Question

Which of the following statements about sewage treatment is false? (a) In primary treatment, large-sized particles are filtered through screens and residual water is subjected to sedimentation, (b) Secondary treatment involves aerobic digestion of the organic waste. (c) In tertiary treatment, the waste water is treated with lime to remove phosphate followed by coagulation. (d) Coagulation can be carricd out by passing ozone through waste water.

Short answer

Statement (d) is false.

Step-by-step solution

- Understand the Question

Identify which statement about sewage treatment is false among the four given options.

- Examine Primary Treatment

Statement (a) describes primary treatment. In primary treatment, large particles are indeed removed using screens, and residual water is subjected to sedimentation. This statement is true.

- Examine Secondary Treatment

Statement (b) describes secondary treatment. Secondary treatment involves aerobic digestion where bacteria consume organic waste. This statement is true.

- Examine Tertiary Treatment

Statement (c) describes tertiary treatment. Tertiary treatment often involves chemical processes like adding lime to remove phosphates and followed by coagulation. This statement is true.

- Check Coagulation Process

Statement (d) claims that coagulation can be carried out by passing ozone through wastewater. Coagulation typically involves adding chemicals to promote clumping of particles, not by using ozone. This statement is false.

Key concepts

Primary Treatment

Primary treatment is the initial stage in the wastewater treatment process. It aims to remove solid materials from sewage before it undergoes further treatment.
During this stage, the sewage flows through screens that filter out large debris such as sticks, leaves, and trash.
After screening, the water moves into a sedimentation tank where the smaller, suspended particles settle at the bottom due to gravity. This process is also called settling.
Thus, the primary treatment reduces the amount of suspended solids and organic matter in the wastewater, preparing it for the secondary treatment stage.

Secondary Treatment

Secondary treatment is designed to remove the remaining organic matter and nutrients from the wastewater after primary treatment.
This stage relies heavily on biological processes to break down organic waste.
Typically, in this process, microorganisms like bacteria are utilized to consume the organic pollutants.
The most common method is aerobic digestion where microorganisms digest the organic material in the presence of oxygen, converting it into carbon dioxide, water, and biomass.

• This step may involve aeration tanks where air is pumped through the sewage to provide the necessary oxygen.
• Eventually, the mixture is moved to a secondary clarifier where the treated water is separated from the bacterial biomass (called activated sludge).

Tertiary Treatment

Tertiary treatment is an advanced cleaning method that follows primary and secondary treatments. Its goal is to polish the water to a higher quality, removing residual pollutants that the previous stages did not eliminate.
This step often involves more sophisticated chemical and physical processes.
Common techniques include adding lime to remove phosphates, and coagulation to aggregate fine particles.

• Chemicals like alum or iron salts are frequently used during coagulation to help particles clump together so they can be removed more easily.
• Another method in tertiary treatment is filtration through sand filters or activated carbon filters to remove finer particles and organic compounds.
• Disinfection, often using chlorine or UV light, is also common to kill any remaining pathogens before the water is discharged or reused.

Overall, tertiary treatment ensures that the treated water is safe for release into the environment or for reuse purposes.

Coagulation Process

Coagulation is a crucial part of the wastewater treatment process, typically placed within the tertiary treatment.
The aim is to aggregate fine particles into larger clusters so they can be more easily removed by sedimentation or filtration.
This process begins by adding chemical coagulants, such as alum (aluminum sulfate) or iron salts, which neutralize the charges of particles in the water. When the charges are neutralized, the particles can come together to form larger clumps called flocs.

• Aid of flocculants may be employed to further enhance the growth of these flocs, making sedimentation or filtration more effective.
• It's important to note that while ozone is effective for disinfection in wastewater treatment, it is not used for coagulation.

This misconception is a common error, underscoring the importance of understanding how each treatment step and chemicals are specifically used in sewage treatment.