Basic Wastewater Operator Practice Exam

The primary effluent to a RBC system is 350 mg/L BOD. If the primary unit removes 83%, the secondary unit removes 50%, and the final unit removes 25%, what is the BOD leaving the final unit of the RBC train?

1. 290 mg/L

2. 60 mg/L

3. 30 mg/L

4. 22.5 mg/L

The correct answer is: 22.5 mg/L

To determine the BOD leaving the final unit of the RBC (Rotating Biological Contactor) train, we need to calculate the BOD at each stage of treatment, starting from the primary effluent concentration of 350 mg/L. In the primary unit, which removes 83% of the BOD, we first calculate the amount of BOD removed. Mathematically, this is done by multiplying the initial BOD concentration by the percentage removed: BOD removed by the primary unit: 350 mg/L * 0.83 = 290.5 mg/L Now, to find the BOD remaining after the primary unit, we subtract the amount removed from the original concentration: BOD remaining after primary unit: 350 mg/L - 290.5 mg/L = 59.5 mg/L Next, we apply the removal efficiency of the secondary unit, which removes 50% of the remaining BOD. So, we calculate the amount removed by the secondary unit: BOD removed by the secondary unit: 59.5 mg/L * 0.50 = 29.75 mg/L To find the BOD remaining after the secondary unit, we subtract the amount removed from the concentration just before it: BOD remaining