MBfR TECHNOLOGY OVERVIEW
Applied is commercializing a membrane biofilm reactor (MBfR) technology that destroys oxidized contaminants that are not treatable with Applied’s HiPOx technology. Target compounds are perchlorate (ClO4־) and nitrate (NO3־). Other oxidized contaminants that can be treated with the MBfR technology include bromate, chromate and other heavy metals, selenate, radionuclides and some chlorinated organic solvents.
MBfR technology removes oxidized contaminants from water through microbial catalyzed reduction— transferring electrons to the contaminants at microbial “transfer points.” The key ingredients to such a process are an oxidized contaminant (electron acceptor), suitable microorganisms to serve as a transfer point, and a source of electrons. In the case of the MBfR, the electron source is hydrogen gas (H2).
Hydrogen gas is ideal because it is the least expensive electron donor source available. Historically it has not been widely used in water treatment operations due to explosion hazards associated with the potential presence of hydrogen gas bubbles in the treatment process. This is where Northwestern University’s MBfR technology represents a scientific breakthrough.
The MBfR is designed to transfer hydrogen gas and electrons to oxidized contaminants without forming bubbles. This is accomplished by delivering hydrogen gas to the inside of a special hollow fiber membrane where the gas diffuses, under pressure, through the wall of the membrane.
The fiber's wall is a special composite membrane that contains a 1-µm thick nonporous, hydrophobic polyurethane layer sandwiched by micro-porous polyethylene walls. The dense polyurethane layer allows a slightly pressurized gas to diffuse through the membrane without forming bubbles.
During water treatment, oxidized contaminants are present in the water on the outside of the membrane. A biofilm naturally forms on the outside of the membrane, as this is where the oxidized contaminant and hydrogen molecules come in contact. When the oxidized contaminant and the hydrogen molecules that have diffused through the membrane wall come in contact, the microbes in the biofilm first oxidize the hydrogen gas and then reduce the oxidized contaminants to obtain energy for growth.
The oxidation of hydrogen gas is captured in the following equation:
H2 → 2H+ + 2eˉ
The subsequent microbial reduction of nitrate and perchlorate occur as follows:
NO3ˉ + 2.5H2+ H+ →0.5N2 + 3H2O
ClO4ˉ + 4H2 → Clˉ + 4H2O
Nitrate contaminants are converted to nitrogen gas and water. Perchlorate contaminants are converted to chloride ions and water. Experiments have shown that the effluent perchlorate or nitrate concentration and percent contaminant removal can be controlled easily and rapidly by adjusting the H2 pressure.
The hollow fiber membranes have a high specific surface area. The high specific surface area means a high density of biofilm can form. This will reduce the required residence time for contaminated water as it passes along the outside of the MBfR and reduce the overall size and footprint of the MBfR treatment unit for a given flowrate. This will translate to lower capital and operating costs for the system.
The MBfR is highly efficient, using nearly 100% of the applied H2 gas to reduce the oxidized contaminants. This both eliminates the risk of H2 off-gas explosion hazards seen with other hydrogen-based systems and translates to exceptionally cost-effective operation. Hydrogen gas has other major advantages as an electron donor. It is non-toxic to humans. In addition, hydrogen gas evolves from water that has an open surface, thereby eliminating a residual that contributes to oxygen demand, biological instability, or disinfection byproducts.
Because it is hydrogen gas, not water, which passes through the hydrophobic hollow fiber membrane, the system is not susceptible to bio-fouling. While an excess biomass is produced, the volume of material produced will be far less than for heterotrophic systems due to the low true yield (autotrophy) and the low detachment rate (high SRT). This will translate to lower total operating costs.
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