The sewage works calculator

A small plant treats the sewage of a town. You will write the sums for the flow, the settling tank and the BOD, and test whether the final effluent is within a limit.

The idea. One m³ is $1000$ L. A tank that holds the sewage of $h$ hours has the volume flow $\times\, h/24$. Each stage removes a share of the BOD that it receives.

Your tasks.

1. Finish flow_m3. It gives the cubic metres of sewage in a day.
2. Finish tank_m3. It gives the volume of a settling tank for the sewage of some hours.
3. Finish bod_out. It gives the BOD after primary and secondary treatment.
4. Finish can_release. It gives True if the BOD is at most the limit of $30$ mg per litre.