The issue with rigid polypropylene tanks is the potential stress loading. A number of people over the years have used them for solar storage. Unfortunately, they can stress crack over time. A generalization is that plastic can tolerate temperature or pressure, not both together. That is a broad generalization, that you might overcome with expensive plastics, which we cannot afford. I have sold tank liners for rigid tanks that have stress cracked from heat stress. The bottom of any tank over two feet deep is taking a tremendous load. An 820 gallon tank has about 7,000 pounds of water in it. All that weight loads the lowest area of the tank walls.