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<Sourcebook of Alternative Technologies for Freshwater Augumentation in Africa>

2.2 WATER QUALITY IMPROVEMENT TECHNOLOGIES

2.2.1 Denitrification of Groundwater

Technical Description

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 acid.

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.

A
Figure 37
B
Figure 37
C
Figure 37
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.

Costs

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 $0.33
running cost /m3 $0.07
treatment cost /m3 $0.40

Suitability

This technology is suitable for the reclamation of nitrate-contaminated groundwater.

Advantages

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.

Disadvantages

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.

Information Sources

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, South Africa.

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.

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