Alum sludge dewatered in a lagoon for a year may still contain more than ______ of the original water.

• A. 75%
• B. 80%
• C. 90%
• D. 95%

Answer: (C)

The key idea is that sludge dewatering in a lagoon can reduce water only so much. Lagoo­ns remove mainly free water through drainage and evaporation, but the majority of water in alum sludge is bound or held in the sludge’s porous structure. Even after a year, this bound water keeps the moisture very high, so the sludge can still contain roughly ninety percent of the original water. That’s why the best estimate is about 90% water remaining: solids content is usually only a few percent, meaning most of what you started with is still water. Lower figures would imply more drying than lagoons typically achieve, while higher figures are less commonly reached in practice.