Sewage Treatment Plant Working Process
What is a Sewage Treatment Plant and STP Work Process?
A Sewage Treatment Plant (STP) is designed to treat wastewater and remove contaminants before discharging the treated water back into the environment. Wastewater, commonly referred to as sewage, includes household water from toilets, sinks, showers, and industrial effluent. The treatment process aims to reduce pollutants, such as organic matter, solids, nutrients (like nitrogen and phosphorus), pathogens, and toxic substances, to acceptable levels to protect public health and the environment.
Components of a Sewage Treatment Plant
Preliminary Treatment:
This stage involves removing large debris and coarse materials that could damage equipment or hinder subsequent processes. It includes:
Screening: Large objects like rags, sticks, and plastics are removed using screens.
Grit Removal: Grit chambers allow sand, gravel, and other heavy particles to settle at the bottom for easy removal.
Primary Treatment:
This stage focuses on removing suspended solids and organic matter through sedimentation:
Primary clarifiers: large tanks where the wastewater is held, allowing heavy solids to settle at the bottom (forming sludge) while oils and grease float to the top for removal.
Secondary Treatment: In this stage, biological processes are employed to degrade the dissolved and suspended organic matter:
Aeration Tanks: Wastewater is mixed with air to encourage the growth of aerobic bacteria that consume organic pollutants.
Activated Sludge Process: The bacteria form flocs, which can be easily separated from the water in secondary clarifiers.
Secondary Clarifiers: Like primary clarifiers, these tanks allow the biological flocs to settle, separating the treated water from the biological mass (activated sludge).
Tertiary Treatment:
This advanced treatment stage further purifies the effluent to meet specific regulatory standards:
Filtration: Sand or membrane filters are used to remove any remaining suspended particles.
Disinfection: Methods like chlorination, ultraviolet (UV) irradiation, or ozonation are employed to kill pathogens.
Nutrient Removal: Additional processes, such as biological nutrient removal (BNR), are used to reduce nitrogen and phosphorus levels.
Sludge Treatment:
Thickening: Sludge concentrated by removing excess water.
Digestion: Biological or chemical processes stabilize the sludge, reducing its volume and odor.
Dewatering: Further water removal is done to produce a semi-solid product.
Disposal: The treated sludge can be incinerated, landfilled, or used as fertilizer, depending on its composition and local regulations.
How Does a Sewage Treatment Plant Work?
The functioning of an STP can be understood through the sequential steps involved in the treatment process, each building on the previous one to ensure that the final effluent meets environmental standards.
1. Preliminary Treatment
The journey of sewage through an STP begins with preliminary treatment. This stage is essential for protecting the downstream equipment from damage and clogging. As wastewater enters the plant, it first passes through bar screens, where large debris like plastic, rags, and wood removed. These materials are typically transported to landfills.
Next, the wastewater flows into a grit chamber, where the velocity of the water is reduced to allow heavier inorganic particles, such as sand and gravel, to settle. The removal of grit is crucial because it can cause excessive wear and tear on pumps and other mechanical equipment if left untreated.
2. Primary Treatment
After preliminary treatment, the wastewater enters the primary treatment stage. Here, the main goal is to remove a significant portion of the suspended solids and organic matter. The wastewater is directed into primary clarifiers, which are large, quiescent tanks where the water is held for several hours.
During this time, heavier solids settle to the bottom of the tank, forming sludge. Meanwhile, lighter materials, such as fats, oils, and grease, float to the surface and are skimmed off. The settled sludge is collected and sent for further processing in the sludge treatment section.
The effluent from the primary clarifiers, now with significantly reduced solid content, moves on to the secondary treatment stage.
3. Secondary Treatment
Secondary treatment is the heart of the sewage treatment process, where biological processes employ to remove the remaining organic matter and nutrients. The most common method used in STPs for this purpose is the activated sludge process.
In the aeration tanks, the primary-treated effluent mixed with air (oxygen) and returned activated sludge from the secondary clarifiers. This mixture fosters the growth of aerobic bacteria, which consume the organic pollutants in the wastewater. These bacteria form aggregates known as flocs, which are heavier and easily separated from the treated water.
After sufficient aeration, the mixture flows into secondary clarifiers, where the bacterial flocs settle to the bottom. A portion of the settled sludge recycled back to the aeration tanks to maintain the bacterial population, while the excess sludge directed to sludge treatment facilities.
The water exiting the secondary clarifiers is significantly cleaner but may still contain dissolved nutrients, pathogens, and some fine suspended particles.
4. Tertiary Treatment
Tertiary treatment is an advanced stage of purification, used to achieve higher quality effluent that meets stringent regulatory requirements. This stage varies depending on the specific needs of the region or the discharge standards.
One common tertiary process is filtration, where the effluent passes through sand or membrane filters to remove any remaining suspended solids. This step enhances the clarity of the water and prepares it for disinfection.
Disinfection is critical for killing any remaining pathogens in the water. Chlorination is a widely used method, where chlorine is added to the effluent. However, alternatives like UV irradiation and ozonation are also used, particularly in regions where chlorine discharges must be minimized.
In areas where nutrient pollution is a concern, additional processes may be used to remove nitrogen and phosphorus. These nutrients, if left untreated, can contribute to eutrophication in water bodies, leading to algal blooms and oxygen depletion.
The final effluent, now fully treated, can be safely discharged into rivers, lakes, or oceans, or even reused for non-potable purposes like irrigation and industrial processes.
5. Sludge Treatment and Disposal
The sludge collected during the primary and secondary treatment stages contains a mixture of organic and inorganic materials. This sludge is further processed to reduce its volume, stabilize it, and prepare it for safe disposal or reuse.
Sludge treatment typically begins with thickening, where excess water is removed to concentrate the solids. This is followed by digestion, where biological or chemical processes break down the organic matter, reducing Odors and pathogens. Anaerobic digestion is a common method, producing biogas that can be used as an energy source.
After digestion, the sludge undergoes dewatering, further reducing its water content to produce a semi-solid product. The final sludge can be incinerated, disposed of in landfills, or used as a soil conditioner or fertilizer, depending on its composition and local regulations.
Conclusion
STP plant is a complex system designed to manage and treat wastewater, protecting both public health and the environment. Through a series of carefully controlled processes, STPs remove contaminants, reduce pollution, and ensure that the treated water can be safely returned to the environment or reused. As urban populations grow and environmental regulations become stricter, the role of STPs in sustainable water management becomes increasingly vital.