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A Sewage Treatment Plant (STP) is a complete and scientifically engineered system used to purify wastewater and make it suitable for reuse in applications such as gardening, agriculture, flushing, and other non-potable purposes. The quality standards for treated sewage water are strictly regulated by the Pollution Control Board, making STPs mandatory in most industries, residential complexes, housing colonies, and commercial buildings. Modern STPs are equipped with high-precision equipment, automation, and advanced treatment technologies to ensure consistent performance and environmental compliance.

STP is often confused with ETP (Effluent Treatment Plant), but there is a clear distinction between the two. While an ETP is mainly used to treat industrial wastewater containing chemicals and process residues, an STP is specifically designed to treat domestic sewage and mixed effluents from residential and commercial sources. The working principle of an STP is based on controlled aeration that promotes the growth of beneficial bacteria, which naturally break down organic waste and harmful pollutants. The objective is to produce significantly cleaner, eco-friendly water that is safe for discharge or reuse.

Depending on capacity and design, a sewage treatment plant can serve a single commercial facility or an entire residential community. The treatment process is carried out in multiple stages—primary, secondary, biological, and tertiary—each playing a critical role in removing solids, organic load, pathogens, and residual impurities. In Bangladesh, MHT Filtration stands as one of the leading sewage treatment plant service providers, working successfully in collaboration with various organizations to enhance wastewater quality and promote sustainable water recycling.

How Does a Sewage Treatment Plant (STP) Work?

A Sewage Treatment Plant (STP) works through a step-by-step purification system that removes physical waste, organic pollutants, and harmful microorganisms from wastewater, making it safe for discharge or reuse. The entire process is divided into well-defined treatment stages, each with a specific role in improving water quality.

1. Primary Treatment – Physical Separation

In the first stage, raw sewage enters the STP and passes through screening units where large materials like plastics, cloth, stones, and solid debris are removed. After screening, the wastewater flows into a grit chamber where heavy sand and grit settle at the bottom. It then enters a primary settling tank (clarifier) where suspended solids and sludge gradually settle, reducing the initial pollution load of the water.

2. Secondary Treatment – Biological Breakdown

This is the most critical stage of the STP. The partially treated water is transferred into aeration tanks where air is supplied continuously. This oxygen supports the growth of beneficial bacteria that consume dissolved organic waste. These microorganisms convert harmful pollutants into harmless by-products like carbon dioxide, water, and biomass. The water then passes into a secondary clarifier where the biological sludge settles, and clear treated water flows forward.

3. Tertiary Treatment – Advanced Purification

In this advanced stage, the water is filtered through sand, carbon, or membrane systems to remove any remaining fine particles. Disinfection is then carried out using chlorine, ultraviolet (UV) light, or ozone to destroy harmful bacteria and viruses. This ensures the treated water meets safety and environmental reuse standards.

4. Sludge Treatment & Disposal

The sludge collected from the primary and secondary clarifiers is thickened, dewatered, and safely disposed of or reused as manure after proper treatment. This step prevents environmental contamination and supports waste-to-resource practices.