What Is Secondary Wastewater Treatment?

Secondary wastewater treatment is a crucial stage in the sewage treatment process, designed to remove dissolved and suspended organic matter that primary treatment cannot eliminate. This step plays a key role in industrial wastewater treatment systems, municipal plants, and decentralized treatment projects worldwide.

How Secondary Treatment Works

The main principle of secondary wastewater treatment is the use of biological processes. Microorganisms break down organic pollutants in an oxygen-rich environment. To supply sufficient oxygen, modern plants commonly rely on aeration systems, especially fine bubble diffusers, disc diffusers, and aeration tubes. These devices ensure efficient oxygen transfer, lower energy consumption, and stable biological performance.

The most widely used secondary treatment technologies include:

1. Activated Sludge Process (ASP)

The activated sludge system uses an aeration tank equipped with fine bubble aerators to maintain high oxygen levels. Microorganisms consume organic matter, forming flocs that later settle in a clarifier. This process is widely used in municipal sewage treatment and many industries.

2. Moving Bed Biofilm Reactor (MBBR)

MBBR technology uses biofilm carriers suspended in the aeration tank. Combined with fine bubble aeration, it offers higher treatment capacity, smaller footprint, and easier operation. It is ideal for upgrading overloaded WWTPs and industrial facilities.

3. Extended Aeration

A simple but stable form of activated sludge using long aeration times. This method benefits from energy-efficient aeration systems such as diffuser membranes and aeration pipes.

Why Secondary Treatment Is Important

Secondary wastewater treatment helps achieve:

• Reduction of BOD and COD

• Effective removal of organic pollutants

• Stable effluent quality for discharge or further tertiary treatment

• Improved system efficiency with advanced aeration technologies

High-performance aeration is especially important because it can account for 40–60% of total energy use in a treatment plant. Upgrading to modern fine bubble diffusers or disc diffusers greatly improves oxygen transfer efficiency while reducing maintenance costs.

Applications in Industry and Municipal Plants

Secondary treatment systems are widely used in:

• Industrial wastewater treatment (food, chemicals, textile, paper, electronics)

• Municipal sewage treatment plants

• Upgrading old WWTPs to meet stricter standards

• Decentralized containerized wastewater systems

With the right aeration design and diffuser selection, operators can significantly improve performance without excessive investment.