The bilgewater of Armed Forces vessels is often impacted with complex oil-in-water (OW) shipboard emulsions that result from on-board cleaning and mechanical operations. These emulsions are strongly stabilized against phase separation by a complex mixture of surfactants and particulates, which increases the time and energy inputs that are required to break the emulsions and separate the organic and inorganic components prior to bilgewater discharge. To address the need for a better understanding of the formation and stability of shipboard emulsions, the following three objectives were investigated in this project: (1) understand emulsion composition and generation including the timeline and processing route for emulsion formation and the components/chemicals within on-board bilgewater; (2) measure and identify the surfactant-oilwater (SOW) chemical and physical parameters that influence the emulsion formation, phase behavior, and stability; and (3) construct binary and ternary phase diagrams relevant for real-world shipboard emulsions using model emulsion formulations as a starting point.

This work was based on fundamental principles of surfactant and emulsion science to provide a detailed picture of the bilgewater SOW phase space. The project team drew inspiration from the field of enhanced oil recovery where the models of hydrophilic-lipophilic difference (HLD) and net average curvature (NAC) have revolutionized the selection of surfactants to maximize oil extraction. The HLD and NAC are semi-empirical models firmly based on thermodynamic principles and together they offer a practical approach to surfactant selection as well as the effects of water salinity, oil type, and temperature on the SOW phase space. This work aimed to utilize the HLD and NAC principles to determine the region of the SOW phase space relevant to shipboard bilgewater.

This project successfully identified and measured fundamental parameters that control the formation and stability of SOW emulsions that are generated aboard marine vessels with a specific focus on the evolution of fine emulsion droplets (diameter less than < 20 microns) which are difficult to separate from bilge water using traditional water treatment methods. The experiments probed the spontaneous emulsification of components common in bilgewater. The project team demonstrated and measured stability of SOW emulsions across a range of compositions and energy inputs to simulate the impact of process formation on emulsion stability in order to provide insights on how to prevent or efficiently destabilize SOW emulsions.

This project found that in systems with single surfactants, for water-soluble oils, nanoscale emulsions formed spontaneously by diffusion of oil molecules into the aqueous surfactant solutions and subsequent swelling of surfactant micelles with oil. In systems with mixed surfactants, spontaneous emulsification was observed in many different oils, even the non-aromatic one (e.g., paraffin oil). This result indicates many oils collected in bilge water could spontaneously emulsify when an appropriate HLD_mix was reached. In trying to predict spontaneous emulsion behavior with a HLD model, in the SLES-Span-80 (Sodium Laureth Sulfate, Polysorbate 80) system, the emulsion behavior mostly agreed well with the HLD prediction. It was found that when the HLD value is close to 0, Spontaneous Emulsification (SE) is more likely to happen, however, when the HLD value increases from negative to positive (by changing the salt / surfactant concentration), A Water/Oil emulsion is more likely to be formed, and vice versa. The spontaneous emulsion is formed due to the ultra-low interfacial tension at the interface. When there is no salt in the system, span-80 micelle swelling is also playing a role.

Overall project outcomes can be used in the future to develop strategies to avoid the formation of fine stable emulsions, including those that are generated spontaneously, as well as to develop new methods to minimize and remove emulsified oils from bilge water. The outcomes of this work can be used by Department of Defense (DoD) operators to rapidly assess the physicochemical nature of specific shipboard emulsions within a vessel’s bilgewater as well as provide guidance on the best practices to destabilize the emulsions. Additionally, the outcomes of this work will provide improved guidance to DoD operations managers for optimizing their detergent formulations and/or cleaning procedures in order to facilitate end-of-use OW separations prior to bilgewater discharge that are effective, but less time and energy intensive.