Cargo Airships Are Happening

One challenge I've heard of is: If you carry 100 tons of cargo from point A to point B in an airship, for the airship to return to point A, it needs to take on another 100 tons of new cargo (or ballast), or it needs to vent (or compress) lifting gas, in order to maintain the correct buoyancy. I wonder what the best approach is here, & how it affects the economics? Is water ballast safe & cheap enough, or is there a better way?

Yeah although typically they used water ballast, which is cheap and easy to find.

One thing worth considering is going back to hydrogen as a lifting gas. Not only is it a better lifting gas than helium and much cheaper, it could be used as fuel.

An airship that burned its own lifting gas would have the curious property of getting heavier the further it traveled. This could be countered by dual-fueling it and also have engines that burned heavier-than-air fuel like kerosene or propane. The hydrogen engines could burn the lifting gas at the same rate as the kerosene engines burn the ballast-fuel.