Nitritation and Autotrophic Nitrogen Removal in a Hollow-Fiber Membrane-Aerated Biofilm Reactor

Kevin R. Gilmore1, Nancy G. Love1,2, Barth F. Smets1,3

Department of 1Civil and Environmental Engineering and 2Department of Biological Sciences, Virginia Tech, Blacksburg, VA 240613Department of Environment and Resources, Danish Technical University, Lyngby, Denmark

Abstract and IntroductionSpecialized wastewaters, such as those encountered in confined space station or planetary base environments, can be characterized by high reduced nitrogen concentrations. In such situations, where water re-supply from earth is prohibitive or impossible, bioregenerative technologies will be essential for sustaining long-term manned space exploration. A hollow-fiber membrane-aerated biofilm reactor (HFMBR) was applied to treatment of a high nitrogen wastewater designed to mimic a concentrated urine waste. Membrane aeration provides a means for bubbleless aeration, which is critical for operation in a microgravity environment. In addition, the membranes provide a substratum for growth of an ammonia-oxidizing biofilm. A conceptual model of the membrane-aerated biofilm is shown. Oxygen mass transfer in the reactor was evaluated using clean water transfer tests, and aeration rates were similar to values published in literature for similar systems (1, 2) The mass transfer coefficient was found to be influenced primarily by intra-lumen gas pressure but very little by gas flow rate. Oxygen transfer efficiencies were, however, influenced by both lumen pressure and gas flow rate. The system was then operated for 90 days. Stable nitritation (conversion of ammonia, NH3, to nitrite, NO2

-) occurred for 60 days. During that time, oxygen consumed was less than stoichiometrically predicted. A nitrogen balance showed 18% of the influent nitrogen was unaccounted for in the liquid and gas effluents. This unaccounted nitrogen was presumably comprised of N2 and N2O gases, the only species not measured. After 70 days, nitrite oxidizing bacteria (NOBs) appeared to successfully colonize the reactor, as evidenced by the appearance of nitrate (NO3

-). Colonization of NOBs presents a challenge to the ultimate goal of the project: denitritation of an organic wastewater fraction using NO2

- as the electron acceptor instead of the traditional pathway via NO3

-.

Hollow-Fiber Membrane Aerated Biofilm Reactor System (Modeled after (3))

Membrane Module: 98 SilasticTM fibers 3.18 mm O.D. 292 mm length Total membrane area = 0.288 m2

Continuous on-line monitoring for pH and oxidation-reduction potential (ORP)

Automatic pH control controlling base addition (NaHCO3)

Dual-tank anaerobic feed system, sparged and pressurized with 95% N2 / 5% CO2 gas mix

Air mass flow controller, delivering air to membrane lumens

Base feed system, controlled via automatic pH controller

Compressed air to membrane lumens to provide oxygen for nitritation

95% N2 / 5% CO2 gas mix for sparging and pressurizing feed to maintain anaerobicity

Refrigerated effluent composite sampling

External flow-through instrumentation cells for pH, ORP, and DO

Constant temperature room, maintained at 30ºC

Oxygen mass transfer coefficient (KOL) was found to be independent of air flow rate but highly dependent upon intra-lumen air pressure.

Oxygen transfer efficiency, however, was a function of both air flow rate and intra-lumen pressure.

Conceptual model of nitritation and autotrophic nitrogen removal in a membrane-aerated biofilm.

Stable nitritation was sustained through day 60 of operation, after which nitrite-oxidizing bacteria (NOBs) colonized the reactor system.

Engineering SignificanceThe application of this process to wastewaters high in reduced nitrogen offers the potential for minimized aeration costs by nitrifying only to nitrite. Furthermore, if stable denitritation can be established, additional cost savings will be realized by the reduced carbon source requirement for denitritation as compared to denitrification from nitrate. The deterioration of stable nitritation poses a challenge for continuation of the research and introduction of an organic carbon wastewater component. The evolution of harmful gaseous species such as NO2 and NO are an important observation of the research to date. Should this technology be employed in space mission environments, provisions must be made to monitor for and remove these harmful gases. In earth-based treatment applications, the generation of N2O, which is expected to account for much of the unrecovered nitrogen, is of concern due to its potency as a greenhouse gas. Operation of such systems to minimize emissions will be important for environmentally sustainable wastewater treatment.

References1. Hibiya, K., A. Terada, S. Tsuneda, and A. Hirata. 2003. Simultaneous nitrification and denitrification by controlling vertical and horizontal microenvironment in a membrane-aerated biofilm reactor. J. Biotech. 100:23-32

2. Hummerick, M. 2005. Personal Communication.

3. Rector, T., J. Garland, R. F. Strayer, L. Lanfang, M. Roberts, M. Hummerick. 2004. Design and Preliminary Evaluation of a Novel Gravity Independent Rotating Biological Membrane Reactor. SAE Technical Paper Series 2004-01-2463. 34th Intl. Conf. on Environ. Systems. SAE, Colorado Springs, CO.

4. Voss, M. A., T. Ahmed, M. J. Semmens. 1999. Long-term performance of parallel-flow, bubbleless, hollow-fiber-membrane aerators. Wat. Env. Res. 71 (1):23-30.

Acknowledgements: The primary author would like to thank Dr. Jay L. Garland of Dynamac Corporation/NASA Kennedy Space Center for advising on this project, Kristina Reid-Black of Dynamac Corporation/NASA Kennedy Space Center for assistance in constructing the membrane module, Jody Smiley and the laboratory of Linsey Marr for assistance in measuring NOx and other gaseous nitrogen species.

R2 = 0.9847

R2 = 0.983

0%

20%

40%

60%

0 20 40 60 80

Air Flow, cc/min-m2

OT

E, %

1 psi

2.5 psi

5 psi

7 psi

7.5 psi

10 psi

0

1

2

3

4

0 2 4 6 8 10 12

Lumen Pressure, psi

KO

L,1

04 c

m/s

ec

16 cc/min-m2

32 cc/min-m2

64 cc/min-m2

Influent Nitrogen Partitioning

NO2-,

0.3%

NH3,

99.7%

NH3

NO2-

Effluent Nitrogen Partitioning, Days 24-58

Unknown, 17.9%

NO2-,

73.8%

NH3, 4.9%

NO2, 1.0%

Biomass, 2.0%

NO3-, 0.4%

NO, 0.1%

NH3

NO2-

NO3-

Org. N

NH2OH

N2

NO

NO2

N2O

Biomass

Unknow n

Nitrogen BalanceCritical to understanding the metabolic processes in the system is the tracking and accounting of nitrogen species in the influent and effluent. During the stable nitritation period, approximately 18% of the influent nitrogen could not be accounted for and was presumed to be N2 and N2O. The gaseous species NO and NO2 are both toxic, and evolution of these species is an important consideration for application of this technology to confined environments, such as space missions. Current efforts underway to analyze for both N2 and N2O are expected to show generation of N2O, a potent greenhouse gas.

Bulk Liquid

Membrane Lumen

AirO2

NH3 NO2-

Nitritation

NH3 + O2 NO2- Biofilm

Autotrophic Denitrification

NO2- NO

NO2

N2O N2

Gases: NO, NO2, N2O, N2

Gases: NO, NO2, N2O, N2

0

200

400

600

800

0 20 40 60 80 100

Day of Operation

NH

3-N

, NO

2- -N, N

O3- -N

, mg

/L

Feed NH3-N Eff NO2- Eff NO3-

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