California Wastewater Treatment Operator Practice Exam

How much dissolved oxygen should be in a conventional activated sludge system if good effluent is being produced?

1. Below 1 mg/l

2. 2 to 4 mg/l

3. Above 6 mg/l

4. Varies based on temperature

The correct answer is: 2 to 4 mg/l

In a conventional activated sludge system, an adequate dissolved oxygen level is crucial for the treatment process to be effective. Dissolved oxygen levels between 2 to 4 mg/l are considered ideal for the system to support the growth of aerobic bacteria responsible for breaking down organic matter efficiently. This range ensures that the bacteria have the necessary oxygen to carry out the biological treatment processes effectively. Option A, which suggests a dissolved oxygen level below 1 mg/l, would typically indicate an oxygen-deficient environment that may lead to incomplete treatment and poor effluent quality. Option C, indicating dissolved oxygen levels above 6 mg/l, could indicate an excess of oxygen that may be wasteful and unnecessary for the treatment process, potentially leading to higher energy consumption without additional benefits for the treatment efficiency. Option D mentions that dissolved oxygen requirements vary based on temperature, which is correct as temperature can influence the oxygen solubility in water. However, for the purpose of achieving good effluent quality in a conventional activated sludge system, maintaining a dissolved oxygen level within the range of 2 to 4 mg/l is the most suitable choice.