Sand drying beds
Explanation
Wastewater treatment generally occurs in stages: preliminary (screening), primary (settling), and secondary (biological treatment). The Activated Sludge Process is a secondary treatment method where microorganisms consume organic matter. The resulting biomass, known as activated sludge, settles at the bottom of secondary clarifiers. Excess sludge must be processed (dewatered and stabilized) to reduce its volume before disposal.
Option Analysis:
- Sand drying beds Sand drying beds: Correct. This is a common, low-cost method for dewatering sludge. The sludge is spread over a bed of sand and gravel with an under-drainage system. Water is removed through filtration (drainage) and evaporation, leaving behind a dried sludge cake suitable for disposal or use as manure.
- Bar screens Bar screens: Incorrect. Bar screens are employed in the preliminary treatment stage to remove large floating solids (e.g., rags, plastics, wood) that could damage pumps or clog pipes. They are not used for processing sludge.
- Surface skimming Surface skimming: Incorrect. Skimming is a mechanical process used in sedimentation tanks to remove floating materials such as oil, grease, and fats (scum) from the surface of the water, not for processing settled sludge.
- Chemical precipitation Chemical precipitation: Incorrect. This involves adding coagulants (like alum or lime) to wastewater to convert dissolved substances (such as phosphorus or heavy metals) into solid particles that settle out. While this process generates chemical sludge, it is not a method for drying or processing the biological activated sludge.
Key Takeaway:
Sand drying beds are the primary physical method for dewatering activated sludge in many sewage treatment plants, facilitating the separation of water from solid biomass through drainage and evaporation.