The Three Stages of the Activated Sludge Process: How Microorganisms Remove Organic Carbon and Nutrients from Wastewater

The wastewater treatment plant operator stands at the control panel of a municipal facility, holding the authority to dictate the biological fate of millions of gallons of incoming sewage. By adjusting just three primary variables—the volume of oxygen pumped into the aeration basins, the rate at which settled biomass is returned to the system, and the volume of excess sludge wasted—the operator controls a vast, living ecosystem. When the next shift begins, these daily adjustments will determine whether the microscopic workforce within the tanks consumes the organic carbon and nutrients efficiently, or whether the system falls out of balance, risking permit violations and environmental harm.[1][6]

This living system is the activated sludge process, a biological treatment method developed in 1913 by engineers Edward Ardern and W.T. Lockett in the United Kingdom. Today, it remains the global standard for municipal wastewater treatment. The process relies on a dense, suspended culture of microorganisms—bacteria, protozoa, and metazoa—that metabolize the organic pollutants present in raw sewage. According to the Maine Department of Environmental Protection's 2009 process control manual, "The activated sludge process is a biological wastewater treatment process which speeds up the work of the bacteria by bringing air and sludge heavily laden with bacteria into close contact with the sewage."[1]

The mechanism unfolds across three distinct stages, beginning with the aeration basin. Here, raw or primary-treated wastewater mixes with a concentrated culture of microorganisms, forming a biological soup known as mixed liquor. Blowers inject atmospheric air or pure oxygen into the basin, maintaining a dissolved oxygen concentration typically between 1.5 and 2.0 milligrams per liter. This oxygen is the critical electron acceptor that allows aerobic bacteria to rapidly oxidize organic carbon, converting biochemical oxygen demand (BOD) into new bacterial cells, carbon dioxide, and water.[1][5]

Within this aerated environment, the microbial community forms dense, sticky aggregations called floc. This flocculation is essential for the subsequent stages of the process. As noted in a 2014 analysis by Water Tech Online, "Microorganisms are the workers in the wastewater treatment process," and their ability to clump together allows them to be separated from the treated water. The mixed liquor suspended solids (MLSS) concentration in the aeration tank is carefully maintained, usually between 1,500 and 3,000 milligrams per liter, to ensure a sufficient population of these biological workers.[2]

The second stage occurs in the secondary clarifier, a large, quiescent settling tank. As the mixed liquor flows out of the aeration basin and into the clarifier, the turbulence ceases. Gravity takes over, pulling the dense biological floc to the bottom of the tank, while the clear, treated effluent rises to the surface and flows over weirs for final disinfection and discharge. The efficiency of this stage is often measured by a 30-minute settleability test, which operators use to calculate the Sludge Volume Index (SVI), a key indicator of floc health and settling characteristics.[1][5]

The third and most critical control stage involves the management of the settled biomass, divided into Return Activated Sludge (RAS) and Waste Activated Sludge (WAS). Because the microorganisms at the bottom of the clarifier are starved and highly active, a significant portion—often 20 to 50 percent of the total flow—is pumped back into the aeration basin as RAS. The Carmel Area Wastewater District explains that this return process continuously reseeds the incoming wastewater with a hungry population of bacteria and protozoa, ensuring immediate and rapid degradation of new organic material.[4]

However, because the bacteria constantly multiply as they consume carbon, the total biomass in the system continuously increases. To prevent the clarifiers from overloading and solids from washing out into the environment, operators must remove a specific volume of this biomass daily as WAS. A 2025 guide by AMCON INC. details that this wasted sludge is then routed to separate solids handling facilities for thickening, digestion, and eventual disposal or beneficial reuse as biosolids.[5]

The balance between the incoming food (BOD) and the microbial population is quantified as the Food-to-Microorganism (F/M) ratio. Operators typically target an F/M ratio between 0.1 and 0.5 pounds of BOD per pound of mixed liquor volatile suspended solids (MLVSS) per day. If the ratio is too high, the bacteria reproduce rapidly but fail to form the sticky polymers necessary for flocculation, resulting in cloudy effluent. If the ratio is too low, the starving bacteria may break apart the floc, or filamentous bacteria may proliferate, causing a condition known as sludge bulking where the biomass refuses to settle.[1]

Beyond basic carbon removal, modern activated sludge plants are increasingly tasked with removing nutrients—specifically nitrogen and phosphorus—that cause eutrophication in receiving waters. This requires advanced biological nutrient removal (BNR) configurations that manipulate the presence or absence of oxygen. To remove nitrogen, operators establish an aerobic zone where nitrifying bacteria convert toxic ammonia into nitrate, followed by an anoxic zone (devoid of dissolved oxygen but containing nitrate) where denitrifying bacteria strip the oxygen from the nitrate, releasing harmless nitrogen gas into the atmosphere.[3][6]

Phosphorus removal relies on a specialized group of bacteria known as polyphosphate-accumulating organisms (PAOs). A comparative study on side-stream enhanced biological phosphorus removal notes that these organisms must first be stressed in a strictly anaerobic environment (lacking both dissolved oxygen and nitrate). In this anaerobic zone, PAOs release stored phosphorus and absorb volatile fatty acids. When subsequently moved to an aerobic zone, they consume oxygen and take up significantly more phosphorus than they released, effectively trapping the nutrient within the cellular biomass that is eventually wasted from the system.[3]

Managing these competing biological demands requires precise control over the Sludge Retention Time (SRT), or the average number of days a microorganism spends in the system before being wasted. Carbon-consuming bacteria reproduce rapidly and can thrive at an SRT of 2 to 3 days. However, nitrifying bacteria are slow growers, often requiring an SRT of 8 to 12 days to maintain a stable population, particularly in colder water temperatures where biological activity slows down.[1][6]

The complexity of the activated sludge process lies in its dynamic nature. The microbial ecosystem is constantly shifting in response to changes in influent flow, organic loading, temperature, and the presence of toxic compounds. Operators must continuously monitor microscopic indicator organisms—such as stalked ciliates, free-swimming ciliates, and rotifers—to gauge the health of the sludge. A dominance of stalked ciliates typically indicates a stable, well-settling floc and a highly efficient treatment process, while an abundance of flagellates suggests a young, inefficient sludge.[2][4]