Massachusetts Wastewater Practice Exam

For effective coagulation in wastewater treatment, the coagulant must function best within what type of conditions?

1. Low pH conditions

2. Neutral pH conditions

3. High pH conditions

4. Variable pH conditions

The correct answer is: Neutral pH conditions

Effective coagulation in wastewater treatment primarily occurs under neutral pH conditions, typically around a pH of 6 to 8. At this range, the charge characteristics of particulates in wastewater come into balance, allowing for optimal interaction between the coagulant and the suspended solids. When the pH is neutral, coagulants such as aluminum sulfate (alum) and ferric chloride work efficiently by neutralizing the negative charges on the particles. This neutralization reduces the electrostatic repulsion between particles, facilitating their aggregation and subsequent removal from the water. In contrast, at low pH conditions, coagulants could lead to the solubilization of metal ions, which may hinder coagulation effectiveness. Similarly, high pH conditions can precipitate coagulants too effectively, or alter the solubility of hydroxides, also compromising the treatment process. Variable pH conditions can complicate the coagulation process, leading to inconsistent coagulation performance. Neutral pH conditions ensure more stable and predictable interactions between the coagulants and the particulates, enhancing the overall efficiency of the coagulation process in wastewater treatment.