Good question, and lets work through each scenario.
If you are required to treat 32 tons/day at 20 day retention time, then you must have a minimum of (32 x 20) 640 tons (640 m3) per 20 day cycle.
Answer #1 is ~ 363 m3. This will not meet the minimum requirements, so it is eliminated.
Answer #2 is 640 m3. This meets the minimum requirements, but does not allow for system poisoning, or tank maintenance. The built up solids will need to be removed at some time, so you will need more than 640 m3.
Answer #3 is 1239 m3. This provides a safety factor of about 2. If the system becomes poisoned, then you have storage to regenerate new microorganisms and/or slowly dilute the poisoned system. To be on the safe side, I would choose #3.
I hope this helps.