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G. Denitrification and aromatic compound removal from freshwater ecosystems

Many freshwater reservoirs receive nitrogen compounds transported in the systems as a consequence of different forms of human activity. These activities include: runoff from agriculture areas, fish culture ponds, and atmospheric fallout as well as point source inputs of pollutants from community and industrial wastewater discharges. Autochthonous microflora remove nitrogen through the denitrification process, and are one of the biological methods that can be used to remove nitrogen compounds from, primarily, eutrophic ecosystems. Denitrifying bacteria can be found naturally in sediments, surface waters, soils, and municipal and industrial wastes. Their numbers in freshwater and sediments rapidly increase with the diminishing redox potentials and increased availability of oxygenated nitrogen compounds. Nitrate denitrifying bacteria can follow one of the three metabolic pathways resulting in the formation of gaseous nitrogen. Two of these pathways do not result in the accumulation of nitrite, while one pathway results in a transient accumulation of toxic nitrite (Błaszczyk 1999). Maximal denitrification rates occur most often at the end of spring or during summer, and appear to vary mainly with temperature. Denitrification is especially evident in reservoirs where a significant portion of the nutrient supply comes from mineralisation of organic matter within the sediments.

Methods

Denitrification rates in lake bottom sediments are difficult to measure because they are the net result of several reactions. Nitrification, denitrification, N-fixation, and nitrate reduction to ammonia occur simultaneously in an aquatic ecosystem. Some methods for measuring denitrification rates in sediments include:

• The acetylene inhibition technique is a standard addition method as acetylene inhibits the nitrification process (Seitzinger et al. 1993).
• The ¹⁵N tracer method (Nielsen 1992, Seitzinger et al. 1993).
• The N₂ flux method, requiring a long period of pre-incubation of the sediment (Seitzinger 1987).
• The in situ chamber method in which the denitrification rate is calculated from the total N₂ flux out of the sediment, measured directly by gas chromatography (Tomaszek 2000).
• The occurrence of denitrifying bacteria, determined by means of the most probable number and plate counting methods, requiring the identification of denitrifying bacteria according to API 20NE system (Biomerieux) (Błaszczyk 1997).

Empirical relationships

There are both temporal and spatial variations in the denitrification process (Bednarek et al. 2002). The principal relationships include:

• Temporal variations in the denitrification rate determined by temperature, resulting in a linear increase in denitrification within the temperature range of 5 to 35ºC.
• Variations in the denitrification rate consequent to the occurrence of anaerobic or nearly anaerobic conditions.
• pH-related variations in denitrification rates between values of 7.0 and 8.2.
• Variations in denitrification rate due to redox potentials of less than + 100 mV.
• Rate variations as a result of the availability of oxygenated nitrogen compounds.
• A positive correlation [r = 0,86; p<0,05] between the organic matter content of the sediments and denitrification rate, and between sediment structure (as described by the quantity of organic carbon) [r = 0,84;p<0,05] and denitrification (Figure 7.8).

Fig. 7.8. The positive correlation between denitrification rate and organic carbon content of the sediments

The denitrification process is the most efficient in inundated areas and in constructed wetlands. These areas, therefore, provide the sites for processes that significantly reduce the nitrogen content of waters entering reservoirs and freshwater systems.

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