Not a reply from the guy you requested, however I have some experience of the ballast system you are describing.
A sphere is the best vessel for withstanding pressure, however they are tricky to build, and to incorporate into scale models. They are also totally unnecessary for our application.
A closed cylinder is more than sufficient for a sealed ballast tank.
Generally, 3mm (1/8") acrylic/lexan/polycarbonate/PVC tubing is sufficient with flat end caps double that thickness (as they have to withstand an additional shear force) will get the job done. If you want to use domed end caps, then the material thickness can be reduced to the same as the cylinder walls.
You can use other materials, i.e. thin sheet metal. Just depends on your skills, tools and facilities.
Running the pump in freeflood area may be okay, however in my models I prefer to fit everything in the module/WTC- just looks neater and better engineered to me.