Bottom ash is a byproduct of coal combustion. The largest producers of bottom ash are coal fired power plants, which burn a very high volume of coal annually to generate electricity. There are several disposal options for this substance, ranging from recycling to landfilling, and several advisory councils and industry advocacy groups around the world promote methods of disposal which are viewed as more environmentally friendly.
Many byproducts of combustion are generated when coal is burned. Bottom ash and a component known as fly ash consist of coal components which did not combust during the burning phase. Fly ash is light enough that it is carried up the flue with the flue gases, and ideally trapped in filters before reaching the environment. Bottom ash forms clinkers on the wall of the furnace, with the clinkers eventually falling to the bottom of the furnace.
The composition of the ash left over once coal has been burned depends on the impurities present in the coal when it was mined and processed for use as a fuel. As a general rule, power companies try to use coal which is as pure as possible for fuel, so that it will burn efficiently. Impurities which do not burn reduce the overall efficiency and generate more bottom and fly ash which must be disposed of, adding to facility operating costs.
Some furnaces grind the ash and pump it out using a water system, which others are designed for dry handling of the ash. Once removed, the ash can simply be disposed of in a landfill facility which is equipped to handle it, or it can be sold to companies which engage in recycling and reclamation activities. Bottom ash can be used as an aggregate in construction products such as concrete, for example, and in the manufacture of cinderblocks.
As long as bottom ash is handled with care, it should not pose an environmental risk. However, it can contain products of partial combustion such as dioxins, which make it hazardous. If landfilled, it is important to dispose of it in a facility which has been equipped to prevent leaching, ensuring that toxins will not enter the environment. If used as an aggregate, it should only be used in materials for which it has been deemed an appropriate aggregate by construction authorities, and it should be tested to confirm that it will be safe for use.