Agriculture Reference
In-Depth Information
12.2 Microbial Treatment Technology for the Removal
of NO 3 from Groundwater
Many studies have developed various methods—physicochemical, chemical, and
biological denitrification reduction methods—to remove NO 3 from groundwater.
In the physicochemical process, the NO 3 is concentrated or spread, instead of
being removed from the groundwater, whereas in the chemical reduction method,
high concentrations of ammonia cannot be completely removed. In contrast, the
microbial method is gradually becoming a popular groundwater pollution treatment
technology because of its high-efficiency, low-cost, and complete degradation with
no secondary pollution.
The biological denitrification method refers to the denitrification process in
which NO 3 or nitrite (NO 2 ) in water is transformed to nitride and nitrogen
under hypoxic environment by facultative anaerobic bacteria with NO 3 or NO 2
as an electron acceptor, instead of oxygen. The denitrification process comprises
the following steps: NO 3 !
NO 2 !
!
!
N 2 . This process is not only a
major part of the nitrogen cycle in the ecosystem but also a main mechanism of
wastewater denitrification. In the sewage treatment system, more than 50 genera
and over 130 species of denitrifying bacteria exist, and many of them, including
Achromobacter , Bacillus , Brevibacterium , Enterobacter , Lactobacillus ,
Alcaligenes , Micrococcus , Pseudomonas , Spirillum , etc., have been reported.
The main factors affecting the biological denitrification process include the
oxygen concentration, nutrients supply, pH, and temperature. High oxygen con-
centrations can inhibit some or all of the steps of the denitrification process.
Presence of sufficient nutrients is vital for the normal growth of bacteria; among
the nutrients, C, H, O, N, S, and P elements are essential for cellular synthesis, while
minerals and trace elements in groundwater, such as K, Na, Mg, Ca, Fe, Mn, Zn,
Cu, and Co, are necessary for the growth of bacteria. The optimum pH value for the
denitrification process is 7.0-8.0; at low pH, methane bacteria dominate, whereas at
high pH, NO 2 accumulation occurs. Furthermore, the effect of temperature on the
denitrification process is very significant; low temperatures (0-5 C) reduce the
denitrification rate (except for some psychrophiles), whereas a rise in the temper-
ature to 10 C can result in a twofold increase in the denitrification rate.
The heterotrophic biological denitrification process requires the addition of
organic carbon as one of the nutrients for the denitrifying bacteria. Methanol,
ethanol, and acetic acid are the most frequently used organic carbon sources for
the denitrification process. In 1979, the flowing sand bed technique was employed
to study the removal of NO 3 in a river and proved the feasibility of this method for
the treatment of groundwater. Since then, various techniques, such as the one-way
submerged membrane reactor, continuous flow membrane reactor, rotating bio-
reactors, upflow sludge blanket reactor, Dentropur biological denitrification
process, and other groundwater denitrification reactors, have been developed for
groundwater treatment. However, a disadvantage of heterotrophic biological denitrifi-
cation technology is that it requires the addition of methanol, ethanol, glucose,
NO
N 2 O
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