Compared to conventional fossil fuel-burning plants, geothermal systems have proven to emit less environmentally-harmful materials. Since geothermal plants do not actually burn any chemicals, they don't produce gases like methane and carbon dioxide that are believed to be responsible for the greenhouse effect. In fact, geothermal plants produce such a small amount of air emissions that compared to coal-burning facilities their carbon footprints are nearly invisible. Whereas a coal-burning facility might produce around 4 lbs. of nitrogen oxide (believed to be toxic to the environment) per megawatt hour of power produced, geothermal plants produce virtually none. From an emissions perspective, geothermal systems have a minimal impact on the environment.
Because geothermal systems extract steam from the ground, you might suspect that there is a net loss in water in the reservoir over time. This is true; however, many plants actually recycle the used water by pumping it back into the reservoir to be reheated, reducing this loss significantly. This helps promote the longevity of the reservoir and has very little environmental consequences. Even waste water that is minimally treated can be fed to a geothermal plant to inject back into a reservoir, providing a cyclic supply of steam for the generation of electricity.
In theory, nonpotable water could be pumped into a geothermal reservoir and then extracted as a potable condensate, potentially offering interesting opportunities to couple water clarification with power generation.
Compared to other energy technologies like solar stations, coal plants and even wind farms, geothermal energy systems are said to use far less land. Since a geothermal plant is built right on its source of energy, there is no need for large trucks to transport fuel to the plant. There is no need for fuel storage, either, which also significantly reduces the land area of the plant. The less land a power-generation station uses, the more environmentally friendly it is, since less land processing leaves more of the environment in its natural state.
Geothermal systems do produce sludge waste that needs to be properly disposed of. The channels for this disposal, however, are well-equipped to break the waste down in a manner that does not harm the environment. Also, research into how to extract impurities from this waste for other practical uses is currently happening in many sectors of geothermal development.