The best soil for a landfill is clay because clay is less permeable than other types of soil. Materials disposed of in a landfill can be further secured from leakage by solidifying them in materials such as cement, fly ash from power plants, asphalt, or organic polymers. Refuse is also burned in incinerators. It is more expensive but a safer method of disposal than landfills. Numerous thermal processes recover energy from solid waste.
Companies burn in-plant wastes in conventional incinerators to produce steam.
Pyrolysis, a process of chemical decomposition, produces a variety of gases and inert ash. Garbage burned in incinerators has poisoned air, soil, and water. Communities near incinerators have objected to them because of fears about possible emissions. Yet another method is the pumping of hazardous wastes into deep wells. There is a strong opposition to this method because of the apparent explosions and even earthquakes that have resulted from waste injection techniques.