Massachusetts Wastewater Practice Exam

Which wastewater treatment method does not depend on particle size?

1. Ultrafiltration

2. Dialysis

3. Reverse osmosis

4. Precipitation

The correct answer is: Precipitation

The chosen method, precipitation, operates based on the chemical reaction between solutes in wastewater rather than relying on the physical size of particles. In precipitation, unwanted substances are transformed from a dissolved state into a solid form, which can then be removed from the wastewater. This process is influenced by factors such as chemical concentrations, pH levels, and the presence of specific reagents rather than the size of particles. In contrast, methods like ultrafiltration, dialysis, and reverse osmosis are all size-selective filtration techniques. Ultrafiltration separates particles based on size using membranes with specific pore sizes, making it effective for removing larger macromolecules and colloidal particles. Dialysis utilizes the diffusion across a semipermeable membrane to separate molecules based on size and concentration gradients, thereby allowing smaller ions and molecules to pass while larger ones are retained. Reverse osmosis relies on pressure-driven filtration through a membrane that permits only water molecules and small solutes to pass, effectively blocking larger contaminants. Understanding these differences emphasizes why precipitation stands apart as it relies on chemical processes rather than physical characteristics such as particle size.