In a conventional activated sludge process, if the return activated sludge (RAS) is not provided, the most likely consequence would be
- AIncrease in oxygen transfer efficiency
- BDecrease in microbial concentration in the aeration tankCorrect
- CImprovement in sludge settling characteristics
- DElimination of excess sludge production
Solution
Concept
Role of Return Activated Sludge in Activated Sludge Process
Formula
NONE
Step-by-step solution
1. The aeration tank relies on maintaining high mixed liquor suspended solids (MLSS). 2. Biomass naturally flows out of the aeration tank into the secondary clarifier. 3. If RAS is stopped, biomass settled in the clarifier is not recycled to the aeration basin. 4. The microbial concentration in the aeration tank rapidly decreases due to continuous washout. Answer: B
Explanation
In an activated sludge process (ASP), Return Activated Sludge (RAS) recycles settled active biomass from the secondary clarifier back into the aeration basin. Without RAS, active microorganisms are continuously washed out with the effluent, leading to a severe decline in MLSS (microbial concentration) and failure of biological purification.
Why the other options are wrong
A) Oxygen transfer is a function of aerators and bubble contact, not enhanced by lack of RAS. C) Settling characteristics deteriorate due to loss of healthy floc structure. D) Halting RAS disrupts biological treatment rather than safely eliminating sludge.
Shortcut
No RAS = No biomass recycle = Aeration tank MLSS washes out and decreases.
Common mistake
Assuming RAS only affects oxygen levels or sludge settling rather than microbial population.
Correct answer: B) Decrease in microbial concentration in the aeration tank