Wastewater treatment is a systematic, multi-stage process designed to safely return used water back to the environment. This process removes pollutants generated by domestic, commercial, and industrial activities before discharge into rivers, lakes, or oceans. The goal is to protect aquatic ecosystems and human health. While the primary phase focuses on physical separation of large debris and solids, the secondary stage represents the core purification step. This biological treatment targets the finer organic material that remains suspended or dissolved, preparing the water for final cleaning and safe release.
Defining the Secondary Stage
The secondary treatment stage shifts focus from the physical separation used in primary treatment. Primary treatment removes large debris and about half of the total suspended solids, but leaves behind significant fine organic matter that can pollute receiving waters.
Secondary treatment is specifically designed to eliminate this remaining dissolved and colloidal organic matter. The main objective is to substantially reduce two key quality indicators: Total Suspended Solids (TSS) and Biological Oxygen Demand (BOD). BOD quantifies the dissolved oxygen required by microorganisms to break down organic pollutants. A high BOD signifies a high concentration of organic waste, which would deplete oxygen in natural water bodies, harming aquatic life. By lowering the BOD and TSS, the secondary stage prevents environmental oxygen depletion and cleans the water for further purification.
The Biological Engine
The purification achieved in the secondary stage relies on a biological process rather than mechanical means. This stage harnesses microscopic organisms, primarily bacteria, protozoa, and fungi, that consume the organic waste and metabolize the complex compounds remaining in the wastewater.
The microorganisms treat the pollutants as food, breaking down substances like sugars, fats, and proteins into simpler, stable compounds. This metabolic activity is typically an aerobic process, requiring a constant supply of dissolved oxygen. The end products are largely harmless, consisting of carbon dioxide, water, and new microbial biomass, often referred to as sludge. Converting dissolved organic matter into solid biological mass makes the pollution easier to physically separate from the purified water.
Primary Methods of Secondary Treatment
The biological reaction is housed within specially engineered systems designed to maximize contact between the microbes, the organic waste, and oxygen. These systems are broadly categorized based on whether the microbial community is suspended in the water or attached to a fixed surface.
Suspended Growth Systems
The most common suspended growth system is the Activated Sludge Process, utilized in many medium and large-scale treatment plants. Wastewater flows into large aeration tanks where air or pure oxygen is continuously injected to maintain an aerobic environment. The microscopic organisms form floc-like colonies, a mixture of water and microbes called mixed liquor, which are kept in suspension by the vigorous aeration.
After several hours of biological activity, the mixture flows into a secondary clarifier, where the dense microbial floc settles to the bottom. A portion of this settled sludge, which contains the active microorganisms, is recycled back to the aeration tank to maintain a high concentration of working microbes, while the clean water flows onward.
Attached Growth Systems
The second major category involves attached growth systems, such as Trickling Filters and Rotating Biological Contactors (RBCs). Trickling filters consist of a bed of material, such as plastic fill media or rock, over which the wastewater is continuously sprayed. A slimy layer of microorganisms, known as a biofilm, grows on the surface of this media. As the wastewater trickles down, the biofilm absorbs and metabolizes the organic pollutants.
Rotating Biological Contactors operate on a similar principle, using large, closely spaced plastic discs mounted on a horizontal shaft that slowly rotate. The discs are partially submerged in the wastewater, allowing a biofilm to grow on the exposed surfaces. As the discs rotate, the microbes alternate between feeding on the organic matter and absorbing oxygen from the air. In both attached growth methods, the excess biofilm periodically sloughs off the media and is separated in a final settling tank, preparing the water for disinfection or a final tertiary polishing stage.