N-Nitrosodimethylamine (NDMA) is a potent carcinogen and an emerging groundwater and drinking water contaminant in the United States. NDMA contamination of groundwater at military and aerospace sites stems largely from the former use and disposal of liquid rocket propellants containing 1,1-dimethylhydrazine (1,1,-DMH or UDMH). This compound, which is a major component of the propellant Aerozine-50, contains NDMA as a chemical impurity and has also been observed to oxidize to NDMA in natural environments. NDMA is also formed as a disinfection byproduct in drinking water and wastewater treated with chloramines.
There is presently no federal maximum contaminant level (MCL) for NDMA in drinking water. However, the chemical is a potent animal carcinogen and a suspected human carcinogen, so its presence in drinking water represents a public health concern. In December 2006, the California Office of Environmental Health Hazard Assessment (OEHHA) established a public health goal (PHG) for NDMA in drinking water of 3 parts per trillion (ppt or ng/L) based on risk calculations. The development of a PHG is a key step in the establishment of an MCL for an unregulated contaminant in California. In addition, the U.S. Environmental Protection Agency (USEPA) recently added NDMA and four other nitrosamines to its Unregulated Contaminant Monitoring Rule 2 (UCMR 2). As a result, many large water utilities are now required to monitor for NDMA.
The most widely used treatment technology for removing NDMA from groundwater to ng/L concentrations is ultraviolet irradiation (UV). This approach is effective, but it is also expensive, requiring pump-and-treat infrastructure and a medium- or low-pressure UV system. In addition to capital costs, the energy input to reduce NDMA concentrations by one order of magnitude is approximately ten times that necessary for standard disinfection of viruses and other waterborne pathogens. At some military sites, reductions in NDMA concentrations of 4 log orders (e.g., from 30 μg/L to 3 ng/L) may be required to meet treatment objectives. Less expensive in-situ or ex-situ treatment alternatives are not presently available.
The objective of this project was to study NDMA biodegradation and to explore potential in-situ and ex-situ bioremediation strategies for this contaminant.
The following research areas were examined: (1) the pathways of NDMA oxidation by propane- and toluene-oxidizing strains were determined; (2) NDMA biodegradation was quantified in microcosms prepared from aquifer samples and in other natural environments under both aerobic and anaerobic conditions; (3) indigenous strains capable of aerobic, co-metabolic NDMA metabolism were isolated and identified from aquifer samples; (4) the technological viability of NDMA treatment using an advanced bioreactor design was assessed; (5) the possibility of treating both NDMA and chlorinated solvents in a single bioreactor was examined; and (6) the potential for in-situ treatment of NDMA and commingled NDMA and trichloroethene (TCE) using biostimulation and bioaugmentation was evaluated. The data provide fundamental information on NDMA biodegradation and a comprehensive evaluation of potential in-situ and ex-situ NDMA bioremediation options in the field.
Initial experiments revealed that propanotrophs and specific toluene-oxidizing strains were capable of metabolizing NDMA and that these bacteria utilize different routes of metabolism. The toluene-oxidizer Pseudomonas mendocina KR1 was observed to oxidize NDMA primarily through a demethylation pathway, forming N-nitromethylamine (NTMA) and formate as terminal products. The enzyme toluene-4-monoxygenase (T4MO) was determined to be responsible for the initial oxidative step through studies with a T4MO clone. In contrast, the propanotroph Rhodococcus ruber ENV425 was found to transform NDMA primarily via a denitrosation route, producing nitric oxide, nitrite, nitrate, formate, methylamine, and dimethylamine as degradation products. Unlike strain KR1, R. ruber ENV425 also produced a significant quantity of carbon dioxide during NDMA biotransformation. Strain ENV425 was observed to use NDMA for N in the absence of other available sources, but neither strain (ENV425 or KR1) was capable of utilizing NDMA as a sole source of carbon and energy. Rather, they each appear to transform NDMA co-metabolically during growth on propane (ENV425) or toluene (KR1) as primary substrates.
