Aeration is used to operate activated sludge process units and is perhaps the most frequently used process to remove biochemical oxygen demand (BOD) from wastewater. Successful BOD removal in an activated sludge process depends on studying and controlling some basics – such as wastewater sources and quantities, wastewater characteristics, and needed execution in any required preliminary and primary treatment. After performing these operations, the primary treated wastewater entering the activated sludge system achieves BOD removal based on the details below.

Biological basics

The activated sludge process is a biological wastewater treatment operation. This means that treatment occurs through many different microorganisms using pollutants as a food source. It is a suspended growth process – since the organisms are suspended in wastewater rather than attached to a medium, as they are in a trickling filter or rotating biological contact process. Since this is a biological process, understanding some basic biology is useful.

Some organism terms and definitions that should be known are:

  • Anaerobic – needs no dissolved oxygen (DO) or nitrate oxygen
  • Aerobic – must have DO
  • Facultative – can exist with or without DO
  • Heterotrophic – consumes organics in the wastewater
  • Autotrophic – able to use inorganic compounds as a food source

The microorganisms

The activated sludge process relies on harvesting a population of millions of microorganisms with different characteristics – mostly aerobic, facultative and heterotrophic bacteria suspended in the wastewater – as the wastewater travels through a reactor known as an aeration tank. This suspension, referred to as the mixed liquor or mixed liquor suspended solid, is supplied oxygen and kept mixed by bubbling air through the entire aeration tank.

These are naturally occurring organisms. They do not need to be supplied from an outside source. As the organisms feed on the organic pollutants in the wastewater, the pollutants are converted to more organisms known as biomass and some spin-offs.

The amount of biomass produced is approximately 0.65 pounds for each pound of BOD removed in this secondary treatment process. While an individual bacterium is not visible, the bacteria stick to one another to make a biological mass, which may be seen easily as a brown colored floss.

The process

The interior of a typical cell contains reproductive characteristics and food storage mechanisms. Fencing the cell is the membrane that keeps the organism together and through which dissolved food may pass. The cell wall is coated with a slime layer that is used to trap particles.

Bacteria cells suspended in wastewater contain both soluble and particulate organic pollutants. Soluble organic pollutants pass through the cell membrane, referred to as absorption, and are used as a direct food source. Particulate organics cannot pass through the membrane, and instead adhere to the slime layer, referred to as adsorption.

The organism starts to create enzymes that are released through the membrane to change the particulate into a soluble, allowing the converted material to fall out through the membrane where it is also used as food. In this way, the organism removes soluble and particulate organics from the effluent.

Likewise, oxygen must be supplied to the organisms as they metabolize the organics and new bacterial cells are created. Byproducts of metabolism include ammonia, carbon dioxide and water.

Three steps

Biological wastewater treatment frequently takes place in three steps.


First, food from the wastewater is transferred to the cell. Adequate mixing and treatment (detention) time are essential to ensure that the organisms contact the nutrient source.


Conversion occurs as the organisms metabolize the food supply, converting it into new cells. For this to occur, the food supply must be a usable type and in a usable form. Some compounds are easily degraded by the bacteria, while others are metabolized more slowly. Some pollutants may not be metabolized until the organisms become acclimated to the pollutants by producing the proper enzymes. A proper DO environment must be present. Aerobic organisms will not efficiently remove pollutants in an anaerobic environment. The nutrients must also be properly balanced for conversion to occur. Like other life forms, the organism needs nitrogen and phosphorus, among other minor nutrients, to metabolize food and grow new cells. The proportion of carbon to nitrogen to phosphorus is generally efficient with a ratio of 100:5:1

Flocculation & separation

During this phase, the microorganisms stick together to form bigger particles that will fall out of the treated wastewater in the secondary clarifier. Flocculation occurs when mixing allows the organisms to contact one another but does not cause conditions so turbulent that the flocculated material is pulled apart. The settling and compaction of the floc particles depend on their density, size and configuration as well as the efficiency of the clarifier.

The settling is affected by the abundance of filamentous bacteria – those that form strings as they arise rather than forming floc. An excessive growth rate of these bacteria may cause a bulky condition in which the mixed liquor does not compact easily, causing more intensity in the clarifier. This circumstance may be caused by many factors – among which are improper DO and environment and nutrient imbalance – and may result in solids loss in the clarified effluent. After an adequate detention time, the mixed liquor flows from the aeration tank to a secondary clarifier in which the biomass descends from the wastewater by gravity and the effluent travels to the next treatment process.

Settled solids

The settled biomass is returned to the treatment process to provide organisms that continue to remove pollutants. This returned biomass is referred to as return activated sludge.

Since this is a living and maturing process, the biomass will reach a stage at which it has too much volume. The quantity of biomass in the process is restrained by removing, known as wasting, a determined amount of the biomass each day. This removed biomass is waste activated sludge.


This column discussed the basics of operating an aerated, activated sludge treatment process. In the next column (which can be found on, more details will cover bulking and restraining filamentous organisms and controlling factors – such as loading, DO, wasting, return activated sludge and solids settling.

Known in the industry as “Wastewater Dan,” Dan Theobald, proprietor of Environmental Services, is a professional wastewater and safety consultant/trainer. With more than 24 years of hands-on industry experience operating many wastewater treatment processing units, he is anxious to share his knowledge with others.