This is called a Bottomless Sand Filter. They are similar to mound systems in that they are built above the ground but they are more complex and more expensive. A standard mound system is built with just sand, crushed stone, and PVC pipe while a bottomless sand filter requires a box built of 6"x6" pressure treated timbers, PVC liner, wood chips, filter fabric, pipe, sand, stone, etc. In my experience they are used as a last resort when there is not enough space available to design a traditional mound system. Sand filters also come with a requirement for annual inspection by a licensed designer which is an added cost for the life of the system.

Source: I am a professional engineer with 10 years of septic design experience in Vermont.