Older oil has achance of becoming emulsified. These firms have special vacuum trucks that pump out materials with the consistency of anything from liquid slurry to solid dirt. The bulk liquid is shipped to a licensed treatment facility where the oils, solids and heavy metals are separated from the water. The treated water can be discharged to the sewer. Choose a reliable vendor by making sure its equipment is right for your situation.
Some equipment requires sludge to be a pumpable slurry.
A lot of water may be needed to break up compacted sludge and to rinse out the truck's tank at the treatment facility. Both will involve extra time and expense. Specialized equipment is available to handle sludges more efficiently without requiring the use of extra water. This may cost more, but you may save money if the sludge buildup is excessive. Make sure the vendor vacuums out all of the sludge in each chamber.