Basic Wastewater Operator Practice Exam

What is the most appropriate method of reducing the F:M ratio?

1. Increase wasting

2. Decrease wasting

3. Reduce BOD from influent flow

4. Add polymer to RAS to increase settling

The correct answer is: Decrease wasting

Reducing the F:M (Food to Microorganism) ratio is essential in maintaining an effective biological treatment process in wastewater management. The F:M ratio indicates how much food, in the form of biochemical oxygen demand (BOD), is provided in relation to the amount of microorganisms available for treatment. A high F:M ratio can lead to excessive growth of microorganisms, causing operational issues, while a low ratio may result in the microorganisms starving. Increasing wasting, as mentioned in another option, would actually remove some of the biomass and could inadvertently reduce the population of microorganisms too much, potentially leading to insufficient treatment. On the other hand, adding polymer to the return activated sludge (RAS) can improve settling characteristics, but it does not directly affect the F:M ratio. By decreasing wasting, the amount of biomass in the treatment system is maintained or increased. This allows the existing microorganisms to efficiently consume the available BOD, thereby lowering the F:M ratio. Efficiently managing the F:M ratio supports a balance where microorganisms are adequately fed, ensuring effective treatment and stability in the biological processes. Therefore, decreasing wasting is a direct and effective method for achieving a desirable balance between the food supplied and the microorganisms available for treatment.