Biological Wastewater Treatment Plant

Biological Wastewater Treatment Plant

A biological wastewater treatment plant is a facility designing to treat wastewater using natural microorganisms to break down organic matter and remove pollutants. It is one of the most common and effective methods of treating municipal and industrial wastewater before its safe release into the environment or its reuse for non-potable purposes.

What is the purpose of biological wastewater treatment?

The purpose of biological wastewater treatment is to treat wastewater effectively and environmentally responsibly, removing or reducing pollutants and contaminants to make it safe for discharge into the environment or for reuse. Wastewater, which includes domestic sewage and industrial effluents, contains a variety of organic and inorganic substances, pathogens, and other contaminants that can be harmful to human health and the environment if not properly treated.

Biological wastewater treatment utilizes naturally occurring microorganisms, such as bacteria, fungi, and protozoa, to break down and metabolize organic matter and pollutants present in the wastewater. The microorganisms act as “biological agents” that consume organic compounds as their food source, converting them into biomass and simpler, stable compounds like carbon dioxide and water.

The biological treatment process typically involves several stages:

Preliminary Treatment

The incoming wastewater undergoes preliminary treatment to remove large solids, debris, and grit through processes such as screening and grit removal.

Primary Treatment

Solids settle to the bottom of primary sedimentation tanks as sludge, while relatively clear water moves on to the next stage of treatment.

Secondary Treatment (Biological Treatment):

Biological WWTP is the secondary treatment process, where microorganisms are employing to convert it into biomass & CO2. There are two main types of secondary treatment processes:

  • Activated Sludge Process: In this process, the wastewater is mixed with a population of aerobic microorganisms in aeration tanks. These microorganisms use the organic matter in the wastewater as food, breaking it down through biochemical reactions. The resulting mixture of treated wastewater and activated sludge then enters a secondary sedimentation tank, where the sludge settles, and a portion of it is recycled back to the aeration tank, while the excess sludge is removed as waste.
  • Biofilm Processes: In biofilm-based systems, microorganisms grow on the surface of a fixed media or inside a submerged biofilm reactor. The wastewater passes through the biofilm, allowing the microorganisms to degrade the organic pollutants

Tertiary Treatment

The tertiary treatment stage is employing to further remove any remaining contaminants, such as nutrients trace metals, and pathogens. Tertiary treatment can involve various processes like filtration, chemical precipitation, and disinfection.


After biological and tertiary treatment, the treated wastewater may undergo disinfection to eliminate harmful pathogens before discharging or reusing.

Key Features of biological wastewater treatment plant:

Aeration System

Biological treatment relies on aerobic microorganisms that need oxygen to break down organic matter. Therefore, most biological WWTPs are equipped with aeration systems to supply oxygen to the microorganisms. Aeration can be achieved through diffused air systems, mechanical aerators, or surface aeration devices.

Biological Reactors

Biological treatment processes typically occur in dedicated reactors, such as activated sludge basins, sequencing batch reactors (SBRs), or moving bed biofilm reactors (MBBRs). These reactors provide an environment for microorganisms to grow and interact with wastewater, facilitating the breakdown of organic pollutants.


After the biological treatment, the mixture of treated wastewater and microorganisms enters clarifiers or sedimentation tanks. These tanks allow the biomass (activated sludge or biofilm) to settle at the bottom while the clarified water moves on for further treatment or discharge.

Sludge Treatment Facilities

Biological treatment generates sludge as a byproduct, which contains the accumulated biomass from the treatment process. Sludge treatment facilities, such as sludge thickeners, digesters, and dewatering units, are incorporating into the WWTP to process and manage the sludge.

Effluent Quality Monitoring

To ensure the treated wastewater meets required regulatory standards, biological WWTPs are equipped with effluent quality monitoring systems. These systems continuously measure various parameters, such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids, nitrogen, and phosphorus levels.

Nutrient Removal Systems

In some cases, advanced biological treatment processes are adding to remove nutrients like nitrogen and phosphorus from the wastewater. This prevents nutrient pollution in receiving water bodies, which can lead to harmful algal blooms and other environmental issues.

Scum Removal

Floating scum, consisting of grease, oils, and other non-biodegradable materials, can accumulate on the surface of biological treatment units. WWTPs often include mechanisms for scum removal to maintain efficient treatment.

pH Control

The pH level of wastewater can influence the activity of microorganisms. WWTPs may include pH control systems to adjust the pH and optimize the biological treatment process.

Process Automation and Control

Modern biological WWTPs has often equipped with advanced process automation and control systems. These systems monitor various parameters and adjust operational parameters to ensure stable and optimal treatment performance.

Disinfection Systems

As a final step, treated wastewater might undergo disinfection to eliminate harmful pathogens before it is discharged or reused. Common disinfection methods include chlorination, ultraviolet (UV) disinfection, and ozonation.

Odor Control

Biological treatment processes can produce odorous gases. WWTPs may include odor control systems to mitigate unpleasant smells and prevent odor emissions to the surrounding environment.


The design and features of a biological wastewater treatment plant depend on factors such as the size of the plant, the type of wastewater treated (municipal or industrial), effluent quality requirements, and the available budget. Continuous research and technological advancements aim to improve the efficiency and sustainability of biological treatment processes in wastewater treatment plants.

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