Activated sludge is a biological process used globally in municipal and industrial wastewater treatment to clean water before it is returned to the environment. This method cultivates a living ecosystem of microscopic organisms within large tanks to consume and remove organic pollutants from the water. The activated sludge process is a highly efficient way to transform contaminated water into a cleaner effluent that meets strict environmental standards.
The Biological Basis of Activated Sludge
Activated sludge is essentially a dynamic, cultivated biomass, not merely a physical process. The “sludge” itself is a dense, brownish suspension of water, organic matter, and a diverse community of microorganisms, primarily aerobic bacteria, protozoa, and rotifers. This living community is the active agent of purification, and its composition determines the efficiency of the treatment plant.
The bacteria within this mixture naturally excrete extracellular polymeric substances (EPS), which act as a sticky binding agent. This allows the bacteria to aggregate and form visible, porous clumps known as “flocs.” These flocs are the functional units of the activated sludge process, trapping suspended organic particles while the bacteria metabolize dissolved organic matter.
Floc formation is directly tied to the operational health of the system, particularly the “sludge age,” or the average time the microbes remain in the system. When the sludge is healthy, the flocs are dense and settle well, indicating the microbial community is actively consuming the pollutants. Protozoa and rotifers graze on free-swimming bacteria and loosely attached particles on the edges of the flocs, which helps to maintain a robust and well-settling floc structure.
How the Aeration Basin Cleans Water
The core of the activated sludge process takes place in a large vessel called the aeration basin or reactor. Wastewater containing organic contaminants is introduced into this tank and mixed with the concentrated, active microbial floc. The mixture of wastewater and biomass is often referred to as mixed liquor suspended solids (MLSS).
Air or pure oxygen is continuously introduced into the basin through diffusers or mechanical surface aerators. This aeration serves two main purposes: it supplies the oxygen necessary for aerobic microbes to metabolize pollutants, and provides the continuous mixing required to ensure microbes contact the wastewater.
The aerobic bacteria consume the dissolved organic material, using it as a source of energy and carbon for growth and reproduction. This biological oxidation converts the contaminants into three harmless byproducts: new microbial cell mass (more floc), carbon dioxide, and water. By transforming the pollutants into a solid, biological form (the floc), the process makes the contamination removable in the subsequent stage.
Clarification and Sludge Recycling
After biological treatment, the mixed liquor flows into a secondary clarifier, which is a large, quiet settling tank. The purpose of this stage is to physically separate the treated water from the newly formed, heavy microbial flocs. Quiet conditions and sufficient retention time allow gravity to pull the dense activated sludge to the bottom, forming a concentrated layer.
The cleaned water, now called the effluent, flows over weirs at the top of the clarifier and is discharged to the next treatment step or the environment. The collected sludge is a highly active, concentrated biomass. A portion of this settled material is continuously pumped back to the front of the aeration basin in a process known as Return Activated Sludge (RAS).
This recycling step ensures a high concentration of active microorganisms is available to immediately begin consuming incoming contaminants. The remaining, excess sludge that is not returned is removed from the system as Waste Activated Sludge (WAS) for further treatment and disposal.
The Environmental Impact of Activated Sludge Treatment
The activated sludge process is a highly effective method for preventing water pollution by removing the bulk of the contaminants before discharge. The two most significant pollutants removed are Biochemical Oxygen Demand (BOD) and suspended solids. BOD measures the amount of oxygen consumed by microorganisms when breaking down organic matter.
By removing organic matter, the process drastically reduces the BOD of the effluent. If untreated wastewater with a high BOD were released into a river, the natural microorganisms in the river would rapidly consume the organic matter, depleting the dissolved oxygen in the water. This oxygen depletion can suffocate fish and other aquatic life, leading to habitat destruction and ecosystem collapse.
The physical separation in the clarifier also removes suspended solids, particles that cause cloudiness and deposit sludge on riverbeds. By successfully reducing both BOD and suspended solids, activated sludge treatment ensures that the water released into natural waterways is clean enough to support a healthy aquatic ecosystem, safeguarding public health and the natural environment.