IMPACT OF DYNAMIC AGEING ON THE FORMATION AND STABILITY OF MODEL BILGE WATER OIL-IN-WATER EMULSIONS
Ships, including those in the US Navy, collect oily wastewater in their bilge due to onboard cleaning and mechanical operations. Oil-in-water emulsions (O/W) are present in bilgewater, and their filtration is difficult due to surfactants provided by cleaning products. Despite cleaning efforts, over 457,000 tons of oil are discharged into the ocean every year. An often overlooked aspect of bilgewater emulsions is their evolution, as the ship's movement at sea provides extra energy that can further emulsify the collected oil. This work aims to understand the effects of motion on model bilgewater emulsions by tracking their evolution in dynamic (rocking motion) and static conditions. The model bilgewater emulsion comprises mineral oil, deionized water with 0.42 M NaCl to mimic the salinity of seawater and sodium lauryl ether sulfate and Triton X-100, as commonly found anionic and nonionic surfactants, respectively. A rocker is used to simulate a ship motion; 15 mL of emulsion were placed in 50 mL centrifuge tubes to mimic partially filled bilgewater tanks. Emulsions were characterized via laser diffraction and optical microscopy. Model bilgewater emulsions with either SLES or Triton X-100 at concentrations above 100 ppm and 500 ppm, respectively, show long-term stability in static (no-rocking) conditions up to 20 days of observation. These concentrations represent the minimum surfactant concentration needed to obtain stable emulsions under static conditions. Under dynamic conditions, the minimum surfactant concentration to obtain stable emulsions increases to 500 ppm and 1000 ppm for SLES and Triton X-100, respectively. These results mean that the ship motion can induce drop coalescence in unstable emulsions with lower surfactant concentrations. However, the drop size distributions for stable emulsions under dynamic conditions show further emulsification as the average drop size decreases. The ship motion can help further reduce the size of the emulsion drops to diameters < 2.8 µm, which are significantly harder to filter out using current methods. A bilgewater tank partially filled will likely show a higher amount of sloshing than a filled one. To understand the effects of bilgewater storage volume on emulsification, a series of dynamic experiments were carried out with samples that contained up to 100% of the centrifuge tube volume occupied by the model bilge water emulsion. Even when 100% of the centrifuge volume is occupied and sloshing is eliminated, the oil moves due to density differences, and the shear stress between fluids can induce the generation of drops < 2.8 µm. In summary, this work shows that the ship motion provides enough energy for emulsification once a minimum surfactant concentration is reached regardless of the volume of emulsion stored in the tanks. The data suggests that the best way to mitigate stable bilgewater emulsion formation is by reducing surfactant concentration.