Massachusetts Wastewater Practice Exam

When treating the same wastewater stream, which alkaline substance will produce the least amount of sludge?

1. Hydrated lime

2. Magnesium hydroxide

3. Quick lime

4. Caustic soda

The correct answer is: Caustic soda

Choosing caustic soda as the alkaline substance that produces the least amount of sludge when treating wastewater is correct due to its chemical properties and reaction mechanisms involved in wastewater treatment processes. Caustic soda, or sodium hydroxide, is highly soluble in water and dissociates completely into sodium ions and hydroxide ions. This complete dissociation leads to a more straightforward reaction with acidic components in the wastewater, resulting in less precipitate formation compared to other alkaline substances. The absence of significant precipitate formation means that there is minimal sludge generated during the treatment process. In contrast, hydrated lime, magnesium hydroxide, and quick lime can produce more sludge due to their less soluble nature and the formation of various insoluble compounds during their reactions in the wastewater treatment process. For example, when hydrated lime is used, it reacts to form calcium carbonate, which can create solid precipitates that contribute to greater sludge volume. Similarly, magnesium hydroxide can react to form magnesium salts that can also lead to sludge formation, and quick lime may react with moisture in the wastewater forming compounds that settle out as sludge. Thus, the reason caustic soda is identified as producing the least amount of sludge lies in its ability to effectively neutralize acids without forming significant solid byproducts