You are right.
The initial volume is implicitly set by the [b]initial mass of water[/b] (the mass vector M is a derived state variable: you can set initial states in the Variables tab page of the Block Details).
Conversely, calculating the initial mass of water from the volume (which volume: min or max?) would also not be ideal - unless of course one would add a 3rd parameter, next to "V_Max" and "V_Min", e.g. "V_0". In which case, you could add the following [b]initial equation[/b]:
[code]state.M[IndexOfSolvent] = parameters.V_0 / parameters.WWTPSpecificVolume[IndexOfSolvent][/code]