NU 1999-063
INVENTOR
Bruce Rittmann*
SHORT DESCRIPTION
Prototypes of an autohydrogenotrophic hollow-fiber membrane-biofilm reactor to reduce nitrate and perchlorate pollutants in water
BACKGROUND
Many of the emerging water contaminants share a common characteristic: they are chemically oxidized. The classic examples are nitrate (NO3-) and nitrite (NO2-), which are increasingly polluting waters subject to agricultural fertilizers. Because nitrate and nitrite cause methemoglobinemia in infants, their U.S. drinking-water standards are 10 and 1 mgN/L, respectively. A more recently discovered oxidized pollutant is perchlorate (ClO4-), which mainly comes from rocket fuel. Perchlorate affects thyroid function and has a recommended action level of only 4 μg/L. A large number of other contaminants also fall into this chemically oxidized class, including chlorinated solvents, bromate, selenite, heavy metals and radionuclide metals. Interestingly, microbes may be used to catalyze the reduction of such oxidized pollutants, detoxifying them into reduced innocuous products. The key to microbial reduction is ensuring all components of the reduction reaction are in proximity—the bacteria themselves, the oxidized contaminants, and an electron donor substrate.
ABSTRACT
Northwestern researchers have developed a hollow fiber membrane bundle reactor to clean drinking water in a cost efficient manner. Unlike current membrane bioreactors (MBRs) that are used to treat wastewater, the Hollow-Fiber Membrane-Biofilm Reactor (HFMBfR) is the optimal environment for bringing together several components that effectively neutralize the oxidized contaminants. In a typical MBR, the membrane is essentially a solids separator yielding a solids-free effluent and concentrated sludge for recycling. Thus, the MBR application is the same as microfiltration and ultrafiltration of drinking water- namely, producing a particle-free permeate. In contrast, the HFMBfR removes oxidized contaminants chemically by bringing three critical components in proximity: (1) the oxidized contaminant; (2) the capable bacteria; and (3) H2 as the electron donor. The novel design of the HFMBfR allows for high efficiency of the electron donor, hydrogen gas, and also minimizes the formation of excess biomass. The researchers have built a laboratory-scale prototype for studies on nitrate and perchlorate reduction. The membrane fibers, which provided 750 cm2 of surface area for biofilm attachment, were installed in a tube reactor about 1 m long. Feed and recycling flow rates during the experiments were fixed. The recycle controlled the liquid flow velocity for good mass transport and to prevent fiber clumping. The system was seeded initially with Ralstonia eutropha, but a diverse mixed culture will be developed over time. The researchers have also designed and built a pilot scale HFMBfR, further demonstrating that the reactor is technically and economically feasible for reduction of nitrate and perchlorate. Given that H2-oxidizing microorganisms have the potential to reduce many oxidized contaminants, they are beginning a systematic study of how well the HFMBfR reduces bromate, selenate, chlorinated solvents, and other oxidized contaminants as well.
APPLICATIONS
ADVANTAGES
PUBLICATIONS
Rittmann B, Nerenberg R, Lee KC, Najm I., Gillogly TE, Lehman GE & Adham SS (2004) Hydrogen-based hollow-fiber membrane biofilm reactor (MBfR) for removing oxidized contaminants. Water Science and Technology: Water Supply. 4: 127-133.
IP STATUS
Issued US Patent No. 6,387,262
Schematic (left) of how the H2 diffuses through the sandwich wall of the composite membrane fibers, while biofilm naturally forms on the outer surface, where the oxidized contaminant is available from the water phase. Bundles of hollow fibers are potted together and pressurized with H2 gas (right).