Wastewater treatment is the systematic process of cleaning water before it is returned safely to the environment. This multi-stage process removes contaminants to protect aquatic life and public health. Secondary treatment follows the initial physical screening and settling of large solids (primary treatment). This stage focuses on removing dissolved and suspended biological matter remaining in the water stream. It primarily utilizes a controlled biological process with microorganisms to achieve purification.
The Primary Target: Organic Matter and BOD
The central objective of secondary treatment is removing biodegradable dissolved and colloidal organic contaminants remaining after the initial settling stage. These organic compounds originate from sources like human waste, food residue, and soaps. The process relies on cultivating massive populations of specialized aerobic microorganisms, including bacteria and protozoa, in large aeration basins. These organisms consume the organic carbon compounds, breaking them down into simpler byproducts such as carbon dioxide and water.
The success of this biological activity is measured by the reduction of Biochemical Oxygen Demand (BOD). BOD quantifies the dissolved oxygen microorganisms consume while decomposing organic material. High BOD in discharged water rapidly depletes oxygen in natural waterways, which harms aquatic life. The secondary treatment process substantially lowers the BOD level, serving as the main performance metric for this stage.
The systems used to house these microbes, such as activated sludge basins or trickling filters, are engineered to provide a sufficient supply of oxygen. In activated sludge systems, air is bubbled directly into the water to keep the microorganisms active and growing. The microbes incorporate the organic pollutants into their own cellular structure, effectively converting the dissolved contaminants into biological solids. This conversion is what makes the subsequent physical separation step possible.
Removal of Suspended Solids
The biological process of consuming organic matter results in the creation of new, suspended solids. As the microorganisms grow and reproduce, they aggregate together with the non-biodegradable solids into clumps called biological floc, or activated sludge. This floc is a complex, active ecosystem of living biomass that has absorbed and trapped the contaminants from the water. This aggregation is a necessary outcome of the biological activity, as it transforms dissolved pollutants into a form that can be physically separated.
The water flows into a secondary clarifier, a large sedimentation basin. Gravity acts on the biological floc, causing the heavy clumps to settle out of the water column. The resulting settled material is the secondary sludge. This sludge is either returned to the aeration tank to treat incoming wastewater or sent for disposal. This physical separation is fundamental to achieving clean effluent.
Advanced Secondary Techniques: Nutrient Reduction
Beyond organic carbon and suspended solids, modern wastewater treatment often integrates specialized biological and chemical steps into the secondary stage to target nutrients, specifically nitrogen and phosphorus. These nutrients, if discharged in high concentrations, can fuel excessive algal growth in rivers and lakes, a phenomenon known as eutrophication. The removal of nitrogen is a two-step biological process requiring different environmental conditions.
First, nitrifying bacteria convert ammonia, a form of nitrogen found in wastewater, into nitrates under aerobic conditions (nitrification). Following nitrification, the water is moved to an anoxic zone, where oxygen is intentionally absent. Here, denitrifying bacteria convert the nitrates into nitrogen gas. This harmless gas escapes into the atmosphere, effectively removing the nitrogen from the water.
Phosphorus removal can be achieved through Enhanced Biological Phosphorus Removal (EBPR), which selects for Phosphorus Accumulating Organisms (PAOs). PAOs are unique microbes selected by alternating the water between anaerobic (no oxygen or nitrates) and aerobic zones. These organisms are forced to release phosphorus in the anaerobic zone and then absorb a much larger quantity of phosphorus when later exposed to oxygen. This excess phosphorus is stored inside their cells as polyphosphate. The PAOs are then removed with the secondary sludge, taking the phosphorus out of the water stream.
The Final Stage of Secondary Treatment and Pathogen Reduction
The water leaving the secondary treatment stage is substantially cleaner, having had the bulk of its organic matter and suspended material removed. While the primary goal is to reduce BOD, this biological process also significantly reduces the population of pathogenic organisms, such as bacteria and viruses. The reduction occurs because the microbes consume the organic matter that many pathogens rely on for food and survival. Furthermore, many pathogens become physically trapped within the biological floc and are removed when the sludge settles in the secondary clarifier.
Conventional secondary treatment systems are expected to remove at least 85% of both BOD and total suspended solids. Many modern facilities achieve greater than 90% removal efficiency. Although this stage significantly reduces pathogens, the treated water is not considered safe for immediate return to sensitive environments. Therefore, the secondary effluent is directed to a final disinfection step, such as chlorination or UV treatment, to eliminate any remaining disease-causing organisms.