Nitrate addition

Nitrate addition is used for in situ oxidation of biode gradable organic substances and reduced substances. Nitrate improves the redox conditions in the deep

Figure 7 Aerator developed for oxygen supply of hypolimnion without disturbing thermal stratification.

water and at the sediment surface and can enhance the P fixation on iron.

Scientific background. For thermodynamic reasons the decomposition of organic matter is mainly realized by denitrifying bacteria and not by iron and sulfate reduc tion as long as high concentrations of nitrate are available. Boosting the rates of decomposition of organic matter by denitrifying bacteria can limit the availability of organic matter for iron and sulfate reduc tion so that P mobilization of iron bound P is restricted. Additionally, some genera of denitrifying bacteria oxidize reduced iron species leading to an enhanced P binding capacity.

Techniques. A solution of calcium nitrate can be added directly into the deep water by a mixing apparatus or injected into the sediment with a harrow like device, at dosages between 19 and 170 gm 2 NO3 N. In some cases nitrate was added in granular form, which permits a more gradual release of nitrate and extends the time of nitrate availability. Nitrate addition is often supported by the addition of an iron salt. Simultaneous addition of calcium hydroxide as a buffer prevents the increase of pH from the hydrolysis of iron salts. A one time addition of nitrate is not expected to lead to long lasting effects, due to the fast consumption of nitrate resulting from the large pool of degradable organic matter in the sediment and its con tinuous supply by sedimentation. A positive effect on the P binding in the sediment of shallow lakes was observed when nitrate continuously entered the lake from external sources.

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