Xd 053 N03 N 032 N02 N 019 Do414

For instance, for a N03" value of 25 mg/l of nitrate-N, 0,5 mg/l nitrite-N and 3,0 mg/l dissolved oxygen, the methanol requirement can be calculated to be 64,1 mg/l from equation (4.13). The M/N ratio, which is the mg of methanol per mg of initial nitrate nitrogen concentration, is therefore 2,57 (64,1 / 25), which Is only 4 percent greater then the requirement for nitrate alone.

Most experimental data is expressed in terms of the C/N ratio, which is the mg of carbon per mg of C per mg of initial nitrate-nitrogen concentration. The ratio includes the requirements for nitrite and oxygen, which are usually small relative to the nitrate requirement.

Values of the C/N ratio required for complete denitrification range from 1,5 to 5.

Table 4.4 show C/N ratio for different types of carbon sources used to perform denitrification. It has been suggested that column denitrification systems require a lower C/N ratio than suspended growth systems due to the higher concentration of biomass maintained in the column systems.

Higher biomass levels produce longer solids retention times and reduce organism yields due to increased endogenous metabolism. In turn this lower yield would result in less carbon required for synthesis and reduced C/N ratio.

In general, a C/N ratio of 2 to 3 will enable "complete denitrification" (95 % removal of nitrate) and this value may be used for design purposes when methanol is used as the carbon source for denitrification. Fig. 4.1 show the C/N ratio using methanol as carbon source as a function of the denitrification, in two different studies for submerged filters. The dotted line is the theoretical C/N ratio needed for total denitrification.

Table 4.4 C/N ratio for different types of carbon sources used to perform denitrifi-cation.

Organic matter

C/N optimum

Unit

as internal source

3-3,5

kg BOD/kg N

4-5

kg COD/kg N

in sludge

1,5-2,5

kg BOD/kg N

2,9-3,2

kg COD/kg N

Methanol

2,3-2,7

kg MeOH/kg N

3,5-4,1

kg COD/kg N

Acetic acid

2,9-3,5

kg HAc/kg N

3,1-3,7

kg COD/kg N

0 0

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