What is a Sewage Treatment Plant & How does it work?

A sewage treatment plant, also known as a wastewater treatment plant, is a facility designed to treat and purify wastewater or sewage before it is discharged into the environment. The primary objective of a sewage treatment plant is to remove contaminants, pollutants, and harmful microorganisms from the wastewater, making it safe for release back into rivers, lakes, or oceans, or for reuse in various applications. Here’s a general overview of how a sewage treatment plant works:

  1. Collection and Preliminary Treatment: The first step in the process is the collection of wastewater from residential, commercial, and industrial sources through a network of underground pipes. The wastewater enters the treatment plant and goes through preliminary treatment. This stage involves the removal of large objects, such as debris, stones, and plastics, using screens and grit chambers. It may also involve the removal of grease and oil using skimmers or grease traps.
  2. Primary Treatment: After preliminary treatment, the wastewater enters the primary treatment phase. In this step, the wastewater flows into large sedimentation tanks called primary clarifiers. The velocity of the flow is reduced, allowing heavy solids and organic matter to settle at the bottom as sludge, while lighter materials like oils and grease float to the surface. The settled sludge is removed and sent for further treatment, while the clarified wastewater continues to the next stage.
  3. Secondary Treatment: The next stage is the secondary treatment, which focuses on the biological breakdown of organic matter and the removal of dissolved pollutants. The wastewater enters large aeration tanks where air or oxygen is introduced to support the growth of aerobic bacteria. These bacteria consume organic matter, such as human waste, food particles, and other biodegradable materials, converting them into carbon dioxide, water, and additional bacterial biomass. This process is called activated sludge treatment. The mixture of wastewater and activated sludge is thoroughly mixed and aerated to optimize the microbial degradation process. The treated wastewater then moves to the next phase.
  4. Secondary Clarification: Following the biological treatment, the mixture of treated wastewater and activated sludge enters secondary clarifiers. These tanks allow the settled sludge to separate from the liquid. The sludge settles at the bottom of the clarifiers and is recycled back to the aeration tanks as return activated sludge to maintain the microbial population. A portion of the excess sludge is continuously removed and sent for further treatment or disposal.
  5. Tertiary Treatment (Optional): In some cases, additional treatment steps may be included to further improve the quality of the effluent. Tertiary treatment involves advanced processes such as filtration, disinfection, and nutrient removal. Filtration can be done using sand filters, membrane filters, or other filtration technologies to remove any remaining suspended solids or fine particles. Disinfection is carried out to eliminate harmful bacteria, viruses, and pathogens. Common disinfection methods include chlorination, ultraviolet (UV) disinfection, or ozonation. Nutrient removal, particularly nitrogen and phosphorus, may be required in environmentally sensitive areas to prevent eutrophication of receiving waters.
  6. Effluent Discharge or Reuse: After the treatment process is completed, the final effluent is discharged into surface water bodies, such as rivers or oceans, where it undergoes dilution and mixing. Alternatively, the treated wastewater can be further treated for specific reuse applications like irrigation, industrial processes, or groundwater recharge, depending on the local regulations and the quality requirements.
  7. Sludge Treatment: Throughout the treatment process, various forms of sludge are generated, including primary sludge and excess activated sludge. Sludge treatment involves additional processes such as thickening, dewatering, and stabilization. These processes reduce the volume of sludge, remove excess water, and treat it to make it suitable for disposal or beneficial use, such as agricultural fertilization or energy generation through anaerobic digestion.

Sewage treatment plants employ a combination of physical, biological, and chemical processes to effectively treat and purify wastewater. The specific design and treatment processes may vary depending on the capacity of the plant, the characteristics of the incoming wastewater, and the local environmental regulations. The aim is to protect public health, safeguard the environment, and promote sustainable water management.