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2.2 WATER QUALITY IMPROVEMENT TECHNOLOGIES
2.2.1 Denitrification of Groundwater
Denitrification is the process whereby nitrogen is removed from water.
When employed in water quality improvement technologies, denitrification
treats water to reduce its nitrate-nitrogen content to potable levels.
There are three principal approaches to nitrate removal: ion exchange,
chemical reduction and biological denitrification. The first two are
well-documented in various publications (Gauntlett, 1975; Metcalf, 1975)
and are not, therefore, further described here.
With biological denitrification, aerobic heterotrophic bacteria, under
anaerobic conditions, utilise the oxygen molecules that, together with
nitrogen, form nitrate. This oxygen is used in place of dissolved
elemental oxygen. Removal of the oxygen molecules converts nitrate to
nitrite, ammonia or nitrogen gas. This process is common in water-logged
soil and other aquatic environments. Within the organisms, the
nitrate-derived oxygen acts in the same manner as elemental oxygen, as an
acceptor for electrons and hydrogen. Chemical energy to drive the process
is added in the form of organic carbon, methyl alcohol, ethanol and acetic
When this process is used to treat water, there is need to create an
environment for the bacteria that mimics the soil conditions in which this
process occurs naturally. This environment is generally created in
biological reactors, like attached growth reactors (packed columns,
rotating disc units and fluidized bed columns), suspended growth reactors
and underground reactors for the treatment of nitrates, in order to bring
the denitrifying bacteria into contact with the water to be treated
(Figure 37). Generally, also, the chemical energy needs to be added
artificially to the system to stimulate the denitrification process. Thus,
a denitrification plant, in its most basic form, comprises an injection
well for adding nutrients, a biological reactor, and a pumping well for
the abstraction of treated water.
Extent of Use
This technology has been used in projects in South Africa.
Operations and Maintenance
Denitrification technologies have high energy requirements associated
with the suspended growth reactors. Electrical energy is required in order
to keep the bacterial floc in suspension by stirring. Chemical energy
sources are also required in many systems. The use of methyl alcohol as a
carbon source has economic and operational advantages because of the
resulting low solids production (Dahab, 1988).
With underground biological denitrification, problems may arise as
aquifer pore spaces clog with biological matter. In such cases, the
recommended approach is to undertake denitrification in an above-ground
biological reactor with underground recirculation of the treated water as
a secondary treatment.
|Figure 37. Groundwater denitrification unit (Latimela,1993).
(A) Sectional view of underground denitrification.
(B) Sectional view of aboveground denitrification with groundwater recharge.
(C) Plan view of sections.
Level of Involvement
This technology requires specialised skills and knowledge to operate
effectively. Thus, while communities can participate by funding the units
and their installation costs, government participation is required to
design, install, operate and maintain the systems.
Biological denitrification has been found to be cheaper than ion
exchange (Latimela, 1993). The treatment costs for a design capacity of 20
m3/day were found to be $0.26 /m3. The capital and operating costs for
biological denitrification for a 10 m3 /day capacity were found to be:
|capital cost /m3
|running cost /m3
|treatment cost /m3
This technology is suitable for the reclamation of nitrate-contaminated
Denitrification technologies remove nitrate from the waters to reduce
the risk of methaemoglobinaemia in infants and others. Use of biological
treatment methods do not require regeneration of media and, hence, there
is no problem of brine disposal. With underground denitrification, both
denitrification and secondary treatment are performed in situ, reducing
the need for infrastructure.
Because this is a biological system, there will be fluctuations in
quality. In some cases, there may be a sensitivity within the service
population to bacterial toxins. A large bacterial population, free of
pathogens, has to be developed. If the system breaks down, this bacterial
mass may be lost. Should this happen, no further treatment of water is
possible until the bacterial population is reestablished.
Further Development of the Technology
Further studies are essential to determine the potential of aquifer pore
spaces to clog with biological matter under operational conditions, and to
identify suitable remedial measures to overcome this problem short of
reconstructing the system as an above-ground reactor. Development is also
needed to ensure a more constant output water quality from these systems.
Dahab, F. and Y.W. Lee 1988. Nitrate Removal from Water Supplies Using
Biological Denitrification. Journal of the Water Pollution Control
Federation, 60 (9):1670-1675.
Latimela, O.N. 1993. Denitrification of Ground Water for Potable
Purposes. Water Research Commission Report No. WRC 403/1/93, Pretoria,
Pelczar, M.J., E.C.S. Chan and N.R. Krieg 1986. Microbiology,
5th Edition. McGraw-Hill Book Company, Boston.
Gauntlett, R.B. 1975. Nitrate Removal from Water by Ion Exchange.
Water Treatment Examination, 24 (3), 172-193.
Metcalf and Eddy, 1979. Wastewater Engineering: Treatment, Disposal
and Reuse, 2nd edition. McGraw-Hill Book Company, Boston.