7.1.2 Sewage Treatment Plants Explained

Key Concepts

• Primary Treatment
• Secondary Treatment
• Tertiary Treatment
• Sludge Treatment
• Disinfection
• Effluent Quality

Primary Treatment

Primary treatment involves the physical removal of large solids and debris from sewage. This process includes screening, grit removal, and sedimentation. Screens capture large objects, while grit chambers remove sand and gravel. Sedimentation tanks allow heavier solids to settle, which are then removed as sludge.

Example: A bar screen at the entrance of a treatment plant captures plastic bottles and rags, preventing them from entering the treatment process.

Analogy: Primary treatment is like a strainer in the kitchen that catches large food particles before liquid passes through.

Secondary Treatment

Secondary treatment focuses on biological decomposition of organic matter. This is typically achieved through activated sludge processes or trickling filters. Microorganisms break down the organic content, reducing the BOD (Biochemical Oxygen Demand) and producing a clearer effluent.

Example: In an activated sludge process, microorganisms in the aeration tank consume organic matter, which is then separated from the water in a secondary clarifier.

Analogy: Secondary treatment is akin to composting, where microorganisms break down organic waste into simpler, less harmful substances.

Tertiary Treatment

Tertiary treatment, also known as advanced treatment, further purifies the water by removing nutrients, pathogens, and other contaminants. Techniques include filtration, disinfection, and nutrient removal (nitrogen and phosphorus). This stage ensures the water meets stringent discharge standards.

Example: A sand filter removes fine particles, and UV disinfection kills remaining pathogens, producing water that can be safely discharged or reused.

Analogy: Tertiary treatment is like a final rinse after washing dishes, ensuring no soap or food residue remains.

Sludge Treatment

Sludge treatment involves managing the solid byproducts of sewage treatment. This includes thickening, digestion, and dewatering. Thickening reduces the volume of sludge, digestion breaks down organic matter, and dewatering removes excess water, making the sludge easier to handle and dispose of.

Example: Anaerobic digestion in sludge treatment breaks down organic matter, producing biogas that can be used as an energy source.

Analogy: Sludge treatment is similar to composting yard waste, where organic material is broken down and reduced in volume.

Disinfection

Disinfection is the final step in sewage treatment to kill pathogens and ensure the water is safe for discharge or reuse. Common methods include chlorination, ultraviolet (UV) radiation, and ozonation. Disinfection ensures the treated water does not pose a health risk.

Example: Chlorination introduces chlorine into the water to kill bacteria and viruses, ensuring the water is safe for discharge.

Analogy: Disinfection is like applying hand sanitizer to kill germs, ensuring the water is free from harmful microorganisms.

Effluent Quality

Effluent quality refers to the standards and parameters that treated wastewater must meet before discharge. These include levels of BOD, suspended solids, nutrients, and pathogens. Meeting these standards ensures the water does not harm the environment or public health.

Example: A treatment plant must ensure the effluent has a BOD below 20 mg/L and a total suspended solids level below 30 mg/L before discharging it into a river.

Analogy: Effluent quality standards are like the grades a student must achieve to pass a course, ensuring the water is safe and meets required criteria.