The objective of this project is to utilize surfactant degrading bacteria strains for preferential surfactant consumption, in optimized potential of hydrogen (pH) and salinity conditions, for improved oil/water partitioning. Typical vessel-board surfactants will be used to enrich a bacterial population containing known surfactant degrading species that are tolerant to high-salinity conditions. Parameters including surfactant concentration limits, oil mix properties, pH and salinity will be determined to result in optimized conditions for shipboard de-emulsification using bacterial biodegradation.
Bilge water is a collection of seawater, oil, surfactant, and various other contaminants present on a ship. Bilge water continues to collect water and contaminates while at sea, so it must be purged to maintain available weight and volume requirements. However, the water must be separated from the oil and surfactant before being released back into the environment. Oil and water can be separated using gravitational measures, but the addition of surfactants makes this much more difficult. Surfactants are composed of both hydrophobic and hydrophilic components; in bulk liquids such as bilge water, the surfactant molecules aggregate and form micelles with the hydrophilic heads in the water phase and the hydrophobic tails in the oil phase, as illustrated in Figure 1. These micelles reduce the surface tension between the water and oil phases, forming a system that is stable and difficult to separate.
Water from a ship being returned to the ocean must comply to MARPOL 73/78, the international convention for ship pollution, and, when in US waters, the Clean Water Act. MARPOL 73/78 states that <15ppm of oil may be released in international waters while the Clean Water Act regulates contamination at <15 mg/L of oil within 12 nautical miles of US shores of <100 mg/L of oil beyond the 12 nautical mile limit. Ships separate the water from the oil and other contaminants in bilge water so that the cleaned water can be released back into the environment. An Oil Water Seperator (OWS) is most commonly used; the OWS utilizes gravity and the density difference between water and oil to skim oil from the surface of the bilge water. Similarly, a centrifuge system might be used. Centrigues utilize gravity in the same way as OWS systems, but utilize centrifugal force to enhance the effect of gravity. However, due to surfactants, neither OWS or centrifuge systems bring the contamination down to the lawful limits. Instead, there are several additional polishing steps that might be taken, including absorbtion/adsorpton, coagulation/flocculation, flotation, ultrafiltration with membrane technologies, and biological treatment. Absorbers, adsorbers, and membranes require media which must be removed and replaced; this produces another problem for storage of both new and used media. Coagulation and floculation methods introduce additional chemicals to break up the emulsions. Similar to filtration media, these chemicals require storage space onboard the host ship. Biological treatments utilize microorganisms such as bacteria to metabolize the contaminates. A better understanding of the inherent emulsions present will provide the pathway to a more efficient solution.
A successful outcome of this project will identify the factors that drive oil-in-water emulsions in shipboard bilge water for a potential cost savings to the government on the amount of time and energy required to run an oil-water separator on a ship. A decreased operation cost can be realized through the use of a low-cost gravimetric separation and integration of specific gravity measurements for a targeted system design in place of expensive filtration systems.