For in-situ or ex-situ biological treatment of NDMA to be practical, μg/L concentrations of the nitrosamine must be reduced to low ng/L levels. Very few compounds have such stringent treatment requirements, and biodegradation processes are rarely considered for such applications. However, batch experiments with strain ENV425 revealed that, when grown on propane, the bacterium could reduce NDMA from 8 μg/L to less than 2 ng/L (the MDL for NDMA). Based on this initial finding, a laboratory reactor system was constructed to evaluate the potential for ex-situ biological treatment of NDMA. The membrane bioreactor (MBR) was seeded with R. ruber ENV425 and received propane as the primary growth substrate. Oxygen was provided to the MBR in excess. At influent NDMA concentrations ranging from approximately 8-80 μg/L in water, effluent concentrations of the nitrosamine were generally below 10 ng/L during more than 150 days of MBR operation. These laboratory data suggest that ex-situ biological treatment of NDMA to ng/L concentrations is both feasible and sustainable. The addition of TCE to the reactor resulted in a significant increase in NDMA in the reactor effluent, most likely due to cell toxicity from TCE-epoxide. Thus, the joint treatment of NDMA and chlorinated solvents, such as TCE, in a single reactor is unlikely to be successful. However, technologies such as air stripping can easily be implemented to remove chlorinated solvents from a groundwater stream prior to NDMA treatment in a bioreactor. Further studies are required to evaluate the cost and performance of a propane-fed bioreactor for NDMA treatment at the field scale.
In addition to pure culture and bioreactor studies, microcosm experiments were performed to evaluate the potential for in-situ remediation of NDMA. Aquifer samples were collected from military (or former military) sites in Colorado, New Jersey, and California for use in these studies. NDMA biodegradation was also evaluated in samples of surface soil, sludge, pond sediment, and manure. NDMA mineralization (conversion of 14C-NDMA to 14CO2) was observed in surface soil, sludge, manure, and other organic-rich environmental samples under aerobic conditions. NDMA degradation in these samples is hypothesized to be largely co-metabolic, with organisms growing on native organic compounds and fortuitously oxidizing NDMA. No bacterial strains capable of growing on NDMA as a sole carbon and energy source were isolated from any of the environmental samples.
In aquifer samples, pre-incubation with propane and oxygen for 2-3 weeks resulted in the rapid mineralization of approximately 50 μg/L of 14C-NDMA to 14CO2. Significant NDMA mineralization was also observed in some samples receiving yeast extract, but not in those receiving oxygen only or in killed controls. Further studies with large-scale microcosms prepared from aquifer samples revealed that native propanotrophs were able to reduce NDMA concentrations from μg/L to ng/L concentrations after biostimulation with propane and oxygen. Nocardioides spp. were enriched on propane and isolated from two of the three aquifers. After growth on propane, each of these cultures (three total) readily degraded NDMA. In addition, the impact of TCE on NDMA degradation was generally less pronounced in the aquifer microcosms than observed in the MBR study. In fact, both NDMA and TCE were biodegraded simultaneously in propane-amended samples from the New Jersey site. Even at an initial concentration of 1 mg/L, TCE did not impact the rate or extent of NDMA biodegradation. The results suggest that in-situ addition of propane and oxygen may be a viable remedial option for NDMA in groundwater at many sites with this nitrosamine.
For future remedial applications, the key findings of this project are as follows: (1) a variety of propane-oxidizing bacteria are capable of degrading NDMA to innocuous products, (2) these bacteria are widely distributed in groundwater aquifers and can be stimulated through the addition of propane and oxygen, and (3) biodegradation of the nitrosamine to low ng/L concentrations is feasible. The data provide potential options for both in-situ and ex-situ biological treatment of NDMA. For ex-situ treatment, propane-fed bioreactors hold promise. The design, cost, and long-term performance of this technology remains to be determined at field scale. For in-situ treatment, application of propane and oxygen to groundwater is suggested as a possible remedial alternative. These gases can be applied through several methods including air- and propane-biosparging, groundwater recirculation with aboveground propane and oxygen addition, bubble-free gas injection systems, and trenches with air and propane injection lines. Field demonstration of one or more of these techniques for NDMA treatment is required. However, if successful, this in-situapproach is likely to be both cost effective and widely applicable at DoD sites. (Project Completed – 2008)