Principal Investigators Danielle Paynter and Jared Church provide a fundamental understanding of bilgewater emulsions that is advancing solutions to ease the challenges of managing oily wastewater generated on ships as well as the development of PFAS-free firefighting foams.

By Laura Mack

Oils, fuels, and lubricants are critical for ship and naval air operations. Water and liquid wastes accumulated from these components will drain into the bilge, or the lowest compartments of the ship, forming bilgewater. Shipboard crews must treat bilgewater, which can be challenging if emulsions have formed.

“When you have oil in the presence of a surfactant, [the oil] will break into small droplets. The surfactant then creates a barrier around those droplets so they can’t bump into each other and coalesce,” Danielle Paynter, a chemical engineer with the Naval Surface Warfare Center (NSWC), Carderock Division, explained.

Rather than settling into two distinct phases, emulsions will stabilize and remain in a continuous phase, with one liquid dispersed throughout the other. This looks similar to how an oil and vinegar salad dressing would appear shortly after being shaken versus mayonnaise, which would remain stable over time. Emulsions can cause significant wear-and-tear on the oil pollution abatement systems that ships have to ensure oil does not go overboard. It takes much longer to treat emulsified bilgewater with current treatment and monitoring systems, which must run often to accommodate incoming bilgewater and maintain storage capacity. Oily wastewater that cannot be separated out is offloaded at land-based facilities for treatment, adding another layer of costs and logistics.

Figure 1: Principal Investigators Danielle Paynter and Dr. Jared Church, from Naval Surface Warfare Center, discuss characteristics of a prepared bilgewater emulsion. (U.S. Navy photo by Nicholas Brezzell)

Instead of focusing on improvements to treatment systems, SERDP Principal Investigators considered how they might address the root of the problem and prevent emulsions from forming in the first place. 

Ms. Paynter had been conducting laboratory demonstrations on different types of equipment for oil pollution abatement systems and found that, although emulsions are commonly studied across other disciplines like food science, there is little information about them in the context of bilgewater. Ms. Paynter and her Co-investigator Dr. Jared Church, an environmental engineer with NSWC Carderock, responded to a SERDP Statement of Need (SON) requesting projects that would build a fundamental understanding of emulsions and how they form in bilgewater. 

“I kept having questions like, are we designing the experiments correctly? Are there testing conditions that we should consider? Are there new ways of treating these emulsions?” Ms. Paynter explained. “If we understand [emulsions] and how they form, can we generate new treatment options and new paradigms for bilgewater management that will help alleviate maintenance and other problems that we see?” 

They focused on determining two types of stability that cause issues for ships. Kinetic stability is the ability for emulsions to stay in small droplets. These types of emulsions can pass through widely fielded gravity-based treatment technologies and can quickly degrade secondary ultrafiltration or sorbent treatment systems, resulting in more frequent and costly replacements. Coalescence stability, or the ability for an emulsion to stay in a cream layer, can also create issues by fouling tank level indicators or other oil pollution abatement equipment. 

The team aimed to find specific parameters that define how stable an emulsion is and can predict how stable other emulsions will be. To select testing parameters, they reviewed and selected the products most often used on ships that could likely end up in the bilge. They separated the cleaners into different types of surfactants and pulled out the chemicals that they could use as baselines for experiments. 

Ms. Paynter and Dr. Church developed a model that could predict emulsion stability based on a series of parameters, including the type of oil, surfactant, and pH. They worked on a collaborative team with Naval Research Laboratory (NRL) and University of Central Florida, where researchers looked at the impact of suspended solids, salt, pH, and temperature changes to find any differences in emulsion stability when adjusting each variable. 

Figure 2: Danielle Paynter prepares a simulated bilgewater emulsion to study its characteristics and stability. (U.S. Navy photo by Nicholas Brezzell)

Critical micelle concentration is a common parameter for characterizing surfactants, which was one indicator Ms. Paynter and Dr. Church used to determine emulsion stability. 

“If you measure [critical micelle concentration] in the presence of an oil that’s representative of a bilge oil using what’s called an interfacial tension measurement, that pretty closely determines your coalescent stability,” Dr. Church explained. “As you get above that critical micelle concentration, you’ll see a very strong cream layer that doesn’t break down. If you’re below that, then you would see oil separation.” 

By measuring interfacial tension with hydrocarbon and a target surfactant, the team discovered that they could determine which products would be preferred for use shipboard. Dr. Church is building off these findings under a Naval Sea Systems Command (NAVSEA) project to help identify bilge cleaner characteristics that would mitigate the formation of emulsion droplets. 

However, Ms. Paynter and Dr. Church discovered that cleaners alone may not resolve bilgewater emulsions; there are additives in fuels that can also cause stable oil and water emulsions. 

“It’s a surprising outcome because it’s much harder to regulate the types of oils that go into ship system,” said Dr. Church. They worked closely with NRL to determine the types of compounds that contribute to stability and looked at additives that could help destabilize emulsions in the bilge. Since it may not be possible to regulate oil on ships, the team explored proactive methods, such as increasing salinity of bilgewater or increasing the temperature of the treatment system, that crews could exercise to destabilize the emulsions and improve separation efficiency. 

Multiple projects were funded under this SERDP SON to advance emulsion science. The project teams, who were often running similar experiments, collaborated to help fill knowledge gaps and work through challenges together. 

“The way that this SON was put out and the management of it was really excellent. Oftentimes laboratories don’t have the resources [to fund multiple projects]. They’ll get a single project and work in isolation, trying to discover things alone,” said Ms. Paynter. “The collaboration and knowledge sharing on this was amazing; that doesn’t happen enough in research.” 

As a result of this teamwork, Ms. Paynter and Dr. Church made the connection with another Principal Investigator that their work could assist in the development of firefighting formulations that are free of per-and-polyfluoroalkyl substances (PFAS). PFAS were widely used as surfactants in aqueous film forming foams to produce stable foams that rapidly spread across fuel fires. The Department of Defense (DoD) has been working towards developing an alternative firefighting formulation due to the potential harmful health and environmental effects from PFAS exposure. Dr. Cari Dutcher at the University of Minnesota is using the fundamental knowledge about surfactants generated from Ms. Paynter’s and Dr. Church’s project to analyze the impact of PFAS surfactants and non-PFAS surfactants on interactions at the fuel-water interfaces. 

“Essentially the same thermodynamic systems apply when you’re looking at oil-water systems versus fuel-air systems, where you have a foam blanket that is generated and put over a fuel. The transport in the processes that happen at the interface are essentially the same,” said Ms. Paynter. Research on how surfactants transport can also assist in the ongoing research for an effective PFAS-free firefighting foam similarly to how they determine the parameters for stable and unstable emulsions. 

Dr. Church, Ms. Paynter, and their team have built an essential foundation for understanding surfactants and bilgewater emulsions, propelling science toward more advanced solutions not only for bilgewater management but also PFAS-free firefighting foams. Their contributions and collaborative efforts will inform and guide how the DoD resolves key environmental challenges across Naval vessels and installations. 



The Strategic Environmental Research and Development Program (SERDP) and the Environmental Security Technology Certification Program (ESTCP) harness the latest science and technology to improve the Department of Defense’s environmental performance, reduce costs, and enhance and sustain mission capabilities. The programs respond to energy and environmental technology requirements across the military services. SERDP and ESTCP are independent DoD programs managed jointly to coordinate the full spectrum of research and development efforts, from the laboratory to field demonstration and validation. For more information, visit Follow us on TwitterFacebook, and LinkedIn.