A system and method for cleaning oil tanks comprising: a bio-reactor for producing bacterial cultures containing bio-emulsifiers from bacteria, air, water and sources of utilizable carbon, nitrogen and phosphate; a first pump for pumping the bacterial cultures from the bio-reactor through a first pipe into the oil tank; at least one spraying nozzle connected to the first pipe for spraying the bacterial cultures onto the oil tank's floor and walls under anaerobic conditions; and a second pump for pumping fluid out of the oil tank through a second pipe into a receiving container.