
- Tertiary Treatment: Advanced treatment (filtration, disinfection, RO, UV, chlorination).
- Sludge Handling: Digestion, dewatering, and disposal.
What are the common technologies used in STPs?
ASP (Activated Sludge Process), MBBR (Moving Bed Biofilm Reactor), SBR (Sequencing Batch Reactor), MBR (Membrane Bioreactor), and Natural methods (oxidation ponds, constructed wetlands).
What is BOD & COD in wastewater?
- BOD (Biochemical Oxygen Demand): Amount of oxygen needed by microorganisms to decompose organic matter.
- COD (Chemical Oxygen Demand): Amount of oxygen needed to chemically oxidize both organic & inorganic matter.
Operation & Maintenance Questions
What are common problems faced in STPs?
Foaming, sludge bulking, odor issues, power failure, improper aeration, or non-compliance with effluent standards.
How is sludge managed in an STP?
Sludge is thickened, digested (anaerobic or aerobic), dewatered (centrifuge, filter press), and then used as manure or safely disposed of.
What are STP effluent discharge standards in India (CPCB guidelines)?
BOD ≤ 10–30 mg/L, COD ≤ 50–100 mg/L, TSS ≤ 20–30 mg/L, depending on whether it is discharged to land, surface water, or reused.
Practical Questions
What is the capacity of an STP?
- It depends on the population/sewage generation. For example:
- Residential buildings: ~135 liters per person per day.
- Hotels, hospitals, industries: higher water use.
How is treated water reused?
For toilet flushing, gardening, HVAC cooling towers, construction, or groundwater recharge.
What is the typical power consumption of an STP?
1–2 units (kWh) per 1,000 liters of sewage treated (depends on technology).
Environmental & Legal Questions
Is STP mandatory?
Yes, CPCB & local pollution control boards mandate STPs for residential apartments, commercial complexes, and industries generating >10 KLD wastewater.
What happens if sewage is not treated?
It causes water pollution, spreads diseases, damages aquatic life, and attracts heavy penalties/legal action.
Interview / Exam Questions
Difference between STP and ETP?
- STP (Sewage Treatment Plant): Treats domestic wastewater (from toilets, kitchens, bathrooms).
- ETP (Effluent Treatment Plant): Treats industrial wastewater (chemical, pharmaceutical, textile, etc.).
What is the life of an STP?
Civil structures: 40–50 years, Mechanical/Electrical equipment: 10–15 years (with maintenance).
