Which alum sludge dewatering method can attain at least 20% solids by weight?

• A. Drying lagoon
• B. Vacuum filters
• C. Sand drying beds
• D. Freezing and thawing method

Answer: (B)

Raising the solids concentration in alum sludge relies on removing as much water as possible, and vacuum filtration does this most effectively among the options. It uses suction to pull water through a filter, separating the liquid from the solids and leaving a compact, drier cake. That suction-driven separation can reliably reach 20% solids by weight or higher, depending on the sludge characteristics, because the pressure differential forces more water out than gravity or passive methods alone.

Drying lagoons depend on sun and wind to evaporate water, so they’re highly weather-dependent, slow, and typically produce variable, lower solids content. Sand drying beds mix evaporation with drainage through a bed, but their performance is also climate-dependent and gradual, making it hard to guarantee high solids without long residence times. Freezing and thawing uses energy to freeze water and then thaw it, which is inefficient for practical dewatering and generally does not quickly yield high solids content.