Biological denitrification by heterotrophic bacteria is common in
the wastewater industry in the U.S. and in drinking water
processing in Europe. To facilitate heterotrophic denitrification,
organic compounds such as methanol, ethanol and acetic acid are
added to provide a carbon source for the bacteria. The resulting
organic carbon residual may create problems with chlorination. The
addition of these carbon compounds is expensive and results in
added sludge production. This study focused on the use of
autotrophic hydrogen oxidizing bacteria for denitrification. The
method transfers hydrogen gas to solution via microporous hollow
fiber membranes. Typically, gases are supplied to a system using
conventional bubble diffusers. The conventional bubble aeration
system has a low gas transfer efficiency, and, as a result, the
cost of dissolving the required amount of gas is very high. In this
study, microporous hollow fiber membranes were employed to supply
hydrogen gas to hydrogen oxidizing autotrophic bacteria. Laboratory
scale membrane modules were constructed and mass transfer studies
were carried out to develop the design correlations for hydrogen
gas transfer. A mixed culture was obtained and acclimated for batch
denitrification studies. Both Sodium carbonate and carbon dioxide
were used to deliver inorganic carbon. Bench scale continuous flow
biofilm reactors containing plastic media were operated to remove
nitrate from water. The required hydrogen gas was supplied at a
constant rate via gas transfer modules, containing sealed end
microporous hollow fiber membranes. The reactors were optimized for
removal of nitrate and nitrite by varying the recycle ratios and
hydraulic detention time. Experimental results indicated the
presence of hydrogen oxidizing denitrifiers in wastewater sludge.
Adequate pH control was possible and the pH averaged around 6.95.
Gas transfer studies indicated that hydrogen transfer was primarily
controlled by liquid film diffusion. Hydrogen gas was successfully
delivered to the reactor via the hollow fiber membrane gas transfer
module. Nitrate and hydrogen concentration measurements indicated
that the system did not experience hydrogen limitations at
detention times of 3.25 hours or greater. The use of hollow fiber
membrane module appears to be a viable technology for transferring
hydrogen gas to water. The research results in this report provide
valuable information for pilot and full-scale studies for the
water/wastewater community focusing on membrane processes for
autotrophic denitrification.
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