Denitrification

Denitrification can be viewed in some ways as a reversal of nitrification; however, although the denitrification does go through a two-step biochemical transformation, the end product of the denitrification is not ammonia or organic nitrogen; rather, it is inert gaseous nitrogen (Liu et al., 2003).

Denitrification can only be operated under anoxic conditions when the free oxygen level is very low, but not necessarily zero, and when a carbon source, such as methanol or settled sewer (which has low dissolved oxygen), is available. The biochemical reaction characterizing the denitrifica-tion process is brought about by a wide range of bacterial genera, mostly facultative anaerobes such as Pseudomonas (P. fluorescens, P. aeruginosa, P. denitrificans) and Alcaligenes, with Achromobacterium, Denitrobacil-lus, Spirillum, Micrococcus, and Xanthomonas often present in wastewater streams (Painter, 1970; Tiedje, 1988).

The overall stoichiometric equation for denitrification using methanol as the carbon source is the following (Equation 2.13) (McCarty et al., 1969):

0.065C3H7O2N + 0.76CO2 + 2.44H2O

The maximum specific growth rate of nitrifying bacteria or denitrifiers (^m) is affected by nitrate and methanol concentrations, temperature, and pH. The growth rate of denitrifiers, is represented as a double Monod expression (Equation 2.14):

f D 1

f M 1

l kD + D

l km + D j

where D is the nitrate concentration (mg/l), kD is the half-saturation constant for nitrate (mg/l), km is the half-saturation constant for methanol (mg/l), and M is the methanol concentration (mg/l). Denitrification rate is related to the growth rate of denitrifiers as the following (Equation 2.15):

where qD is the nitrate removal rate (mg NO3-N mg VSS_1d_1) and YD is the yield (mg VSS per mg NO3-N removed). Table 2.3 lists some values of the constants in Equation 2.14.

Table 2.3. Common values for various Monod kinetic constants applicable to the denitrification process (adapted from Henze et al., 2001).

Monod Kinetic Constant

Value

20°C, organic matter

3-6 day"1

20°C, methanol

5-10 day"1

ks, MeOH

5-10 COD mg/l

ks, COD

10-20 COD mg/l

ks, NO3

0.1-0.5 O2 mg/l

YMeOH

0.5-0.65 mg COD/mg COD

ycod

1.6-1.8 mg COD/mg NO3"-N

Further Reading

Grady, C.P.L. and H.C. Lim. 1980. Biological Wastewater Treatment: Theory and Applications. New York: Marcel Dekker. Barnes, D. and P.J. Bliss. 1983. Biological Control of Nitrogen in Wastewater Treatment. London: Spon.

References

Belser, L.W. 1979. "Population ecology of nitrifying bacteria." Annual Reviews of Microbiology 33: 309-333.

Henze, M., Harremose, P., Jansen, J.L.C., and Arvin, E. 2001. Wastewater Treatment: Biological and Chemical Processes, 3rd edition. Berlin: Springer.

Hultman, B. 1973. "Biological nitrogen reduction studies as a general microbiological engineering process." In Environmental Engineering, Linder, G. and Nuberg, K., eds. Holland: D. Reidel.

Liu, S.X., Hermanowicz, S.W., and Peng, M. 2003. "Nitrate removal from drinking water through the use of encapsulated microorganisms in alginate beads." Environmental Technology 22:1129-1134.

McCarty, P.L., Beck, L., and St. Amant, P. 1969. "Biological denitrification of wastewaters by addition of organic materials." In Proceedings of the 24th Industrial Waste Conference. Purdue University, PP1271-1285.

Painter, H.A. 1970. "A review of the literature on inorganic nitrogen metabolism." Water Research 4: 393-450.

Tiedje, J.M. 1988. "Ecology of denitrification and dissimilatory nitrate reduction to ammonium." p. 179-244. In Environmental Microbiology of Anaerobes, Zehnder, A.J.B., ed. New York: John Wiley & Sons.

Food and Agricultural Waste Water Utilization and Treatment

Sean X. Liu

Copyright © 2007 by Blackwell Publishing

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