What is “sludge”?
Sludge is a soft, wet mud made up of a mixture of liquids and solids. The term is often used to refer to dirty oil or the byproducts of the oil refining process. Heavier solids drift to the bottom of the tank and form a sediment that gets thicker with time.
Removal
The sludge handler must remove any unconsumed oil from the residential oil tank to get to the sludge. This is done by pumping oil. Sludge must be removed in a separate pumping operation. After the sludge has been pumped, a thin layer will remain on the bottom of the tank. Processing and removing sludge is best left to a professional.
Safety
Oil tank sludge is a hazardous material and must be handled according to the Environmental Protective Agency (EPA) regulations for disposal. The contractor decommissioning and removing an old oil tank will cut the empty tank into pieces for transportation and disposal. Home oil tank sludge is flammable, and a spark from a metal-cutting saw could ignite the sludge, so specialists use a non-sparking metal nibbler to do the job.
Source: http://www.ehow.com/how_7832323_remove-sludge-fuel-oil-tank.html
