STP vs ETP: Key Differences and How to Choose the Right Treatment Plant

Both STPs (Sewage Treatment Plants) and ETPs (Effluent Treatment Plants) treat wastewater, but they serve fundamentally different purposes. Understanding these differences is crucial for regulatory compliance and effective waste management. This guide explains when to use each system and how to make the right choice.

What is an STP (Sewage Treatment Plant)?

STPs treat domestic sewage—wastewater from toilets, kitchens, bathrooms, and laundry. The waste is primarily organic (biodegradable) with consistent composition. STPs are mandatory for residential complexes above 10,000 sq.m (as per local building bylaws), hotels, hospitals, educational institutions, and commercial buildings.

What is an ETP (Effluent Treatment Plant)?

ETPs treat industrial effluents—wastewater from manufacturing processes. This includes heavy metals, chemicals, oils, acids, alkalis, and complex organic compounds. Each industry generates unique effluents requiring customized treatment solutions. ETPs are mandatory for industries classified as Red/Orange category by CPCB.

Key Differences at a Glance

Waste Source: STP handles domestic/sanitary waste; ETP handles industrial process waste. Treatment Complexity: STP uses standard biological processes; ETP requires industry-specific treatment trains. Regulatory Authority: Both must comply with CPCB/SPCB norms, but discharge standards differ. Capital Cost: ETPs typically cost 2-5x more than equivalent capacity STPs due to specialized equipment.

STP Treatment Processes

Common STP technologies include: Activated Sludge Process (ASP), Sequential Batch Reactor (SBR), Moving Bed Biofilm Reactor (MBBR), and Membrane Bioreactor (MBR). These biological processes efficiently break down organic matter, achieving BOD removal of 95%+ and producing reusable treated water.

ETP Treatment Processes

ETPs employ multi-stage treatment: Physical (screening, oil separation, equalization), Chemical (neutralization, coagulation, precipitation), Biological (for biodegradable organics), and Tertiary (filtration, carbon adsorption, advanced oxidation). For zero liquid discharge, evaporation and crystallization may be added.