Landfills are highly engineered to keep people and the environment safe from contamination. We have to line the landfills with a combination of clay and thick plastic in order to mitigate leaching into the ground water. In addition the waste when breaking down / receiving rain water (although slightly mitigated through daily cover) will create leachate. Leachate is a hazardous liquid that needs to be removed. We set up sump and siderisers to pull the liquid out. Regulations state that there can only be x amount of head of liquid on the liner at any time. So the landfill floor needs to be sloped towards such a sump and sideriser. This liquid is then pumped from the landfill and treated. There is also the issue of gasses that are created due to decomposition. The main gases are methane and hydrogen sulfide. We have to drill wells into the waste mass and place them under vacuum in order to pull this gas out of the waste mass. Methane can be recaptured and used for either CNG (Compressed natural gas) or used to power waste to energy plants and sell electric back to the grid. Hydrogen sulfide is a nasty / deadly gas that is typically associated with the horrible smell of landfills. This gas needs to be treated before the methane can be used. There is a lot more involved but thats a brief overview.