Question

Explain what is meant by primary, secondary, and tertiary treatment of sewage.

Short answer

Primary sewage treatment involves the removal of large solid particles using physical methods, reducing the overall load on the sewage system. Secondary treatment involves biological processes to remove suspended and dissolved organic matter, with microorganisms called "activated sludge" consuming the organic material. Tertiary treatment is the final stage, removing pathogens and remaining pollutants to produce high-quality effluent using biological, chemical, or physical methods. These three stages protect human health, the environment, and water resources by treating wastewater effectively.

Step-by-step solution

Primary Sewage Treatment

Primary treatment is the first stage in the sewage treatment process. This stage involves the removal of large and solid particles from the sewage using physical methods. The main purpose of primary treatment is to reduce the overall load of the sewage system. In this stage, raw sewage enters the treatment plant and passes through screens, which remove large objects like twigs, leaves, and plastic. After screening, the sewage flows to a sedimentation tank, where heavier particles like sand and gravel settle to the bottom, while lighter materials like oil and grease rise to the surface and are skimmed off.

Secondary Sewage Treatment

Secondary treatment is the second stage in the sewage treatment, which aims at removing suspended and dissolved organic matter from the sewage. In this stage, biological processes are used to break down the organic matter. The sewage from the primary treatment is pumped into aeration tanks, where microorganisms consume the remaining organic material. These microorganisms are called "activated sludge." The aeration process provides oxygen to support the growth of these microorganisms. After this process, the activated sludge is separated from the treated sewage by allowing it to settle in a secondary clarifier. The treated water can then be discharged into a local water body or proceed to the tertiary treatment stage.

Tertiary Sewage Treatment

Tertiary treatment is the final stage in the sewage treatment process, which involves the removal of pathogens, nitrogen, phosphorus, and other remaining pollutants to produce high-quality effluent. This stage employs biological, chemical, or physical processes depending on the required effluent quality. Some common tertiary treatment methods include filtration, membrane processes, lagooning, denitrification, phosphorus removal, and disinfection by chlorination or ultraviolet light. The resulting treated water can be discharged into sensitive environments, used for irrigation, or even for indirect potable reuse. In conclusion, sewage treatment is a vital process performed in three stages - primary, secondary, and tertiary - to remove pollutants in wastewater and protect human health, the environment, and water resources.

Key concepts

Primary Treatment

Primary treatment is all about getting rid of large, easily removable particles from wastewater. Think of it as a "pre-cleaning" step. This involves using physical methods like screening and sedimentation.
First, raw sewage flows into the treatment plant where screens block big debris such as leaves, branches, and plastic. After the debris is removed, the sewage passes into a large tank called a sedimentation tank.
Here, heavier solids settle at the bottom to form sludge, while oils and greases float to the top and are skimmed away. This process reduces the load on subsequent treatment stages and prepares the sewage for further treatment by making it more manageable.

Secondary Treatment

Secondary treatment dives deeper into cleaning the water by dealing with organic matter left after primary treatment. Here, biological processes take center stage, utilizing naturally occurring microorganisms.
Wastewater from the primary treatment is pumped into aeration tanks. These tanks provide oxygen which helps microbes, often referred to as "activated sludge," thrive and multiply.
These microorganisms consume the organic matter, effectively cleaning the water. Afterward, the mixture of microorganisms and treated water goes into a secondary clarifier. Here, the sludge settles down, and the clearer water is either released or moved to tertiary treatment for further purification.

Tertiary Treatment

Tertiary treatment is like the final polish for wastewater treatment, focusing on removing pathogens and any left-over pollutants to ensure the highest quality of water.
This stage involves various methods, including chemical, biological, and physical processes, depending on the desired purity level. Some common methods include filtration, which traps remaining particles; disinfection, using chlorine or ultraviolet light to kill pathogens; and nutrient removal, which captures nitrogen and phosphorus.
The aim here is to produce water that can safely enter local ecosystems, be used for irrigation, or sometimes even re-enter the potable water supply through indirect methods.

Wastewater Management

Wastewater management is the overarching system that collects, treats, and disposes of wastewater, ensuring it doesn't harm the environment or human health. It's like an invisible guardian of our water resources.
Management begins with the proper collection and transportation of sewage to treatment facilities. Once at the facility, sewage goes through the three treatment stages mentioned earlier, each crucial to reducing pollutants.
After treatment, safe and responsible disposal ensures that water returns to the environment without causing harm, completing the cycle where water can be reused safely.

• Effective management reduces water pollution.
• Plays a role in preventing waterborne diseases.
• Contributes to sustainable water use.

Environmental Protection

Environmental protection is not merely a concept in sewage treatment—it's the goal. Treatment processes are designed to safeguard natural resources.
The processes reduce harmful contaminants that could damage ecosystems if left untreated, protecting waterways and wildlife from hazards like nutrient pollution.
By producing cleaner effluent water, sewage treatment minimizes the risk to aquatic life and helps maintain the quality of recreational waters, thereby supporting biodiversity and conserving ecosystems.

• Reduces stress on local environments.
• Prevents eutrophication, a process that can lead to dead zones.
• Promotes healthier ecosystems and communities.