Most domestic sewage is treated in large tanks with bacteria that eat the contamination and then settle out of the water, leaving clean water behind. The sludge is then processed and either landfilled or used on farmland as a fertilizer. Some industrial wastes are also treated this way, and some are treated using chemicals to precipitate out whatever contaminants are in the water. Agricultural wastes are usually animal manure which can be safely plowed into soil as a fertilizer. Septic tanks are only used for individual houses, never for large quantities of waste and never for industrial or agricultural waste.