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Salt Inhibition Effects on Simultaneous Heterotrophic/Autotrophic Denitrification of High Nitrate Wastewater
Aminzadeh, B.; Torabian, A.; Azimi, A. A.; Nabi Bidhendi, Gh. R. & Mehrdadi, N.
Abstract
Denitrification of high-nitrate high-salinity wastewater is difficult due to plasmolysis
and inactivation of denitrifiers at high salinity conditions. In this study, the effects of salinity and
empty bed contact time (EBCT) on simultaneous heterotrophic and sulfur based autotrophic
denitrification of synthetic wastewater were evaluated in an up flow packed bed reactor .The reactor
was filled with granular elemental sulfur particles with diameters of 2.8-5.6 mm and porosity of 40%.
The initial culture was prepared from sludge of Shahrak-e-ghods domestic wastewater treatment
plant. The influent nitrate concentration and EBCT were 600 mg NO3-N/lit and 16 h respectively.
First, the stoichiometric fraction of nitrate removed by heterotrophic denitrification (with methanol
as organic carbon source) supplied enough alkalinity to compensate the autotrophic alkalinity
consumption, was determined 60%. Then, salt concentration was gradually increased with NaCl
from 0% in the feed. The Process kept high nitrate removal efficiency (>99%) even at 3.5 % NaCl.
During these changes the alkalinity variations were insignificant which showed the microbial
population ratio of acclimated autotrophic to heterotrophic denitrifiers had no any significant changes
with NaCl concentrations up to 3.5% in the feed. At 4 and 5% NaCl, the efficiency drastically
decreased to 78% and 48%, respectively. Similar behavior was also observed for methanol removal
efficiency, effluent turbidity as an indirect determinant of biological mass and sulfate production.
The effects of flow rates on denitrification of synthetic high nitrate high salinity wastewater with
3.5% NaCl under mixotrophic condition were also investigated by increasing the flow rate from 7.06
lit/day to 70.6 lit/day with corresponding EBCT 20 to 2 h. Denitrification efficiency was close to
100% at EBCT of 20 to 8 hr, but decreased to 79% and 39% when the EBCT was 4 and 2 h, respectively.
The decrease in effluent sulfate concentration (as an indicator for autotrophic denitrification) and
the increase in effluent alkalinity (as an indicator for heterotrophic denitrification) and pH at EBCT
of 4 and 2 h were considerable correspondingly. These results imply that the population ratio of
autotrophic to heterotrophic denitrifiers depends on EBCT.
Keywords
Autotrophic, Heterotrophic, Denitrification, Saline wastewater, Nitrate, Sulfur, Packed bed reactor
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