Understanding the Sequence of Sewage Treatment Steps

Sewage treatment is a multi-step process designed to remove contaminants from wastewater. Understanding the correct order of these steps is crucial. Let's break down the provided statements and place them in their logical sequence as part of secondary sewage treatment.

Analyzing the Sewage Treatment Steps

We are given four statements describing parts of the sewage treatment process:

• A. Primary effluent passed in aeration tank
• B. Activated sludge is formed
• C. Filtration and sedimentation of sewage
• D. Aerobic microbes grow into flocs

Let's consider the typical stages of sewage treatment.

Primary Treatment: Initial Cleaning

The first stage of sewage treatment usually involves removing large and suspended solids. This is typically done through physical processes like filtration and sedimentation.

• Statement C describes "Filtration and sedimentation of sewage". This aligns with the initial steps of primary treatment, where solid materials settle out (sedimentation) or are screened out (filtration). The liquid remaining after primary treatment is called primary effluent.

So, step C must come first among the given options.

Secondary Treatment: Biological Process

After primary treatment, the primary effluent moves to the secondary treatment stage, which primarily uses biological processes to remove dissolved and suspended organic matter.

• Statement A says "Primary effluent passed in aeration tank". This is the beginning of the aerobic biological treatment stage. The primary effluent is introduced into large tanks where air is supplied.
• Statement D says "Aerobic microbes grow into flocs". In the aeration tank (where the primary effluent is passed, step A), aerobic bacteria and other microbes are encouraged to grow. These microbes consume the organic pollutants in the effluent. As they grow, they clump together, often with fungal filaments, forming masses called flocs. This process happens within the aeration tank. Thus, D follows A.
• Statement B says "Activated sludge is formed". After the aeration tank, the mixture of treated water and flocs (microbes) goes to a sedimentation tank. Here, the flocs settle down. This settled material, rich in active microbes, is called activated sludge. A portion of this activated sludge is recycled back to the aeration tank to maintain a high concentration of microbes, while the excess is removed for further processing. This step follows the growth of flocs and their subsequent settling. Thus, B follows D.

Sequencing the Steps

Based on the analysis, the logical sequence of these specific steps in sewage treatment is:

1. Filtration and sedimentation of sewage (Primary Treatment) - C
2. Primary effluent passed in aeration tank (Start of Secondary Treatment) - A
3. Aerobic microbes grow into flocs (During aeration) - D
4. Activated sludge is formed (Settling of flocs after aeration) - B

So, the correct sequence is C > A > D > B.

Conclusion on Sewage Treatment Sequence

The steps provided, when arranged in the order C, A, D, B, accurately represent a flow through the primary and secondary stages of sewage treatment, starting with solid removal, followed by aerobic biological treatment using microbes in aeration tanks to form flocs, and finally settling these flocs to produce activated sludge.

Revision Table: Sewage Treatment Steps

Additional Information on Sewage Treatment Processes

Sewage treatment involves several stages to clean wastewater before it's discharged or reused.

Primary Treatment: This is the initial physical stage. It involves screening to remove large debris, grit removal (sedimentation of sand and gravel), and primary sedimentation where organic solids settle out as primary sludge. This step removes about 50-60% of suspended solids.

Secondary Treatment: This stage uses biological processes to remove dissolved and colloidal organic compounds that were not removed in primary treatment. It commonly involves aeration tanks where microbes (bacteria and other microorganisms) decompose the organic matter. This process requires oxygen, hence the term "aerobic". The microbes form flocs, which are then separated from the treated water in secondary sedimentation tanks.

Activated Sludge Process: The steps A, D, and B described in the question are part of the activated sludge process, a common method for secondary treatment. In this process, the microbes forming the flocs are continuously mixed with the incoming wastewater and aerated. The settled flocs (activated sludge) are partially returned to the aeration tank to maintain a high concentration of microbes, speeding up the decomposition process.

Tertiary Treatment: After secondary treatment, tertiary treatment (also called advanced treatment) may be performed to remove remaining pollutants like nutrients (nitrogen and phosphorus), heavy metals, and pathogens. This can involve filtration, disinfection (using chlorine, UV light, or ozone), and other specific processes depending on the required water quality.