I guess they would rather drill them than dig them? If you are designing for gravity only, reinforce them like they were rectangular (two mats) and design worst case for 1' section.
I don't know how big your footing is, but I would probably want an orthogonal rebar layout versus a radial layout. With an orthogonal layout I would inscribe a square and then design as a rectangular footing.
8 feet dia to 32 feet dia tanks and high up to 50 feet. So footing can be 0ne or two feet larger. No pedestal. Tanks are directly mechanically anchored to circular slab on grade.
For the 8 ft diameter tanks I would expect to see a square footing, not round. Forming and rebar fab would be less for a square footing. The square would probably be cheaper for tanks up to about 20 to 25 feet in diameter. After that I would expect a hex or octagonal shape. Again forming costs would be lower.
Typically, a circular tank footing would have rebar laid out in a square grid. Sum moments and shears about a straight cross section to design; also check shear on circular sections outside the tank. With no seismic or wind moment, the controlling section is normally at the slab center. With large moments, the controlling section can be nearer the toe of the slab. Most economical shape can vary depending on the contractor. See ACI-318 App. D, latest revision, for anchor bolt design. I've not seen any design aids online.
