Alternative Electron Donors and the CN Relationship

As shown in section 4.3, (equations 4.1 and 4.2), the denitrification process needs an electron donor to be accomplished.

A variety of compounds that can substitute for methanol as a carbon source have been evaluated experimentally and described in the literature. Table (4.5) shows the wide variety of carbon sources which have been used experimentally other than methanol and internal carbon.

The selection of an electron donor depends upon three factors which will be discussed in this section: availability of the electron donor, the reaction rate, and costs. The combination of a high reaction rate and moderate costs is achieved by the use of methanol.

Denitrification rates achieved with waste water organics, also called the internal carbon source, are approximately one third of those achieved when methanol is employed as the electron donor; this is because the availability of the electron donor is one of the most important factors controlling the activity of the denitrifiers. If the availability of the electron donor fluctuates, then the performance of the denitrification will also fluctuate, yielding a lower denitrification rate.

Denitrification reactors must, therefore, be proportionately larger using an internal carbon source than when methanol is used.

Volatile acids have also been used as a carbon source for denitrification. (Climen-hage 1982). In studies of nitrate reduction in waste water generated in the manufacture of nylon, is was found that a mixture of C1 to C5 volatile acids was very effective as a carbon source for denitrification.

It is also possible to use inorganic compounds as electron donors. Hydrogen and sodium sulphide have been used in these experiments (Kurt et at. 1987).

Some of the alternative carbon sources cause greater sludge production than others. About twice as much sludge is produced per mg of nitrogen reduced when saccharose is used, than when methanol is employed, because the yield coefficient of the bacteria using the first carbon source is greater.

On the other hand, acetone, acetate and ethanol produced similar quantities of sludge to that produced when methanol was employed.

Methanol has certain advantages over carbon sources in waste water. It is free of contaminants such as nitrogen, and can therefore be used directly in the process without taking special precautions that must be made for the use of a system with an internal carbon source. Using a external carbon source produces a consistent quality, while waste water sources may vary in strength and composition, either daily or seasonally, which complicates both process control and optimization. Use of waste water sources will require regular assays of the source to check its purity, and strength and its biological availability.

The disadvantage of using methanol is its cost, and this alone advocates the necessity of economic comparisons of alternate carbon sources.

Denitrification is considered to be a heterotrophic process, conducted by microorganisms that require a reduced organic substrate for energy and cell synthesis.

Heterotrophic denitrifying microorganisms can use a variety of organic carbon sources, while most of the published reseach regarding the denitrification of water, presumes the use of methanol, ethanol and acetic acid.

Figure 4.2 show the denitrification reaction rate as a function of temperature for different carbon sources. The more easily degradable the carbon source, such as methanol is, higher is the reaction rate. Heavily degradable endogenous carbon has a low reaction rate, especially at low temperature.

Carbon Sources For Denitrification
Fig. 4.1 C/N ratio using methanol as carbon source, in two different studies indicated as x and o, as a function of the denitrif¬°cation efficiency, for a submerged filter. The dotted line is the theoretical amount (Source: Henze and Harremoes 1978)

The stoichiometric relationships for these substrates have been formulated as follows:

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