Activated sludge is a fundamental component in modern wastewater treatment, purifying water before its return to the environment. It refers to a biological floc, a collection of microorganisms, primarily bacteria, that consume and break down organic pollutants in wastewater. This process mimics and accelerates natural decomposition within a controlled environment. The primary purpose of activated sludge systems is to remove dissolved and suspended organic matter, along with nutrients like nitrogen and phosphorus, from municipal and industrial wastewater.
The Biological Basis of Activated Sludge
The “activated” aspect of activated sludge stems from its vibrant microbial communities. These are predominantly composed of aerobic bacteria, thriving in oxygen-rich environments, alongside other microorganisms like protozoa and fungi. These microbes consume organic pollutants in the wastewater as their food source, converting them into carbon dioxide, water, and new microbial cells.
As these microorganisms grow, they produce sticky substances that cause them to clump together, forming visible, brown, gel-like particles known as flocs. These flocs are a heterogeneous mixture of living organisms, dead organic material, and inorganic particles. The ability of these microbes to form flocs allows the biological material to settle efficiently, separating from the treated water. This system continuously adapts to the incoming wastewater, making it an effective biological treatment method.
The Activated Sludge Process Explained
The activated sludge process begins after preliminary treatment removes larger solids. Wastewater then enters an aeration tank, where it is mixed with activated sludge. In this tank, air or pure oxygen is continuously introduced, often through diffusers or mechanical aerators, to maintain dissolved oxygen levels suitable for the aerobic microorganisms. This aeration ensures the microbes remain suspended and have the oxygen needed to break down organic matter.
The mixture of wastewater and activated sludge in the aeration tank is known as mixed liquor. As the mixed liquor undergoes biological treatment, it flows into a secondary clarifier, also called a settling tank. Here, the activated sludge flocs, denser due to microbial growth and pollutant consumption, settle by gravity, separating from the treated water.
A portion of this settled sludge, known as Return Activated Sludge (RAS), is continuously pumped back to the aeration tank. This recycling maintains a high concentration of active, healthy microorganisms, ensuring a consistent population to treat incoming wastewater. The remaining excess sludge, Waste Activated Sludge (WAS), is removed for further processing, balancing the microbial population and preventing overgrowth.
Managing Treated Water and Sludge
After the activated sludge settles in the secondary clarifier, the clarified water, known as effluent, is discharged. This treated water has significantly reduced levels of organic pollutants and suspended solids, meeting environmental discharge standards before being returned to natural water bodies or used for other purposes.
The excess Waste Activated Sludge (WAS) requires further management due to its high water content and biological activity. This sludge undergoes a series of processes to reduce its volume and stabilize its organic content. Initial steps involve thickening, which removes a significant amount of water to increase the solids concentration. This can be achieved through methods like gravity thickening or dissolved air flotation.
Following thickening, the sludge is dewatered, further reducing its moisture content, and may undergo digestion, a biological process that breaks down remaining organic matter and reduces pathogens. The final treated sludge, often referred to as biosolids, can then be disposed of safely, such as in landfills, or beneficially reused, for instance, as a soil amendment in agriculture, depending on regulations and quality.