Wastewater treatment is mainly carried out through two major methods:

• STP (Sewage Treatment Plant)
• ETP (Effluent Treatment Plant)

Both treat wastewater but differ in the type of water treated and end-use.

1. STP (Sewage Treatment Plant)

STP is used for treating domestic wastewater generated from residential buildings, apartments, hotels, hospitals, offices, and institutions.

Sources of Sewage:

• Black Water → Wastewater from toilets (contains fecal matter, urine, pathogens, organic matter, nitrogen, phosphorus).
• Gray Water → Wastewater from bathrooms, washbasins, kitchens, laundry (less contaminated, contains soap, grease, detergents).

STP Treatment Process:

1. Preliminary Treatment
   1. Screening, grit removal, oil & grease trap.
   1. Removes plastics, rags, stones, sand.
2. Primary Treatment
   1. Sedimentation tanks allow suspended solids to settle.
   1. Produces primary sludge.
3. Secondary Treatment (Biological)
   1. Aeration Tank / MBBR / SBR: Microorganisms break down organic matter (BOD, COD reduction).
   1. Secondary clarifier separates biomass (activated sludge).
4. Tertiary Treatment
   1. Filtration (sand, carbon, UF membranes), disinfection (UV/chlorine/ozone).
   1. Produces clean water safe for reuse in flushing, gardening, cooling towers.

Key Point: STPs treat black water + gray water and make it reusable for non-potable purposes.

2. ETP (Effluent Treatment Plant)

ETP is used in industries to treat trade effluents before discharge/reuse.

Sources of Industrial Effluents:

• Trade Waste → Non-hazardous industrial wastewater (food processing waste, textile dye water, sugar mill effluents, paper pulp wastewater).
• Hazardous Effluents → Contains heavy metals, toxic chemicals, solvents, acids, alkalis, oils (from pharma, chemical plants, tanneries, electroplating, refineries).

ETP Treatment Process:

1. Preliminary Treatment
   1. Screening, equalization, oil separation, neutralization.
2. Primary Treatment
   1. Chemical coagulation, flocculation, sedimentation.
   1. Removes suspended solids, color, some metals.
3. Secondary Treatment (Biological)
   1. Aerobic / anaerobic systems depending on wastewater.
   1. Reduces organic load (BOD, COD).
4. Tertiary / Advanced Treatment
   1. Activated carbon, reverse osmosis, MEE (Multi-effect evaporator), stripping, advanced oxidation, membrane bioreactors.
   1. Removes toxic chemicals, heavy metals, dissolved salts.

Key Point: ETPs are designed to handle industrial hazardous effluents and comply with Pollution Control Board norms before discharge

Comparison – STP vs ETP

Aspect	STP (Sewage Treatment Plant)	ETP (Effluent Treatment Plant)
Source of Wastewater	Domestic sewage (black water + gray water)	Industrial effluents (trade waste + hazardous effluents)
Contaminants	Organic waste, soap, grease, pathogens	Chemicals, heavy metals, dyes, oils, toxins
Treatment Focus	Biological + disinfection	Chemical + physical + biological + advanced
Reuse Options	Flushing, gardening, cooling tower, construction	Process reuse, safe discharge, zero liquid discharge
Regulation	CPCB & local municipal norms	CPCB & SPCB industrial effluent standards

In Short:

• STP → Treats sewage from toilets, kitchens, bathrooms (black water + gray water).

• ETP → Treats industrial wastewater (trade waste + hazardous effluents).

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