Sewage Treatment Primary Secondary And Tertiary

Sewage treatment is an essential process for protecting public health and the environment. Untreated sewage contains harmful pollutants, including pathogens, organic matter, and nutrients, which can contaminate water sources, spread diseases, and damage ecosystems. Sewage treatment plants employ various processes to remove these pollutants and produce treated effluent that can be safely discharged or reused. The typical sewage treatment process involves three main stages: primary, secondary, and tertiary treatment.

Primary Treatment: Removing Solids and Preparing for Further Treatment

Primary treatment is the first stage of sewage treatment, focusing on removing solid materials from the wastewater stream. This process relies on physical separation techniques, such as screening and sedimentation, to remove large debris, grit, and settleable solids.

Screening

Screening is the initial step in primary treatment, where large objects like rags, sticks, plastics, and debris are removed from the sewage. This is typically achieved using bar screens, which consist of closely spaced bars that trap these materials. The screenings are then collected and disposed of in landfills or incinerated. Removing these large objects prevents them from clogging or damaging downstream equipment.

Grit Removal

After screening, the sewage flows into grit chambers, where heavier inorganic solids like sand, gravel, and silt are removed. These materials, collectively known as grit, can cause abrasion and wear on pumps and other mechanical equipment. Grit chambers are designed to slow down the flow of sewage, allowing the heavier particles to settle to the bottom. The collected grit is then removed and disposed of.

Sedimentation

The next step in primary treatment is sedimentation, where the sewage is held in large tanks called primary clarifiers or sedimentation basins. These tanks are designed to slow the flow of sewage even further, allowing settleable solids, such as organic matter and fecal solids, to settle to the bottom. The settled solids, known as primary sludge, are then scraped from the bottom of the tank and sent for further treatment or disposal. Meanwhile, the clarified effluent, which is still high in organic matter and other pollutants, flows on to the secondary treatment stage.

Primary Sludge Treatment

Primary sludge, the material removed during sedimentation, requires further treatment to stabilize it and reduce its volume. Common sludge treatment methods include:

• Anaerobic Digestion: This process involves the decomposition of organic matter in the absence of oxygen by microorganisms. Anaerobic digestion reduces the volume of sludge, kills pathogens, and produces biogas, which can be used as a renewable energy source.
• Aerobic Digestion: This process involves the decomposition of organic matter in the presence of oxygen by microorganisms. Aerobic digestion also reduces the volume of sludge and kills pathogens but does not produce biogas.
• Sludge Dewatering: This process removes water from the sludge to reduce its volume and make it easier to handle and dispose of. Common dewatering methods include belt filter presses, centrifuges, and drying beds.
• Sludge Incineration: This process involves burning the sludge to reduce its volume and destroy pathogens. Incineration can also recover energy from the sludge.

Secondary Treatment: Removing Dissolved and Suspended Organic Matter

Secondary treatment is the second stage of sewage treatment, focusing on removing dissolved and suspended organic matter that was not removed during primary treatment. This stage relies on biological processes, where microorganisms are used to consume and break down the organic pollutants.

Activated Sludge Process

The activated sludge process is the most common type of secondary treatment. In this process, the effluent from primary treatment is mixed with a culture of microorganisms, known as activated sludge, in an aeration tank. The aeration tank is supplied with air or oxygen to promote the growth of the microorganisms. The microorganisms consume the organic matter in the sewage, converting it into carbon dioxide, water, and more microorganisms.

After the aeration tank, the mixture of treated effluent and activated sludge flows into a secondary clarifier or settling tank. In this tank, the activated sludge settles to the bottom, forming a sludge layer. A portion of this settled sludge is recycled back to the aeration tank to maintain a high concentration of microorganisms. The remaining sludge, known as waste activated sludge, is sent for further treatment or disposal. The clarified effluent from the secondary clarifier is then sent to the tertiary treatment stage.

Trickling Filters

Trickling filters are another type of secondary treatment process. In this process, the effluent from primary treatment is sprayed over a bed of rocks, gravel, or plastic media. A layer of microorganisms, known as a biofilm, grows on the surface of the media. As the sewage trickles down through the media, the microorganisms in the biofilm consume the organic matter in the sewage.

