N-Nitrosodimethylamine (NDMA) is present in groundwater and drinking water from industrial, agricultural, water treatment, and military/aerospace sources. NDMA is a suspected human carcinogen and an emerging groundwater contaminant that has been detected at a number of Department of Defense (DoD) and National Aeronautics and Space Administration (NASA) sites involved in the production, testing, and/or disposal of liquid propellants containing unsymmetrical dimethylhydrazine (UDMH). NDMA was a common contaminant in UDMH-containing fuels (e.g., Aerozine-50) and is also produced when these fuels enter the environment through natural oxidation processes. Currently, the most effective treatment technology for NDMA in groundwater is pump-and-treat with ultraviolet irradiation (UV). However, this approach is expensive because it requires high energy input to effectively reduce the levels of NDMA to meet regulatory requirements.
The objective of this project was to demonstrate and validate the use of an advanced bioreactor design, a fluidized bed bioreactor (FBR), in the field for the ex situ treatment of NDMA from part-per-billion (µg/L) influent concentrations to low part-per trillion (ng/L) effluent concentrations. The demonstration was conducted at the NASA White Sands Test Facility (WSTF) in Las Cruces, New Mexico. The capital and operational costs of the FBR were subsequently compared to those of an existing UV system for NDMA treatment at the WSTF facility.
Previous studies revealed that the propanotroph Rhodococcus ruber ENV425 and other similar strains are capable of biodegrading NDMA to low ng/L concentrations while growing aerobically on propane. Based on this observation, and laboratory bioreactor tests, a field-scale FBR was designed, constructed, and tested for NDMA treatment. The FBR is an efficient fixed-film bioreactor. It consists of a reactor vessel containing media with a high surface area (usually sand or granular activated carbon [GAC]) to foster the growth of microbial biomass. The high biomass achievable within the FBR bed makes it appreciably more efficient for water treatment than many other types of biological reactor systems. This reduces the reactor size and, subsequently, the cost of treatment. The pilot-scale FBR (1-5 gpm influent flow) was operated for approximately 1 year on the actual site water using coconut shell based GAC media under various operating conditions. The FBR was seeded with ENV425 and subsequently fed propane, oxygen, and inorganic nutrients to promote cell growth and NDMA biodegradation. The treatment of a secondary contaminant from rocket fuel, N-nitrodimethylamine (DMN), within the FBR was also examined. The hydraulic residence time (HRT) of groundwater within the FBR was varied from 60 minutes to 10 minutes, and different challenge studies were conducted to assess the resiliency of the FBR to system upsets.
Based on more than a year of operational data, the FBR treatment system was demonstrated to be an effective means to treat approximately 1 µg/L concentrations of NDMA in WSTF groundwater to less than 10 ng/L at a 10 minute HRT and to less than 4.2 ng/L, the WSTF regulatory discharge limit, at a 20 minute HRT. The system also effectively treated DMN from approximately 0.6 µg/L to less than 10 ng/L at an HRT as low as 10 minutes. The system was observed to be resilient to upsets, including power outages, interruptions in influent groundwater flow, shutdown of propane and nutrient feeds, and presence of low levels of volatile organic compounds (VOCs) in the influent groundwater. In general, effluent NDMA and DMN concentrations either remained at less than 10 ng/L during the system challenge studies (most of which were conducted at the 10 minute HRT), or recovered to this level within a few to several days. The system reliability was high, with a 94% uptime recorded over the duration of the study, and less than 10 hours per week of operator attention was required. In addition, a cost analysis suggested that the FBR would be $900,000 less expensive (roughly 35%) to treat NDMA than a comparable UV system at a 125 gpm flowrate, with the primary savings being related to lower electrical and maintenance costs over a 30-year remedial timeframe.
This FBR technology is currently ready for implementation. A full-scale system can be designed, constructed, and operated based on the results of this demonstration. Although this is the first application of a field-scale propane-fed FBR, the basic FBR technology is mature and has been widely applied for treatment of other contaminants, including nitrate, perchlorate, tert-butyl alcohol, and other organics. Systems are currently operating in the United States at groundwater flow rates as high as 5,000 gpm. Moreover, the electrical demand of a FBR system is anticipated to be about 3 times lower than a comparable UV system for groundwater treatment, making this biological approach more sustainable and energy efficient for NDMA. The implementation of this technology to treat contaminated groundwater, rather than simply relying on energy intensive alternatives, can serve as a new paradigm of water treatment for significantly impaired resources. With quality supplies of water rapidly declining throughout the United States, the implementation of such a biological treatment plant can be effectively used for NDMA contaminant removal. Technology transfer efforts for this project included presentation of the results at several national and international remediation and groundwater conferences, publication of a two peer-reviewed manuscripts on the technology, and presentation of the results directly to NASA, DoD, and commercial aerospace contractors with NDMA contamination in groundwater.