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Potable Water Biotechnology, Membrane Filtration and Biofiltration 483
anoxic conditions utilize organic substrates such as methanol, ethanol, and acetic acid for
the conversion of nitrates to nitrogen. Gaseous organic substrates such as methane and
carbon monoxide can also serve as substrates for denitrification in water. Heterotrophic
denitrification is very efficient in nitrate removal if adequate amounts of organic carbon are
available.
Autotrophic bacteria such as Thiobacillus denitrificans are capable of denitrification (33).
In autotrophic denitrification, hydrogen or reduced sulfur compounds serve as substrates, and
carbon dioxide or bicarbonate serve as the carbon source for cell synthesis (29). Autotrophic
denitrification has been booming recently due to two major advantages compared with
heterotrophic denitrification (34, 35): (a) no external carbon is needed, which reduces the
cost and risk of operation; and (b) less sludge is produced, which minimizes the bacteria in the
effluent.
Both hydrogen gas and elemental sulfur can be used as ideal electron donor for autotrophic
denitrification because they are completely harmless to human health, and no further steps are
needed to remove either excess substrate or its derivatives (35–37).
However, in drinking water, the concentration of biodegradable organic materials is
insufficient for denitrification. The shortage of organic carbon may limit the application of
heterotrophic denitrification. Thus, the reduction of NO3 – (or NO2 – ) requires addition of an
electron donor substrate, and many organic (heterotrophic denitrification) and a few inorganic
electron donors (autotrophic denitrification) are possible.
Hydrogen gas (H 2) is an excellent electron donor for autotrophic choice because of its
clean nature, low biomass yield, and relatively low cost, as well as because hydrogen gas does
not persist in the treated water to create biological instability (32, 38). The hydrogen gas can
form flammable and explosive mixtures with air. It is poorly soluble in water (1.6 mg/L at
20 C), and thus utilization of this gas in water treatment is limited (39).
Elemental sulfur is also used as electron donor for autotrophic denitrification (34). This
application is available as sulfur limestone autotrophic denitrification (SLAD) systems, which
have been studied widely in Europe and USA (35, 40–42). The denitrification efficiency can be
compared with that of the heterotrophic denitrification in the nitrate-contaminated water
treatment. In such processes, limestone is used to adjust the pH and 7.54 mg/L sulfate will be
produced stoichiometrically when 1 mg-N/L NO3 – is removed. However, hardness may
increase in treated water because of Ca 2+ produced by the limestone for pH adjustment (34).
It is reported that there is a wider application of heterotrophic denitrification in comparison
with autotrophic denitrification, especially on a full-scale level. Heterotrophic denitrification
processes possess higher specific volumetric denitrification rates (0.4–24 kg NO3–N/m 3 /day)
than autotrophic denitrification (0.5–1.3 kg NO3–N/m 3 /day). The autotrophic reaction rate is
low; thus, a large volume of reactor is required to achieve sufficient residence time for
denitrification that increases the capital cost. The disadvantage of biological denitrification is
that it requires additional carbon sources for the activity of microorganisms and consequently
some post-treatment operations for the elimination of the process contaminants (22). The
post-treatment processes include filtration through sand and active carbon beds, aeration, and
chlorination. In conventional denitrification processes these post-treatment operations are
usually carried out in sequence (22).