Answer:
The lagoon must be at least 65552 m3
Explanation:
First its we have to know the detention time that its needed for the pollutant to reduce its concentration up to 12 mg/l.
We use the formula of decay:

We can calculte the time as

The flow of the pollutant is 0.3 m3/s, so the daily flow is

The lagoon has to be at least this volume: