This information is specific to Minnesota and may or may not b e applicable to your area.
It almost sounds as though the septic system was never completed; is this a new house? Please have a county or city inspector visit the site to determine if this type of septic system is up to local code.
Again, with limited details, my sense it that the septic system is missing several feet of cover soil. That said, it is certainly possible to plant the edges of the mound to hide it. You should not till or, technically, even walk on the system.
Consider low-maintenance perennials and native grasses, such as little bluestem, prairie smoke, prairie coneflower. These options do not require mowing and will form a nice background to a low hedge surrounding the mound.
