Tennessee Wastewater Operator Practice Exam

If an activated sludge process experiences low DO levels after an increased ammonia load, what is a likely outcome?

1. Reduction in chlorine usage

2. Decrease in effluent quality

3. Increase in chlorine consumption

4. Improvement in biosolid production

The correct answer is: Increase in chlorine consumption

When an activated sludge process is faced with low dissolved oxygen (DO) levels after an increase in ammonia load, a likely outcome is an increase in chlorine consumption. This occurs because low DO levels indicate that the biological treatment process is not operating efficiently. Ammonia is typically oxidized by nitrifying bacteria, which require sufficient oxygen to convert ammonia into nitrate. If the DO levels are low, this process is inhibited, leading to incomplete nitrification. Consequently, higher levels of ammonia may be present in the effluent. To minimize the environmental impact of these elevated ammonia levels, chlorine is often used in the disinfection process to eliminate pathogens. However, chlorine also reacts with ammonia, which increases the demand for chlorine (hence, higher chlorine consumption) to achieve effective disinfection standards while also reducing residual ammonia concentration in the treated wastewater. In summary, the situation described—a low DO level after increased ammonia load—necessitates additional chlorine to manage both the biological and disinfection challenges, leading to increased chlorine consumption.