A Sewage Treatment Plant (STP) treats wastewater (from homes, industries, etc.) to remove contaminants — making it safe to discharge into the environment or reuse.
The process happens in three main stages:
1. Primary Treatment (Physical)
2. Secondary Treatment (Biological)
3. Tertiary Treatment (Chemical/Advanced
1. Primary Treatment (Physical Process)
Purpose: Remove large solids and suspended materials.
Steps:
Screening:
Removes large objects (rags, plastics, sticks) using bar screens.
Grit Removal:
Removes sand, gravel, and heavy particles in a grit chamber.
Primary Sedimentation:
Wastewater is allowed to settle in a large tank.
Solids (sludge) settle at the bottom.
Oils and grease float to the top and are skimmed off.
Outcome:
~30–40% of organic matter removed. Effluent (liquid) goes to secondary treatment.
2. Secondary Treatment (Biological Process)
Purpose: Decompose dissolved and suspended organic matter using microorganisms.
Activated Sludge Process:
Air is pumped into aeration tanks containing microorganisms.
Microbes break down organic pollutants.
Mixture flows to a secondary clarifier — sludge settles at the bottom.
Some sludge is recycled; excess is sent for sludge treatment.
Trickling Filters / Biofilters:
Wastewater trickles over a bed of stones or plastic media covered with microbial film that digests organic matter.
Oxidation Ponds / Aerated Lagoons:
Natural treatment using algae and bacteria in large basins.
Outcome:
Removes ~85–95% of organic pollutants (BOD/COD).
3. Tertiary Treatment (Chemical / Advanced Treatment)
Purpose: Further polish the effluent for safe discharge or reuse.
steps:Filtration: Removes remaining fine particles using sand or membrane filters.
Disinfection: Kills pathogens using:
Chlorination
UV radiation
Ozonation
Nutrient Removal: Eliminates nitrogen and phosphorus to prevent eutrophication.
Advanced Treatments (if ne
Reverse Osmosis (RO)
Activated Carbon Adsorption
Outcome:
Effluent is clear, odorless, and safe for reuse or discharge.