The landfill site is for collecting any lost parts that have fallen through the world, as well as for retrieving sold kilju bottles, although some important items will respawn in the garage pit at the player's home as well. The middle of the landfill site is marked by a sofa, lamppost and toilet.
The sofa is quite useful since it can be placed anywhere and it is possible to sleep on it. The sofa can fit inside the cargo area of the Hayosiko van, so it can be carried around, thus making the van a self-made campervan.
A garbage disposal pit has been added to the landfill site as a part of the 31 July 2017 update. Any empty items thrown into the pit will be destroyed, decreasing the amount of objects loaded into the world and therefore potentially increasing game performance.
The landfill is located to north of the airfield. Take a right at the end of your house lane towards Rykipohja. When you reach the white concrete at the end of the dirt road, turn left and follow the white concrete up to where it joins the runway taxi lane. Travel down this road on the left and you'll see the landfill site.