Massachusetts Wastewater Practice Exam

In the physical-chemical treatment of chromium waste, one of the primary objectives is to facilitate the removal of chromium from the wastewater. Lowering the pH is a critical step because it influences the solubility and the speciation of chromium species present in the solution.

At lower pH levels, soluble chromium species, particularly chromium(VI), may convert to less soluble forms, allowing for easier precipitation or co-precipitation with other chemical agents. This step helps in transforming the contaminants into forms that can be separated from the liquid waste more effectively.

In contrast, raising the pH could lead to the solubilization of chromium species, making them more difficult to remove from the wastewater. The addition of chlorine is primarily used for disinfection and does not directly contribute to precipitating chromium. The precipitation of chrome often occurs later in the process, after adjustments to pH have been made to optimize the conditions for solid formation.

By beginning with lowering the pH, the treatment process prepares the wastewater for subsequent steps aimed at achieving effective removal of chromium.