The treated effluent is collected at the bottom of the trickling filter and sent to a secondary clarifier to remove any remaining solids. The solids that settle in the secondary clarifier are typically recycled back to the trickling filter.

Other Secondary Treatment Processes

Besides activated sludge and trickling filters, other secondary treatment processes include:

• Rotating Biological Contactors (RBCs): RBCs are similar to trickling filters, but instead of spraying the sewage over a stationary media, the media is rotated through the sewage. This provides better contact between the sewage and the microorganisms.
• Stabilization Ponds (Lagoons): Stabilization ponds are large, shallow ponds where sewage is treated by natural processes, such as sunlight, algae, and bacteria.
• Constructed Wetlands: Constructed wetlands are artificial wetlands that are designed to treat sewage. They use plants, soil, and microorganisms to remove pollutants from the sewage.

Tertiary Treatment: Removing Remaining Pollutants and Disinfecting the Effluent

Tertiary treatment, also known as advanced treatment, is the final stage of sewage treatment. This stage is used to remove any remaining pollutants that were not removed during primary and secondary treatment, such as nutrients, pathogens, and trace organic compounds. Tertiary treatment is often required when the treated effluent will be discharged into sensitive water bodies or reused for non-potable purposes.

Filtration

Filtration is a common tertiary treatment process that removes suspended solids from the treated effluent. Various types of filters can be used, including:

• Sand Filters: Sand filters consist of a bed of sand that removes suspended solids by straining them out of the water.
• Multimedia Filters: Multimedia filters use multiple layers of different types of media, such as sand, gravel, and anthracite, to remove a wider range of suspended solids.
• Membrane Filters: Membrane filters use a thin membrane to remove very small particles, including bacteria and viruses.

Nutrient Removal

Nutrient removal is an important tertiary treatment process, especially in areas where excess nutrients can cause problems like eutrophication (excessive algae growth) in lakes and rivers. The two main nutrients that are removed are nitrogen and phosphorus.

• Nitrogen Removal: Nitrogen can be removed through a process called nitrification-denitrification. Nitrification is a two-step process where ammonia is converted to nitrite and then to nitrate by nitrifying bacteria. Denitrification is then carried out by denitrifying bacteria, which convert nitrate to nitrogen gas, which is released into the atmosphere.
• Phosphorus Removal: Phosphorus can be removed through chemical precipitation, where chemicals like aluminum sulfate or ferric chloride are added to the sewage to form insoluble phosphorus compounds that precipitate out of the water. Phosphorus can also be removed biologically using phosphorus-accumulating organisms (PAOs).

Disinfection

Disinfection is the final step in tertiary treatment, where pathogens are killed or inactivated to reduce the risk of disease transmission. Common disinfection methods include:

• Chlorination: Chlorination involves adding chlorine to the sewage to kill pathogens. Chlorine is a powerful disinfectant, but it can also form harmful byproducts.
• Ultraviolet (UV) Disinfection: UV disinfection involves exposing the sewage to UV light, which damages the DNA of pathogens and prevents them from reproducing.
• Ozonation: Ozonation involves adding ozone to the sewage to kill pathogens. Ozone is a powerful disinfectant that does not form harmful byproducts.

Other Tertiary Treatment Processes

Besides filtration, nutrient removal, and disinfection, other tertiary treatment processes include:

• Activated Carbon Adsorption: Activated carbon adsorption removes trace organic compounds from the treated effluent.
• Reverse Osmosis: Reverse osmosis uses a membrane to remove almost all pollutants from the treated effluent, producing very high-quality water.

Conclusion

Sewage treatment is a multi-stage process that removes pollutants from wastewater and produces treated effluent that can be safely discharged or reused. The three main stages of sewage treatment are primary, secondary, and tertiary treatment. Primary treatment removes solid materials, secondary treatment removes dissolved and suspended organic matter, and tertiary treatment removes remaining pollutants and disinfects the effluent. The specific treatment processes used at a sewage treatment plant depend on the characteristics of the sewage, the discharge requirements, and the available resources. Proper sewage treatment is essential for protecting public health and the environment.