Contamination of soils from the various processes is generally a less significant problem when compared to contamination of the atmosphere and the various water systems. Past (pre-1970) production practices may have led to spills on company property that now need to be cleaned up. Natural bacteria that may use the organic chemical products as food are often effective at cleaning up organic chemicals spills and leaks compared to many other pollutants. Many waste materials are produced during the production of organic chemicals, and some of them are recycled through other stages in the process. Other wastes are collected and disposed of in landfills, or they may be recovered by other facilities. Soil contamination including by some hazardous wastes, spent catalysts or coke dust, tank bottoms, and sludge from the treatment processes can occur from leaks as well as accidents or spills on or off site during the transport process.