Texas Wastewater Class B Practice Exam

In the activated sludge system, what does "Hydraulic Wash Out" refer to?

1. Unstable flow of effluent

2. Movement of water exceeding settling velocity

3. Filtration of solids from water

4. Absorption of oxygen by sewage

The correct answer is: Movement of water exceeding settling velocity

Hydraulic washout refers to the scenario where excess water flow through an activated sludge system can lead to the movement of mixed liquor, which includes organisms and solids, at a rate that exceeds the settling velocity of those solids. This situation results in the unwanted removal of treated biological mass from the system, potentially compromising the treatment process and reducing the efficiency of nutrient removal. While unstable flow of effluent, filtration of solids from water, and absorption of oxygen by sewage are related concepts in wastewater treatment, they do not accurately define hydraulic washout. Unstable flow deals more with the consistency and reliability of exfiltration rates rather than the effects of hydraulic overload on solids. Filtration of solids is a different process that focuses on removing particulates from water rather than dealing with conditions within the activated sludge system itself. Absorption of oxygen is a normal metabolic function of the microorganisms but not relevant to the concept of hydraulic washout.