Remediation of Floating Vegetable Oil Spills by Sedimentation Followed by Anaerobic Biodegradation

Floating vegetable oil can be effectively removed from the water surface and the water column as negatively buoyant oil-mineral aggregates by addition of a dense mineral, such as clay. In bench-scale experiments, it is possible to remove virtually all of the floating oil by addition of a sufficiently large dose of clay (>10 g clay/g oil). Once present in the sediments, vegetable oil can be completely transformed to harmless end products (e.g., carbon dioxide and methane) by naturally occurring microbial populations. Transient production of toxic intermediates (probably free fatty acids), measured as a reduction in sediment EC₅₀ using the Microtox Solid-Phase Test (SPT), was observed during anaerobic biodegradation. The EC₅₀ is the sediment concentration required to reduce light emission by bioluminescent bacteria by 50%. Addition of amorphous ferric hydroxide as an alternative electron acceptor stimulated the rate of vegetable oil biodegradation, but had no effect on the Microtox EC₅₀. Alternative factors, including clay and calcium, were tested for their ability to reduce the transient toxicity by reducing the bioavailability of the free fatty acids. Calcium, and especially calcium plus clay, reduced the rate and extent of vegetable oil biodegradation but had no effect on the sediment toxicity as measured by the Microtox SPT. Only oil biodegradation significantly reduced the sediment toxicity.