Introduction to Activated Sludge Process–Sewage Treatment

Discover how the activated sludge process transforms wastewater into clean, reusable water through advanced biological treatment. This comprehensive guide explains the working principles, operation methods, aeration techniques, and key design factors that ensure efficient sewage purification — a critical foundation for drinking machine water systems and municipal wastewater treatment facilities.

los activated sludge process is a cornerstone of modern biological wastewater treatment technology. It uses a community of microorganisms suspended in water — known as activated sludge — to decompose organic pollutants in sewage, producing purified effluent suitable for further treatment or reuse.

Developed in the early 20th century and continuously improved since, this method remains the most efficient and widely adopted biological treatment process in municipal sewage treatment plants, industrial wastewater systems, and even drinking machine purification setups that rely on biological pre-filtration.

To accelerate the dissolution of oxygen essential for microbial respiration, pure oxygen aeration systems emerged in the 1970s. These systems replaced standard air with high-purity oxygen, improving efficiency and reducing tank size requirements.
Most facilities use surface aeration devices to enhance oxygen transfer, ensuring microorganisms receive a continuous oxygen supply to maintain metabolic activity and water purification performance.

Successful operation of an activated sludge system depends on two critical parameters:

1. Activated sludge concentration (MLSS – Mixed Liquor Suspended Solids):
   This defines the mass of microorganisms in the aeration tank.
   Adjusting MLSS regulates the load rate y biological activity, ensuring optimal pollutant degradation.
2. Oxygen supply:
   Oxygen is required for microbial respiration. Insufficient aeration leads to incomplete degradation and the formation of foul-smelling anaerobic zones.

Common Operational Issue: Sludge Bulking

Sludge bulking occurs when the sludge fails to settle properly in the secondary clarifier due to excessive water content or the overgrowth of filamentous bacteria.
This condition causes sludge washout, deteriorating effluent quality and reducing biomass in the aeration tank.
Operators must identify the cause promptly — often by adjusting aeration intensity, nutrient ratios, or return sludge rates — to restore balance.

Microorganisms in the activated sludge absorb and break down organic compounds in wastewater, converting them into carbon dioxide (CO₂), ammonia (NH₃), and new microbial biomass.
The process also involves adsorption of suspended solids and colored substances, improving overall clarity and reducing biochemical oxygen demand (BOD).

Typical Process Flow:

1. Raw wastewater enters the aeration tank.
2. It mixes thoroughly with activated sludge y aeration air.
3. Microorganisms consume organic pollutants for energy and growth.
4. The mixture flows into the secondary sedimentation tank, where sludge settles.
5. Part of the sludge is recycled back into the aeration tank to maintain microbial activity, while excess sludge is removed for further treatment.

Under proper conditions, the BOD₅ and suspended solids removal efficiency can exceed 90%.
If followed by rapid filtration, the residual BOD₅ can be as low as 1–2 mg/L, achieving near-drinking water quality standards.

The activated sludge process includes several design variations to accommodate different wastewater types and treatment goals:

(1) Conventional Activated Sludge Process

The standard model featuring uniform aeration along the tank. It provides stable performance and is ideal for municipal sewage.

(2) Multi-Point Inflow Activated Sludge Process

Here, wastewater enters the aeration tank at multiple points, distributing organic load evenly.
This enhances oxygen utilization and reduces localized overloads.

(3) Adsorption-Regeneration (Contact Stabilization) Method

An advanced variant where microorganisms rapidly adsorb pollutants during a short contact phase, followed by a regeneration phase that allows them to digest absorbed matter fully.
This approach improves organic removal efficiency y sludge stability.

(4) High-Load Activated Sludge Method

Operates with a sludge loading rate >1.0 kg BOD₅/kg MLSS, offering lower capital costs y smaller reactor volumes but moderate treatment efficiency (BOD removal ≈70%).
Suitable for industrial pre-treatment or cases where high-quality effluent is not critical.

(5) Extended Aeration Method

Uses a low load rate (<0.1 kg BOD₅/kg MLSS) y long aeration times (≥24 hours).
It produces minimal excess sludge, offers stable operation, and requires less frequent sludge disposal, making it ideal for small communities and decentralized treatment systems.