Question

Describe primary, secondary, and tertiary sewage treatment.

Short answer

Primary treatment removes solids, secondary degrades organic matter, and tertiary further purifies sewage.

Step-by-step solution

Understanding Primary Treatment

Primary treatment involves the mechanical process of removing large solids from sewage. This is achieved using screens to trap debris and a settling tank where solids settle at the bottom as sludge, and greases float to the surface. This step removes approximately 40-60% of suspended solids.

Exploring Secondary Treatment

Secondary treatment involves biological processes to degrade the organic matter left in the sewage after primary treatment. This is typically done using aeration tanks where microorganisms digest organic pollutants, converting them into carbon dioxide, water, and energy. This step can remove up to 85% of organic waste.

Examining Tertiary Treatment

Tertiary treatment is an advanced stage that further purifies sewage to remove specific pollutants. Methods include chemical additives for nutrient removal, filtration for residual solid removal, and disinfection to eliminate pathogens. This step is crucial for ensuring that the water released is environmentally safe.

Key concepts

Primary Treatment

Primary treatment is the initial phase in the sewage treatment process. It focuses on the physical separation of solid waste from the water. This is important because it reduces the amount of organic load present in the sewage, making further treatment more effective. During this stage, screens are first employed to catch large debris such as leaves, sticks, and other refuse. After this, the sewage flows into a settling tank. In this tank, gravity plays a key role. Most of the suspended particles, or solids, settle down at the bottom forming sludge. At the same time, oils and grease, being lighter, float to the top and can be skimmed off.

• Removal of large solids is mechanical.
• Effectively removes 40-60% of suspended solids.
• Prepares sewage for more intensive secondary treatment.

This primary phase is vital for making sure that the subsequent biological processes work more efficiently. It also prevents damage and clogging of the equipment used in later stages.

Secondary Treatment

Secondary treatment comes after primary treatment and involves biological methods to break down organic matter remaining in the sewage. This step is crucial because organic materials are the major contributors to pollution and oxygen depletion in water bodies. An aeration tank is often used for this process. Here, air is pumped into the water to provide oxygen which is essential for the microorganisms that digest the organic pollutants. These microorganisms consume organic matter, converting it into carbon dioxide, water, and energy, which diminishes the organic load.

• Involves biological processes.
• Uses microorganisms for degradation.
• Can remove up to 85% of organic waste.

The result of secondary treatment is a significant reduction in biochemical oxygen demand (BOD), ensuring the water is much cleaner and safer to return to the environment or to use further in the tertiary phase.

Tertiary Treatment

Tertiary treatment involves advanced and specialized methods to further clean the sewage, making it nearly as pure as natural water. This stage is necessary for removing any remaining contaminants that secondary treatment couldn't fully eliminate, such as nutrients and pathogens. Various processes can be part of tertiary treatment:

• Chemical additives: Used to remove nutrients like nitrogen and phosphorus, which can lead to eutrophication in water bodies.
• Filtration: Used to trap any remaining small solids.
• Disinfection: Eliminates pathogens to ensure the water is safe for release or reuse.

This step is critical for producing environmentally safe water that meets regulatory standards, whether the water is to be discharged into natural water bodies or reused in agricultural or industrial settings. It represents the final step in sewage treatment and epitomizes the goal of sustainable water management.