STP - Sewage Treatment Plant

💧 Sewage Treatment Plant (STP) – Complete Overview

A Sewage Treatment Plant (STP) is a facility designed to treat wastewater and make it safe for discharge into the environment or for reuse. STPs are essential for maintaining public health, protecting water bodies, and supporting sustainable urban development.

⚙️ Working Principle of STP

STPs operate through a multi-stage process to remove physical, chemical, and biological contaminants from sewage:

1. Preliminary Treatment

• Removes large debris like plastics, rags, and stones using bar screens and grit chambers.

• Prevents damage to downstream equipment.

2. Primary Treatment

• Wastewater is held in primary clarifiers where solids settle as sludge.

• Sludge is removed for further processing.

3. Secondary Treatment

• Biological treatment using aeration tanks where microorganisms break down organic matter.

• Secondary clarifiers separate treated water from biological sludge.

4. Tertiary Treatment

• Disinfection using chlorine, UV light, or other methods to kill pathogens.

• Sand filters or advanced filtration systems polish the water for final discharge or reuse.

5. Sludge Handling

• Sludge is thickened, dewatered, and treated in digesters, drying beds, or screw presses.

• Final sludge may be reused as fertilizer or disposed of safely.

🛠️ STP Equipment

Common equipment used in STPs includes:

• Bar Screen – Removes large solids and debris.

• Grit Chamber – Separates sand, gravel, and heavy particles.

• Primary Clarifier – Settles solids and collects sludge.

• Aeration Tank – Facilitates biological treatment with microorganisms.

• Secondary Clarifier – Settles remaining solids after biological treatment.

• Sand Filter – Removes fine particles from treated water.

• Disinfection Unit – Uses chlorine or other chemicals to kill pathogens.

• Ultraviolet Disinfection System – Uses UV light for chemical-free disinfection.

• Sludge Drying Beds – Dewater sludge before disposal.

• Sludge Digesters – Break down sludge biologically, producing methane.

• Screw Press – Mechanically dewaters sludge using a screw mechanism.

🔄 Pumps Used in STP

Pumps are essential for moving wastewater and sludge through various stages:

• Centrifugal Pumps – Transfer raw and treated water; also used for sludge movement.

• Positive Displacement Pumps – Handle thick sludge and high-pressure applications.

• Submersible Pumps – Lift raw sewage from influent chambers; used in aeration tanks.

• Air Operated Diaphragm Pumps – Move sludge and dose chemicals during disinfection.

• Screw Pumps – Transport viscous sludge; ideal for dewatering and thickening.

• Progressive Cavity Pumps – Handle abrasive, high-solid sludge and slurry.

Pump selection depends on flow rate, pressure requirements, fluid characteristics, and plant layout.

Testing of STP

Regular testing ensures that the STP operates efficiently and meets environmental standards:

1. Flow Test

• Measures inflow and outflow rates.

• Ensures the plant can handle peak loads.

2. Chemical Oxygen Demand (COD) Test

• Assesses organic matter in wastewater.

• Indicates treatment effectiveness.

3. Biological Oxygen Demand (BOD) Test

• Measures oxygen required by microorganisms to break down organic matter.

• Lower BOD indicates better treatment.

4. Total Suspended Solids (TSS) Test

• Measures solid particles in water.

• Evaluates sedimentation and filtration performance.

5. Total Dissolved Solids (TDS) Test

• Measures dissolved substances in water.

• Indicates overall water quality.

6. pH Test

• Monitors acidity/alkalinity.

• Ensures optimal conditions for biological treatment.

7. Dissolved Oxygen (DO) Test

• Measures oxygen levels in aeration tanks.

• Critical for microorganism activity.

8. Chlorine Residual Test

• Checks remaining chlorine after disinfection.

• Ensures water safety for reuse or discharge.

9. Sludge Volume Index (SVI) Test

• Evaluates sludge settleability.

• Helps optimize clarifier performance.

10. Coliform Count

• Detects bacterial contamination.

• Ensures treated water is safe for discharge or reuse.

✅ Conclusion

A well-designed and properly maintained STP:

• Protects water bodies from pollution

• Enables safe reuse of treated water

• Reduces health risks and environmental impact

• Supports sustainable urban infrastructure

From mechanical screens to advanced biological digesters, every component plays a role in transforming wastewater into a resource. Regular testing and the right equipment ensure that STPs operate reliably and meet regulatory standards.