Chlorine is sometimes used in sewage treatment:
To avoid bulking of activated sludge
Chlorine is a chemical commonly used in various stages of sewage treatment. The question asks about a specific purpose for which chlorine is sometimes employed in this process.
Understanding Chlorine Use in Sewage Treatment
Sewage treatment involves removing pollutants from wastewater to make it safe for discharge or reuse. Different stages employ various physical, chemical, and biological processes. Chlorine is often used as a disinfectant, especially towards the end of the process, to kill harmful microorganisms. However, it has other applications as well, depending on the specific needs of the treatment plant.
Chlorine's Role in Activated Sludge Process
The activated sludge process is a biological treatment method where microorganisms consume organic pollutants. A common operational problem in activated sludge plants is "bulking."
What is Activated Sludge Bulking?
Sludge bulking occurs when the activated sludge fails to settle properly in the secondary clarifier. Instead of forming dense, settleable flocs, the sludge becomes light and voluminous, often floating or washing out with the treated effluent. This leads to poor effluent quality because solids are not separated effectively.
Bulking is frequently caused by the excessive growth of filamentous bacteria. These bacteria grow in long strands that interfere with the formation of compact flocs, causing the sludge volume index (SVI) to increase significantly.
How Chlorine Helps Prevent Bulking
Chlorine, typically added in small, controlled doses directly to the return activated sludge line or aeration tank, can effectively control the growth of these filamentous microorganisms. By targeting the surface growth of the filamentous bacteria, chlorine disrupts their structure and reduces their ability to cause bridging between flocs. This allows the sludge flocs to settle more compactly, thereby avoiding or mitigating sludge bulking.
It's important to note that the chlorine dose must be carefully controlled. Too high a dose can harm the beneficial floc-forming bacteria needed for the biological treatment process itself.
Analyzing the Options
- Option 1: To avoid flocculation. Flocculation, the process of forming larger particles (flocs) from smaller ones, is a desirable outcome in activated sludge to aid settling. Chlorine's primary role related to sludge settlement is improving floc settling, not avoiding flocculation.
- Option 2: To increase biological activated sludge. Chlorine is a disinfectant and would generally kill microorganisms, not increase the amount of activated sludge biomass.
- Option 3: To avoid bulking of activated sludge. As discussed, this is a primary use of chlorine (in controlled doses) in activated sludge plants to control filamentous bacteria that cause poor settling or bulking.
- Option 4: To help in grease separation. Grease separation typically occurs in primary treatment through physical means (skimming) or sometimes with chemical addition (coagulants), but not usually with chlorine.
Based on the analysis, the use of chlorine to control filamentous organisms and thus avoid bulking of activated sludge is a well-established practice in sewage treatment.
Revision Table: Chlorine in Sewage Treatment
| Use of Chlorine | Purpose | Stage |
|---|---|---|
| Disinfection | Kill pathogens (bacteria, viruses, parasites) | Effluent (usually final stage) |
| Activated Sludge Control | Control filamentous bacteria, avoid bulking | Activated sludge process (return sludge line or aeration tank) |
| Odor Control | Oxidize odor-causing compounds (e.g., $\text{H}_2\text{S}$) | Various points (influent, sludge handling) |
Additional Information: Sewage Treatment Processes
Sewage treatment plants typically involve multiple stages:
- Preliminary Treatment: Removes large solids and grit (screening, grit removal).
- Primary Treatment: Removes suspended solids and grease through sedimentation and skimming.
- Secondary Treatment: Removes dissolved and colloidal organic matter, usually using biological processes like activated sludge, trickling filters, or RBCs. This is where bulking problems in activated sludge occur.
- Tertiary Treatment (if applicable): Further removal of specific pollutants like nutrients (nitrogen, phosphorus), remaining suspended solids, or dissolved substances using processes like filtration, nutrient removal processes, or adsorption.
- Disinfection: Kills remaining pathogens, often using chlorine, UV light, or ozone. This is typically the final step before discharge.
Controlling activated sludge bulking is crucial for the efficient operation of the secondary treatment stage and for achieving good effluent quality.
Important Questions from Sewage Treatment
Consider the following statements in respect of anaerobic sludge digester:
1. It is less expensive compared to several other methods available.
2. Processing of separable solids impacts the environment to a minimum.
3. Quantity of separated solids requiring disposal is minimal
4. Digested sludge is very readily dewatered able.
Which of the above statements are correct?The activated sludge process is an
Consider the following statements regarding contact stabilization process:
1. Primary settling tank is not required in some cases.
2. BOD removal occurs in two stages.
3. Aeration volume requirements are approximately 50% of those of a conventional – or tapered – aeration plant.
4. Returned sludge is aerated for 30 min to 90 min in sludge aeration tank.
Which of the above statements are correct?- A trickling filter is designed to remove ______
If the moisture content of sludge is reduced sludge is reduced from 98% to 96%, the volume of sludge will have decreased by ______.
